vibration under boost - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 04-11-2008, 10:41 PM   #1
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Default vibration under boost

No clue! from road speed if i step on it the engine generates a vibration when under boost, if i just give it enough throttle to aquire 0.0 psi. To equilize boost and vaccum it purrs no problem. also a vibration at idle... New plugs/wires..fresh oil ..air filter 315 000 kms on relitivly well maintained engine. any additional info needed? Possible problems? Things to check over?
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Old 04-12-2008, 01:01 AM   #2
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I had same problem with my 93 non-sc. I had new plugs and wires too. Turned out to be coil(*). Willwren said this is indicative of ignition... so, per my FSM...
The quick check to find the cylinder is:
  • -Start engine; allow to stabilize; unplug IAC motor (not to exceed 2 min.); use insulated pliers (AND a rubber glove is great too); pull one plug wire off at the coil end, at a time while listening for miss to get worse or stay the same.
    -If the miss gets worse, that is NOT the cylinder
    -If it stays the same, that is the indicator of the bad cylinder
    -You can have more than one.
    -Mine was 2 bad coils. But they had been passing the resistance checks until the morning I went out and it started missing after I started it. That is when the coils completely went out.
    -This is not required, but if you have a volt-ohm meter, you can check across the output side of each coil. They should all be close to equal. Of course, if it reads open, the coil is shot.
Something else that might help find the baddy; when I would pull the wire off a GOOD coil terminal, I would observe lots of arcing from the terminal back to the coil body. On pulling off a wire from a BAD coil, there would be NO arcing. In both cases, the coil would only be bad on one side; the other side worked fine.
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Old 04-12-2008, 02:26 PM   #3
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Ok THX i will try that. is it also a possability that a coil issue would make it hard to start the car on cold/cooler mornings(sitting over night) but start fine on warm mornings? Also if i shut the car off for say 15-20 min.. when i fire it up again it will idle errattic and stall if i dont give it any gas...ideas?
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Old 04-13-2008, 06:42 PM   #4
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I would only be guessing at this point, but the cold vs. warm piece would seem more air/fuel related. Also, my SSE is NA, so I'm not comfortable diagnosing SC things. But if you are missing spark, it may be effecting your start up too. A guess.
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Old 04-13-2008, 07:28 PM   #5
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ya i think maybe it is the FPR for your help. would a faulty coil cause my tach to malfunction?
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Old 04-13-2008, 09:02 PM   #6
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The tach gets a signal from the ICM. Assuming 98 ICM is the same, the white wire on pin E of the ICM is the tach output. So, another thing would be to unplug and clean the ICM male and female connectors. Something like carb cleaner may do the trick.
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