Terrible Rough Idle, Good MPG = *BLOWN SC gasket*..pics pg 2 - 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 03-04-2007, 03:43 AM   #1
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Default Terrible Rough Idle, Good MPG = *BLOWN SC gasket*..pics pg 2

98SSEi:

I've replaced the following as part of a regular tuneup:
Spark Plugs and Wires
Air Filter

Other steps I've done:
CLEANED IAC
SWAPPED IAC WITH KNOWN GOOD FROM A GTP
CLEANED MAF
CHECKED TPS VOLTAGE...its very smooth and sits at 0.43 at idle.
CLEANED THROTTLE BODY
SWAPPED COILS FROM A GTP


I was thinking vaccum leak but it appears all vaccum lines are good, I've gone thru just about all of them I think.

My second thought was a dirty IAC, the passage is clean, the valve looks new. MAf sensor was my next guess...well it looks like its been cleaned too and is shiney.

it got worse after the 400 mile trip today. the car does have a noisey SC coupler but it was like that before and idled smooth. even at 1100 rpm its extremely smooth.


I'm trying to come up with more ideas.

(updated thread)

as mentioned on page 2...when I spray TB cleaner near the base of the SC in the back the engine dies.
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Old 03-04-2007, 11:13 AM   #2
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A few vac lines you can't see are under the MAP rail (above rear valve cover), and under the air box (totally hidden eh) Check with a can of carb or TB cleaner..it'* a far easier way to check.

Look to ensure that no plug wires are touching the O2 Sensor wiring. And above all...change the plugs and wires first.. that'* got a 94% probability of fixing it.
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Old 03-04-2007, 11:55 AM   #3
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Plugs and wires. One thing at a time.

While you're at it, do a full tuneup.
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Old 03-04-2007, 05:10 PM   #4
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On the way to napa and autozone right now. I think I hear a hiss but the coupler is rattling so loud I cant figure it out..time to make the supercharger stop rattling and pull the belt...Update shortly.
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Old 03-04-2007, 10:02 PM   #5
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ok, i pulled the airbox and inspected the vaccum lines and they were good to go, I replaced for good measure. I also pulled the MAP (?) sensor(*) on the back of the motor and both lines were good, t-fitting was good, I tracked the lines back to the main vac port...all look good.

Spark Plug Wires and Plugs were replaced a while back.

idle is "rough" up to about 1100RPM then its smooth as can be.
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Old 03-04-2007, 10:37 PM   #6
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Could the harmonic balancer cause this behavior?
Although all Ive ever read said they were noisy until rpm'd up I was thinking it might be able to cause an issue with the crank sensor.
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Old 03-05-2007, 05:24 AM   #7
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I dont think this is a crank sensor related problem.

It screams vaccum but I dont see any leaks.

Perhaps I should swap out a known good MAF and IAC motor? what else can I do to troubleshoot this one?
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Old 03-05-2007, 08:01 AM   #8
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Follow your own advice up top and remove the SC belt. Then listen for things.
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Old 03-05-2007, 10:50 AM   #9
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Spuds...after reading your ride post on the new car. I'll have to suggest that in fixing up all the leaks and issues. You may come across the problem or at least be able to rule out many items.
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Old 03-05-2007, 12:05 PM   #10
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Considering the high miles, you might want to do this too:

http://www.bonnevilleclub.com/forum/...e=article&k=64
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