99 Z24, IAC & IAT high voltage, bucking on upshifts, AC cuts out from this

[mikestoyshop]

Hello all,

This is my first post here. I've read many of the posts on various forums 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 Z24 with 2.4 motor,automatic, 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 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.

I then took it to Advance Auto parts and had them check the code while the CEL light was still on, Their code 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. 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. 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 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 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 don't want to keep tossing money in on new sensors that may keep coming up as bad on their machines, and I'd still have the same issues.

I did notice something else tonight while tinkering under the hood looking at connections, I see there is a short (maybe 6 inches long)rubber hose that runs from what I believe may be the MAP sensor down into the bottom of the Throttle body. that was somewhat brittle, when I pulled on it a bit it broke apart in my hand. All the parts stores were closed when I found this, but I did come up with a temporary solution hose. Do you think this brittle hose could have anything to do with the IAC and IAT sensors reading Voltages high? Or would this sensor have anything to do with the ac issues when the CEL isn't on??

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!

regards,
Mike

[pappyl]

I have a 99 z24....

Well, strange coincidence or not... maybe couple things going on a once.
- Shifting problems can be caused by vacuum leak.
- Map sensor vacuum line - when that goes car will NOT idle without you on the gas. Did the temporary vacuum line fix things up?
- Check the vacuum line to the fuel regulator as well for leaks

- A/C problem -When the shifting problem starts - when you turn off the A/C does it go away? I'm thinking bad bearings on the compressor causing too much load on the engine.
- Getting high voltage on IAC, then the IAT? I'm thinking you need to check your wiring harness to the firewall - maybe damaged line.

-Would be interested in hearing what you find out...