1998 Isuzu Trooper Bad IAC valves

PENNMEDIC

MEMBER
1998 ISUZU TROOPER
6 CYL
4WD
AUTOMATIC
117,000 MILES

I have a 98 trooper 4x4 3.5l v6 DOHC 24 valve. I had a IACV go bad with a check engine light about 3-4 months ago took it to PEP boys determined it was an IACV and replaced it problem solved or so I thought. Last week out of the blue the engine started idling fast at rest even after warming up, just like before when the IACV went bad. The check engine light comes on and goes off intermittenly. While driving it occasionaly gives out and then surges and sometimes stutters especially when just cruising at steady speed. I took it back to pep boys again due to being under warranty for The IACV and they said everything pointed to bad IAC they replaced it and took out drove it to let the computer reprogram the IACV and 10 minutes later same issue. So I took my trooper and went home and though maybe it was the brand of IACV they were using ( Warner) so I went to NAPA and bought a Echlin IAVC and installed making sure it didn't extend more than the recommended 1 1/8 inch into throtle body. I then took it out on highway and drove it for at least 20 miles @ 55 mph to allow the computer to reprogram it. And the problem still exists. So thus it leads me to believe that it is not an IACV but something else. Nothing else seems to be wrong other than my 4x4 wont work front axle wont engage I think its either electrical or vaccum in nature dont know if it is related. The other issue I know is wrong is my muffler and tailpipe I wasnt sure if this was somehow messing up my O2 sensors and thus causing problems for my IACV etc etc, Can someone help please? P.S I plan to replace the entire exhaust in March including both O2 sensors.

8 Replies

KHLOW2008

MECHANIC
41,814 POSTS

Hi Pennmedic,

Thank you for the donation.

To calibrate the new IAC after installation, cycle the ignition switch ON and OFF a few times and it should be ready.

What was the exact troule codes retrieved for the CEL indicating? Get the latest result and let me know what it says. Autozone provides free scanning services.

When idling is erratic, it could be due to vacumn leakages, insufficient coolant or clogged coolant hoses to the throttle body.

What is the idling speed after the engine has warmed up?

The front axles do not use engine vacumns so it should not be related to the issue.

If the O2 sensor is causing problems, it usually would have some trouble codes stored in PCM. The muffler has minimal impact on the O2 sensor though it can cause incorrect readings.

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Saturday, February 20th, 2010 AT 11:49 AM

PENNMEDIC

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It will calibrate even though I have already driven it all that way and have switched on and off to start several times?

I did not get the new codes I assumed they were the same as before seeing was same problem I will get them tomorrow at advanced auto parts and let you know.

Not insufficient coolant I checked and filled the resivour and check the radiator and it was full @ cold engine.

@ rest park neutral etc. @ 2500 then sometimes falls back to 1000 or less which is normal for it.

I was told by someone and read on message boards that it has a GM trans that has VSM on the front axle that engages the 4x4 and that some time they get clogged and need cleaned out. That is why I thought it may be related.

And what is the PCM and where is it and how do I read it and is the same as the OBDII?

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Saturday, February 20th, 2010 AT 4:21 PM

KHLOW2008

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41,814 POSTS

If the new and previous codes are different, please let me know what they are. It could be due to electrical connections faults or a bad PCM.

When IAC fails, it can short out the PCM for the circuit.

The idling speed should be 750 after the engine has warmed up and if it is more than 1000, there could be a vacumn leaking somewhere. Cold start of 2000 rpm is normal.

VSM does use vacumns to engage and disengage but it has its own pump motor. Improper engagement could be due to the clogging of the lines, contaminations of the wireharness terminals or a failing motor.

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Sunday, February 21st, 2010 AT 2:37 AM

PENNMEDIC

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OK so I went to advance and no codes the check engine light went off and nothing. But I think I made some progress on my 4x4 I was up under the car looking for a screw from the IAC valve I dropped ( dohhhh) no luck its MIA, anyway I happened to look up at the area where I believe the VSM are located for the front axle and the engagement mechanism is and there is this thing that looks like a plug sensor that has two wires running into it and they are both broke off from the plug thing this maybe my problem with the 4x4 we will see. And I took out the IAC and adjusted it out a bit still under the 1 1/8 inches and put it back in and turned the key on and off without starting motor multiple times and then even drove it again for the steady speed for 8 mintues and nothing still same problem it just cant be the IAC but something strange while it was running I unplugged the IAC and no change in engine idle or anything.

Another questions where is MY PCM is it part of my ECM and why cant I find a repair manual for a 98 trooper I found them up to 96 but not 98?

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Sunday, February 21st, 2010 AT 2:29 PM

KHLOW2008

MECHANIC
41,814 POSTS

If the idling is still erratic, check for vacumn leakages. Any additional air getting past the throttle body would cause the erratic idling. It occurs when the idling speed is higher than expected.

The PCM is located behind center of dash below the radio.

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Sunday, February 21st, 2010 AT 3:48 PM

CARADIODOC

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Hi pennmedic. I wanted to reply to your latest question but if I do, it will appear to other people that you received a reply so they won't read it. That would do you a disservice.

As for your question about donations, thank you very much for your past donations, but think of them as a tip, not a requirement. It's true, you are less likely to get a quality reply the first time without a donation but that's not the reason. Most of us put a lot of time into researching solutions, then we type for quite a while. About half the people who don't make a donation never respond back or have relatively simple questions that are already resolved. When someone posts a second time, we know it won't be a waste of time to do a lot of research and typing.

When someone makes a donation, it shows they are going to remain involved in finding a solution to a difficult problem. It lets us know we won't be wasting our time. People tend to value something less if it's free, but isn't it neat that you CAN ask a question without being held hostage for bucks. By making a previous donation, we know you aren't going to disappear.

Some people make donations after the problem is solved. That's why we try to answer as many questions as possible as correctly and quickly as possible. I would like to spend as much time with other questions, but just in my specialty areas, that means reading over 200 new questions each day. I have to pick those that I think can do the most good in the least amount of time.

Hope that clears things up. I wish I could answer your last post, but I'm a genius in other areas. :)

Thanks bunches for visiting us and giving us the chance to help.

Caradiodoc

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Friday, March 12th, 2010 AT 9:15 PM

LRJEROME

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Hi I just had the same problem with my 98 trooper and the problem was the fuel pump didnt have enough pressure. I know this sounds weird that it idles high with out the proper pressure but thats what mine did and that was how the problem was fixed

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Tuesday, March 16th, 2010 AT 1:54 AM

PENNMEDIC

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I figured out what the problem was it is a bad connector or pigtail which is a gm style 3 pin pigtail was loose and making bad connection when you took it off the vehicle would settle down to normal idle or when you jiggled it when it was idling fast it would then settle down to normal idle and while driving sometime when you hit a bump would idle fast then hit another bump would settle back down so I have come to the conclusion this is the problem I found and ordered new pig tail for 8.00 and will try it what can it hurt hey?

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Tuesday, March 16th, 2010 AT 6:41 AM