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#610094 - 06/03/05 01:15 AM Surging idle (stalls at stops) on '91 2.8  

Been dealing with an inconsistent idle since we acquired the vehicle. It has gotten worse and now stalls at stops. The idle varies between 500-1200. Seems to smooth out a little once it warms up, but it's still bad enough to stall frequently.

Can't find any vacuum leaks or air leaking around intake manifold. TPS tests OK w/ a multimeter. Chilton says to read the four pins on the IAC valve. Should get 20-80 ohm on "one set" of pins and infinite on all others...

We get 50 ohms on one pair of pins, 50 on the other pair. And infinite resistance on any other combination of pins. Does anyone know if that sounds OK. I'm confused as to what Chilton's means by "one set" ???

Any other ideas? This idle problem used to be just a nuisance, now it's becoming a major pain in the ...

Thanks in advance!

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#610095 - 06/03/05 01:27 AM Re: Surging idle (stalls at stops) on '91 2.8  
Wayne  Offline
Roll Me Over
Joined: Dec 2001
Posts: 2,651
Tucson, Dallas/FW or Virginia ...
You hit a bunch of them, but here's my recently expanded list.

Edit: WHen I corrected "whores" to "wires" (darn spell check messed up "wores") a minute ago it changed the number. Hope this works.

Last edited by Wayne; 06/03/05 01:33 AM.

[color:"white"]• 04 Rodeo DI •[/color] 75k mi, body damage on the 1st weekend I got it.

#610096 - 06/03/05 02:39 AM Re: Surging idle (stalls at stops) on '91 2.8 [Re: Wayne]  

Thanks for the link! We're working our way through a lot of those items.

I stopped at a local AutoZone and tested a new IAC valve. It tests exactly the same as our old one. So I'm ruling that out as the cause. The little opening, or jet or whatever you call it, wasn't very dirty but we cleaned it out anyway.

#610097 - 06/05/05 10:59 PM Re: Surging idle (stalls at stops) on '91 2.8  
JohninLV  Offline
Getting the Wheeling Fever
Joined: Jan 2003
Posts: 90
Las Vegas NV
It seems I have to adjust my IAC valve twice a year, once in the spring and once when the weather cools off in the fall.
Found the Thermac valve on air cleaner broken and actuator (motor) leaking on one of my troopers.
A three wire heated O2 sensor helps some also.
JOY in Paradise

3 90 Trooper V-6's, 5 speed, 4.3L TBI, 31" BFG AT's, Fender Flairs, Bored intake, 1.6 roller rockers, MSD Coil, Jacobs Omni ignition. Heavy duty torsion bars, Rancho 9000 Shocks CAN'T SPELL, TO OLD TO LEARN, GOD BLESS YOU IF YOU CAN.

#610098 - 06/07/05 01:49 AM Re: Surging idle (stalls at stops) on '91 2.8 [Re: JohninLV]  

How do you adjust the IAC valve? All I know how to do is test it w/ a multimeter.

How did you determine the Thermac valve was broken? I'm assuming that's the little switch thing that is located inside the air filter housing.

How did you go about switching to a 3 wire sensor? Replacing the sensor is high up on my list of next steps.

We're still stumped here. Vehicle will hardly start. It is definitely trying to run very rich...

#610099 - 06/07/05 04:03 AM Re: Surging idle (stalls at stops) on '91 2.8  
Jim_Paget  Offline

Roll Me Over
Joined: Aug 2000
Posts: 4,868
Mount Vernon, WA, USA
Here is the procedure for adjusting the IAC (provided to me by John Young from Vegas:

IAC adjustment procedure

1. Remove air cleaner

2. Plug any vacuum line ports as required

3. With IAC valve connected, ground diagnostic terminal (that’s the ALDL connector under the center console, most auto parts stores sell a jumper for GM diagnostic work, that's how you read your flash codes )

4. Turn on ignition, DO NOT START ENGINE. Wait at least 30 sec. (this allows IAC pentle to extend and seat in throttle body)

5. With ignition ON, disconnect IAC valve electrical connector.

6. Remove ground jumper from diagnostic connector (ALDL) and start engine.

7. Remove the plug that covers the idle stop screw (I can't remember if the Holley has one)

8. Adjust the idle stop screw, to obtain 700 RPM (+ or -25) in neutral. 500-550 in drive for automatic.

9. Turn ignition OFF and reconnect IAC valve electrical connector.

10. Unplug any plugged vacuum line ports and reinstall air cleaner.

It seems that I have to go through this procedure several times a year. Three V-6 troopers - Three sets of 1.6 rocker arms installed - one Dyna max muffler and cat. Every mod seems to change the idle somewhat.

Jim Paget
88 YJ with a few changes


#610100 - 06/07/05 05:16 AM Re: Surging idle (stalls at stops) on '91 2.8 [Re: Jim_Paget]  

Thanks Jim. Now that I see the steps, I do recall seeing it in a post on here somewhere. Another thing on the to-do list for tomorrow!

#610101 - 06/07/05 11:21 PM Re: Surging idle (stalls at stops) on '91 2.8  
JohninLV  Offline
Getting the Wheeling Fever
Joined: Jan 2003
Posts: 90
Las Vegas NV
Link to Three Wire o2 conversion.


The Thermac valve is the flaper inside the air cleaner snorkal. A round vaccume motor sits on top of it and opens and closed when intake air is cold sending hot air to engine. linkage can wear out, diapham can leak.

try to get factory KT shop manual and 87 GM Truck manual on EBAY.

3 90 Trooper V-6's, 5 speed, 4.3L TBI, 31" BFG AT's, Fender Flairs, Bored intake, 1.6 roller rockers, MSD Coil, Jacobs Omni ignition. Heavy duty torsion bars, Rancho 9000 Shocks CAN'T SPELL, TO OLD TO LEARN, GOD BLESS YOU IF YOU CAN.

#610102 - 06/08/05 02:14 AM Re: Surging idle (stalls at stops) on '91 2.8 [Re: JohninLV]  
Richard Saylor  Offline
Roll Me Over
Joined: Aug 2002
Posts: 3,007
Lockhart, TX *****
I hope you can figure this out. Mine's the same way. I am willing to bet that whatever is causing your's is the same that's causing mine.

Richard <img src="/forums/images/graemlins/patriot.gif" alt="" />

Had an 89 Isuzu Pickup
Had a 94 Mitsubishi Eclipse
84 Honda XL185S
Had an 89 Isuzu Trooper w/ 3.4 V6
01 Toyota Tundra SR5 V8 4Door Access Cab

#610103 - 06/08/05 07:25 AM Re: Surging idle (stalls at stops) on '91 2.8 [Re: Richard Saylor]  

Adjusted the IAC and set the idlescrew so we had about an 800 RPM idle. I thought maybe that was going to do it, but as soon as we turned the steering wheel -poof- idle drop towards 500 and it stalled. Had a heck of a time getting it started again. Engine had near zero power while running, we could barely get the vehicle rolling <img src="/forums/images/graemlins/frown.gif" alt="" />

Had my son pull off the alternator, going to bench test it tomorrow. I also found a thread today where someone's problem was similar to ours. Last post in that thread says he replaced ignition module and it was resolved!!!

I'm not sure what to try next. I bought an oxygen sensor and socket today, but haven't put it on yet. Started wondering if this is happening with a cold engine and we're not getting any codes - should this sensor even be in question? Can't decide whether to take it back and focus on other things...

Did notice that the pintle(sp?) was making a noise while resetting the IAC (while engine was off but ignition turned to on and waiting 30 seconds...), but read here somewhere that sound is normal while adjusting.

I'm also going to pull all the spark plugs tomorrow and regap them. They were replaced a few days ago as part of the troubleshooting process. I let the auto parts guy convince me they needed to be gapped at .032" instead of the prescribed .045". The .032 was on his computer screen and I thought maybe platnum plugs needed to be gapped differently for some reason. I'm starting to lose it...

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