Hunting Idle With Air Conditioner On

Hi Everyone,

I have a solid, running car except for one thing. The car idles perfectly with the A/C off, but not with the A/C on. With A/C on, the idle goes from it's normal 800 rpms, down to 450 rpms and bounces cyclicly between the two. This will go on indefinitely or until I turn the A/C off.

I bought the car already built, so my technical knowledge is limited. I know that the PCM can control the A/C and bump up the idle when the compressor clutch engages. However, my car's A/C is independent of the PCM.

I am going to test the IAC this weekend, but I imagine that it's ok since it holds the idle well with the A/C off. Strangely, the throttle position sensor shows a slight opening off the throttle blades with the A/C on. With it off, it shows that the blades are closed. I'm not sure why this is happening.

Hopefully, someone here has dealt with this issue before and can help.

Welcome. Where/who did you purchase your car from?
Does your car still have a stock throttle body? Larger aftermarket units can have issues with base idle setup.
For a stock throttle body the first thing to check, using a program like DataMaster, is the IAC counts. With the engine at operating temp the counts should be 30-35. If the counts are too high then the system could run out of adjustment range when the AC kicks on? Also check the TPS is functioning correctly, which can be checked in DataMaster too.

Does the LT PCM have an A/C select input pin like the LS? I applied a signal from the Porsche A/C system (I think +12v) to that pin when the A/C was turned on so the LS PCM could adjust engine parameters, worked like a champ.

I bought the car from Al (sbwrench). It's a great car - well put together and a lot of extras.

I cleaned the MAF sensor and the problem cleared up a bit. The idle still fluctuates, but now only between 700 and 950 rpms. I think that I have a couple of bad injectors though. I got an IR thermometer from Harbor Freight and checked each header tube. The front, driver's side tube is about 100 degrees less than the others on that side. My LTerm fuel trim is consistently rich on that side (113). The O2 sensor seems to stay down in the 300 - 400 range on DataMaster.

On the passenger side, the 3rd tube back from the front of the engine is 100 to 150 degrees hotter than the rest on that side. The O2 sensor on that side fluctuates normally, and the LTerm fuel trim is around 125. So, does that indicate that the injectors aren't flowing the same?

I'll have to run a new DataMaster file and recheck the TPS readings.

I don't really know about the A/C thing, though

Clean all the injectors and replace the O ring seals while you're at it. The idle problem can be fixed if you will find the AC request wire on the ECU and hook it to the same wire as the compressor, this will let the ECu know when the compressor is on and it'll help control the idle.

Thanks for everyone's input. I will try these things and hopefully that will take care of my issues.

Al is a great guy, and IIRC you got yourself a great car for a good deal....

As Eric mentioned, hooking up the AC request wire may solve your issues......my car was able to stabilize the idle without the AC request signal though; it has since been hooked up.
Split LTFT could be injectors, O2's, intake leaks around the ports, un-matched heads if they are ported heavily; sending the injectors out for cleaning is a very good starting point.
Did you determine the IAC counts and type of throttle body? Also you may want to consider checking/setting up the base idle.

Well, I had the injectors cleaned today and they were in bad shape. Now they all flow the same, (much more than the car is tuned for) so I'll have to see what the DataMaster log shows. Of course, when I reinstalled the injectors I managed to crimp a fuel line. It burst when I turned on the fuel pump. I'll get a replacement tomorrow.

I am in the process of getting the car tuned by mail, so I'll have to get that redone to compensate for the injectors, so it may be a few days before I know if this solved my problem. I have a lot of variables going on right now, so I'll update you sometime next week when things get narrowed down a bit.

As far as the throttle body goes, I have a 58mm BBK. My IAC is reading between 55 and 59 at idle.

Thanks for helping me out. I'll take any advice that you can give me!

Okay, once you get a new base tune let us know.
BTW, I have had good luck with base tunes from PCM 4 less.
Also, what size injectors do you have?

The injectors turned out to be 42# Lucas. Based on the part numbers on the injectors, all of the info posted on the web, and the datalogs, we thought that they were 38#. So, back to the drawing board as far as the tune is concerned.

I had one other car tuned by PCM 4 Less, but this time I went with Solomon at LT1PCMTuning.com. He has supposedly tuned over 1000 LT1's, so I'll see what he can do. If it doesn't work out, PCM 4 Less is my go to backup.