1989 Mariner 135 V6 overheating

[caithnessdiver]

My trusty Mariner 135 suddenly started setting off the overheat alarm recently after years of uneventful service. I researched the matter on iBoats and replaced the thermostats and poppet valve as these seemed to be the most likely culprits. I ran the engine with the flushing 'headphones' fitted and at 3000rpm, after about 10 mins running, the exit temperature of the water at the telltale was 66deg C. This is slightly higher than the recommended 61deg C, but not too much higher. There was only a dribble of water at the telltale on startup, but this rapidly increased to a strong flow as the engine warmed. The impeller in the water pump is fairly new, only done a few tens of hours since new.

On the first trial on the water, the engine was still overheating and my pal in the boat said that the telltale was "bl**dy hot!!" when he checked it. I didn't have a thermometer with me to check the temperature at the telltale. After 15-20 secs at lower revs, the overheat alarm stopped and I was able to cruise at a little over 3000rpm, which is actually a slight improvement over the situation before I replaced the thermostats and poppet valve. I didn't think to ask my pal to check that there was still water coming out of the telltale when I was cruising at speed-absence would have suggested that the poppet valve was bypassing too much.

Any advice or insight regarding the way ahead would be very welcome. My money would have been on the poppet valve, but that has been replaced with a new one and I am a bit mystified

 

[Chris1956]

Check the compression. Perhaps you have a leaking head gasket? That motor should never overheat in neutral on a flusher, unless the motor is not getting water, for some reason. Check the water intakes for blockage?

 

[caithnessdiver]

The engine didn't overheat (66 deg C) at 3000rpm with the flusher or at 3000rpm out on the sea, but did at higher revs on the sea. The flow from the telltale was strong when in neutral or using the flusher, but I didn't think to check the flow when under load at speed.

 

[Chris1956]

A friend of mine has a 200 Merc of about that age. His motor will overheat at near full throttle as well, and we changed the 'stats and poppet valve. I am therefore very interested in the outcome of this thread. A leaking head gasket is a possibility, I would think.

 

[Dukedog]

did you replace JUST tha impeller or did you do tha whole pump? oem? water pressure gauge?

 

[caithnessdiver]

It was just the impeller that was replaced (OEM). (The old one didn't seem to be very worn btw.) There is a good strong telltale but I haven't been able to check yet if it is still strong when the alarm goes

 

[caithnessdiver]

Finally managed a sea trial and the telltale was strong when the overheat alarm went off, so the poppet valve is probably not bypassing. Next step is a compression test. As the cylinder head had never been removed since new, I have a sinking feeling about trying to get the bolts removed if I need to replace the head gasket

 

[caithnessdiver]

All 6 cylinders are showing 100psi +/- 10psi on a compression test so all seems well. I took the water pump apart and the impeller looks fine, but I will replace it anyway. Running out of options!!

 

[Dukedog]

tha "very first thing" with an unknown heating problem is a "complete" water pump kit (oem)... tha stainless plate and cup in tha housing can be worn and not be able ta tell just by lookin' at it.. this eliminates tha most probable cause of heating problems... ya can't tell by lookin', feelin' tha tale!.....jmo