Mercruiser 888 overheating

[RCAnderson]

Hello Iboaters ! Haven't logged on in a while due to forgotten password issues, but anyhow... Due to some less that fortuitous circumstances, my 1 year old rebuilt engine seized solid, due to water intrusion through the carb. I obtained a 'good running' replacement, and engine that tested well for compression, coolant leak down, and oil pressure while running. This was supposedly a fresh water cooled engine, but further investigation has pretty well convinced me otherwise. There is no thermostat in the engine right now. The engine is a Mercruiser 888, which is a Ford 302 with 351 heads. Full closed cooling system, which has a larger heat exchanger than my old engine. I have heard of Merc saying that they want a belt driven raw water pump when using a full system, but my old engine ran fine with a smaller heat exchchanger, so I don't think that is the problem.

On to the issue.

This engine overheats if run above 2000 rpm for any length of time.
Does not overheat on muffs. Runs about 110 degrees with no thermostat in.
Put in a length of clear tubing on the raw water side of the heat exchanger, showed bubbles in the tube when in the water, replaced raw water pump, including base, and that eliminated the bubbles, and the clear tube shows good flow.
Took end caps off exhaust manifolds, cleared out a bunch of rust flakes.
Removed engine circulation pump, flushed block, seems to be flowing fine.
Removed heat exchanger, had it hot tanked and pressure tested. It was dirty.
Put clear tubing between the intake side of the heat exchanger and the coolant outlet of the exhaust manifolds, this showed bubbles in the freshwater side of things.
Replaced exhaust manifolds and risers with the used ones off my old engine that were known to be working fine.
Now not showing any bubbles in the clear tubing, but still overheating.
Checked the exhaust shutters with a piece of welding rod today, they are intact and functioning.
I am seeing about a 50 degree difference between cool wear into the block, and hot water coming out of the manifold.
I am seeing about a a 30 degree difference between raw water coming into the heat exchanger and the water going out.
I have not melted the exhaust bellows.
I am thinking that I have either a blown head gasket, a cracked head, or a cracked manifold.

Are there any other tests or anything anyone can think of that might narrow down this problem ? I am not averse to pulling the heads off to have a look, but if I don't need to, I'd rather not.

Thanks in advance

​Ryan

 

[RCAnderson]

I have been running the engine with no thermostat while troubleshooting, but it appears that running with no thermostat may, in some cases actually CAUSE an overheat, I am going to put it in and try it.

 

[Bondo]

Ayuh,..... Yer T-stat plays a smaller role, as ya got closed coolin',... Much more like a car,....

Have ya tried pressure testin' the closed system,..??

It sounds yer hittin' the bases,....

 

[RCAnderson]

Yes I pressure tested the whole system before the engine went into the boat. It held 15 psi for 5 days, so I was not expecting any problems. I am trying to think of a way to get the engine up to temperature while the boat is out of the water. I have the drive off, so if I don't go too crazy, I can run the engine with no raw water for a few minutes, which will certainly get the temp up. I am thinking I might I might see a pressure loss if I put the coolant system tester on when the engine is hot.

 

[RCAnderson]

So I put the a 165 degree thermostat in, and connected the cooling system pressure gauge to the fill neck on the heat exchanger, but didn't pressurize it. Started the engine on muffs, and waited to see something happen. The thermostat is functioning, as you can watch the temp go up and down just after the thermostat housing. The pressure in the cooling system slowly came up to about 10 lbs, and would drop to 6 lbs once the thermostat opened. It did this over and over again, definitely repeatable. I was thinking that if it was a cracked block or head, or blown head gasket, that I would see significantly more pressure than that. So I put a 14 lb cap on it and hoped for the best.

Tried it on the water today, and it is still overheating. I am now thinking there must be some kind of restriction in either the block or the heads. I can't really get in very well to get a temp gun on the rearmost cylinders to see if there is a hot spot on the engine.

If anybody has any ideas at all, I am all ears.

 

[pureinsanity]

I may be way off base here for I have never dealt with a closed system in a boat before (my 888 is a open system). But from what I am reading it sounds just like a car when the radiator is restricted in some way. I have personally had this happen on 2 cars. You said the exchanger was dirty, could it still have a restriction? Can they put it on a flow meter and test it? Again this was on a car, I had one overheating so I took the rad to a shop. All they did was basically put a hose in it and water come out so they said it was fine. Put it back in and still overheated. Took it back out and to another shop. This shop put it on a machine and flow tested it and said it was restricted. They rodded the rad and no more problem. In both cars I could baby them at low RPMs and keep the temp manageable but if you tried to run at any speed (RPM) they would heat up. So you can see my comparison here to your issue. It may be worth another look.

 

[RCAnderson]

I don't think you are off base at all. I did have the heat exchanger hot tanked, which supposed to get it clean as a whistle. I think I will remove it again, along with the engine circulation pump and thermostat housing (since the gaskets are only $5) and try to back flush the block and heads and see if anything comes out that shouldn't.