7.4 Mercruiser running hot at rpm and burning up exhaust boots

[Lennyd123]

On the first real trip this year after finally getting my mechanic to pull the engine to correct a vibration problem from a flywheel that was installed in the wrong positions my 7.4 B1 Merc was running very well, but when running at speed it quickly filled the bilge with smoke, and upon inspection it had melted the lower rubber exhaust boot.

This is a recently rebuilt "rebuild" that had to be redone due to either the rebuilder or original marina not getting things exactly right and nearly everything except the transom assembly is now new or rebuilt. So yes it has a new impeller, the correct intake thick wall hose, and the circulating pump though approx 3 yrs old now has seen less than 5 hours run time.

It seems my engine is not getting enough water in through transom, and though it runs cool up to around 1500rpm (145f) it gets hot on the gauge quick @ 3000 (approx 170-180) and when I ran it good @ 4000 it shot up to 200f but even with the toasted boot the engine itself did not overheat (thankfully). It does cool down quickly once it is run slow

I did change the risers as they were starting to close up a bit, were cleaned out once before in 09, and were over ten years old etc as a precautionary measure, but I guess you need to get in enough water before you need to be worried about getting it out

I am thinking it is not enough water based on what I had seen coming through this hose in the past when having to change out the impeller as it is much less than before. It is not a trickle, but just a slow steady flow etc.

By recommendation of my current mechanic I have already ordered the small hose that goes from the transom to the gimbal and the grommet or flange that holds it in, but I have also been told that these do not normally go bad, and that it is very likely that there may be some growth or obstruction inside the drive itself.

Since it is looking like I will have to be doing this myself due to my mech being busy I wanted to reach out to you guys to get your opinions on what to expect etc. I hear the hose is a real pain if there is much corrosion (this is a 89 so it would not surprise me) and that more than one has been converted to a thru hull intake.

I have also never split the cases on a Bravo before and wanted to learn what to expect, and if there are any seals or other parts needed to re assemble it?

I have a few days as I am waiting on borrowing a trailer to haul it, but since I am borrowing it I want to be sure I have as many parts as I can before removing the drive since I can not keep it on the trailer too long either.

One odd thing was that it did seem to get enough water not to burn the starboard boot, and only the port one. Any ideas on this?

Thanks in advance!!

 

[04fxdwgi]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

View attachment 104624

On the first real trip this year after finally getting my mechanic to pull the engine to correct a vibration problem from a flywheel that was installed in the wrong positions my 7.4 B1 Merc was running very well, but when running at speed it quickly filled the bilge with smoke, and upon inspection it had melted the lower rubber exhaust boot.

This is a recently rebuilt "rebuild" that had to be redone due to either the rebuilder or original marina not getting things exactly right and nearly everything except the transom assembly is now new or rebuilt. So yes it has a new impeller, the correct intake thick wall hose, and the circulating pump though approx 3 yrs old now has seen less than 5 hours run time.

It seems my engine is not getting enough water in through transom, and though it runs cool up to around 1500rpm (145f) it gets hot on the gauge quick @ 3000 (approx 170-180) and when I ran it good @ 4000 it shot up to 200f but even with the toasted boot the engine itself did not overheat (thankfully). It does cool down quickly once it is run slow

I did change the risers as they were starting to close up a bit, were cleaned out once before in 09, and were over ten years old etc as a precautionary measure, but I guess you need to get in enough water before you need to be worried about getting it out

I am thinking it is not enough water based on what I had seen coming through this hose in the past when having to change out the impeller as it is much less than before. It is not a trickle, but just a slow steady flow etc.

By recommendation of my current mechanic I have already ordered the small hose that goes from the transom to the gimbal and the grommet or flange that holds it in, but I have also been told that these do not normally go bad, and that it is very likely that there may be some growth or obstruction inside the drive itself.

Since it is looking like I will have to be doing this myself due to my mech being busy I wanted to reach out to you guys to get your opinions on what to expect etc. I hear the hose is a real pain if there is much corrosion (this is a 89 so it would not surprise me) and that more than one has been converted to a thru hull intake.

I have also never split the cases on a Bravo before and wanted to learn what to expect, and if there are any seals or other parts needed to re assemble it?

I have a few days as I am waiting on borrowing a trailer to haul it, but since I am borrowing it I want to be sure I have as many parts as I can before removing the drive since I can not keep it on the trailer too long either.

One odd thing was that it did seem to get enough water not to burn the starboard boot, and only the port one. Any ideas on this?

Thanks in advance!!

My '92 7.4L got hot on the port side also (but not that hot and the engine temp stayed at 160). The problem I found was the check valve ball in the thermostat housing "T", feeding the riser had stuck in the bore, choking off the water flow to the riser. Pulled out the check ball, cleaned everything up and was good to go. Your situation sounds a bit more dire and has some sort of restriction causing lack of flow thru the raw water stystem, causing whole engine to heat up.

Looking at your pic, I'm having a difficult time seeing if your shutter is in place in the "Y" pipe. Hoses being cooked as they are, I would plan on new shutters in exhaust also. I did find some old ones down inside my "Y" pipe, partially restricting the exhaust flow and, luckily, was able to fish them out with a wire. Runs much better now.

I found a Mercruiser article that said the raw water pump should pump about 7 1/2 qts of water in 15 seconds at 2,000 RPM, when pump discharge is disconnected from engine (don't do this longer than the 15 seconds recommended). If yours does that volume, then blockage is between pump and exhaust at transom, if it doesn't, there is a restriction in the raw water feeding the pump, providing pump is good.

The hose from drive unit to the transom was suspect (but was new), so, while drive was off, I put a wire up thru it and it was clear. When the wire hits the transom, it'll stop and can't make the 90 degree bend at the transome fitting. Get the gasket for the elbow inside on transom plate, pull off hose, pull off elbow and inspect (stick finger in there to see if it's clear), put on new gasket and reinstall if all is good. They say lots of corrossion can happen at that elbow, but mine is made from polymer and can't see how it can corrode. PS: Kind of tough getting in back there and being a contortionist helps....... Mine was easier to get to from Port side of engine and risers / exhaust hoses were off.

I just did the drive removal, split the lower from upper and re-assemble. Process is quite easy and all O-rings are available in a kit from Sierra for about six bucks, plus you'll need drive oil and a pump to re-fill drive. Oh yeah, have the manual handy and read everything before you start.

This evolution is like a search and destroy mission, checking every component / passage, one at a time. Don't get frustrated if you don't find something right away. PS: Even check the inlet side of power steering cooler on the port side of engine, and fuel and oil coolers if you have them. If an impeller had failed at some time, pieces of the old impeller could be lodged inside the cooler tube nest, but they are usually ouside the tubes, blocking flow thru cooler.

 

[Lennyd123]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

Wow I just lost a seriously long winded reply to this.

The important parts were to say thanks, advise that the problem is definitely before the pump, that the check balls were cleaned up earlier in the year, the hook up or elbow on the inner transom plate is very accessible on this set up, and that I decided not to remove it until I confirmed that there problem was there as I was told the bolts can be a real problem and snapping them is not uncommon.

Since you have done the drive before were there any problem areas or special attention needed? It does look pretty straight forward and just remove four nuts, remove growth, install new o-rings, and install the four nuts again. Is this correct?

The hose that mounts on the gimbal seems a bit more involved due to the really strange design. I know there is a tool to remove and install the small bushings that hold the hose in, but if these are clogged or damaged from corrosion in the aluminum of the transom mounting point it looks like it will be a serious pain in the you know what.

I hope that it is just growth in the drive as that hose looks like it would make me nuts lol. Then I read about people just going to the thru hull pick up, and keep thinking how that really makes more sense than trying to feed a 1-1/4" hose leading to the pump with the 5/8 or 3/4 hose inside the transom assembly.

Maybe I should be asking about proper mounting positions in the hull for the thru hull, and any problems typical to this model like wood in the hull bottom or something that would make that process a total mess (this boat is over 20yrs old and searay is not known for doing the best job sealing up the wood in the hull are they now lol)

 

[Lennyd123]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

Well it took a little time, but was able to haul it and remove the drive and inspect the hose between the gimbal and transom as well as the internal passage through the drive.

It was looking good at the back end as except for some cooked up remains of the butterfly in the exhaust in the drive all was clear.

Once I pulled the the two bolts from the inside my problem was obvious. What used to be the ID of the hose had become what looked like a solid piece of rubber. Couldn't even push a 3/8 drill into it (was going to try to just ream it out lol) and am going to see if it is not so bad that it will be a real PITA to replace, and if it gets ugly once removing the plastic pushing in the hose it is looking like through hull time.

Would figure the only original part left (the transom/gimbal assy) has to give me trouble.

 

[04fxdwgi]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

Glad to hear you found the problem, even though it could render the drive pickup useless. Can be fixed, but I can see it being a royal pain in the arz with the engine in place and a thru hull may be the way to go. Bravo's did, at one time, use a transom mounted pickup for the hi performance setups and the factory parts are still available. May be the way to go. My boat is a trailer boat and I get nerveous about any thought of a thru hull and transom space is getting scarce since I put on the Bennett 12x16 Sport Tabs and have the sonar transducer and such down there.

Fortunately for me, the previous owner had the entire gimbal assy replaced 6 months before selling the boat and all the stuff back there was replaced brand new (including the coupling) when the engine was out.

Hope you get her back together and enjoying the water in a quick fashion.

 

[HT32BSX115]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

Glad to hear you found the problem,

He may not have found all the problems though.

You have an oil cooler that is in the raw water circuit before the T-stat housing. It has VERY small tubes/holes in it that will become blocked by raw water pump impeller fragments.


Depending on how much debris is in there it can be partially or fully blocked.

Mine was so plugged that I am amazed that any water got past it. You have to remove it to clear it out.


Regards,


Rick

 

[Lennyd123]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

He may not have found all the problems though.

You have an oil cooler that is in the raw water circuit before the T-stat housing. It has VERY small tubes/holes in it that will become blocked by raw water pump impeller fragments.


Depending on how much debris is in there it can be partially or fully blocked.

Mine was so plugged that I am amazed that any water got past it. You have to remove it to clear it out.


Regards,


Rick

First it kind of sucks that the only parts outside of the hull itself and interior that are not new is the transom and gimbal ring assemblies.

Of course the one remaining part is going to create grief.

Anyhow I did pull have the cooler rebuilt (I guess tanked and resealed as it did have a leak) last year when the re rebuilt engine was installed, and yesterday pulled the hose leading to it and found nothing but a couple strands of seaweed. I did not pull it and check each individual tube.

MORE IMPORTANTLY I did do some work and testing on it this morning .

Was able to get a uni bit on a angle drill to remove enough of the hose and bushing that it should flow enough water now, BUT in the process also made a small hole or something that some water will come out if you remove the drive and apply a hose to the hose in the outside of the gimbal. It flows through both ways and enough to make me happy it would get enough water, BUT there is another issue in that if run on the ears it is not exhausting any real amount of water when running, but with the drive removed and the water hose pushed against the inlet in the gimbal it works fine.

My thinking is that the small hole is causing it to suck air and lose whatever suction is needed to keep the pump primed and water flowing up through the drive and into the hoses etc.

Was originally going to just epoxy or silicone the ID of the hose so it would seal the hole etc, and just launch it till winter and then deal with either the proper hose replacement or like many have advised just convert to the thru hull intake.

I am not sure that I shouldn't just go pick up the damn thru hull parts now and get it over with today. That way it will be finished and over with.

I guess the real problem that I have is somehow trying to keep the system original and that the thru hull may be a turn off to potential buyers when it comes time to sell (actually was moving up in 05 when this all started so I am more than ready except for the funds lol).

Much as I like this boats set up and the amount of room top sides etc I am starting to really think my initial thoughts when the motor was seized back in 05 of just selling it off as is and getting what I wanted then may have been the best thing I ever did. Too late now, but unless I have all the proper tools (hoists etc) to remove an engine and do the job myself in the future I am pretty sure that will be the way to go.

Making the final decision now to go with the thru hull or just repair the hose best as I can and get her wet till winter time. Hope I get it right this time!!

 

[HT32BSX115]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

My thinking is that the small hole is causing it to suck air and lose whatever suction is needed to keep the pump primed and water flowing up through the drive and into the hoses etc.

If you have ANY hole in the raw water line that can suck air, IT WILL. and you will lose prime.

Most Bravo installations also have the pump at or slightly below the water line (mine is below). If there's a hole in the raw water line, it will leak into the bilge (and possibly sink the boat!!)

 

[Lennyd123]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

I think your right on it sucking air, and that is the only thing that makes sense to why it is moving plenty of water one way and not when on the ears etc.

The raw water pump looks to be just below the water line, but the hole is outboard of the transom or actually in the transom assy and would therefore would not put water into the bilge.

Now with how things have gone with this previously I would not be surprised if the inlet or neck that the hose leading to the pump attaches to ended up leaking, but if so hopefully not enough to sink anything.

 

[Lennyd123]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

Can anyone advise if it would be ok to use a bronze ball valve as opposed to a true sea ****?

I ask because it seems no one local stocks the seacock, and I want to be sure they are not just selling me etc when they advised it would be OK and that everyone uses them etc.

I do want to be able to shut off the flow as it makes sense for safety, but I also figure I will install a hook up for a garden hose to use as a flush to remove salt from the system and also so it can be run on land.

 

[04fxdwgi]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

Can anyone advise if it would be ok to use a bronze ball valve as opposed to a true sea ****?

I ask because it seems no one local stocks the seacock, and I want to be sure they are not just selling me etc when they advised it would be OK and that everyone uses them etc.

I do want to be able to shut off the flow as it makes sense for safety, but I also figure I will install a hook up for a garden hose to use as a flush to remove salt from the system and also so it can be run on land.

Bronze is the way to go..... BUT, the valve should be installed at the thru hull intake. The sea cocks are generally used because there is normally a raw water strainer in the bilges and there needs to be a means of closing the intakes to clean the strainer. If there is a hose between the thru hull and valve, there is also the potential of developing a leak in the hose and no way of shutting it off while in the water, turning the boat into a submarine. I know, all sterndrives have that situation and don't sink on regular occassions because of it, but why not add the safety margine.

On another note, I installed a manual flushing manifold in the 1 1/4" hose, between the transom fitting and raw water pump, so I can flush engine without the use of the muffs on the drive unit. Have tried 3 sets of muffs and none would seal on the drive unit enough to force the water through the drive and plumbing to the pump to pull a good prime. It's a no brainer now, with everything done thru the engine bay and can even run the engine with the drive unit removed. Makes adding antifreeze a snap when winterizing. Another plus is, that if I have to (in the event of a pump failure), I could swap out the raw water pump (carry a complete spare) with the boat in the water by closing the 1 1/4" ball valve, isolating the raw water intake to the pump. May come in handy if pump fails and am a ways away from the trailer.

 

[04fxdwgi]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

Another note: You don't need all the hoists, gantry and such. When I need to yank an IO / inboard engine, I do all the disconnect work, then I hire a tow truck with an adjustable boom on it for an hour to lift the motor out, nice and easy. Re-hire to lower the motor back in. Cost less than a "C" note each way, is safe and easy.

 

[Lennyd123]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

Another note: You don't need all the hoists, gantry and such. When I need to yank an IO / inboard engine, I do all the disconnect work, then I hire a tow truck with an adjustable boom on it for an hour to lift the motor out, nice and easy. Re-hire to lower the motor back in. Cost less than a "C" note each way, is safe and easy.

Had actually tried that route a few years back when my mech at the time was proving he was an unreliable flyby etc, but after a few calls I was thinking either I was not finding the right place or no one was really interested.

Maybe was just bad timing and they were busy, but who knows.

In response to an earlier post on the plastc fitting on the transom that the 1-1/4 hose clamps to not corroding, it is not that fitting but the ID of the hole just behind it where the transom to gimbal hose goes through that was corroded and shrinking the hose ID to nearly nothing.

Back to seacocks, I would prefer to do it right over acceptable even though cash is a bit tight right now.

Upon reviewing several diff websites it seems there are a couple different schools of thought on how to do this.

From my mech/maybe no longer friend who swears the whole idea is wrong and if I insist on goin thru hull that I have to use a high speed screen pu, or the through transom unit used on TRS to those saying since its a sterndrive all I need is a bronze thru hull and a hose barb fitting on the bilge side.

I am starting to think that like most things in life the right or honest answer is someware in between

My current intent is to go with a 1'' scoop thru hull with either a matching sea **** or ball valve w same threads or adapter base and a 90deg 1'' to 1-1/4 hose fitting.

Any thoughts? Is 1'' enough since much larger than the orig 3/4 hose, or just go w the larger 1-1/4?

 

[04fxdwgi]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

On the wrecker / hoist issue, I have to admit I cheated. I drove a wrecker for 9 years and had them at my disposal. BUT, many a customer called upon us to haul engines. Even did a few on inboard boats, along side a sea wall (lots of sea walls south of Boston) with the boats in the water. Saved them a ton of money not needing to haul the boats, but marina operators seem to have a problem with that.

Some may argue, but the 1 1/4" hose is overkill. Follow the water routing thru the stern drive unit and it can get down to 3/4" ID in some places, telling me the 1" would suffice with capacity to spare. I believe your thinking of a 1" sea **** / ball valve with a reducer to step up to the 1 1/4" hose inlet to the pump would work just fine, especially after removing all the twists, turns and transitions of the passages in the drive. Are you going to put a raw water strainer before the pump the inlet? Seems as though the thru hull will let alot more crap into the raw water than the Bravo pickups would and a strainer may be money well spent in keeping crap out of the pump impeller.

PS: I was thinking about the same thing, but am in a quandry about the thru hull placement so as not to clash with my trailer bunks, Bennett Sport Tabs and sonar transducers. The routing thru the drive, although time tested, proves to be a point of problems from time to time, especially changing that friggin' hose when the time arrives. How does the saying go? "The part most prone to failure is always the least accessible"? Make everything easy to get at and nothing fails. Right? NOT. But at least the fix will be an easy one.... Now if I could only figure out how to get to that engine coupling without pulling the engine out.....

 

[Lennyd123]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

Seems to be good reasoning that working in the towing business would be a help lol, but then again having friends in the marine business did not do much good so who knows

I am looking at using Groco for the set up and some assortment of the following http://www.groco.net/00-scks-valves/ibvf-series.htm http://www.groco.net/00-scks-valves/sck-sys-models.htm http://www.jmsonline.net/thru-hull-scoop-1w-nut.htm http://www.jmsonline.net/flanged-seacock-1.htm

Pretty sure I will wait until the winter haul to install the strainer as I think it is a good idea, but still am not sure it is really needed. Plus it will add about $250 to the install and and that will be a bit tough right now.

I am seeming to have trouble finding the adapter blocks that allow a ball valve to be used as a seacock, and though I like the idea of being able to replace the valve without having to remove the through hull etc I am not sure I will own the boat long enough to have to worry about it.

IS there some kind of maint schedule or life span on these valves?

 

[04fxdwgi]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

Only maintenance schedule I can think of is tooperate it now and again to keep it freed up.. They are built so they can be taken apart if needed. Just add a little plumber's grease to em if you do and a good bronze and stainless steel ball unit should last life of the boat.

I don't think you would need the big "basket strainer" in this installation. There are some good, smaller in-line strainers (that are used as origional equipment on lots of boats) available for under a hundred, that should work just fine.

http://www.foreandaftmarine.com/108-906066.htm

 

[Lennyd123]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

since I plan on installing a fresh water flush out into the system I figure it will get used fairly often, and that sounds like a good thing.

I do have a new concen as you mention the SS ball as some of the seacocks I am looking at (the more affordable groco one specifically) says it has a chrome bronze ball.

Will that be a problem?

Is the ball just going to rust and seize in salt water in a week?

I would like to order the parts I cant get local later today so I can get them etc so I really need to know if the non SS ball is a problem source etc.

Actually I picked that valve over their full flow first due to overall size, then due to price, and it being a flanged mount seacock just made it appear a better choice over the ball valves I had seen, and even considered that SS maight be q potential source of galv corrosion within the valve as well.

Think I better figure this out before I pull the trigger as I dont want to have to replace this valve and deff not in a year or so.

 

[04fxdwgi]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

since I plan on installing a fresh water flush out into the system I figure it will get used fairly often, and that sounds like a good thing.

I do have a new concen as you mention the SS ball as some of the seacocks I am looking at (the more affordable groco one specifically) says it has a chrome bronze ball.

Will that be a problem?

Is the ball just going to rust and seize in salt water in a week?

I would like to order the parts I cant get local later today so I can get them etc so I really need to know if the non SS ball is a problem source etc.

Actually I picked that valve over their full flow first due to overall size, then due to price, and it being a flanged mount seacock just made it appear a better choice over the ball valves I had seen, and even considered that SS maight be q potential source of galv corrosion within the valve as well.

Think I better figure this out before I pull the trigger as I dont want to have to replace this valve and deff not in a year or so.

1st....Chrome plated Bronze won't rust and I suppose will be just fine and, as you suspect, the S/S version could possibly cause a galvanic thing to happen under the right circumstances. No matter what one does, there are plus' and minus' to everything. I am like you, trying to "over anylize" everything to the point somethings never get done or spend way more time / cash than I need to. HAHAHAHA.

2nd..Just go for it and get out and enjoy the boating for what is left of the season......

 

[Lennyd123]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

Ordered everything today and should have it all in by mid next week, and should be all hooked up before the weekend so just need to arrange for a launch.

Went with the thru hull scoop, and flanged valve with the chrome brass ball, and a 90 to attach the hose.

Planning on adding on something inline between the pump for allowing easy flush out of the system with fresh water.

I have to agree on getting it done and enjoying the rest of the season, and being the fall offers some of the best boating around here with the reduced traffic on the water and smaller crowds at most all destinations.

I hope this will finally be the last thing with this re-power and I can finally just go out and run it. The new engine feels like it has plenty of power and should be great, but so far this may be the first time I can run it at any real rpm for more than a minute or two.

Do you guys have any ideas on if I really will need to add a strainer on this set up?

 

[04fxdwgi]

Re: 7.4 Mercruiser running hot at rpm and burning up exhaust boots

"Do you guys have any ideas on if I really will need to add a strainer on this set up?"

Probably the answer is a resounding no...... There are more boats w/o strainers than with. Will it help? Maybe yes, maybe no. Depends on where you boat and what crap is in the water. Just a matter of do you want one or not.

I am adding one to mine only because I tend to over do things for my own piece of mind, but at times, tend to make things overly complicated where they should be pretty simple. Will probably never need to clean it in 5 years of use. Never been a fan of "value engineering" (cheapening), but am guilty of doing just the opposite. A little OCD..... and like to tinker (too much).