94 Skylark Custom 3100 erratic idle/stalling

1994 Buick Skylark Custom
3100 SFI
230,000 miles
Auto 4sp


What has been done:

"Replaced" cylinder head gasket(nearest radiator)
" " cylinder head bolts
" " Lower intake manifold gasket
" " Exhaust manifold gasket(nearest radiator)
" " Upper Intake(plenum) gasket twice
" " Both rocker panel(valve covers) gaskets
" " Thermostat with a "fail open" type (when/if it fails, it will fail in the open position)
" " Upper and lower radiator hoses
" " Engine Mount(upper anchor)
" " serpentine belt
" " belt tensioner
" " one rocker arm and exhaust pushrod
" " All spark plugs and wires
" " All fuel injector o-rings
(All gaskets are fel-pro)

Labor: Took apart, cleaned, lubed where necessary, installed gaskets and such, and torqued bolts to spec per Alldata and Haynes manual

!st startup after assembly; fuel squirts from fuel return line...its the line that threads into the fuel rail. Found that a missing o-ring was culprit.

2nd startup after reassembly; starts up, idles very rough, but ran well enough to back vehicle out of driveway, but bad clanging sound from engine, no fuel leaks, oil leaks, or coolant leaks. Found that during assembly the 4th cylinder ex and in pushrods were switched...sigh...found exhaust pushrod pushed through rocker arm. Pulled valve cover, extracted pushrod and rocker arm, lubed, and replaced the exhaust pushrod and rocker arm.

3rd startup after reassembly; Engine idles very rough, RPMs bounce from 1500 to 1000, and stalls when braking to a stop. When stopped, it stalls. Can restart, but need to press down on gas pedal. No smoke. Found throttle body was a bit separated from plenum. Threads stripped a bit. Replaced with longer bolts to make a better seal between plenum and throttle body.

4th startup, starts, runs again very rough. After a few minutes of idling erratic, it smoothes a tiny bit, but takes the RPMs down and eventually stalls. Each time trying to start immediately, it starts and runs down the RPMs. It reminds me of a vacuum leak, but all vacuum hoses are tight and in operation. Anyhow, I pull plugs for some observation and find them totally carboned up. Changed plugs and wires.

5th startup, starts, idles a bit smoother but nothing near what it should. Again, after a while RPMs plummet stalling out the car and immediate starts will start but plummet down to stall. I am able to keep the car running by keeping my foot on the acelerator.
No black smoke. No leaks.

Am planning on doing a scan on the computer although I've had no check engines, checking the throttle body plate tiny hole for any blockages that might have occurred , changing out the IAC valve and throttle position sensor. It runs like it has a big vacuum leak, to me. I was advised to use a lot of gasket sealer on the plenum because there were problems in the past with vacuum if the 3100 plenum were not sealed well.

Any ideas on this problem I am having??



I'd gladly donate if I can get this problem solved. :D
Do you
have the same problem?
Sunday, October 21st, 2007 AT 4:48 AM

2 Replies

I would make sure that you got all your pushrods in the right locations. I would also suspect that if that one pushrod pushed through the rocker arm that it has probably damadged the valve. I would inspect that valve or at least do a compression test and a leak down tests. Have you put a vacuum gauge on it? How did the car run before the intake repair?

Was this
Sunday, October 21st, 2007 AT 7:33 AM
Thank you Cly for your input. I believe that you are correct as the exhaust rumbles when trying to keep the idle steady around 1000 to 1500. Higher idles run smoother. As to your question I put a vacuum gauge on it about 10 minutes ago. I had to keep the engine running, but at low RPM(abt 1000) the vacuum registered at 8 in Hg and at high(abt 3500) 15 in Hg. The exhaust was being put through the ringer at the lower RPMs.

The car ran smooth with a little light hiccup here and there.

I don't have the leak down tester nor the air compressor to to that test. I do have a compression gauge -- just tested #4 cylinder, no compression, which is where my problem was at with the pushrod/rocker arm. Checked #2 cylinder to make sure I was operating the gauge right and came up with 80 psi.

Thanks for your help!

Was this
Sunday, October 21st, 2007 AT 12:18 PM

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