I have a nice, 1996 Olds 88 that runs well except that it intermittently stalls/surges when the AC is turned on. It tries to stall mostly at idle but it feels like the engine is lacking power at high speeds. The surge happens at idle (stop signs!) whether in forward or reverse. Don't feel surge when at high speeds. This is an intermittent problem and happens regardless of ambient temperature when the AC in turned on. In addition to the stall/surge, sometimes I hear what might be a slight "Knocking" coming from the fuel tank area. Had the fuel pump/sending unit and pressure sensor replaced about 2 years ago.

I have checked the freon level and it is good. I replaced the PCV valve and cleaned the MAF sensor and Throttle Control Sensor. I have also checked the fuel pressure with a gauge and it is within spec.

Any thoughts would be appreciated. Thanks in advance!

How many PSI was the fuel pressure showing with vacuum removed from the fuel pressure regulator?

Also, please check for proper functionality of the idle air control valve.

Clean the throttle body and IAC and test again.

Hello- thanks for your quick reply. I did the fuel pressure test last fall and it was within spec with vacuum tube removed. I also cleaned the idle air control valve and checked that the solenoid was working correctly. Do you know if this vehicle'* computer regulates idle when the AC is on and could the computer be bad?

I did clean the throttle body and IAC. I would think that if one of those was the problem, it would happen when the AC was not engaged?

The computer tells the IAC to open when the AC is requested. A dirty/sticky or bad IAC will cause problems.

Thanks for your help! I will check/clean/replace the IAC and see what happens.

What was the fuel pressure reading?

Was the current issue happening last fall when this test was performed?

The IAC isn't a solenoid . . . so I wonder what was done to test it.

The vehicle'* computer uses the IAC valve to regulate the engine'* speed. If there'* an additional load from the transmission or power steering or electrical system and etc. that slows the engine down a bit, then the IAC opens a little to compensate. It also helps it idle at the same speed at higher and lower altitudes and in cold or hot weather. The IAC won't move at all (such as during the test) if the PCM is unable to control it.

Not necessarily. They often get stuck in the position where they spend the most time, at idle. It could be that it is stuck in a position that is good enough for free idle and idling in gear, but not open enough to compensate for the air conditioning compressor as well.

Thanks for everyone'* help with this. The problem started last summer and I tried many things to fix it. I borrowed a fuel pressure gauge from O'reilly Auto. I don't remember the fuel pressure reading(*) but I followed the instructions and the results came back as normal. If I remember right, I tested the IAC (in place so I wouldn't shoot the plunger out) by giving it 12 volts and listening for the valve to be actuated. Yesterday I ordered a new IAC from Rock Auto and when I get it installed I will post an update. Thanks again!