1990 OMC Cobra 4.3L All cylinders bouncing from120 to 0 psi after repairs

[sikemd]

I just had a warped valve replaced on one side of my v6 and we only pulled the cylinder head on the side that was broken. Replaced all the gaskets and put it back together, it ran good for about the first hour then started running running poorly again. The engine was never taken passed 3500 RPM. After we finished the repairs all cylinders had ~180psi. Now I just tested the first 2 cylinders on both sides, the gauge jumped to 120psi than back to 0 and continuously bounced between 120 and 0. The first plug on the side we did not do any work too appeared to be wacked by a valve, it had a nice dent in it. We did not take that side apart at all. Any ideas here?

 

[sikemd]

Re: 1990 OMC Cobra 4.3L All cylinders bouncing from120 to 0 psi after repairs

I want to add that someone has suggested the manifolds could be the culprit in this. I was told to check the exhaust portion of the manifold for rust, and now that I think about it the manifold that we removed was quite rusted in the area that connects back to the Y pipe. It is a single piece original manifold and I would expect the other side is rusted as well. I am now led to believe that water in the cylinders was/is the cause of the original valve deformation as well as potentially blowing out both head gaskets this time around.

 

[HT32BSX115]

Re: 1990 OMC Cobra 4.3L All cylinders bouncing from120 to 0 psi after repairs

Howdy,


Welcome to iboats!



Well, if you think you have manifold problems (batwings, right?) and you have evidence of water intrusion on one side from internal leaking, continuing to run those manifolds will eventually destroy the engine.

A leaking riser/manifold (usually) isn't a problem when running because the pressures produced by the exhaust simply pushes all the water out with the exhaust gases.

The manifold is completely full up to the elbow when running.

When you shut down, any water in the manifold/riser will begin leaking back into exhaust passages and directly into any exhaust valves that remain open after the engine stoos turning.

THEN when you crank the engine you will have a hydrolock. If the valve is hot when the water hits it, it may warp enough to stick or break and/or cause valve sticking (causing the intermittent 0 compression)

If you did not take the other head off, you probably have the same problem on that side too.

I would say you're where you'll need to replace both manifolds with a center riser KIT and probably do at least a valve job on both sides.


Regards,


Rick