Newly rebuilt 1986 350 OMC Overheating at WOT

[cgalt]

I'm seeking advice for next diagnostic/troubleshooting step for my overheating problem.

I installed a rebuilt long block replacing a freeze damaged block. The damaged engine ran and did not overheat, but suffered water in oil and hydro-locking problems.

The new long block is a later roller cam block, and has flat top pistons installed. Other than the bare long bock, most parts were re-used.

The new engine is operational and running smoothly. The engine makes more power than the old one and revved up to 5200 WOT with my 17 pitch prop and started overheating.

I changed up from a 17 to a 19 pitch prop now it maxes out at 4600 RPM.

I replaced the thermostat, installing it correctly. It seems to function correctly at idle and at medium cruising speeds.

I inspected the impeller and it looks perfect.

I removed the transom bracket water hose from the thermostat housing and started the engine, water flow is good.

The engine is still overheating when run over 3800-4000 RPM, or even around 3600 after about 10 minutes. At 3500 it hovers just under the red zone. If I back off to idle, or even to 3000 RPM, the engine cools down to a normal operating temperature with the gauge in the middle of the green.

I don't believe the temp sender or gauge is off, because when it reads hot, the engine starts smelling hot.

Because the water flow checks out, I am assuming the problem is on the engine side not in the outdrive (which,btw only has 385 hours on it).

The ignition timing has been set by hand without a light to the smoothest idle operation by ear.

The belts are tight and working correctly.

I haven't checked for exhaust gas in the water line yet, but again, the problem wasn't there before and nothing has been changed with the outdrive.

The exhaust manifolds and risers are remaining fairly cool until the engine is overheating. The intake manifold seems to be getting really hot.

The intake manifold gasket was installed without restrictors because the old engine didn't have them installed.

The engine, being new, obviously will have clear passages. The intake manifold is clear. The exhaust manifolds and risers are clear except for one of the passages to the riser on the port manifold. (This blockage did not make the old engine overheat.) riser gaskets were carefully installed and should not be blocking anything.

Charles Galt

 

[Blk-n-Blu]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

Here is a link to some OMC manuals. Thanks to Don S. I would check the timing rather than setting by ear.
http://forums.iboats.com/showthread.php?t=309093

 

[cgalt]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

Here is a link to some OMC manuals. Thanks to Don S. I would check the timing rather than setting by ear.
http://forums.iboats.com/showthread.php?t=309093

Thanks for the link to the manuals, I had a service manual but these are great to have!

I guess I'll go buy a timing light... I doubt this is going to be the solution but at $30 it's a reasonable next step, heck it might even solve the problem!

Any other ideas out there?

 

[Blk-n-Blu]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

Verify that it actually is overheating, with a different gauge.

 

[Dakota47]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

Change the IMPELLER , looks can be deceving. my 19 ft ski boat with omc 4.3 would run 175-180 degrees and last week i changed the Impeller ( looked new) i bought the complete kit ,Housing and all. i installed it and now she runs @ 160 all day long.. oh yea, check the power steering cooler for water flow.

 

[a70eliminator]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

Get the ignition timing set with a timing light, too advanced and the engine will run hot, especially at the manifolds.

 

[fossill]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

I assume the rebuilt came from a reputable and reliable rebuilder. Usually symptoms like yours can be caused by combustion gasses getting into the coolant either from a faulty head gasket or a crack in the cylinder head causing an overheat condition under high load.

 

[scoflaw]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

Flat top pistons are usually high compression. What heads (cc's) what pistons? What cam? Do the math. Late timing will have the fuel charge still burning when the exhaust valves open. Check the heat range on your spark plugs as well.

 

[Bifflefan]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

Get the ignition timing set with a timing light, too advanced and the engine will run hot, especially at the manifolds.

That is backwards.
To advanced will cause spark knock, To retarded will cause manifold over heating.

 

[Bifflefan]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

Flat top pistons are usually high compression. What heads (cc's) what pistons? What cam? Do the math. Late timing will have the fuel charge still burning when the exhaust valves open. Check the heat range on your spark plugs as well.

Even with small head chambers of the past you will only get about 9 to 1 with flat tops. Thats not high compression.

Spark plug heat range has no effect on the engine temp.
Read this Spark Plug Tech

 

[a70eliminator]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

That is backwards.
To advanced will cause spark knock, To retarded will cause manifold over heating.

Ok thats right

 

[scoflaw]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

Flat tops are a term for a piston that has no dish. A flat top piston can have valve reliefs with different volumes. A cam shaft can create more compression or bleed it off, depending on when the intake valve closes. Have the block or the heads been shaved? All this matters. Like I said. You have to do the math to see where your compression ratio is at. As far as the plugs are concerned. Since when does'nt a cooler plug make a temperature difference in the combustion chamber, which is where the heat is created in an internal combustion motor. Why would they make different ranges ifthe did nothing. Did you read that article Biffle?

 

[cgalt]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

Thanks all! I went out and bought a timing light at Harbor Freight for $15 and set the base advance to 10 degrees. It was around 20-25 degrees. I clearly should have done that to start. Good thing I was filling up with 93 octane fuel. I'm going to try to eyeball how much the full advance is but the light I bought doesn't have an advance adjustment and the markings on the timing cover only go to 12. I guess I'm looking for about 28-32 full advance. I expect that since I didn't change the distributor, once I set the base the curve should be fine. What I don't know is whether I should make any adjustment to the advance curve since I have changed up to a higher compression of 9.5 with the flat tops from I think 8.5 stock.

I hope to take her for a test ride this evening. If I still have a problem, I'll replace the impeller next.

 

[cgalt]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

Flat top pistons are usually high compression. What heads (cc's) what pistons? What cam? Do the math. Late timing will have the fuel charge still burning when the exhaust valves open. Check the heat range on your spark plugs as well.

76cc heads, 9.5:1 compression. 286 duration cam with near stock lift. The timing was too advanced, so I'm going to test and see how she does now.

 

[cgalt]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

I assume the rebuilt came from a reputable and reliable rebuilder. Usually symptoms like yours can be caused by combustion gasses getting into the coolant either from a faulty head gasket or a crack in the cylinder head causing an overheat condition under high load.

I can but hope. It's a local rebuilder that builds over 1000 engines a year and gave me a 2 year warranty. It's a lower cost rebuilt long block with reused crank, rods etcetera rather than new parts, costing me $1250 rather than $1700+ for an engine remanufactured with new parts. It should be done right but time will tell. If it holds up well I'll recommend the builder because the price was right and the service was really excellent.

 

[scoflaw]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

Sounds like a lot of duration. Was that at .050 or .020? Thru hull?

 

[cgalt]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

Get the ignition timing set with a timing light, too advanced and the engine will run hot, especially at the manifolds.

This may be it. Base advance was over 20 degrees.

 

[cgalt]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

Sounds like a lot of duration. Was that at .050 or .020? Thru hull?

I don't know the exact specs. I allowed the builder to convince me that he knows what he is doing on the cam selection. He said that this cam has the maximum duration that will be reliable with no water ingression. He said they've put it into many marine 5.7 engines for years with no problems.

I can say that so far I haven't had any water ingression, and it idles smoothly even if I take it all the way down to 450rpm, which I think is great!

No I don't have though hull exhausts.

 

[cgalt]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

Flat tops are a term for a piston that has no dish. A flat top piston can have valve reliefs with different volumes. A cam shaft can create more compression or bleed it off, depending on when the intake valve closes. Have the block or the heads been shaved? All this matters. Like I said. You have to do the math to see where your compression ratio is at. As far as the plugs are concerned. Since when does'nt a cooler plug make a temperature difference in the combustion chamber, which is where the heat is created in an internal combustion motor. Why would they make different ranges ifthe did nothing. Did you read that article Biffle?

When I did my research I found that a flat piston in this engine with 76cc heads gives 9.5:1 compression. I don't know about the valve relief displacement. I don't know about how much the cam could be changing it, but it is a long duration cam. The heads have not been shaved.

I will try to find out the cam spec.

 

[cgalt]

Re: Newly rebuilt 1986 350 OMC Overheating at WOT

Well, after adjusting the base ignition timing to 10 degrees advance I took her out for testing, warmed her up and found that even at WOT for a few minutes it is no longer overheating into the red zone, but still gets pretty hot, with the gauge heading up to just below the red zone. I think I will also get another temp gauge with degrees marked.

Next, I think I'll change the impeller regardless of whether it looks like it needs replacement and see what happens.