Overheating cylinder, mercruiser 7.4L

[BrettRosolen]

Hi Guys,

My mechanics have discovered a significant overheating problem centred around one end cylinder on my 7.4L MPI mercruiser engine and they actually aren't sure what's causing it.

Pump is working, heads are good, gaskets are good, risers and manifolds are new, and the rest of the engine is cool enough.

Is it possible that a piece of something is stuck in the block? How does one check the block for that, let alone clear it? Could it be something else?

Thanks for any thoughts!

[Lepke]

Is the engine heat exchanger cooled or does it use ocean water?
You could have some blocked water passages.
If it was my engine, the first thing I would do is run a good flush of block using something that dissolves rust like Rydlyme.

[BrettRosolen]

Thanks and yes, it uses a heat exchanger. I guess that means no broken impeller pieces have caused a blockage.

I should mention the engine is 20 years old, but has low hours.

[rslifkin]

How has the issue been traced to that cylinder? Is it showing higher temps at the exhaust runner before the water jacketed section of the manifold? Is it showing spark plug damage from overheating?

If there's no apparent cause for the issue, I'd be inclined to switch that cylinder's fuel injector with another and see if the problem moves. A clogged or failing injector causing that cylinder to run lean will cause it to get hot under load.

[David Mathis]

If you have raw water cooling, could the engine cooling system have taken on mud or sand? If so, while the boat is in the water, remove the block drains located on the side of the engine, one at a time, and start the engine. If water comes out in a forceful stream, that side is clear. Now do the other side. If one side has little or no water, sediment is blocking the flow of cooling water. If the flow is weak or non-existent, use a small screwdriver to break up the packed sediment. Wiggle the screwdriver until the water flows forcefully with the engine idling then shut off the engine, replace the plug, and do the same on the other side. The force of the water will push out the sediment. I've done this with a couple engines, both boats had gotten grounded in sand and the operator tried to drive out. We were surprised at the amount of sediment found in the bilge.

If this isn't the problem, the information previously posted about the possibility of a faulty injector causing lean running resulting in overheating. is logical. The overheated sparkplug is an indicator of this problem.

[BrettRosolen]

Thanks for these thoughts.

Can check the plug, yes.
It's a heat exchange system so unlikely to be sediment.

I always thought a faulty injector meant the rest of the engine gets hot, rather than that cylinder, but swapping them and seeing if it moves sounds like a plan.

I guess my greatest concern is that something is wrong with that cylinder/piston or the block itself ... like a worst case scenario.

[Bowdrie]

Originally Posted by BrettRosolen

Hi Guys,
My mechanics have discovered a significant overheating problem centred around one end cylinder on my 7.4L MPI mercruiser engine

This probably won't help you, so informational only.
The 7.4/454 Chevy uses a firing order in which the #5 & 7 cylinders are adjacent firing.
The #5 cylinder is still "heat-loaded" when the #7 fires.
It's the most common place for overheating on those engines.
The racers came-up with some ways of forcing more water to that area, I believe it was thru the use of head gaskets where the water flow was more restricted to the cooler running cylinders so that more flow went to the "hot spots".
Anyway, I hope the mechanics can sort it out.

[Lepke]

The coolant side needs a flush from time to time just like a car. I'd do that first. Excessive rust on the water jacket can cause heat problems. Also if the cylinder is running lean it can run hotter. Make sure the engine is getting enough air (no restriction of intake air) and the heat exchanger is clean.

If you have a temperature measuring gun take the exhaust port temps when hot and running.


On some racing engines we ran a coolant line to the back of the block so cooling was more even. But you shouldn't need to do that with a marine engine.

[BrettRosolen]

Originally Posted by Bowdrie

The #5 & 7 cylinders are adjacent firing.
The #5 cylinder is still "heat-loaded" when the #7 fires.
It's the most common place for overheating on those engines.

I hadn't noticed that so thanks!

[BrettRosolen]

Originally Posted by Lepke

The coolant side needs a flush from time to time just like a car. I'd do that first. Excessive rust on the water jacket can cause heat problems. Also if the cylinder is running lean it can run hotter. Make sure the engine is getting enough air (no restriction of intake air) and the heat exchanger is clean.

If you have a temperature measuring gun take the exhaust port temps when hot and running.


On some racing engines we ran a coolant line to the back of the block so cooling was more even. But you shouldn't need to do that with a marine engine.

Thanks for all that info. I'll be sure to check all that!

[FPNC]

Check coolant ratio. If 50/50 is exceeded, your engine may over heat.

Most marine applications can use 40/60. Which will provide better heat transfer and cool the engine better. I believe 40/60 is good to -12°F.

[rslifkin]

Originally Posted by BrettRosolen

I always thought a faulty injector meant the rest of the engine gets hot, rather than that cylinder, but swapping them and seeing if it moves sounds like a plan.

I guess my greatest concern is that something is wrong with that cylinder/piston or the block itself ... like a worst case scenario.

If an injector is totally failed and barely if at all spraying fuel, then yes, you'd get a cold cylinder (and a noticeable misfire). But if it's just a little weak, then you can get a lean / hot cylinder under higher loads (but all will likely seem ok near idle).

A compression test won't hurt and will give a good idea of whether there's major mechanical damage to that cylinder or not.

[FPNC]

Originally Posted by rslifkin

If an injector is totally failed and barely if at all spraying fuel, then yes, you'd get a cold cylinder (and a noticeable misfire).

It can also spray too much fuel and cause the oil around the piston rings to washout and increase friction. Eventually damaging the cylinder and piston. I've seen injectors so bad that gasoline was coming out around the exhaust manifold gasket and in diesels, the piston would start to melt.

[rslifkin]

Originally Posted by FPNC

It can also spray too much fuel and cause the oil around the piston rings to washout and increase friction. Eventually damaging the cylinder and piston. I've seen injectors so bad that gasoline was coming out around the exhaust manifold gasket and in diesels, the piston would start to melt.

That's also possible, but if the injector were flooding one cylinder it would cause a noticeable poor / rough idle, likely with some smoke.