1988 Pontiac Grand Prix 2.8L V6.  Was running fine.  Check engine light comes on.  Use paper clip jumper and get trouble code 44 and 45.  I replaced O2 sensor and check engine light is not coming on, yet car is now stalling out when it warms up.  Cold, it is fine.  Warm, runs fine when not idling.  Idling, car revs up and down until it stalls, unless you put it in nuetral.....

Any takers?

Ok, EGR valve is not moving when I rev up engine.  EGR valve is brand new (replaced in summer).  I am guessing I have a vacuum problem.    I can also smell a fuel mixture in the air in the engine compartment when it is idling.  Anyone have ideas?

Ok guys, I just cleaned the Idle Air Control (IAC) with carb cleaner as described here:

www.automotiveforums.com/vbu...ad.php?t=338811

I installed it, ran it, and it still idles kind of funny, but it is a little better in that it only died on me once instead of 10 times, but it is still running crappy (idle almost quits when I come to a stop). I am thinking about replacing the IAC to see if that works. Should I?

Quick question, was it a single wire O2? An O2 only commands the computer to go lean or rich on the fuel, depending on what it reads, but won't cause a car to surge at idle.

Typically an IAC motor that's going bad would give you problems when it's cold, not just the motor but outside temp, but yours gets worse when hot. In my experience the cold weather causes them to hang up or stick.

The EGR valve; and the test for them (and there were at least four or five different designs used) usually require a few steps, but IIRC they mostly all used a seperate EGR Controller that works through the P/N switch and ECM to stop the vacum source when in P/N. This is why most of them won't move up and down with a throttle blip. If they do move in P/N from idle to 2,000rpm and back to idle there is a problem and it's often not the EGR itself, but the P/N switch.


Pulse Width EGR valves are also common, and they can be tested quickly by pushing up on diaphragm at idle(in park) and the engine rpm will go down -or stall- if it's working. This EGR valve is also designed not to move between 2,000 rpm and idle, if it DID move with a blip of throttle it is not working right. This is usually a problem in the P/N switch too, or a short.

Good chance of it being an EGR related component or the valve is stuck slightly open. Gotta love GM's crappy EGR system.

Your problem also sounds like a vacum leak to me, you can't always hear them though, you should check your vacum hoses, especially at the MAP sensor if equipped. I had one in the shop very recently with a possible bad EGR, but it also had a constnt lean reading on the O2's. I was able to make sure the EGR was fully closed and I unplugged it to eliminate it until I found the other problem.
It was difficult to track down but I ended up finding the vacum leak on the throttle body butterfly valve shaft. The seals were leaking badly causing the surge in idle and while it didn't stall, it always came close. A simple vacum leak could cause the idle surge and stalling.



Hope this helps.

Quoted: Quick question, was it a single wire O2? An O2 only commands the computer to go lean or rich on the fuel, depending on what it reads, but won't cause a car to surge at idle. Typically an IAC motor that's going bad would give you problems when it's cold, not just the motor but outside temp, but yours gets worse when hot. In my experience the cold weather causes them to hang up or stick. The EGR valve; and the test for them (and there were at least four or five different designs used) usually require a few steps, but IIRC they mostly all used a seperate EGR Controller that works through the P/N switch and ECM to stop the vacum source when in P/N. This is why most of them won't move up and down with a throttle blip. If they do move in P/N from idle to 2,000rpm and back to idle there is a problem and it's often not the EGR itself, but the P/N switch. Pulse Width EGR valves are also common, and they can be tested quickly by pushing up on diaphragm at idle(in park) and the engine rpm will go down -or stall- if it's working. This EGR valve is also designed not to move between 2,000 rpm and idle, if it DID move with a blip of throttle it is not working right. This is usually a problem in the P/N switch too, or a short. Good chance of it being an EGR related component or the valve is stuck slightly open. Gotta love GM's crappy EGR system. Your problem also sounds like a vacum leak to me, you can't always hear them though, you should check your vacum hoses, especially at the MAP sensor if equipped. I had one in the shop very recently with a possible bad EGR, but it also had a constnt lean reading on the O2's. I was able to make sure the EGR was fully closed and I unplugged it to eliminate it until I found the other problem. It was difficult to track down but I ended up finding the vacum leak on the throttle body butterfly valve shaft. The seals were leaking badly causing the surge in idle and while it didn't stall, it always came close. A simple vacum leak could cause the idle surge and stalling. Hope this helps.

Thanks for the response.  I think I figured it out.  I took out and cleaned the IAC valve and that seemed to help---ALOT!

But it still surges a little.

I might end up having to replace it to see if that helps.  The part is $50, so it is an expensive test.....

Before you spend the $50, unplug it, it should go into the default setting and then you can see if it's the problem.

Also, I probably have one lying around in the shop that you can have. It's used, but it works. I took it off a car that blew a head gasket before it got junked.

Let me know, I'll send it out to you if you want, it's just sitting in a drawer full of sensors etc. I have a few IAC's so it's no problem.

Quoted: Before you spend the $50, unplug it, it should go into the default setting and then you can see if it's the problem. Also, I probably have one lying around in the shop that you can have. It's used, but it works. I took it off a car that blew a head gasket before it got junked. Let me know, I'll send it out to you if you want, it's just sitting in a drawer full of sensors etc. I have a few IAC's so it's no problem.

What do you mean 'default setting'?  What should I look for when I unplug it?  Should the car stall or not?

BTW, I just might take you up on that IAC valve....

tag.  Great info, thanks

The default setting is just dropping it into base idle, or mechanical idle. IAC's require resetting once unplugged with motor running. The 2.8L is simple, you just turn the key to the ON position without starting the car and then back OFF. This will reset the IAC.

The IAC is only used to control idle, it works from computer commands which order it to move in 'steps' based on inputs such as coolant temp, engine rpm, and load. If the throttle plate is suddenly closed(when you release the gas pedal) the ECM will move it in or out a certain number of steps so the motor doesn't stall out.

Since it's not stalling right now I would probably leave it where it is, though I will be happy to send you the one I have. The only thing is that you need to check it carefully before installing, there were several different IAC's used on this motor. Yours may be hanging up a little between steps, but I still think there may be a second problem. Hopefully I'm wrong.



Here's a litttle basic info on default settings that may help you further diagnose:

Default setting -or actually default value- is probably the incorrect technical term when applied to IAC's.

Default settings are generally reserved for something like a TPS, MAF, MAP etc. where the computer uses a default value for operation of an unplugged sensor, sometimes using data from other active sensors to control the fuel mixture. It will run, but not as efficiently as it would with a good part connected. Look at it as an emergency setting that will get you home or to a shop, but shouldn't be used for much more.

We will occasionally use default values at the shop to help diagnose cars based on common symptoms. For a general example, if a car with a MAF equipped 2.8L has a rough or unstable idle we go through diagnostic steps to pinpoint the problem. One step is to disconnect the MAF. If this corrects the problem then we know the MAF is bad and we replace it. By doing this, the car goes into default value for the MAF so when it runs normally with it unplugged we know it was bad. This is not the case with IAC's though.



Sorry, I had more for you but my dumb cat jumped on my laptop this morning and screwed up my post. I guess it's feeding time.


Good Luck, LMK if you want the IAC. IM me.

Quoted: Sorry, I had more for you but my dumb cat jumped on my laptop this morning and screwed up my post. I guess it's feeding time. Good Luck, LMK if you want the IAC. IM me.

[raises hand]  I have a question for when you get a minute.  I have a '94 full sized truck (GMC) that keeps throwing O2 sensor fault codes.  How will this effect driveability before I replace it?  I've been waiting to cut off the cat and install true duals on it before I replace the sensor.  With the O2 sensor right before the cat, will removing it cause the O2 fault code to constantly trip due to a change in back pressure (less back pressure)?

Quoted: Quoted: Sorry, I had more for you but my dumb cat jumped on my laptop this morning and screwed up my post. I guess it's feeding time. Good Luck, LMK if you want the IAC. IM me. [raises hand]  I have a question for when you get a minute.  I have a '94 full sized truck (GMC) that keeps throwing O2 sensor fault codes.  How will this effect driveability before I replace it?  I've been waiting to cut off the cat and install true duals on it before I replace the sensor.  With the O2 sensor right before the cat, will removing it cause the O2 fault code to constantly trip due to a change in back pressure (less back pressure)?

A bad O2 will effect your fuel economy without a doubt, but in many cases it also effects driveability.

Now if you plan on removing the Cat you certainly can get a Check Engine Light that comes on every time the car gets a little warmed up. This would be a definite yes if your truck has a second O2 after the Cat, which it probably does. But that's only part of your problem, I'll explain more about the Cat-O2 relationship below. The Chk Eng Lt itself isn't a big deal, however removing the Cat is a bad idea depending on your state inspection process. If a car/light truck that was factory equipped with a Cat comes in for inspection -and had the Cat removed- that's an automatic failure in most states. No way around it legally.

I've seen nice dual exhaust setups for these trucks with a single Cat. The last GM truck (w/dual exhaust) that I worked on had a single Cat after the 'Y' pipe from the manifolds, and the Cat had a large single inlet from the Y pipe and dual outlets that went to dual exhaust pipes, to dual Flowmaster mufflers and straight out the back. I've seen this setup many times, it's very effective power-wise and you keep the Cat and O2's working right. This particular truck belongs to my boss and we also used a high flow Cat.

Cat/O2 relation:
You will get Catalytic efficiency and other trouble codes when you remove a Cat because of how the O2's and Cat work together. The O2 BEFORE the Cat always changes voltage at a different rate of speed (and voltage output) than the O2 that's ON or AFTER the Cat. The front O2 is used for adjusting Air/fuel mixture while the rear O2 is in place to monitor Catalytic efficiency. The Cat is what actually causes the difference in voltage between the two. The computer monitors this and expects to see a difference. If the Cat is removed, then the rear O2(On or After Cat) will generate nearly the same -if not identical- voltage and this trips the Check Engine Light because they shouldn't be the same. The trouble code is normally for 'Catalyst efficiency below threshold' or various O2 codes, or both. The computer will recognize that the Cat is not working properly, if at all, but can't tell if it's physically missing so the rear O2 was put there to make sure the Cat is working right.

Now let's back up to the state inspection for a moment. Even if your mechanic is trying to give you a break on the the missing Cat by overlooking the fact that it's gone, many states require a Tailpipe Emmissions Test every year or so on cars '95 and down unless they are past the 25 year mark, then it's just a safety. Without a Cat, your emmissions levels will be too high to pass, although with some careful tweaking of the vehicle I have heard of them passing without Cats but have never attempted it myself.

So now you can see that by removing the Cat you can create more than one problem. Your best bet is to look at a system like I described earlier.

actually, our state doesn't have any emissions regs.  I did this same thing with my '90 LEO-model Caprice (true duals, no exhaust, O2 sensor on the manifold and it never trips the codes).  Also, this is the only O2 sensor on the truck, right before the cat.  Hopefully all will be well.  Thank you for the info.

No emmissions, that's awesome! We get tailpipe emmissions up to '95, and '96 and up get the enhanced OBDII inspections tailpipe but are much more stringent. A Check Engine Light will fail any car automatically, as will too many OBDII monitors that haven't gone into Ready Status.

All those EVAP codes that people ignored are coming back to haunt them, if you don't get it fixed -and then pass inspection- you won't be able to re-register your car next year. DMV gets all this info on thier screen when you go for renewal, that's how strict they are,

You are absolutely correct, with just a single O2 you are fine with true duals, and you won't get a light.

Good Luck!

Ok, I just replaced the Idle Air Control Valve.  It is still doing it!!!!

(oneshotonekill, I would have taken your offer, but I kind of needed it asap...if it could be fixed, that is)

What are my options now?

Will a dioide or something else help the second sensor's circuit show the ecu an acceptable difference?

Quoted: Will a dioide or something else help the second sensor's circuit show the ecu an acceptable difference?

huh?

I think I know what the problem is.

Did you buy a Bosch universal replacement or the OEM specified O2 sensor?
If you used a Bosch, chances are it is not heating up fast enough. I had this problem on my 2000 Crown Vic. the bosch sensors are made for rear O2 sensor locations, they can take about 5 minutes to heat up properly and start switching. My front O2s need to switch within THIRTY seconds of a cold start as it goes into closed loop.

I got the right Motorcraft O2 sensors for the front sensor locations on my 00 Ford. I hooked up my datalogger and saw that when the car switches to closed loop (took 60 seconds from a cold start), the O2 sensors started switching right then and there. With my Bosch sensors, they took 120+ seconds to switch so when I shifted into drive at under 120 seconds in closed loop, the car stalls out.

The rear o2 sensors should not really switch but rather they should maintain a relatively constant positive voltage. The tech manual states that the rear sensors should switch only about 7 times per 120+ on the front sensors.

here's what you need to do:
* Use OEM replacement O2 sensor(s)

* pull the negative battery terminal for about 5 minutes to reset the KAM (That's Keep Alive Memory). On OBD-II cars (95/96-up), the KAM is used to store idle trim settings. To relearn the idle trim, shift into neutral for about 1 minute, then shift into drive for 1 min, shift back into neutral, turn on A/C and idle for 1 min. Shift into drive w/ the A/C and idle for 1 min.

That is NOT necessary but it is done to relearn the idle trim.

Your IAC is the culprit when you stall coming to a stop.
Your EGR is NOT functional at low loads.

The only time EGR starts working is around cruising RPM/loads. It shuts off at idle and at WOT. Revving in neutral isn't registering enough of a load at the RPM to cause the EGR transducer to kick in.

O2 sensor is a Carquest part.

This thing boggles me.  Well, maybe I didn't reset the car right.  Chilton's says to turn car on (engine off) for 5 seconds, then turn off for 10 seconds, then start it.  This resets the IAC

Quoted: O2 sensor is a Carquest part. This thing boggles me.  Well, maybe I didn't reset the car right.  Chilton's says to turn car on (engine off) for 5 seconds, then turn off for 10 seconds, then start it.  This resets the IAC

You can't reset the IAC. It's an electric motor that opens up based on amount of voltage.

The Carquest O2 is your problem. Return it and get an OEM O2 sensor like yesterday.

A bad IAC will make a loud hoot noise when letting off the gas pedal. Your car will stall out coming to a stop or it will just cause stallouts. It does not matter if it is hot or cold, an internal combustion engine needs air to operate. A faulty IAC will prevent that from happening in neutral.

Now, you said:

1). The only thing you changed is the O2 sensor.
2). After the new O2 sensor was installed, the car stalls out in drive when it is warmed up. Shifting into neutral prevents the stallout.

This rules out the IAC. I doubt it is a vacuum leak because I've driven cars on the road w/ loose vacuum hoses before.

I suspect the O2 sensor. It must be sending weird signals to the PCM when the engine is warmed up. My car with the Bosch O2 sensors would not stall out in neutral, but it would stall out in drive when it is cold.

I bet Carquest sells bosch o2 sensors that are repackaged in Carquest logos. Get a GM/AC Delco O2 sensor.

Welp....I guess Ill try the O2 sensor again.  AC Delco, eh?  Ill order one right now.  Ill update when things come together.  Thanks.

I only buy factory O2 sensors.  Read about way to many Bosch ones causing issues on the Jeep forums.

It's pretty much across the board with sensors.  I replaced my Crank Position Sensor with an O'reilley's Borg Warner one.  It now started, but ran like crap and misfired when halfway thru the warmup cycle.  Replaced it with another - it was the same deal.  Went and got a Mopar unit, ran perfect ever since.

Headlice: You should go to your GM dealer and get a GM replacement O2 sensor for your application. Then reset the KAM on your PCM by pulling one battery terminal for about 5 min.

Quoted: Headlice: You should go to your GM dealer and get a GM replacement O2 sensor for your application. Then reset the KAM on your PCM by pulling one battery terminal for about 5 min.

Not AC Delco?

Quoted: Quoted: Headlice: You should go to your GM dealer and get a GM replacement O2 sensor for your application. Then reset the KAM on your PCM by pulling one battery terminal for about 5 min. Not AC Delco?

I was just guessing AC Delco is GM's supplier, like Motorcraft and Ford. If you can find the AC Delco unit, give that a try. That is more OEM than Bosch.

UPDATE!

Ok, I replaced the CarQuest O2 sensor with an AC Delco O2 sensor and there is somewhat of a difference but not too much.  It runs a little better, however it still stalls out once in a while.  Still not sure what it is...  Maybe I need the GM stock part?

I went through the same routine with my daughter's car recently. After checking and replacing just about every damn sensor on the car, I gave up and took it to a repair shop. Turned out it just needed a set of plugs. Man was I pissed! And the worst part was my wife kept saying, "Maybe it needs new spark plugs."

With the plugs mis-firing, the mixture was too rich for the engine when it was warm, so it ran like crap. Luckily, the place only charged me for a set of plugs,....... but my wife was still right.

check your tuneup parts..spark plugs etc..also check for vacuum leaks..on the intake, induction boots, injector o rings..just use some brakeclean..or carb clean..

Car is running much better for some reason....

Maybe it just took a few days for the new O2 sensor to kick in?????

Hasn't stalled on me in a while now.

Quoted: Car is running much better for some reason.... Maybe it just took a few days for the new O2 sensor to kick in????? Hasn't stalled on me in a while now.

Did you reset the PCM KAM after you installed the new O2 sensor? If so, it may just need some time to relearn the idle trims.

If this doesn't work, see if you can return the AC Delco sensor and go with a GM/factory replacement.

Quoted: Quoted: Car is running much better for some reason.... Maybe it just took a few days for the new O2 sensor to kick in????? Hasn't stalled on me in a while now. Did you reset the PCM KAM after you installed the new O2 sensor? If so, it may just need some time to relearn the idle trims. If this doesn't work, see if you can return the AC Delco sensor and go with a GM/factory replacement.

Not quite sure how to do this.  I know the instructions for resetting the IAC valve- drive your car at 45 mph and it will reset....

Quoted: Quoted: Quoted: Car is running much better for some reason.... Maybe it just took a few days for the new O2 sensor to kick in????? Hasn't stalled on me in a while now. Did you reset the PCM KAM after you installed the new O2 sensor? If so, it may just need some time to relearn the idle trims. If this doesn't work, see if you can return the AC Delco sensor and go with a GM/factory replacement. Not quite sure how to do this.  I know the instructions for resetting the IAC valve- drive your car at 45 mph and it will reset....

On mine - you simply unplug the battery for more than 30 minutes.