1994 Merc 150 new powerhead running hot

[Brich626]

I Was breaking in new powerhead this morning when it started running hot. Ran at different speeds (never going past half throttle). After a 15-20 min run stoooed for

 

[Chris1956]

Re: 1994 Merc 150 new powerhead running hot

OK, why did you get a new powerhead? I presume the old one went bad? Did you fix the issue that caused the old one to fail? Usually carbs, timing, waterpump, fuel pump. Test/replace thermostats and poppit valve. Replace water pump parts. Check water pump base carefully. If it looks melted, replace it. Check water pump cover for scratches. Replace it if cracked or scratched.

 

[Texasmark]

Re: 1994 Merc 150 new powerhead running hot

. After a 15-20 min run stoooed for

What does that mean? How do you know you were running hot? Do you have any earthly idea as to just HOW hot?

 

[CharlieB]

Re: 1994 Merc 150 new powerhead running hot

Incorrect WOT timing will cause timing to be wrong even at mid throttle.

A fuel hose pinched too tight with a zip tie can cause lean running at mid to upper throttle.

You have to check EVERYTHING. ANYTHING can cause heating and toast a motor.

Check and inspect every spark plug and piston dome, color difference anywhere can help lead you to a problem.

Do NOT assume that anything is O.K. until you have checked and made sure.

 

[Brich626]

Re: 1994 Merc 150 new powerhead running hot

Sorry I wasnt able to finish my original post. Had an emergency.
anyways, ran motor at different rpm for 20-30 min. Never over half throttle. Approached idle zone and begin idling. Idled for about 1 min when alarm went off. I quickly turned around and noticed water and steam coming out of "pee" hole and immediately shut it off. Luckily the idle zone was for the boat ramp and I put it on the trailer. Installed new impellar 2 months ago and water pressure is good.

 

[Chris1956]

Re: 1994 Merc 150 new powerhead running hot

Sounds like thermostats are bad. At idle speed the poppit valve will be closed

 

[Texasmark]

Re: 1994 Merc 150 new powerhead running hot

Sounds like thermostats are bad. At idle speed the poppit valve will be closed

I realize this post is a few days old but this is Christmas time and other things are pressing.

Agree with Chris. Engines of which I am familiar (90 and 150 Vs) use stats, one on the 90 triple and one in each bank for V engines, and a high speed popoff valve to control block temperature. Seems Merc decided that you need to get your block up to 143F for low speed operation but once you advance above 2500 rpms, a larger valve aka popoff opens and allows a significant amount of water to flow through the powerhead. If running the engine in cold climates, once above 2500 the stats will close and flow will be controlled by the popoff.

So, if you have OT at slow speeds only, one or both of your stats are stuck shut. If replacing one do both. BTDT on mine and son's engines.

Mark

 

[CharlieB]

Re: 1994 Merc 150 new powerhead running hot

It is also very possible to have sucked up a piece of plastic trash blocking off water flow and toasting your new impeller.

It never hurts to inspect your water pump, (unless you drop the LU on your foot), You must be sure it is still in good condition.

You may remove the stat and start the motor with the stat housing removed to see if you still have good water flow at idle. Note: V motors will only produce water out of the lower stat housing at idle. Plugging that massive leak then the block will fill and flow out of the higher stat opening.