Mercruiser engine overheating only at slow speed

[Costa]

Any advice, suggestions?? I have a 454 mercruiser with Bravo 1 leg. It is overheating at very, very low speed .. idling speed as well. There doesn't appear to be any issue when travelling over 4 or 5 knots up to 20 knots. As well, the antifreeze seems to be disappearing in the past few days and I have to keep topping it up daily. There doesn't seem to be any leakage or at least I don't see any anti-freeze in the bottom of the engine compartment. The mechanics suggested I check out my temperature gage to make sure it is working and it is. The engine has about 240 hours on it and was put in as a reconditioned engine last year.

Any ideas?

 

[Haut Medoc]

Re: Mercruiser engine overheating only at slow speed

Coolant loss on a closed system is usually the sign a bad heat exchanger, & thats where I would start looking.....
Drain the system, remove the exchanger & pressure test it....
It should hold 16 pounds all day long....
If it does not, you have an internal leak....
Sometimes they can be repaired, sometimes not....
Take it to a radiator shop or replace it....
In my experience, once they leak, junk them & get a new one....

 

[cr2k]

Re: Mercruiser engine overheating only at slow speed

Overheating at low speed only is a sign that your seawater impeller is going bad.
They should be replaced every other year at least, if you run the boat or not.

 

[Haut Medoc]

Re: Mercruiser engine overheating only at slow speed

Overheating at low speed only is a sign that your seawater impeller is going bad.
They should be replaced every other year at least, if you run the boat or not.

I agree with the impeller check/replacement...
But it does not explain the coolant loss....
I suspect he not only has a leak, but has corrosion/clogging issues in the exchanger tubes, possibly from marine growth, & that can cause overheat as well...
Best to check both....

 

[HT32BSX115]

Re: Mercruiser engine overheating only at slow speed

Coolant loss on a closed system is usually the sign a bad heat exchanger, & thats where I would start looking.....

If it's a "Full" closed system, the second place to look for leaks is the blocking gasket at the risers/extensions.

If it's leaking, it will allow coolant into the exhaust which also could allow a little to run back into the exhaust on shutdown.

 

[Costa]

Re: Mercruiser engine overheating only at slow speed

Thanks...will check out the heat exchanger and impeller issue.

 

[paulrfrancisco]

Re: Mercruiser engine overheating only at slow speed

I've been battleing the same issue on a 1990 454... The Heat exchanger was pressure tested several times to 15psi and no leaks... but the pressure tester seals from the inner sealing surface of the neck of the H/E. Next I thought it to be a bad cap, so I replaced the cap, and same issue. Next I inspeced the neck sealing surfaces very closely and there were 2 little knicks on the top sealing surface. I filed them out and then the problem was fixed....In retrospect, I was constantly seeing a little bit of antifreeze on the fuel pump housing, but I always attributed it to when I refilled it, it would overflow a little and spill out onto the fuel pump and into the bilge... My filler cap is on the left side right above the fuel pump...​