1988 70hp exhaust gasket cause overheat??

[horiconducker]

Ive been chasing a overheat problem with this motor for longer than i like, but i think I'm getting somewhere now. Ive installed a new brp water pump kit with the tell tale relocation and new thermostat spring, but still have a overheat (verified with temp gun) at higher speeds with a strong tell tale stream. After it happened this weekend i took the thermostat cover off and started the motor with no thermostat cover or thermostat to verify water flow. When i pulled the t-stat out i noticed it has exhaust soot on it .When i first start the engine with no stat or stat cover ii get exhaust flowing out of the thermostat area for a few seconds before water starts gushing out, not good. My question is which area is most like to cause the exhaust leak, exhaust cover gasket, powerhead base gasket, or head gasket? I don't think its the head gasket though, checked compression is 112-115 on all three. Can the exhaust cover gasket on this motor 56ci even leak exhaust into cooling system? I found a picture on here of the exhaust plate removed, but couldn't tell for sure if it could leak exhaust into the cooling system. I know on the 49ci motors there were a more plates and gaskets. Any insight is appreciated thanks ducker

 

[racerone]

That bit of exhaust coming out is normal.-------------Exhaust cover gasket can not cause exhaust leak into the block.

 

[horiconducker]

Doesn't the exhaust heat the cooling water then?

 

[racerone]

Just accept it as fact.

 

[horiconducker]

I'm back to stumped then. One thing I did find strange I had someone drive while I was checking temps, temps stayed at 140 on top of the head by thermostat housing where the omc manual states to ckeck temps but other areas on the block and cylinder head were hot 190ish

 

[flyingscott]

Did you take the nipple off of the end of the pee-hole. The fact that you found the exhaust soot on the thermostat is a symptom as the water flow should have cleaned it off. Did the old waterpump look good or was it in pieces and did you put the 5 blade pump in.

 

[horiconducker]

Yes i took the restrictor nipple off of the tell tale. No chunks of impellers have been missing( I've put the last 3 water pumps in it). Yes its the new 5 or 6 blade pump. I have verified the thermostat/ poppet valve pushes open on the muffs. Temp at idle is 140 ish and increasing the rpms to 2000 temp starts to drop rapidly. On the first start of the day I can run for 10-15 mins at 4000rpms before alarm goes off, it then seems the rest of the day it only takes a few minutes at cruise speed to overheat. I can idle and run up to 2000 rpm all day with no overheat. thanks for any further input ducker

 

[horiconducker]

This problem all started after a sheared impeller key caused a overheat alarm last summer. I have since sheared another impeller key a few weeks ago( done with that brand of water pump kits). Same exact symptoms with both the aftermarket and oem water pump kits.

 

[flyingscott]

Are you replacing the housing and all and is the oring on the housing. You should not be stripping keys that usually happens when the pump gets to hot. Are the intakes clear down in the lower unit.

 

[horiconducker]

Yep replacing entire kit housing and all including the oblong oring for the housing.The stripping key problem seems to be aftermarket part problems brp parts fit much better. Intakes are clear and screens are in place