overheating question, stumped!

dbetterred · #1 06-03-2010, 08:19 AM

I got a new (rebuilt) 150 merc installed this spring. I've been having overheating problems since day one. The motor guy is fixing the problem but, he is getting stumped and i'm getting frustrated.
The motor overheats between about 1300 and 3000 rpm. The pisser works the whole time, until it get too hot and its just steam. Above about 3000 rpm it will cool back down. Below 1300 it will run all day.
It only has the problem while in gear with the boat moving.
So far, everything has been replaced. There is a new powerhead (after the original). New poppet valve. New Thermostats (also tried without). 2 new impeller and housings. Even a different midsection and water tube. Literally everything except the lower unit housing has been replaced.
The boat is at the motor guy right now and he is getting a lower unit for it. I hope to have it for the weekend.
The height of the motor is the same as my old one so that shouldn't be an issue.

Other than overheating the motor runs great.
My motor guy seems knowledgeable and is doing his best to make sure I'm satisfied. I'm just getting frustrated.

Any ideas?

Sorry 'bout the long post.

spareparts · #2 06-03-2010, 12:59 PM

check the gasket between the upper amd lower mid plates, its the part that sandwiches the upper motor mounts. Seen it blow out before and force exhasut into teh cooling water, cost us 2 pistons adn a lot of time before we figured it out

dbetterred · #3 06-03-2010, 02:16 PM

Is that the gasket directly below the powerhead? Or is it one that would have to be replaced if removing the midsection? The motor guy just replaced the midsection and I know he had to run and get a gasket that he didn't have in his shop.
The reason I ask is he replaced the whole midsection and the problem remains.
I'll mention it to him when I pick up the boat, hopefully tomorrow.

spareparts · #4 06-03-2010, 05:21 PM

its teh gasket that you replace when you replace upper motor mounts, if he replaced the entire mid, it should have taken care of it. The description you gave of the problem points directly to the poppit valve, but you said its been replaced, make sure the spring hasn't been altered or cut(old hot rod outboard trick that didn't do anything)