Runing rough at idle in P/N and on decel (11/14) - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 09-16-2007, 01:46 PM   #1
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Default Runing rough at idle in P/N and on decel (11/14)

My idle is rough in park and neutral and my scantool is showing misfires on all cylinders. If I put it in drive, the misfires go away and it idles smooth.
Another thing is, at certain speeds when I decelerate, the rpms come down and bounce between 1500 and 2k.
Those problems will only happen if the car has reached operating temp too.
Other than that the car runs great and pulls hard.
There are no codes either.

Everything I've checked / tried:
Fuel pressure : Good
Vacuum : Good
Fuel Trims : Good
EGR tube has no cracks
Cleaned the MAF
Fuel system cleaner

Swapped:
IACs
ICMs
PCMs
Plugs and wires are new
Tried stock 95' TB with stock MAF and TPS, no change
Edit : swapped EGRs too

I also tried increasing values in the Commanded Airflow vs. RPM vs. Gear Ratio Table. This is what controls the decel rate. This also did not cure the rpm bouncing on decel.

I had to repair my engine harness from my accident too. I soldered and shrink tubed everything and searched hard for any damage. If I see a 95 SE at the yard I'll grab the harness and try it out.

So, any ideas?
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Old 09-16-2007, 02:49 PM   #2
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You wouldn't have any missing grounds would you? Or maybe an unhappy ground?

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Old 09-16-2007, 03:41 PM   #3
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Jeez... after everything you've already tried, I would be mystified.... Don might be on to something with a bad ground?

This is a tough one...
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Old 09-16-2007, 11:21 PM   #4
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Hmm. I know the ground from the battery to the body is good but the engine to battery may not be.
Instead of bolting it to the block I bolted it to the same stud the ICM is grounded to...

Ok, I ran outside after I typed that and relocated that ground to the block under the front header. I sanded it down too.
It still does the same thing when it'* warmed up. :(
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Old 09-17-2007, 09:22 PM   #5
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Only does it in open loop... Interesting..

I am at a loss

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Old 09-17-2007, 10:28 PM   #6
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I had a cracked coil post which caused my SSE to add funny.
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Old 09-18-2007, 12:08 AM   #7
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Quote:
Originally Posted by jr's3800
Only does it in open loop... Interesting..

I am at a loss
Well, the decel problem comes on after the first couple stops when the engine is just heating up.

The misfire in P/N starts when coolant temps are 130* or so and it gets worse as it increases to operating temp (180-185ish)

I'm going to go to the yard this weekend and see if they have a 95 SE. I'll grab the engine harness if they do and I may try adding another ground before the weekend.
(I'm actually on travel till Thursday)

I did have all the coils off a couple times and didn't see any cracks. I'll check agian later in the week. Can't hurt.
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Old 09-18-2007, 02:30 AM   #8
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It'* random/multiple misfires, right? So no way to track a misfire to a different cylinder if you switch the coils around? See if you have more misfire counts on any particular cylinder, then switch coils and see if it changes. I can't imagine all 3 coils failing at once, though

You already tried everything I would have thought to try. Maybe you got a bad bunch of plug wires? Might want to try testing them...
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Old 09-18-2007, 11:12 AM   #9
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Quote:
Originally Posted by big_news_1
It'* random/multiple misfires, right? So no way to track a misfire to a different cylinder if you switch the coils around? See if you have more misfire counts on any particular cylinder, then switch coils and see if it changes. I can't imagine all 3 coils failing at once, though

You already tried everything I would have thought to try. Maybe you got a bad bunch of plug wires? Might want to try testing them...
When the misfires start, they alternate between all the cylinders then all cylinders misfire when it warms up.

I've had a bad spark plug wire before and it would only arc under load. Should I check the resistances?
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Old 09-26-2007, 10:42 PM   #10
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So,
Now when I'm in really heavy traffic,
on my way home from work,
the engine is well heat soaked and coolant temp is as high as it gets (190ish),
AND I'm completely stopped in drive,
the engine will randomly feel like it'* missing and the RPMs will dip down and come back up.
I haven't had this happen in drive before..
This will only happen when it'* hotter out. It'* cooler in the morning and this dosen't happen on the way to work.

I tried shaking various parts of the engine harness while the engine is hot and missing in park but it dosen't effect it at all.

I went to the u-pull today to get another engine harness. This is the second time since my last post I've gone and they still don't have a 95 L36.. It'* really suprising. They always have them..
I'll try agian next week.

Tonight, I added another ground to the front of the block and I sanded the crap out of where the battery connects to the body.

It still does the same thing in park and during decel. It didn't miss in drive but it'* much cooler out.

Any more ideas??
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