Mercruiser 260 Overheating

[Griff]

I have been reading this website for some time and found a great deal of useful information. Maybe someone can help me with a problem I am fighting. Let me start with a little background. I purchased a 21 foot 1985 Wellcraft about 7 years ago. The boat was like brand new, but with an original engine that had a couple minor problems. I am your normal hands on guy, so I instantly started replacing things that could be linked back to the symptoms and the boat ran fairly good for several years. There has always been a nagging problem with overheating, specifically once the engine is shut down. It would run around 165 degrees down the lake, but when I shut it down and then restarted a couple minutes later it would be at 200 degrees plus and it smelled/felt hot. I replace the water pump with new, had the local dealer replace the impeller and still no change. It then hydrauliced and I found water in the #7 cylinder. Enough was enough so I pulled the engine and completed a full overhaul with the best parts. Block was tanked, flushed and checked for cracks. The water jackets were very restricted and took some time to flush clean due to scale. Heads were pressure checked, new seals and milled to true etc.. Pretty much everything else was replaced with new except for the original intake and carb(new holley years before). Engine now runs beautiful with great oil pressure, but it is still heating up after shut down. Exhaust risers are warm while engine is running, but get hot to the touch just minutes after shut down. I really looked at the exhaust manifolds and risers while the engine was apart and found no cracks or internal rust, but my guess is they are creating my overheating problem. They are the original manifolds and risers and are really in good shape externally and internally from what I can see. The boat has always been in fresh water and our season is very short, so they do not have a bunch of time on them. Boat does exhaust water through prop and exhaust in lower unit.
1. Could the manifolds just be plugged not letting enough water to circulated through the engine, but just enough to show water in the exhaust.
2. Is it normal for engines to heat up this much after shut down?
3. Would a faulty/internally cracked manifold effect top end RPM range? I swear the engine revved to 5K when I first bought the boat, prior to the overheating problem. The current cam, valve spring and carb. combination should take it up to 5K, but I am only seeing around 4100 as before the overhaul.
4. Neither Head was cracked at overhaul according to a reputable shop. Could a blown head gasket create a leak so much as to hydraulic the engine, or would this also lead towards a cracked manifold? The engine only did this once prior to overhaul.
5. Engine also pulses at times like it is loosing a cylinder when really hot and under a load. This seems intermittent and really no pattern, but of course engine heats up when this happens. Ignition is all new and fuel pressure is dead on at around 7-8 lbs.
6. The ignition coil was jumping spark a few years ago between the two small terminals. I was told that this could be from a faulty Exhaust manifold. Could this be true.

Would replacing the manifolds be going in the right direction or am I way off base?


All help would be appreciated!

Thanks

[Pevito2]

Where did the water you´ve found on #7 cylinder came from? There are some likely sources of water on a cylinder: a cracked exhaust manifold, a blown head gasket, a cracked block or condensed moisture (less possible). Also , are you sure you don´t have water anymore on #7? It smells like bad risers or exhaust manifold, on a complete overhauling like yours, new ones would be cheap insurance...

[Griff]

I was sold that a blown head gasket from the overheating allowed the water to enter #7 prior to overhaul. Cylinder heads and block checked out good at overhaul, but that still leaves manifolds as a possibility. Still, I only have couple hours on the engine since it was rebuilt, so I can not say for sure that #7 is now dry. I took it for a short spin last night and once again it ran great down the lake at around 165 degrees, but as soon as I shut it down the temp shot up to a little over 200 degrees and the manifolds were hot, same old problem. I can start it up and let it run at around 1500 rpms and the gauge shows the engine cools down to around 160 degrees, but at soon as I shut it down it slowly raises to the 200 degree mark. I also notice that it seems to heat up to 165 degrees quickly. I ordered new manifolds and risers this morning as this has to be the problem and I don't want to tear up what took me all winter to build. Can anyone confirm these symptoms with bad manifolds/risers?

[Pevito2]

Have the #7 checked. Remove the spark plug and check it for moisture, then crank the engine ( unplug the coil , so it won´t start) and see if it splits water from the spark plug hole. If positive , you´ve found your problem. If negative, i would do the same for the rest of the cylinders, just to be sure it is not a bad manifold, if OK, compression test would be a nice option. Also, if there is a leak on the manifolds, the water passes through the rings and accumulate at the oil pan, so your oil level should be increasing ( must be a big leakage to see it happening) and milky.
I´ve seen my temperature gauge going up after shuting down the engine. it happens because when you stop the engine, there is still plenty heat on the block and heads to warm the accumulated water in it. As the hot water tends to move up on the engine, it reaches the temperature sensor and it detects that increase on readings.It is normal since the working temperature, even under load , is within the expected operating range.
Good Luck

[Griff]

Well, I replaced the exhaust manifolds and risers and things are a little cooler running down the lake(about 150 degrees with 165 degree thermostat), but still have that nagging problem with the engine running up to 200+ degrees when shuting it down. There is no water in the cylinders and all have great compression as a fresh overhaul should. I started the engine with the lower unit submerged in a barrel of water instead of using the muffs and briefly pulled the water supply hose from the thermostat housing and we do have water pumping from the impeller. I would say about 1 to 1.5 inch of water shooting out the top. After reattaching the hose the engine stayed at around 160 degrees during this exercise. The power steering heat exchanger is clear and not plugged in anyway. Spark plugs look good, not too lean or rich. One thing I did notice is that one side of the exhaust seemed to be shooting more water out back of the boat than the other side. I would say 50% more. Is that normal? I just have a real hard time accepting that the engine tempeture should rise from 155 to 200 plus degrees (almost 50 degrees) in a matter seconds once it shuts down. It sounds like there has to be some major heat building in the engine to do that. I was always under the impression that 200 degrees was getting on the hot side for an engine. Anyone have an idea or experience with what should be the acceptable temp. range, specifically the max., for a 5.7 mercruiser 260? The temp. sensor is in the front of the intake real close to the thermostat housing. Could it be that the impeller is moving just enough water to the engine to keep the temp. sensor in the intake cool, but not enough volume to keep the rest of the engine cool, and therefore creating too much heat in the block. Or maybe we have enough volume when the thermostat is closed, but not enough when it opens to cool the entire enigne. So when the engine shuts down the heat rises and instantly raises the temp. I did confirm that the impeller was moving water by running the engine with lower unit in barrell and pulling water supply hose, but I have no idea is the amount being supplied is adequate. I did pull the supply hose from the power steering heat exchanger, held it upright and filled it full of water to see it if water would come out the intake in the lower unit and there was a small trickle leaking. Eventhough the impeller was replaced two years ago, it could always need replaced again. Does any of this sound like maybe a bad impeller or cold water volume problem? Just thinking outside the box, could there be an exhaust blockage down steam in the "y" pipe after the risers?

Anyone have some thoughts? No doubt someone has run into a similair situation out there. It has to be something simple!

[gonzo]

Are you sure the temperature guage is accurate? An ifrared thermometer is the best tool for troubleshooting a problem like this. It will tell if there are any hot spots. Do you have manifolds with only one or two hoses?

[Griff]

I am just taking for granted the Temp gauge is working correctly. The engine smells hot. The intake is soo hot that it has actually melted some of the plastic protective covering of the main harness and the overflow fuel tube from the carb to the fuel pump. I filled one of the small indentions on the top of the intake with water and it actually steamed the water out with in a minute or so. The water just fizzled dry quickly, which tells me its above 200 degrees. I didn't think an intake would was suppose to get that hot. I will try to get my hands on an infared thermometer. As for the manifolds, they have one hose on the bottom. Of course the risers have one hose each from the thermostat housing. My next move is to replace the impeller in an effort to make sure I have enough volume of water getting to the engine.

[marshmat]

Well, I'm about as stumped as anyone here....
I agree with Gonzo, don't trust the gauge. An IR temp probe is the way to go.
It strikes me as pretty odd that temp. would be fine while running, and go up when stopped. Checking the impeller certainly won't hurt. I wouldn't be surprised if there's a blockage in the cooling system; I might try running it on a flush fitting on land to see exactly what is coming out of each outlet. But the IR temp probe will be your best bet; take the engine bay cover off and have someone else drive while you point the thing at each component. This is a weird one, indeed.

[comptiger5000]

the temperature gauge tells coolant temperature, so parts of the engine might be much hotter. However, marine engines do get very hot after shutdown, because heat is still radiated into the coolant, but the coolant isn't being cooled.

The same happens to cars, but in both cases convection starts circulating coolant (to some extent), but in a car you have a radiator, in a boat, there's still nowhere for the heat to go.

[tom9876]

I have the same issue. Boat temperature rises substantially after shut down. How did you solve your problem?

[GatorNative]

I'd like to know if the problem was ever solved(?) Does it have a separation plate with a tiny hole, at the base of the riser? If not, the cooling water may be voiding out when the engine is shut down.

I have an overheat problem of my own with a Mercruiser 888 (1972 twin Wellcraft Nova). Under load, the port engine quickly heats up over 200 F.
I have installed new water pumps, tested the manifolds, thermostats are long gone. Could scale in the block and heads be the cause? Any way to treat this, besides tearing it all down?

Thanks for the help.

[Frosty]

When you shut down --ie turn the ignition off the guages are dead and don't read, some types relax and look like they are responding but they are not.

When you shut down the hottest water will rise to the top of the engine where the temp sender is, this is of course is normal and if the engine is off whats the prob.

[PAR]

Bleed the air out of the system. Also check the intake manifold's crossover water passages for scale and plugging. Does the intake have the automotive block off plate(s) in place (if so equipped)? Check to insure the temperature sender is in contact with coolant water, after shutdown, other wise it'll sense block temperature through the threads on the body of the sender. As Frosty mentioned, it's normal for the gauge to read higher temperatures when you shutdown, because heat rises. You check this by re-starting a few minutes later, if she's quickly backing down to the "running" temperature, you know what the issue is. Do NOT run without thermostats, regardless of what people might say, especially if you have a heat exchanger equipped system. The impeller is a "consumable" product, just like an oil filter and should be replaced every year. Even an unused impeller can die in it's bore, just by taking a "set". It's easy to replace and is usually done with gear case lube as part of an annual service.