car bucks when AC comp. engages

[JHZR2]

Hi,

I have a similar issue with my GF's 94 acura integra and my 91 BMW. When sitting, idling, when the AC compressor engages, the RPMs drop significantly, almost as if the engine was about to stall.

The integra will do this both when driving (which is really annoying), and when idling. The BMW will only do this while idling, when driving it is smooth. The integra compressor is original, the BMW compressor has been factory rebuilt, after a retrofit from R12 to R134A. The BMW did it with both compressors.

I tried cleaning the idle air control valve on the BMW, it didnt fix it. I dont know why this happens.

Any ideas?

Thanks!

JMH

 

[Alan]

A vacuum leak can cause that bucking to happen.Something might be loose.

 

[JHZR2]

Its ONLY when the AC compressor engages. NEVER any other time. Wouldnt a vaccum hose cause this more often?

Thanks!

JMH

 

[Alan]

Well you are putting more load on the engine at that moment of compressor engagment than compared to idling.

 

[ediamiam]

but the rpms are suppose to kick up at idle when the ac is turned on. does that in my cars.

 

[JHZR2]

yeah, thats just it. In our other cars, many of which are 4cyl cars (previa plymouth breeze, etc), this does not happen ever.

Thats why it strikes me as odd.

JMH

 

[T-Keith]

Either the load is too high(like a faulty compressor) or the engine is not compensating for the load from either a mechanical or computer related problem.

-T

 

[Quest]

Serveral things I can think of:

(1) Idle air actuator valve maybe dirty or not smooth.

(2) The A/C vacuum actuator (the one that cracks the throttle plate open a bit when A/C engages) is in question (not smooth, vacuum leaks, etc.)

(3)vacuum leak somewhere in your intake section

(4)Retrofitting of R134A puts too much load to your engine as opposed to running R12.

(5)weak ignition spark

(6)incorrect idle RPM settings (with load vs w/o load). In general, Japanese cars have 2 specs. for this part (engine in normal operating temp, no load (no headlights,etc.) idle RPM and check this against with load (idle RPM should be the same after A/C soleniod kicks in, or -50rpm max.).

(7)Alternator/charging system weak? Load test your battery for condition check and also verify that your alternator is giving enough juice when A/C kicks in.

That's all I can think of for now...

 

[Alan]

On vacuum leaks,some cars have vacuum resviors.These can develop cracks and create odd problems.

 

[nel]

JMH,

Check to see if the idle control valve (ICV) is functioning properly first. There's a few ways to do this. On the Integra and possibly the BMW, turn the steering wheel to full stop with the engine running. The power steering pressure switch should tell the engine computer to compensate by opening up the ICV a bit. Does the rpm drop dramatically? If it does, then you'll have to figure out if it's the ICV or something else in the control circuit.

With the AC, I don't know if the compressor is turned on by the engine computer or not. It is on GM cars, but I don't know about yours. Do you have an electrical schematic? If the engine computer doesn't control the AC compressor, then there should be a signal back to the computer as to when the compressor engages. There may be a problem in that circuit. You could also try to disconnect the ICV and see if there's any difference. If not, it probably is the ICV. If there is a difference, then try to look for the other things people have suggested, particularly a vacuum leak. Good luck.

 

[The_Jeff]

It's sometimes in the car's computer logic to bump up the idle as the a/c is turned on. GM (my frame of reference) controls the A/C clutch from the ECM so there may be a brief delay in getting A/C as it gets the idle up (and it can kill the A/C if the car starts overheating). I agree with nel. You may be seeing a reactionary surge when the ECM notices an idle drop but doesn't know why then steps in at the last second to fix the problem.

The integra system is probably cruder; the A/C clutch is on/off where the BMW could have a nicer deal like CCOT that's variable. Check out imcool.com for lots of experts.

 

[JHZR2]

Thanks for the info!

I think that 1 and 2 could be the culprit. Why? because all Ive done is clean the ICV once on the BMW and never on the acura. However, on the acura it bucks when the comp goes on while driving.

3 and 4 I have to doubt. If 3 was true, I think itd be rough all the time. There is no other sign of a vacuum leak. 4 wouldnt relate to the acura at all, as it was factory fill with r134a, and the BMW exhibited this through last summer when it had the factory comporessor and factory fill r12 in there. It cooled and worked fine then, and does now, and the idle near-stall never changed.

5 is one I havent thought of. However, the cars both deliver excellent, above spec MPG. WOuldnt a weak spark cause poor combustion? Also, the acura has been recently tuned up. The BMW is distributorless, but I replace the plugs yearly, and the wires are about 2 years old. The plugs were A-OK and clean.

6 and 7 Ill have to check. Thew BMW has a factory rebuilt bosch alternator in it, less than a year old. We've never had charging issues really, though the integra has a big sound system that the PO put in, and it puts some alternator whine (????) through the front tweeters. Maybe thats a sign of something...

Thanks again,

JMH