Overheating Mercruiser 7.4L MPI

Status
Not open for further replies.

DracoDan

Seaman Apprentice
Joined
Feb 22, 2012
Messages
44
Hey guys, I think I have a bad circulating pump but I wanted to get some outside opinions before I order a part.

Engine: 1999 Mercruiser 7.4/454 MPI (not magnum) with raw water cooling (serial number 0L347xxx) with Bravo 3 outdrive. Actually it's dual engine but only having the problem on port engine.
What I've tried: Rebuilt the raw water pump (it was needed anyway after it it destroyed an impeller last year due to a crack inside)
Symptoms: Engine gradually reaches high temperature while idling, somewhere around 200F (and yes, it's actually overheating, the thermostat housing was HOT!). There is some cooling happening, but it's clearly not where it should be. One of my other fears is that part of the impeller that disintegrated last year is still in the loop somewhere causing a partial blockage, but I had the boat out twice after replacing the impeller (but prior to the full rebuild). The impeller I replaced today didn't have any pieces missing. Also, I took the large hose off the thermostat housing and poured water into it and hot water flowed out of the top of the manifold from the engine. I did this both to check for restrictions and to cool my engine down. Lastly, the weep hole on the circulating pump appears to have serious corrosion, probably from water, but it was dry when I inspected it. The same hole on the starboard engine shows no signs of corrosion.

Thanks for the sanity check, I'm fairly sure I correctly identified the problem, but I'm not a mechanic so I wanted to be sure before I ordered parts.
Dan
 

DracoDan

Seaman Apprentice
Joined
Feb 22, 2012
Messages
44
Google bravoitis

Look for missing impeller parts

Googled, but that doesn't seem like the issue. I say that because the flow of water from the drive is VERY strong, when I went to rebuild the raw water pump the hose was gushing water until I elevated it above the water line. Also, the failure of my raw water pump previously was due to a crack in the housing, likely from the previous owner having overtightened the bolts. That's why I did the rebuild. Also, the previous impeller death was due to the crack causing the plastic "bridge" across the output hole having broken away, leaving a hard edge that the impeller was going across constantly. (kinda hard to explain). When that happened I didn't have a new housing and had to just put a new impeller in. I tried to smooth the edges of plastic to reduce damage to the new impeller, but when I went to replace the housing today the impeller was 95% okay, but there was no reason not to replace it since I was doing the rebuild anyway and I had extra impellers. I tried finding a pic to better explain this but no such luck.
 

tpenfield

Supreme Mariner
Joined
Jul 18, 2011
Messages
14,235
I went down this road last year with the port engine on my F-330. . . it was a lot of 'fun' :rolleyes:

Here is a link to my thread . . .

http://forums.iboats.com/forum/engin...unning-hot-ter

I have a couple of questions . . .

1) First off . . . are you in salt water or un-salted?

2) The engine slowly over heats at idle . . . does it overheat when cruising, or if you raise the RPM slightly above idle?

I chased a cooling issue all over my engine last year, only to find that a small sea shell had grown inside the outdrive in past years and then dislodged, getting stuck at the fitting of the inlet hose in the area of the bell housing (transom assembly). The only evidence of this restriction was about a 15-20% reduced flow of sea water from the port engine versus the starboard engine.

I had checked everything else, and the fact that I had closed cooling (rather than open - raw water cooling) compounded matters. I too checked the re-circulating pump . . . it was perfect.

Anyway, without too much disassembly, you could do a comparison test of the raw water flow from the sea water pump of each engine. . . with a little creative plumbing. I just timed how long the sea water took to fill a 5 gallon bucket and compared that for each engine (assuming warm engine and at idle speed of about the same)

You could also back flush the oil coolers, if not already done so.

My transom assemblies were showing some signs of Bravoitis, but it was not the contributing factor to the over heating . . . it was the restriction at the bell housing caused by the sea shell.

Anyway, cooling system diagnosis and resolution can be evasive. With raw water cooling it is a bit simpler . . .

Key factors are

- Sea water pump
- Inlet hose restrictions
- restrictions at the oil coolers
- restrictions at the exhaust manifolds and/or elbows

Not saying that it is not the re-circulating pump . . . just don't be disappointed if it looks fine.
 
Last edited:

tpenfield

Supreme Mariner
Joined
Jul 18, 2011
Messages
14,235
Googled, but that doesn't seem like the issue. . . . Also, the previous impeller death was due to the crack causing the plastic "bridge" across the output hole having broken away, leaving a hard edge that the impeller was going across constantly. (kinda hard to explain). When that happened I didn't have a new housing and had to just put a new impeller in. I tried to smooth the edges of plastic to reduce damage to the new impeller, but when I went to replace the housing today the impeller was 95% okay, but there was no reason not to replace it since I was doing the rebuild anyway and I had extra impellers. I tried finding a pic to better explain this but no such luck.

Been there, done that. Usually when the 'bridge' breaks off in the plastic housing . . . the impeller is immediately shredded by the sharp edge. I'm not sure how effective the pump is without the bridge intact. Anyway, if you have completely replaced the housing, and impeller, all should be good in that regard.
 

DracoDan

Seaman Apprentice
Joined
Feb 22, 2012
Messages
44
- Sea water pump
Sea water pump has been ruled out.
- Inlet hose restrictions
I don't THINK there are inlet hose restrictions. Like I said when I went to replace the pump (below water line) the inlet connection had a powerful flow of water until I elevated it.
- restrictions at the oil coolers
Sorry if this is a dumb question, but where are the oil coolers on a raw water cooled setup? What do they look like?
- restrictions at the exhaust manifolds and/or elbows
What's the best way to test this?

The temperature seemed to level off around the 200? mark at idle; increasing the throttle to about 2000 rpm (no load) brought it up to around 210? but when I went back to idle it dropped back to around 200?.
 

Scott Danforth

Grumpy old guy who plays with boats
Joined
Jul 23, 2011
Messages
33,421
Pull the drive and look to see if you have bravoitus. Cant assume you dont by your methods. You may not think you have a restriction, however unless you eplore, you CANT rule it out

Follow the raw water path. Suction side of pump should be the power steering cooler, pressure side should be oil cooler.

Pull the exhaust manifolds and risers to look for restrictions
 

DracoDan

Seaman Apprentice
Joined
Feb 22, 2012
Messages
44
Pull the drive and look to see if you have bravoitus. Cant assume you dont by your methods. You may not think you have a restriction, however unless you eplore, you CANT rule it out

Follow the raw water path. Suction side of pump should be the power steering cooler, pressure side should be oil cooler.

Pull the exhaust manifolds and risers to look for restrictions

This is by far the toughest way to troubleshoot for me. The boat is in the water at my marina which doesn't have a lift, and I don't have a trailer for it. Last time I did a short haul it was like $400. Once I've exhausted all other options I will consider this.
 

Lou C

Admiral
Joined
Nov 10, 2002
Messages
6,806
Have you tested the raw water pump output with the boat in the water? If you can't pull the boat, that is a good place to start, if your raw water flow there is good, (look up Merc's specs) then the clear hose test (if the pump is pulling in air) and really checking your manifolds and risers for clogging (hard to do with the boat in the water because its risky, could flood the boat with water) are the next steps. Keep in mind that these raw water systems require a large volume of cool water IN and OUT, they are not pressurized like a closed system so if the raw water flow is lower than normal, it cannot carry away the heat.

If your raw water flow from the impeller is low, then you have to pull the boat. No other way to fix it right. Then go through the whole water intake system, from the outdrive water intakes, to the transom fitting and hoses, oil coolers, etc. Like Ted I had a restriction, that is common on boats moored in salt water, I had barnacles growing in the water intake area of my Cobra drive. To get them all out, I had to split the upper and lower drive housings, remove the plastic water screen (I did not replace it because I always had some barnacles growing on this) clean out the passage, and replace the upper and lower water tube gaskets. And that, is what solved my running hot problem I chased for a season or 2. Not really over heating, but an increase from 160-165 to 185 or so.
 

tpenfield

Supreme Mariner
Joined
Jul 18, 2011
Messages
14,235
This is by far the toughest way to troubleshoot for me. The boat is in the water at my marina which doesn't have a lift, and I don't have a trailer for it. Last time I did a short haul it was like $400. Once I've exhausted all other options I will consider this.

I had a similar predicament . . . $330 to pull the boat and another $330 to go back in. So, I did what I could while the boat was in the water, and it was not until the end of the season after the boat was hauled for the season that I found 'the shell' in the outdrive inlet tube.

My approach is to do the easy stuff first, and you have done some of that already.

Is this salt water or fresh water ???

To answer some of your questions. . .

Here are the various coolers that are in line between the sea water pump and the thermostat housing where the raw water first enters the engine.

7-4-MPI-Cooling.png

I am assuming that since your engine is a 1999, that you have the 'cool' fuel system and not the VST system. So, there is an additional cooler as part of the fuel system. So, you can inspect/backflush etc. those 'coolers' to see if they have any trapped impeller fragments or marine life debris clogging them.

As for the exhaust . . .

If you get to that point, then the water passages for the exhaust manifold that lead up to and through the exhaust elbow can be inspected by draining down the manifolds and taking the elbows off. You will need a new set of gaskets to re-assemble them. The old gaskets will not be reusable.

This really comes into play if you boat in salt water rather than fresh water.
exhaust-passages.jpg

I'd do the coolers first and see where that leads.

FWIW - I tried to inspect for bravoitis while the boat was in the water using an inspection camera/borescope, but I could not really see past the fitting on the inside of the transom.
 

Attachments

  • 7-4-MPI-Cooling.png
    7-4-MPI-Cooling.png
    155.4 KB · Views: 0
Last edited:

Scott Danforth

Grumpy old guy who plays with boats
Joined
Jul 23, 2011
Messages
33,421
At a bit over 1000 rpm, the raw water pump shuld fill a 5 gallon bucket in under 15 seconds. 20 seconds if you have a bit of restriction.
 

Lou C

Admiral
Joined
Nov 10, 2002
Messages
6,806
good to know, that's the first test you should do. If you have good flow, its not pulling in air (clear hose test) and your manifold and riser ports are open (most likely so in fresh water) then you should not have an overheat, as long as you don't have other problems, like leaky head gaskets.
 

tpenfield

Supreme Mariner
Joined
Jul 18, 2011
Messages
14,235
Just looking back at my cooling system issue thread from last year . . . I did a flow test of the raw water through the cooling system at idle and got the following results:

- Port engine took 40 seconds to pump 5 gallons of water. 7.5 gallons/minute

- Starboard engine took 33 seconds to pump 5 gallons of water. 9.1 gallons per minute

The engine idle speed was about the same for each engine (620-640 RPM), but the flow was 20% different as measured, even though the flow looked healthy and sufficient.

The 20% difference in flow caused the port engine to run about 15 F degrees hotter than the starboard engine.

Point being that if you had an even more restricted flow (say 30% reduced), it could result in an overheating situation.
 

DracoDan

Seaman Apprentice
Joined
Feb 22, 2012
Messages
44
I finally had time to work on the boat today, and I was able to improve things slightly and rule some things out.

Changed so far:
  • sea pump and impeller (housing was cracked)
  • circulating pump (major rust around weep hole)
  • oil cooler (>50% of paths blocked by rocks?!)
  • thermostat housing (rusted more than I trust)
  • thermostat and associated parts (too cheap not to)
Results:
  • Engine now idles around 180?, so I'm improving! But it still gets over 210? if I try to go above 2000RPM...
Tests performed:
  • One of the concerns was about the inlet being blocked. I realized I have an engine coolant flush tank and hose (normally used for winterizing) that connects in just before the sea pump. I ran cool water through this tank and the temperatures didn't change at all. So I have ruled out an intake restriction.
  • Checked temperatures around the engine (by hand, forgot my thermometer) and I found something unusual... the port side exhaust elbow is cool to the touch at all times. The starboard side is mildly warm. On my good (starboard) engine both are warmer.
  • Checked flow out of two coolant hoses that go to exhaust manifolds, neither seem to be restricted.
Suspicions at this point:
  • I did not check my fuel cooler, it's going to be a nightmare to get to, but I'm starting to think that's where the blockage could be. What do the insides of that look like? I don't think it could be similar to the oil cooler considering how much had been trapped there and how big the pieces were, wouldn't things get blocked before that?
  • Could it be that the engine block actually has a blockage that's slowing water through it and forcing water to go out the exhaust cooling paths instead? I 'm guessing this could explain the cooler than normal exhaust elbows despite the engine overheating.
  • Is it possible that it's actually the oil overheating for some reason? If so, what should I check on the oil cooling system?
  • Edit: Also, I just realized there's also a power steering cooler hidden somewhere in the back, Could the blockage be here?
Thanks a ton for all the help everyone, I was hoping to take the family out for memorial day, but at this point I think maybe I should buy paddles... ;-)
 
Last edited:

tpenfield

Supreme Mariner
Joined
Jul 18, 2011
Messages
14,235
Since you found some restriction (rocks) in one of the coolers, you should check/clean the other two.

For short money, you could buy an IR temperature gun and be able to sample temperatures at various parts of the engine/hoses, etc. since you have twin engines, you could do some comparisons of the temperature at the same spots on each engine.

Just curious what brand/model of boat you have with twin 7.4's ?
 

DracoDan

Seaman Apprentice
Joined
Feb 22, 2012
Messages
44
Thanks, I think I'm going to try back flushing that section today and see if that helps. I have an IR gun, I just forgot it when I went to work on the boat yesterday.

The boat is a 99 Formula 330 Sunsport.
 

DracoDan

Seaman Apprentice
Joined
Feb 22, 2012
Messages
44
SUCCESS!!! Looks like I had the funnest kind of problem, a compound one!

Turns out the problem was with my thermostat! Well, at least that was my most recent problem, I think the original problem was the clogged oil cooler, but as part of my troubleshooting I replaced the thermostat with a brand new one that is total junk. Despite it being a 160? thermostat (per my order and the markings on it) it starts to open at about 175? and is fully open at 192?! On top of that, it looks like the pin on it is longer and hitting something, preventing it from opening all the way when it's installed. This is exactly why my engine was idling around 180? but overheating under load. It also explains my cool exhaust elbow, since the raw water was going there instead of circulating through the engine.

I tested this by running without a thermostat and the temperatures never rose above 140?!

One part that I don't understand is the part number for the thermostat I received (ordered from **********). I ordered part number 8M0109441 (which matches the old thermostat), but the part I received is 8M6003555, which I can't find hardly any information for. Looking at the two, the only real difference I see is that the 8M6003555 has a longer pin in it (which results in interference it seems).

So yeah, I have a thermostat that doesn't open at the correct temperature AND is unable to open all the way due to it's design... I'm swapping the old thermostat back in tomorrow morning and enjoying my memorial day! Thanks again for everyone's help!
 

jmarines

Petty Officer 2nd Class
Joined
May 8, 2015
Messages
148
Glad you found it. I was going to advise on a new thermostat.. That was my problem.. I installed a 160* when I should have installed a 140* . Took the local marine mechanic to figure it out for me.
 
Status
Not open for further replies.
Top