Working on a 3100, Gordy did the LIM gaskets on it, and now it won't idle well.

We tried a new MAF, cleaning the IAC, were gonna check the coils and plugs, and look for vacuum leaks.

If we unplug the MAF, it idles fine, with it plugged it bounces between 800-1100ish RPMS. It seems to run fine once you give it some throttle.

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Jason Z - Exposed Autos

2001 BMW 330i - Titanium Silver - Sport Package - 3 Pedals - Koni Yellow/H&R Sport
2006 Volkswagen GTI - (gone) Tornado Red - DSG, Stage II~280hp/325tq
1993 Pontiac Bonneville - (gone) Purple Pearl H4U/SLE. Loud

Sending Ed and Jason out for a junkyard (tested) MAF. Everything else checks out. MAF unplugged and it runs smooth, no miss-fires or anything. If we run it with the MAF plugged in, it runs very rough, and usually ends up stalling out. It also throws tons of random misfire codes when it runs rough, but never when the MAF is unplugged.

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Owner / Admin

1995 SSEi / 2002 SSEi / 2001 Sierra C3 DENALI

May have it isolated down to in injector or intake leak on the back side of the engine. We were able to spray a small area with intake cleaner, and the engine idle responded accordingly. It was too hard to see exactly where it might be. Me and Ed just tore the upper intake back off and loosened the injectors. Ed has to go and I am back on kid duty again, so either later tonight or tomorrow I will go at it some more.

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Owner / Admin

1995 SSEi / 2002 SSEi / 2001 Sierra C3 DENALI

Should have called me! I was only 30 miles away. I'm back down in Waseca now though.

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Andrew - owner/operator of Bonnevilles Unlimited

2004 Bonneville GXP | 60k | White Gold Tricoat | custom built supercharged 3800 hot rod | garage queen
1997 Corvette | 55k | Silver Metallic | Z06 wheels | Date-night Hauler/Parts runner
2014 Town & Country Limited | Cashmere Pearl | 95k | Family Hauler
2002 Ram 1500 Quad Cab Sport | Black| 260k | Official Bonneville Hauler

Well, next time you in my neck of the woods, you should just stop by and say hi anyways

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Owner / Admin

1995 SSEi / 2002 SSEi / 2001 Sierra C3 DENALI

I just did 2 of those not too long ago. I do remember there are a couple vacuum lines by the firewall there. Right under the coils.

Yea, there are some going to the MAP right there, but those didn't seem to be it. Vacuum actually seemed to be OK from what we could tell.

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Owner / Admin

1995 SSEi / 2002 SSEi / 2001 Sierra C3 DENALI

You do have the plug wires on correctly right? I found out that the #1 cylinder is actually the passenger side towards the firewall. The last one I did idled like crap and gave the misfire codes until we figured that out and then found to bad plug wires at the same time.

Yup, we checked the cyl and coil arrangements.

When we sprayed below the intake manifold, next to the MAP, it ran fine, so the vacuum Tee or the gasket isn't 100%

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Jason Z - Exposed Autos

2001 BMW 330i - Titanium Silver - Sport Package - 3 Pedals - Koni Yellow/H&R Sport
2006 Volkswagen GTI - (gone) Tornado Red - DSG, Stage II~280hp/325tq
1993 Pontiac Bonneville - (gone) Purple Pearl H4U/SLE. Loud

They called me, (no kidding) but I was too far away to come over, and they didn't have the right kind of beer.

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Click here for mod list for both cars
93 SSEi, 95 SLE (supercharged) 97 Buick LeSabre Limited
PontiacDad at WCBF '04: Cruise control? That's like surrendering!
Comprehensive guide to troubleshooting, rebuilding, and modifying Eaton Superchargers

There is also a Vac port near the alternator on the back of the intake, Other possible leaks are the PCV system( part of it is in the rear valve cover under the ICM, the other part hooks to the PCV Valve in the front valve cover...

How much does it smooth out when you unplug the MAF?

Also read the idle change with spray..... Check all vac locations according to the Vac diagram...

Did you do intake gaskets on this engine or just trouble shooting it?

I redid all the gaskets on it (the known gm leaking engine). With the maf plugged in it gets huge missfires in all cylinders, unplug it, and the idle goes perfectly smooth. Right now I took the upper intake off again to check a few things, and hopefully will get it back on this afternoon to test some more.

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Owner / Admin

1995 SSEi / 2002 SSEi / 2001 Sierra C3 DENALI

What year 3100 is it? 96 and newer I'm assuming. Factory intake or Aftermarket? Does it run extremely rich with the MAF unplugged? The PCM could be toast if the MAF is good. My Father-In-laws 5.0 did the same thing. The PCM was stuck in a full rich loop with the MAF unplugged and become extremely confused when the MAF was in. The PCM was stuck in full rich and was trying to let the MAF do it's job, which led to the irratic idle. We replaced the Intake and MAF with another brand and swapped PCM's. The car runs fine now

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Bonneville Forum COTM Sept. 09
Pontiac Bonneville Club COTM Dec. 09

Talked to my sister-in-laws BF (former ASE mech). I think I will try replacing the IAC next. He has seen many of these fail on the 3.1/3.4 engines before. It would also give similar symptoms when it does.

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Owner / Admin

1995 SSEi / 2002 SSEi / 2001 Sierra C3 DENALI

Spent $25 on a new IAC, no difference (junk yard ones were $20). I followed my ear and found a semi hidden bolt hole on the TB housing without a bolt. I could hear some air getting thru it. Found a bolt from my ssei bolt bucket (I know.. don't say it) in the garage and tightened it up. I couldn't even start it then, it would stumble and die before ever really going. So I unplugged and replugged in sensors in various orders, and all the sudden it was running smooth, and that was with everything plugged in. I started and stopped it a half dozen times with simliar results. Probably take it for a drive later today to see if it just might be fixed (9 months later... lol)

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Owner / Admin

1995 SSEi / 2002 SSEi / 2001 Sierra C3 DENALI