1999 Bayliner 2855 Mercruiser 7.4 major overheating riser

[amitchell01]

Hi everyone.
Out today for the first trip in my new for me 1999 Bayliner 2855 with a pre-MPI mercruiser 7.4 and generation 1 bravo 3 leg.
About 45 minutes into the trip, while engine temps (according to the gauge) never went above normal, I noticed a strong burning rubber smell. I stopped the boat and had a plume of smoke come up from around the engine cover. On opening the engine cover (with a friend at the ready with a fire extinguisher) I noticed that the starboard side exhaust riser was extremely hot (to the point where it has melted the paint on the riser) and the lower rubber boot (below the elbow) was almost totally burnt away. Smoke was pouring from this. (The rubber fitting between the riser and the elbow was strangely unaffected) There was so much water pouring in through the non almost non-existent rubber fitting that the bilge pump could't keep pace so I started bailing while a passing boat towed me to a nearby beach. The port side riser and manifold appeared to be completely normal temperatures.

These are brand new risers, fitted by the previous owner just prior to me purchasing the boat. (The boat was run for 3 months with these risers before being pulled out of the water for sale) The boat also came with new manifolds in boxes - unfitted as yet as the previous owner had ordered them but they arrived a day after I bought the boat.

It's up on dry land now but I'm a newby to mercruiser I/O so I'm a little unsure of where to start. I obviously need to replace the rubber fitting but does anyone have any pointers for what I should be checking in terms of what caused the riser to overheat in the first place? Anything else I should be checking? (I ended up with about a foot of salt water in the transom)

Any help or advice would be appreciated.

 

[achris]

Hello amitchell01 and :welcome: to iboats.

By 'pre-MPI', do you mean EFI or carburetored?... We try not to refer to anything as 'pre-' here, it's open to too many possibilities. Here is where a serial number would be very helpful, because in 1999, all big blocks were MPI. Last carbed 7.4 was 1997, so I'll go with that....

As water flow from the engine it directed to both manifolds (and ELBOWS), the water will generally follow the 'path of least resistance', so I would be looking for something between the thermostat housing and the elbow that has obstructed flow... Rust scale in the manifold would be my first line of inquiry.... Maybe check the hoses between the thermostat housing and manifold and elbow for collapse or blockage.... You also have check-balls in the thermostat housing, check those too....

Do you have a water flow diagram for the engine? If not, post the engine serial number and I'll post up the diagram. Also a photo of the thermostat housing/front of the engine would help to identify which cooling system style you have...

Chris.........

 

[amitchell01]

Thanks achris . Correct - It's a carb fed 7.4. I've had the boat put into a storage facility for now (I was about 10 miles from home when it happened and I couldn't get it back) so I can't get a close up photo of the thermostat housing right now but I've included a photo from the original advertisement. Hopefully it's good enough. The engine serial number is 0(zero)K283922

 

[achris]

Hmmm. A Victorian I see. (didn't live in Bundaberg a few years ago did you?)

That photo is exactly what I needed to see, and the serial number does indeed make it 1997, built on a Gen VI block...

The thermostat housing is the one without check-balls, so check inside the housing for rust flakes that could block the hose going to the bottom of the manifold, and the hose itself. If they are clear, pull the elbow and riser and check the water passages...

Here's your flow diagram.



Chris..........
(in Perth)

 

[amitchell01]

Perfect Chris. Thanks! Yep - Victorian but I haven't lived in Queensland. I'm in Paynesville (Gippsland Lakes) at the moment but it looks like new years eve on the water will have to be put on hold for now
One other question if you don't mind. Looking at the photo I posted, behind the riser there is a cast iron elbow attached to the riser with a rubber fitting. At the other end of the elbow there is another rubber fitting. It's this lower fitting that's burnt - not the one attached to the riser. Are you saying that the cast iron elbow (downstream from the riser) should also be cooled? I couldn't see it in the flow diagram and don't recall seeing any coolant lines going to it (although, admittedly, I was a little busy bailing and trying to plug the hole to pay much attention to cooling lines.... )

 

[achris]

Water flow once the water leaves the engine and is expelled through the thermostat housing to the manifold. Manifold -> Riser (looks like 6") -> exhaust elbow (cast iron) -> rubber tube -> cast aluminium elbow -> rubber tube -> Exhaust pipe (aka Y-pipe).... -> out through the propeller hub....

Water that has finished its job of cooling the engine is expelled through the hose to the bottom of the exhaust manifold. It is pushed up the manifold, up through the water jacket in the 6" riser and into the water jacket of the cast iron elbow. As it leaves the elbow it mixes with the exhaust gasses from the exhaust manifold, cooling them so they don't burn up the rest of the system on their way out... The top rubber tube not being burnt is not unusually in cases of lost water flow, the exhaust gases are not in direct contact with the tube. The lower rubber tube has a very different story. Not water and that will cook up really quickly... As you found out!

All components downstream of the cast iron elbow are prevented from getting cooked by the exhaust gases (or should be) by the water mixing with the exhaust gas and cooling it....

Chris........

 

[tpenfield]

:welcome: amitchell01 Looks like achris has you on the right track. The older style cooling systems had hoses running to both the exhaust manifold and the exhaust elbow (riser) so even if one hose became clogged, you would get some cooling via the second hose.

Unfortunately, your cooling system has the single hose. Looks like your problem lies between the thermostat housing and the fitting at the lower side of the starboard exhaust manifold.

Once you get everything squared away with a new rubber boot, I'd check for any exhaust leaks on the starboard side to verify the exhaust manifold/riser/elbow survived the overheating.

 

[amitchell01]

Thanks for the advice everyone. It's been very helpful. Generally speaking, if the lower rubber boot lets go as mine did, should the flapper valve in the Y pipe prevent the rapid ingress of water I experienced? I'm guessing the valve is going to need replacing as well. In fact I'm considering giving the entire cooling system a complete overhaul (impellers, hoses, everything......) With all the smoke, followed by the water rushing in it gave the whole family a bit of a scare and it isn't something I want to repeat in a hurry.

 

[Scott Danforth]

in addition to the boot, you need to change the exhaust flapper as that is rubber too and all the rubber in your exhaust is good for a maximum temp of 250, so when you lost cooling water flow on the one side, your 1000-1200 degree exhaust burned everything in its path.

 

[tpenfield]

The flapper valve is not water tight and only dampens the 'surge' of water that you would get when coming off plane.

The valve is supposed to be above the water line, so if it is not, that may be something to look into.

 

[achris]

The flapper valve is not water tight and only dampens the 'surge' of water that you would get when coming off plane.

The valve is supposed to be above the water line, so if it is not, that may be something to look into.

He has what appears to be 6" risers, so no, the flappers will be below the water line.... That would mean that a burnt through rubber tube could allow water into the boat... Not a good look

Chris........

 

[Scott Danforth]

especially with the Mercruiser multiple rubber boot exhaust systems.

 

[tpenfield]

He has what appears to be 6" risers, so no, the flappers will be below the water line.... That would mean that a burnt through rubber tube could allow water into the boat... Not a good look

Chris........

Got it. . . Thus the 6" risers to get the elbows sufficiently above the water line. Almost seem like these situations should have an extended y-pipe :noidea:

 

[Lou C]

Usually the extra length is taken up with a longer aluminum elbow in these installations where there is an added riser between the manifold and the exhaust elbow. This to me is one of the big drawbacks to inboards, a major failure in the cooling system can come close to sinking the boat, or at least create an immediate emergency. I had a similar problem (different cause) from a raw water intake hose popping off the transom water hose neck (way back behind the engine) and the Cobra impeller quickly filled the bilge, instead of the engine, with salt water. So I had an overheated engine and water up to the pulleys. After that experience I go over every single one of the cooling hoses at the start of each season. Even with a bad overheat though, the rubber exhaust hoses were not burnt, after I took them off they were slightly scorched on the inside but not bad. The flappers were vaporized and I had to remove the drive to get all of the remains of them out, they were laying in the bottom of the exhaust housing. Pulled em out with a coat hanger wire.

In this case you have adequate flow on one side but inadequate on the other, it could be clogging in the thermostat housing, the water fitting on the bottom of the manifold or the manifold to riser/elbow ports could have been getting clogged by rust. I'm thinking the previous owner knew it was time for a new exhaust system as he had ordered the parts but only installed the risers (or risers + elbows?) if you are in salt water and have a raw water cooled engine figure on replacing these parts every 5-7 years to avoid failures that cause overheating and water leaking into the cyls, probably the 2 major causes of inboard engine failure.

When I had the old style OMC one piece manifold/elbow units on my V6 one of the cooling ports would be clogged after 5-6 seasons, these were on the small side and the new system I installed in the summer of '17 (Volvo style center riser exhaust) has much larger cooling ports between the manifolds and elbows.

 

[Lou C]

Clogged exhaust ports, what you will be looking for....

 

[amitchell01]

Thanks for all the information everyone. With a possible drive removal required to clean out the burnt flapper remains it's sounding like more than I can do in a couple of days so I think I'll have to get a professional to take a look at it. (I'm about 300km from home and have to be back for work in a few days)
Thanks again for all the advice. It's nice to come to a forum as a newby and get great advice and not be called a dickhead for making such a rooky mistake.

 

[Lou C]

It is the nature of I/Os, I keep an IR temp gun on board to monitor exhaust temps. Also, some Volvos came with exhaust temp sensors and they are available in the aftermarket. Might be a good addition, just like the audible warnings for low oil pressure or high water temp.

 

[amitchell01]

One more update.....

I've got the rubber boots which will go on along with two new manifolds. While I've got the risers off I'll give them a check for any blockages and while I'm at it I'm installing new flappers, impeller and thermostat kit. I'll check the thermostat housing for blockages while it's off and have a poke around with some wire to see if any flapper remains are floating around anywhere.
I've washed down the inside of the transom with fresh water and fogged all electricals, the engine sump and all other exposed metals with WD40 (it's all I could get at the time) Throttle cables etc have been sprayed with the little bit of Lanox I had left. (The water didn't get that high but there was a lot of seawater getting splashed around while we were bailing)
Hopefully that'll be enough to get me back on the water

 

[amitchell01]

Also, some Volvos came with exhaust temp sensors and they are available in the aftermarket.

Good info. I was only thinking of this this morning on my way back to get some stuff off the boat. Either exhaust temp gauges or flow gauges reading the flow through the hose going to each manifold. Good to know there's something already available and I don't have to build it myself.

 

[amitchell01]

Oh - and in case anyone is wondering what a burnt out rubber boot liike like, here's mine. There's a green cloth in it that I stuffed down into it to try to slow down the flow of water into the hull.