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Hi all. I need some help and opinions with a real problemo! While the engine is running in my 350, you can pull off the #5 spark plug wire and there is absolutley no change in Rpm. I found this out because the truck started missing at idle and while driving. Here is what I have all done so far. Complete compression check. All cylinders 125psi or greater except for #5 which is 115psi. New plugs, wires, cap, rotor, coil, different distributor base with pickup coil and module and vacuum advance. I made sure the rockers were going up and down on that cylinder as well. When I pull the plug out after 3 mins of idling, it is oil foulded. Rebuild heads were installed 6months ago with new valve seals. The problem came instantly and over nite, nothing gradual. No external vacuum leaks were found either around the carb or intake. Can anyone give me any ideas about the fix, or am I taking the motor apart to look deeper? Thanks to all who read and reply!
How do the rest of the plugs look? (are they fouled also?) The difference in compression does not look that bad. It sounds like your crankcase is pressurizing. Have you checked your PCV system? I have seen engines oil foul very quickly when the PCV system plugs up. I realize the heads are new, but how old is the rest of the engine?
The rest of the engine is pretty tired. The #8 piston has a wrist pin that is starting to wear (noisy) I know the engine has some blow by cuz there is always oil blowing out of the breather......PCV valve is new. Do you think its possible that the one cylinder is sucking some oil from the intake manifold? It's aluminum.
One or two things could be going on here -- Your "rebuilt-heads" could have a "valve-seal" out of place...
Stock GM seals, are small O-rings on the "retainer" end of the valve, with "umbrella-seals:, little cups that "float" on the valve stem...If the rubber O-ring is damaged or not there, oil-fouling will result...
Some "rebuiders" use the higher-quality Perfect Circle type "push-on" seals, that fit on the end of the valve-guide..
The little O-rings in the retainers are not always needed or used with these seals
Somtimes the seal can be damaged when slipping over the valve end (there is a small tool to prevent this) and sometimes, the seal can "pop-off" the end of the guide...and just "float" on the valve stem..
When this happens, you get WAY too much oil down the valve-guide and a fouling plug... -- -- The other thing is what intake-gaskets were used, and how much care in "centering and torquing" the intake manifold..
There are several types of intake-gaskets.. but the BEST are the OEM's and the Felpro 1212's, these are VERY thick and soft gaskets...
You also have to be careful when you mount the intake..make sure it isn't "cocked"
IF the heads were "cut" (machined" sometimes the 'V' between the heads gets too great, and the intake will not go down far enough to seal the ports properly..it bottoms on the block..
In this case the bottom of the intake manifold needs to be machined...
Sometimes running WITHOUT the end-seals and just using Ultra-black silicone will solve it...
Put the manifold down on the gaskets without the end seals and make sure there is clearance between the manifold and the block..
YOu might even check it before you take it apart...try an put a feeler gage or knife between the block and the bottom of the manifold.. -- -- The Valve-seal can be inspected and replace if necessary with the head on the vehicle...
If you pull the manifold, look for oil along the bottom of the gasket under the port...
IF you used those Hard-Blue Intake gaskets... it is almost sure that is the problem...
I found it! I found it! I decided to remove the aluminum intake manifold and replace it with the original cast iron. During this process I noticed that the #5 exhaust spring was cracked and missing a coil! Can you believe this? So I continued with the intake change and swapped out the spring while the head was still on. Put it all back together and fired her up, and voila! Fixed! In all of my haste I guess I missed that. But it makes perfect sence cuz that is the also the cross over to the other side via the intake, so it was always letting exhaust gas in the mixture of the # 5 and of the opposing cylinder. Well at least I found it!
Quote: ... the #5 exhaust spring was cracked and missing a coil! ... But it makes perfect sence cuz that is the also the cross over to the other side via the intake, so it was always letting exhaust gas in the mixture of the # 5 and of the opposing cylinder. ...
Thanks for getting back to us with the results.
Seems like there should've been some popping going on with the broken valve spring.
If I'm following you on the intake crossover, that passage shouldn't mix exhaust gas with intake mixture, unless the intake is cracked. Or was the intake gasket leaking too? ~SS~
Glad you fixed it, but I think it was an oil leak at the bottom of the intake gasket. or possibly a vacuum leak.. - A broken exhaust spring will sometimes (not always, depends on how broken it is) cause a "miss" sometimes not, just a loss of power as the revs go up and some noise..
A broken exhaust spring WILL NOT cause exhaust to go back through the intake.. the only thing that will do that is a worn exhaust cam/lifter, a broken pushrod or rocker, or way out of adjustment...and no lift..The valve NOT opening..
The only way to get exhaust gasses back into the intake, would be with a badly broken INTAKE spring..
Badly broken means near the middle where there is no or virtualy no pressure to close the valve.. broken near the end will usualy close it at idle..