1988 OMC 5.0l overheating issue

[Frozen]

Boat - 1988 Bayliner Trophy 2359
Engine - 1988 OMC 5.0L(GM) Cobra with a closed cooling system.

Start boat and let it idle at dock, doesn't reach temp. 5-10min in no wake zone out of harbor, gets up to temp. Go out and run the boat for awhile temp is right where it should be. Enter a no wake zone and the boat overheats badly.

Replaced impeller in outdrive no difference

Replaced circulating pump(water pump on block) no difference

Removed thermostat seemed to make it worse.

Any ideas? Clog some where? Heat exchanger gone bad?

 

[Bridar]

Re: 1988 OMC 5.0l overheating issue

I don't recall the GM 5.0 having a closed circuit cooling system. Is there a chance this was a custom modification? If so, there's a good chance there's a clogging issue especially if it was converted when the system was used.

 

[jerryjerry05]

Re: 1988 OMC 5.0l overheating issue

If the pumps have been replaced.Then the risers and manifolds are suspect.Are they part of the closed system?
The closed system was an aftermarket addition.
The heat exchanger could be clogged.Don't know what kind you have but the end cap on some can be removed for inspection.

 

[Bridar]

Re: 1988 OMC 5.0l overheating issue

The risers & manifolds are not part of the 'closed side' of the system, so they are exposed raw water. Has the boat always been run in salt water? Are these the original manifolds?

In the 'raw water side' of a closed system, the raw water pump draws cold water in, pumps it through the heat exchanger, through the manifolds (cooling them) and then discharges the water out through the risers & exhaust. Meanwhile, in the 'closed side' of the cooling system, the engine's water pump circulates coolant through the heat exchanger, then through the engine's water jacket and back to the heat exchanger (if running at normal operating temperature). Before the engine reaches normal operating temperature, the coolant in the 'closed side' is simply circulated (bypassed) until the thermostat opens, diverting coolant through the engine block.

It is possible that the 'closed side' of your system is restricted or plugged. This is possible even if the raw water side is flowing properly. If this is the case, your exhaust manifolds will continue to be cooled normally, while the engine is overheating. If the aftermarket addition of the heat exchanger was installed after the first 50-100 hours of operation, then it would be more prone to becoming plugged from pre-existing deposits (already formed in the system).

Your heat exchanger could be partially of completely plugged off. Time to start checking for ease of flow through individual components of your system. Be sure to also check the hose connections on the thermostat housing. They tend to corrode up (on the inside), gradually restricting flow. If you have power steering, be sure to check for flow through the cooler.

 

[indybleck]

Re: 1988 OMC 5.0l overheating issue

Are you relying on your dash gage or are you using a thermal gun to check the temp? Make sure your gages are good, along with the sending unit. What is badly? How high is the temp?

 

[jerryjerry05]

Re: 1988 OMC 5.0l overheating issue

The manifolds can be part of the closed system and so can some risers.
Most OMC's the manifolds can be part of the closed system.The stock risers can't.

 

[MRS]

Re: 1988 OMC 5.0l overheating issue

Not sure if it works the same mine is open system but same thing happened to mine. It ended up that the pick up hose on the outdrive was split. Something else to check. Hope it helps..

 

[Frozen]

Thanks for all the info and help guys. Turned out to be a bad head gasket so I was losing coolant.