Hard Brake - Now won't Idle with HVAC on
 

Hi all. I'm a new vet owner (2 wks.) I have a '88 Z52 with 50k mi. on it. I am looking forward to years of enjoyment with this car. It was running great till last Sat. I had to brake abruptly, though not extreme. After that, my car won't idle if the air or heat is turned on. I'm talking a rather violent right/left shake under the hood. The idler jumps up and down. I had the alternator tested. It was defective. I replaced it and even had it re-tested but did not fix problem. I'm looking to take to a vet guy for repair next week. When I try to google the problem, I get it could be the IAC, ECM, MAF, ICM, EGR or other 3 letter things I know nothing about. I'm extremely mechanically inclined, just not so much under the hood. I don't understand how the braking hard would cause this. I sure would appreciate any advice. Any ideas?

It sounds like a good sized vacuum leak to me. Start with the basics. Check all of your vacuum lines.
If necessary get a vacuum line plug kit and remove and plug lines one at a time. Could be a bad booster.
Does anything change when you shut off the heater controls completely? They're also vacuum controlled if I remember correctly.

As far as the heater controls, it would not idle whether the AC was all the way down or if the heater was all the way up. If I needed to drive, I either would keep the RPM up or push the HVAC off button when I had to come to a stop. Also I could unplug the compressor. You suggested a good sized vac leak. I think you are definitely on to something. After doing a little research on my vacuum lines, I decided to simply go out and inspect the best I could on the lines I could find. First thing I touched was a split line on the cruise and it fell off. I fixed it and tested the idle. At first I thought it was completely fixed. Then I could tell it was still a bit rough. Probably always was since I got the car and I just ignored it. Tells me I still have a leak. As I backed out of the driveway for a test run, it seemed worse. So now I will take it to the shop next week and have them first zero in on a vac leak. That will save a lot of troubleshooting. Thank You so much for steering me in the right direction.

Welcome to the forum.

Does the A/C compressor turn freely by hand with the engine off?

Yes, the hub at the far end turns freely. I am hoping that's not the problem. I guess I don't understand why it even kicks in and draws power when you only have the heat on as high as it will go while testing.

The defrost system runs the a/c to dry the air and clear the windows much faster.

It sounds like it's doing that even though defrost is not selected.

Your reply got me thinking, so I just went out and checked that again. It actually only drops the RPM and idles rough when the Auto, Bi-Level or Defrost buttons are selected. It is fine when the Off, Econ or Htr buttons are selected. So yes, it's when the compressor runs. Does that mean bad? Or still maybe a vac leak?

I believe the computer is supposed to compensate for the compressor load and open the throttle to maintain the correct idle speed. On my carbureted engine, it has a solenoid that kicks the throttle open a little bit when the compressor is turned on. The computer controlled cars are a little more sophisticated than that.