Unexpected vapor

Have a mercruiser 305 and just rebuilt the heads. Started for the first time and noticed vapor coming out of the carb. By chance I pulled the oil fill cap off and noticed pressure had built up under the rocker arm covers and more vapor. Also engine reved about 500 more. Put cap back on and engine rev drops. What is the cause/solution?

Re: Unexpected vapor

Few questions first

1. Why did you rebuild the heads to begin with?
2. What year/model engine.
3. Did you follow the OEM service manual (or any manual for that matter) for disassembly, inspection, and reassembly?
4. Have you ever done this kind of work before?

Re: Unexpected vapor

1. Found 3 bent pushrods from stuck valves.
2. 2000 GM 305 5.0L
3. Yes, followed manual
4. I have some personal experience but limited.

Re: Unexpected vapor

should be a breather some where.

Re: Unexpected vapor

Why were the valves stuck?

Was there rust in the cylinders? Has the engine ever been stuck from rust and wouldn't turn over? But with enough force it finally moved?
Were the cylinders worn out, pitted from rust?
Were the exhaust manifolds and risers ever checked?

I would strongly suggest a compression test, then a leakdown test to determine which cylinder and what in that cylinder is causing the blow by.

Re: Unexpected vapor

Exaust manifolds leaked back into the cylinders, hydro lock. (salt water) Cranked to force water out plug holes as best I could. Did not try to start the engine for a couple months, sprayed oil in spark plug holes and would turn it over every once in a while. Drained oil and changed filter. Installed new manifolds and risers. Fired up the engine and heard loud tapping. Stopped, pulled valve covers and noticed the bent rods. Pulled heads. Tapped out 3 stuck valves. Sanded, lubed and lapped valves and established shiney seats. Nothing bent, no pitting and all move easily. Some cylinder walls had minor rust marks, no pitting. Wiped them out and sanded lightly with black wet paper.
Checked compression today. It's all over the place from 40 to 180, but sounds like it runs and idles ok. Checked valve lash again. Seems right.
Can you walk me through the leakdown process? Any other thoughts?

Re: Unexpected vapor

Chris58 said:

Tapped out 3 stuck valves. Sanded, lubed and lapped valves and established shiney seats. Nothing bent, no pitting and all move easily.

Click to expand...

That is not what I consider rebuilding heads.

Some cylinder walls had minor rust marks, no pitting. Wiped them out and sanded lightly with black wet paper.
Checked compression today. It's all over the place from 40 to 180, but sounds like it runs and idles ok. Checked valve lash again. Seems right

Click to expand...

I'm sorry, but you need to find a mechanic

Re: Unexpected vapor

Tapping out stuck valves could well have messed up the valve guides which could account for some of the pressure.

Also could have the head gaskets on wrong.

Valve lash? There should not be any lash, they are hydraulic lifters.
Factory settings are 1 turn down from zero lash with lifters full of oil. If you go too far the valves won't seat, if too little they won't open.

You need a mechanic.