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99 Z24 weird shifting, A/C, CEL problems, I'm stumped

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99 Z24 weird shifting, A/C, CEL problems, I'm stumped

Old August 22nd, 2010, 12:58 AM
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Question 99 Z24 weird shifting, A/C, CEL problems, I'm stumped

Hello all,

This is my first post here on this forum, I've asked the same question on another forum but didn't get any helpful info there. I've read many of the posts on the forum and can't find anything similar enough to my issue. I'll try to be as detailed as possible. Hopefully it's not so long that it will bore anyone.

I have a 99 Cavalier Z24 coupe with 2.4 motor (guess they all have that?), automatic transmission, 120,000 miles on the car, we're the original owners, and overall it runs great. Just recently had some check engine light issues, transmission shifting hard between gears and weird air conditioning issues simultaneously, that I will attempt to describe.

The check engine light has been on for about 3 weeks, with the only exceptions being the times when I would disconnect negative battery cable overnight to clear the stored codes when I'd do some sensor replacing that I'll describe below. In the mornings I could then drive the car about 10-15 minutes and then light would come back on.

First thing I noticed about 3 weeks ago was that our CEL had come on while driving it probably about 20 - 30 minutes or so. When I came to some stop and go driving through some local traffic lights, I also noticed that the A/C compressor was kicking on and off like it was low on freon. The AC would blow out cold sometimes, and sometimes it would blow out warm with no rhyme or reason to it, it did this for the rest of the 30 minute trip home. When I got home, I verified that the electric cooling fan was running like it should, I got a gauge to check the freon, it was borderline between low and normal area, so I added one small can of freon. It did seem to help the cycling on and off of the compressor... so I thought it was now all taken care of.

I then took it to Advance Auto parts and had them check the code while the CEL light was still on, Their code reader came back with only one code, not sure of the code number, but it was for the IAC - Idle air control valve was reading a high voltage. I then took it to Autozone and had them check it, same reading on their scanner.
I bought a new IAC at Autozone, came home, took the old IAC out, and disconnected the negative battery cable overnight to clear the stored codes.

In the morning I installed the new IAC onto the throttlebody and connected it back up. I then reconnected negative cable, turned key to on position for 5 seconds, turned it off for 10 seconds (they say this is to adjust the pintle on the new IAC) and then I started the car, let it warm up for about 10 minutes. All seemed great.... No CEL, and car seemed to sound good just like normal. I shut it off and let it sit for a few hours. Later in the day I then drove it around for about maybe 30 minutes, the car ran perfect, no CEL, but I noticed my A/C was only blowing out hot air during this whole time instead of our normal nice cold A/C. All of a sudden my CEL comes on and my AC immediately starts blowing out nice cold air like I've always been used to with this car..... I was thrilled until I came to a stoplight, and when I started to pull away from the light, I noticed some very hard upshifting or bucking between gears. I drove home and CEL light stayed on, but ac was nice and cold this whole time. I disconnected the neg battery cable when I got home to see if I could duplicate this issue the next day... same things again happened that I describe above at roughly the same intervals. So I figured the IAC was not the problem or maybe I had gotten a defective IAC.

The next day I go to auto zone to get it rescanned while light is on, and instead of getting the code for the IAC again, this time it gives out a new code for the IAT sensor and High Voltage ( some called it the Air Charge Sensor, others called it the Intake Air Temperature sensor) I buy the new IAT sensor, go home, do the same as with previous sensor, disconnecting, sitting overnight, reinstalling in morning. I drive it and all seems great again.... this time I have AC, not cold, but it is working somewhat...after about 20-30 minutes driving the CEL comes on and the same rough upshifting or bucking between gears starts again, and the AC gets nice and cold immediately. Car drove normally except when I had to slow down and go again, rough upshifts, but a/c ran perfect throughout the whole ride home.

I'm now at a loss on what to do next. Any ideas?? I've read on here about TPS sensor, but haven't read any of those posts talking about the AC issues like I am having. A/C has always worked great, nice and cold, I've had to add Freon maybe twice over the 10 years but that's about it. Car is in overall good mechanical condition, runs great with no loss of power, no backfiring, hesitations, stumbling like you'd expect in an older car. The plugs were changed about 10,000 miles ago, it's had it's regular oil changes, air filter, Throttle body was cleaned with Gumout spray cleaner about 2 weeks ago and didn't seem overly dirty.

I'm wondering if I should go and get it rescanned again after installing the new IAC and new IAT sensors and see if another new code pops up. I don't want to keep tossing money in on new sensors that are coming up as bad on their machines, and I'm still having same issues. I just can't imagine that out of nowhere various sensors all go bad at the same time.

I'm hoping someone on here can maybe point me in the right direction.

In a nutshell:
1)Car was running fine, ac was always cold.
2)Out of the blue A/C starts running warm, CEL light comes on, Hard shifts.
3)small can of freon added, IAC and IAT sensors replaced, shifting is normal.
4) CEL light is back on, A/C is nice and cold, hard shifting again
5)Any ideas

Sorry this post is so long, but I felt it best to include as much info as I thought about. Any ideas would be greatly appreciated!

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mikestoyshop is offline  
Old August 29th, 2010, 3:36 AM
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Default New Updated Scanner testing info..Need Help Please


I don't actually feel that both the two sensors (IAC & IAT) I changed did anything to correct the problem, the car has run exactly the same before and after these changes. AC is doing the same thing as before I changed out these sensors....running warm when CEL light is off, and running nice and cold immediately when the light comes on..... The transmission shifts perfect before the light comes on and immediately starts the bucking when the light comes on.

I don't see how these 3 things can be related, but they are.. It's not just coincidence because the same exact thiings happens each and every time.

Just the other day I ran new tests on the OBDII myself with a new Actron 9580 code scanner I just bought. Same results as before when I had it tested. High voltage on the IAT circuit. P0113 code.

I checked for continuity between pcm connector plug and the IAT sensor connector harness. No problems there on either wire so it should not be a short or broken wire of any kind. The sensor is brand new. I get the 5 volts its supposed to have on the feed side of the connector. Nothing on the other which is ground.

I cleared the code using the scanner. I then checked it again with the scanner since the CEL light was off, no code showed up since it was cleared. I then ran the scanner while the car was running and it shows that the IAT is reading at -140 degrees, which is the default reading it gets when the voltage is high, (no CEL light on yet at that point though).

While I had it out for a test ride, the CEL came back which I assumed it would since the scanner showed the sensor was reading the -140. Same P0113 code.

I even pulled the pcm out yesterday from under the front corner, it looked clean, I sprayed compressed air all over the connectors and the pins, then I used some canned electrical contact cleaner and let that sit and dry out. Put the pcm back in, connected it all back good and snug, went through the same testing and warming up, and the light is back on.

I tested the IAT connector wiring to the PCM by jumping the 2 terminals on the IAT harness connector (I disconnected connector from the sensor). Then I jumped the 2 terminals on the connector with a jumper wire, the reading on the code scanner changed from -140 degrees to 304 degrees, From what I've read around the net, this is supposed to tell you that the circuit and the PCM are good. It shows me that the temperature readings are capable of changing, but it is only the two default extremes that the computer gives. I believe that if I were to leave the jumper in, the computer wold give out the P0112 code for IAT low voltage? That wouldn't help me either.

I am really needing to get this issue taken care of and I feel I am so close,but I just don't know what to try anymore. Can anyone help?....Please...

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Old September 10th, 2010, 5:54 AM
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Have you looked for a vaccum leak? or tried to run the car without the IAC plugged in? I know I had all kinds of problems with the IAC on my 2.2L when I first got the car. I replaced it and got a bad IAC from autozone, it had fallen apart inside the intake (you may want to check this!) and so the pintle was stuck so no air could bypass the IAC therefore stirring up all kinds of trouble.
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Old July 9th, 2013, 12:40 AM
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I have an 02 Cavy Z24 2.4L 5 speed with 162,000. Ive had no problems untill about a month ago when i turned my ac on and the compressor just kicked on and off. I figured the compressor was shot or needed an R-134a charge. Just this past Friday my CEL came on. I got home hooked my OBD-II up and the error " P0113 - Intake Air Temperature Circuit High Input" came up. But for the most part the symptoms/issues are almost identical. The only differences are: 1) my ac compressor rapidly cycles on and off with an interval of 5 seconds or less with or without my check engine light on. 2) i do not have the hard shifting issue... i guess due to the fact of having a manual transmission, but i do have a "surging" issue that occurs when the ac compressor cycles on and off. 3) using my obd-II (after CEL came back on after being off for only a few bours) earlier today I checked the IAT temp and instead of a constant minimum or maximum it is semi-rapidly changing from the normal outside temp between 77 and 82 to between -40 and -44. And since my CEL came back on the ac compressor works with no issues...

Ive read on other forums that the possible issues can be/are caused by: faulty IAT sensors; loose wiring in the sensor connection or wiring harness; low ac refridgerant...?; or worst case scenario... a faulty/bad PCM
Don Meadows is offline  
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