mercruiser4.3 low compression

[leoleo]

Hi, mercruiser 4.3 V6 alpha g2. Low compression in two cylinders maybe water inside. 1300 hours, 1990. Any tips on how bad can it be ? From best possibility to worst ?

[mmwjr]

What’s the compression numbers for all 6 cylinders? Have you performed a leak down test to determine if it’s a valve or ring issue?

[leoleo]

Normally 10 bars. Two of them are below 5. Seems that water enter in the engine. I'm not sure which one. Could you emphasis the different scenarios ? Faithfully

[mmwjr]

5 bars is around 72psi. To me this is either a leaky intake or exhaust valve. A leak down test will allow you to hear either through the carburetor ( intake) or removed riser (exhaust) to isolate which valve is leaking. My assumption is the exhaust manifold/riser is bad and allowed water to back flow into the cylinder which resulted in a tupiled exhaust valve. The cylinder head will require rebuilding if this is the case.