2000 Pontiac Montana intermittent high idle

cozwurth

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#1
Hello, all.
I have a 2000 Pontiac Montana (3. 4L) with about 168K miles that has an intermittent high idle problem. It happens whether the engine is cold or warm, and there is no SES light on and no trouble codes. I took it to a trusted shop today to have him take a look at it with his scanner to see if he could find what's wrong. When the idle was high (2. 5K+) he said that all of the sensors looked to be in the 'normal' range, except for the IAC. I don't remember the exact numbers he told me, but he said that the computer seemed to be commanding the IAC to open up, which was causing the high idle.
The TPS was at . 53-. 55 at idle, even during the 'high idle'.
The CTS started at a little above ambient (about 70-72°F) and climbed slowly as the engine warmed up. (it had been driven a couple of hours before)
The EGR sensor said that it was closed, no flow.
The IAT was at about 104°F
The IAC, if I remember correctly, was at about . 2-. 3 while idling normally, and . 7-. 8 while idling high.
When this thing decides to idle high, it sometimes take 3-4 times of shutting it off and restarting to get it back to normal.
Also, when starting (warm or cold) if it is going to go into the abnormal 'high idle', it will briefly stumble first. Kinda hard to describe, but something like this. . . crank - light off to 1100 RPM for less than 1 second, stumble down to about 500 RPM for a split second, then zoom - off to 2K-2. 5K. I'm at a loss here. Any ideas?

Dave
 

billr

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#2
Can you find the port in the throttle-body where air enters the IAC passages? It would be interesting to plug that with your finger and see if the idle drops to normal. That would kind of prove it really is IAC air causing the high idle and not a vacuum leak somewhere. When the scanner was on it, did you run the engine long enough for it to fully warm up, and did the CTS then read something appropriate, like 190-210F?
 

cozwurth

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#3
Bill,
I will look for the port to see if I can get to it, but I doubt I can get to it while running without disconnecting the intake plumbing. IIRC it is in the throttle body itself and the computer will get angry if it does not see a reading from the MAF. I don't know exactly how long he left it running and hooked up to the scanner, but he did say that the CTS readings were normal. He didn't see any spikes or dips in the reading as it warmed up. I just don't see how the computer could 'command' the IAC to be open that much without setting a code for something. Is it possible that there is a problem with the IAC itself, but it's not bad enough to set a code? I am planning on pulling the IAC to inspect the pintle and the port to see what I can see.

Thank you,
Dave
 

Gus

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#4
Well, we weren't there to see the scan data either.......but the readings you give for the IAC don't jive with an IAC problem......

Need to also see the O2 sensor readings and the block learn readings when this happens......normal counts for an IAC are usually 10-15 counts....if the idle needs to be increased(because of power steering need, a/c on, etc.) the counts will go up to increase the idle speed.....

Not sure of the "." in front of your data, but they seem low like the PCM is commanding it low to bring the idle down....usually if there is a vac leak, a dead indication is the IAC will read zero counts.....

So we have several possibilities: PCM, vac leak, intermittant IAC problem.......determining factor is to see those IAC counts, O2, and block learn readings when the idle is high......also there is an IAC tool that can be used to open/close the pintle.....hard to believe a code isn't set when idle is high, and it shouldn't be......
 

cozwurth

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#5
Gus, I have some clarification on the IAC counts. I just called the shop back to get a little more info. What he told me was that the IAC counts were in the mid 20's to upper 30's while warm, no load, and idling at about 600-700 RPM. When the idle went high, up to about 2. 5K, the IAC counts were in the mid 70's to low 80's. In both cases he said that the O2 was switching lean/rich normally. He did not remember the exact counts for that, but he said that it was nothing out of the ordinary. He offered to let me come by the shop to use the scanner so that I can get a better baseline, and hopefully get more info while it is idling high. That will be a good oportunity to get the block learn readings, too. Is there any way that the IAC can get 'out of synch' with what the PCM is telling it to do?

Thank you,
Dave
 

billr

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#6
Yes, the IAC motor (IAM) can get out of sync if it is cruddy and sticking. At key-on the motor is driven for a while to a mechanical stop (usually full open), then the PCM keeps count of the steps that it commands the IAM to make. If it is binding in some way the motor can "stall' and lose steps, so the PCM thinks it is in a position different than it is. I don't think that is your problem, though...
 

cozwurth

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#7
I was able to spend an hour or so with the scanner this evening. My friend let me borrow his Matco Determinator scanner. This afternoon at the shop the IAC really acted strange. We used the scanner to move the pintle position around. We started off with IAC counts in the upper 30's to low 40's at 650-700 RPM idle speed. We opened up the pintle to 100%, which took the engine speed up to about 3K RPM's. When we terminated the test, the idle dropped back to 650-700 RPM's again, but the IAC count stayed at about 71. We messed around with the IAC tests and the IAC counts varied from the mid 30's to a high of about 71 when the engine speed dropped back to the normal idle speed. We could not get it to idle up (act up) on its own.

This evening I went out to the van again and wrote down some baseline numbers. The idle speed was normal 650-700 RPM's unless I had the IAC opened with the scanner.

Key On, Engine OFF, after sitting for about 4 hours: RPM=0, ECT=95°, IAT=82°, BLM=2, TPS=0. 55V
Initial start-up: RPM=930-975, ECT=100°, IAT=73°, IAC=57, BLM=0, TPS=0. 55V
5 min run time: RPM=750, ECT=158°, IAT=68°, IAC=42, BLM=0, TPS=0. 55V, O2=0/1/2 (O2 stayed mainly on 0, but changed to 1 or 2 briefly several times)
abt 8 min run time: RPM=700, ECT=187°, IAT=70°, IAC=35, BLM=0, TPS=0. 55V, O2=0/1/2

IAC test (position) results:
10% - 25 count - 500-600 RPM, ECT=190°
20% - 51 count - 950-1000 RPM, ECT=192°
30% - 76 count - 1580-1620 RPM, ECT=195°
40% - 102 count - 2350 RPM, ECT=196°

IAC test (RPM) The header on the screen had "IAC RPM STATE" with an RPM in (). I'm not exactly sure what this means. . . is this what RPM the engine should be at at a given IAC position?
Began test at 650 RPM, IAC=32, ECT=205° (ECT climbed slowly until the fans kicked in, then dropped normally)
430 RPM - IAC=20 (700 RPM)
450 RPM - IAC=22 (750 RPM)
500 RPM - IAC=25 (800 RPM)
525 RPM - IAC=28 (850 RPM)
550 RPM - IAC=29 (900 RPM)
600 RPM - IAC=31 (950 RPM)
620 RPM - IAC=33 (1000 RPM)
650 RPM - IAC=35 (1050 RPM)
700 RPM - IAC=34 (1100 RPM)
740 RPM - IAC=35 (1150 RPM)
750 RPM - IAC=36 (1200 RPM)
775 RPM - IAC=37 (1200 RPM)
820 RPM - IAC=38 (1300 RPM)
850 RPM - IAC=40 (1350 RPM)
890 RPM - IAC=41 (1400 RPM)
When I stopped this test, the idle dropped back to 650-675 RPM, IAC was at 31-32, ECT=217°, BLM=0

I have not pulled the IAC to look at the pintle or the port yet. I do have a known good IAC to swap in to see if any of these values changes any. I know something strange is going on when an IAC was at 70-71, the RPM's were around 2. 5K, and today, after testing at the shop, the engine was idling at about 650-700 with an IAC count of 71. I'll pull the IAC out in the morning to inspect it. I have to return the scanner on Monday morning. Is there anything else I should look for?

Thanks,
Dave
 

Gus

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#8
Pull that IAC out......try the other one you have.......make sure the hole the pintle mates with is clean(use carb cleaner on a rag to get in there........I use cotton swabs).....have run into intermittant problems where the pintle would open and then not close, causing a high idle.......verified this with an external IAC tester.....
 

nickb2

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#9
These little units works more than one may think, constantly correcting and adjusting, I haven't ever cut one a apart, but I assume it is either a worm gear or a rack type drive and most likely mounted on plastic parts. As gus said also, cleaning the pintle seating area, may help. I use an old toothbrush, that toothbrush has cleaned so many throttle bodies and IAC valves it not funny, will never come close to my teeth again. :ROFL
 

cozwurth

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#10
I swapped out the IAC this morning. Since I hade to drive a short distance to the garage, I went ahead and hooked up the scanner to get a few last minute readings on the old IAC.

Cold start (about 39° F ambient)
1060 RPM @ 81 IAC
990 RPM @ 59 IAC
850 RPM @ 43 IAC

After I swapped out the IAC I let the engine idle and watched to ECT & IAC readings:

900 RPM - 25 IAC - 108°
850 RPM - 22 IAC - 120°
820 RPM - 20 IAC - 130°
810 RPM - 15 IAC - 150°
800 RPM - 14 IAC - 160°
700 ROM - 11 IAC - 170°
675 RPM - 8 IAC - 180°

I ran a few IAC position tests, also.
IAC% / IAC count / RPM

10% / 25 / 1025
20% / 51 / 1500
30% / 76 / 2225
40% / 102 / 2700

When I exited out of that test, the IAC count fell to 5, and the RPM's settled out to 650.

I ran a few IAC RPM STATE tests, too.

IAC / RPM / (RPM)

1 / 450 / 700
2 / 475 / 750
6 / 500 / 800
7 / 530 / 850
9 / 575 / 900
10 / 625 / 950
11 / 650 / 1000

When I exited out of this test, the IAC went to 10, with the idle at 650. I'm still not sure what the (RPM) is supposed to be. . .

Anyway, these are drastically different readings from before. The IAC that I took out was pretty cruddy, but not as bad as I thought it would be. I cleaned out the port and the "new" IAC with a toothbrush and a rag with a little brake cleaner on it.

Dave
 

cozwurth

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#12
Nick, I'm thinking that it's fixed. The IAC count values dropped to where Gus said they should be, they seem to be more stable, and the idle seems to be more stable now, too.

A big Thank You to you, Gus, and BillR for helping me out with this.

Dave
 

nickb2

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#13
Ok, happy for that, this was all Gus and Billr, I just hijack threads and like taking credit. :ROFL Apart from that my use is limited here. Gus is a wealth for the GM's of this world I have noticed. I don't know if he worked a dealership for gm or what, but he really know the General Motors. Billr is the resident head splitter, always there to make sure we are on our toes and doling out good advice, he is a wealth for the electrical side of things. Pays to know who is good at what. Used to have a guy here that was a crack at trannys, tried to fill his shoes but they where just to big. I also must say you didn't seem completely (I think some geeks call it Noobie or noobz) ignorant to how to use a scanner and the info it shot back at you. Many folks just give up at the complexity of it sometimes. So applause for an involved. So with that, I'm off to hijack some other thread and boost my ego!! :ROFL
 

cozwurth

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#14
Nick, I'm not a total Noob when it comes to vehicles, but I'm no pro mechanic by any stretch of the imagination. I didn't even stay at a Holiday Inn Express last night. ;) I've learned from necessity, usually because I could not afford to pay a mechanic to patch up my heaps. It helps that I really like to tinker in the garage. I did take a couple of informal EFI classes that were held at the auto hobby shop on base when I was stationed at Fort Meade, MD sometime in '90 or '91. That basic info has stuck with me, for the most part, so I know just enough to get myself into trouble. I was only messing with this because my buddy who runs the shop down the road said that he didn't really have time to troubleshoot such an intermittent problem. It would sometimes go 2-3 weeks between flare-ups. He's the one who let me borrow the scanner and let me pull the IAC off of a junker there at the shop. The thing about this problem was that it did not throw a code, and my code reader does not allow me to look at live data. I was stuck between a rock and a hard place here. This thing is slowly killing me. I bought it knowing that it needed intake gaskets, just like every other 3. 4 on the road. Then the EPC solenoid in the tranny went out. Off to the tranny shop for that and both shift solenoids while they were in there. cha-ching. Then one of the "special" fittings on one of the heater hoses broke, dumping coolant all over the parking lot where my wife works. Then, the R&P started leaking. Then the TPS went out. Then the outer boot on the driver side CV axle started leaking grease through a puncture. I have no idea how the hole got in the boot. Then it lost 4th gear. Off to the tranny shop again, this time for a total re-build. cha-CHING! I had them replace the R&P and the CV axle while they had it apart. Then the other heater hose started leaking through one of GM's wonderful quick-connect fittings, dumping more coolant. All of the remaining heater hoses and fittings got replaced then. The clunk in the steering that I had been hearing occasionally, even after replacing the rack, finally became predictable enough that I could find it. The pass side strut was the culprit. I'll be finishing that particular job up tomorrow, if its not raining. I should probably take a good look at the rear shocks, too. . . Will the joy never end? :(

Dave