I've got a 95 Cavalier with the 2.3L Quad 4. Here's the problem; the car starts and runs great but when it makes the transition from open loop to closed loop the engine revs up and down several times and if you're not paying attention and the car is in gear it could be very scary if not dangerous if you're not solidly on the brakes. Could this be a problem with the idle air control or do you think the problem is in the computer? I've got an OBDII tester and I can watch the tester go from open loop to closed loop at exactly the same time the engine starts to do the racing dance. I might have to spend $160. to rule out the IAC but I thought that maybe someone here might be able to shed some light on this problem but if it's the computer I've wasted $160. on the IAC.

Hoping someone can share some insight.

Vic

Can you post data on what the sensors are reading, any of them appear out of spec?

If you could post data on all your sensors, I could tell you exaclty what is wrong.




- 2004 Cavalier - 124k, owned since new

Can you disconnect/bypass the IACV?


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We all drive in a yellow Cavalier...

Rob S wrote:Can you post data on what the sensors are reading, any of them appear out of spec?

If you could post data on all your sensors, I could tell you exaclty what is wrong.

I'll run a test this weekend and post some figures, thanks.

Vic

Impreza WRX wrote:Can you disconnect/bypass the IACV?

Thanks. Now why didn't I think of that? But if the computer is sending the signal to operate the IACV unplugging it will mask the problem, right?

Vic

UGH...just unplug the 02 sensor..the one in the exhaust manifold..it only takes a few seconds ..unplug it, make sure the wire is away from the manifold and start the car..then come back and tell me if it still does it...ill bet cash its the 02..unpluging it and running the car should tell you..thanks

Wel Scott, thank you and looks like you are are correct. I unplugged the O2 sensor, started the car and the idle speed was stable but here's the catch , that O2 sensor was new only 6 months ago for the emissions test. Does this mean that the O2 sensor is junk and needs to be replaced again?

Vic

Scott, I just had a thought. If I unplug the O2 sensor that prevents the computer from going into closed loop and the problem only occured during the transition from open loop to closed loop. I'm thinking that the problem is more in a worn idle air control valve that is slow at responding to the computer's input during the trasition from open loop to closed. Does this make any sense?

Vic

the problem IS in the 02 ..i just havnt figured out why yet ..but since i was right about the o2 im gonna bet double money and say when you bought the nwe 02 that you used one from auto zone or the like..which would make it a bosch..yes no?? either way you have to replace your o2 with an AC delco ONLY ..i went thru about 6 brand new auto zone o2's on my own car (quad4) before i found out it was the "wrong brand" so the symptom you have is a sign of a bad o2 or the wrong BRAND o2 ... not sure why this matters as of yet but all i know is that as soon as the car changes loops it acts up for a minute or 2 then is fine for the rest of the trip...its annoying and possible dangerous ......but let me know if i was right on my second bet ok??

I'll get a Delco replacement O2 and give it a try and let you know what happens. Do you happen to know if I need a special socket to change the O2 or can I do it with regular tools? Thanks again.

Vic

they make a special socket for 02 sensors..i used a 22mm wrench i think..but i have the HO manifold and it doesnt have an upper heat shield...look at your sensor and if it looks hard to get to then get the socket..if it looks easy try the wrench..thats all i can say

It looks like I'll have to go buy a socket. Thanks Scott.

Vic

Well, no difference Scott. I was really hoping that would be the cure. Now I'm wondering if the coolant temperature sensor may be the problem? If the thermistor has a bubble in it and it's bouncing between open loop and closed loop it just might be the cause of the problem. I just don't know for sure and I hate guessing and throwing parts at it but there's not even a test in the manual for this problem and I even have a GM factory manual. I'm open to any other educated suggestions.

Vic

I was just re-reading over your symptoms and you say its scary when it does it? if its an 02 itll be a loss of power when it acts up...but when yours does it does it actually REV up..like speed up too? if so then it will be the TPS or IAC ....it would really help if you had some kind of scan tool

that sounds like a TPS problem to me. i just recently had a buddy with a similer problem on his cav. its newer but symptoms were alike. get the specs of your car, get it scanned or if worse, you might have to use an ocilloscope. i hope you find the cure to your problem soon buddy


Silly ricer, useless wings are for penguins.

A tps CAN mane your rpms jump up and down..but usually makes it idle high.....but doest have much to do with open or closed loop from what i understand

you may want to try restarting the computer... sometimes that helps. if its frozen or not letting you select shut down, then just hold the power button in for a few seconds until it turns off.... then just turn it on.

scott (section8cav) wrote:I was just re-reading over your symptoms and you say its scary when it does it? if its an 02 itll be a loss of power when it acts up...but when yours does it does it actually REV up..like speed up too? if so then it will be the TPS or IAC ....it would really help if you had some kind of scan tool

I do have an Actron OBII scan tool and it shows the sensors all good. It's an odd problem because it only occurs for a few seconds upon cold startup when the computer system makes the transition from open loop to closed loop. The car starts fine from cold, runs fine for about a minute and a half then it does a rev up and down dance, wether it's in neutral or in gear, then it's fine all day and never misses a beat, good power, idle and performance. It's just that little RPM dance that scares the hell out of my wife. There's got to be a cure for it. All the things that can cause it are PCM, TPS, CTS, IAC, O2S, IAS I believe but I can go broke replacing these parts by guessing at which one might be bad. Does anyone have some positive theories or conclusive tests I can perform to diagnose these sensors to validate their operation or is it all just guesswork?

Vic

did you try running it with the 02 unplugged ? or did you just go buy one? if not start it cold with the 02 unplugged...it still sounds just like an o2

I tried it with the O2 unplugged and the car ran fine but my theory there is that the car will not make the transition into closed loop if the O2 is unplugged. Any suggestions?

Vic

I just noticed that my scan tool has a continuous mode so I'm going to hook it up to the car tomorrow and leave it on from cold start to warm up and see if it pulls any codes. Wish me luck.

Vic

wait wait wait wait....you unplugged the o2 and it ran fine?? you said you bought a new 02..which BRAND did you buy?

Wait you guys do know that 95 cavs are obd1 right. They have the obdII connectors but are still obd1...


My car--->http://www.cardomain.com/ride/2569452

scott (section8cav) wrote:wait wait wait wait....you unplugged the o2 and it ran fine?? you said you bought a new 02..which BRAND did you buy?

Yes, it ran fine at idle with the O2 unplugged. New O2 is AC/Delco, old one looks like Bocsch.

Vic

88 red Legend wrote:Wait you guys do know that 95 cavs are obd1 right. They have the obdII connectors but are still obd1...

If it had the 2.2L engine it would most likely be OBD1 but this is the 2.3L Quad4 engine with a kazillion sensors and it definately reads as OBDII according to my Actron CP9110 scan tool.

Vic