Manager Forum

This tired G van and a 5.7L engine with 185,000 miles on it came in with a misfire complaint. Upon diagnosis, we discovered the two center cylinders, #4 & 6 on the right bank had no compression. Assuming we were dealing with a blown head gasket we proceeded with head removal. The machine shop informed us that both heads were cracked. We found a set of good heads, had them checked for cracks and resurfaced. We installed them along with all the other parts. Got it running but cylinders #1 & 4 are misfiring. We have good spark, 150 psi of compression, 13 lb of fuel pressure, 19-20 inches of vacuum at idle. Customer wanted new spark plugs and wires which we installed. The vacuum gauge flickers between 16 and 20 inches of vacuum when it is idling. Thinking we may have a coil pickup issue, we installed a known good used distributor with no better running. We removed rocker covers thinking maybe the rocker adjustment is too tight or maybe we have a camshaft lobe or two gone but the cam lobes/pushrod lift are all right on the money. Loosened rocker nut and readjusted it with no change. With good spark, good fuel, no check engine light and good compression, what are we missing? I am more frustrated over this one then i have been for a along time. Any suggestions as to where to look?

The bounce in the vacuum gauge would lead me to think bad/sticking valve...something in the valve train. Did you check the lift on the ones misfiring and compare to good cylinders. Dial indicator would tell the story on a bad lobe or lifter. Just my thought, HTH,Jeff.

What will the vacuum gauge show if the intake manifold gasket is leaking underneath? How do I check for that?

I would think a leak along the bottom would show excessive crankcase vacuum,not sure if it would cause a flutter. Maybe check at the dipstick tube. Did you check running compression on the weak cylinders compared to good cylinders? That might give you a indication of weak springs or sticking valve.

I am agreeing with Jeff, I would check running compression and compare it. I have seen weak springs cause the issue. Weird the way it was 4 and 6 first and now 1 and 4. Compare that and see what it tells us.

id also smoke the vac system for leaks. sounds like vac leak or valve train issue to me. I try not to go beyond the job i did and since the work was done, the misfires moved. Let us know what you find.

Since the mis moved Id be double checking the head work. Good luck

Ok. This one is another reminder to go back over your work. The fix was to go back over valve adjustment. Cylinder #1 valves were just a little too tight and apparantly the pulsations in the intake manifold were passing over to cylinder #3. Once we got the #1 valves adjusted properly the motor ran like a top. One thing we are still trying to figure out is how we could have good compression even though the valves were too tight. So - - - - the moral of the story is sometimes we must go back over what we do and recheck our work. But, in the mean time, guys, thanks for the ideas and help. It is helpful to realize that there is expert help available when we need it.

Thanks again!

Thanks for posting the fix. An issue of the forest and the trees? I've lost count of how many times this has happened to me. I had a chev I sent the heads out on and the machine shop had to install deeper seats because I wanted different valves and never adjusted the valve stem length. On your application the compression and fuel spark timing was off. The valve could be opened sooner because of being over tightened then the spark and fuel would come into play. Be thankful it was a noticeable skip and wasn't sent out. You would have trashed cats and a bad valve in about a thousand miles. Looking at this all over again I know the easy way of figuring it out but I would still be banging my head a bit on this one.

Well, after connecting my old trusty vacuum gauge and observing it flicker, I was somewhat thinking it had to be a valve related issue but we needed to prove it. And we did. Trust me, I aint throwing the vacuum gauge away yet.

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