I have a 97 Acura CL and at idle and "D"(rive) the tach reads 750 or so. When the AC compressor kicks on the idle drops to about 600 and there's is a irritating vibration. I've read in the helm manual (for my 97 prelude) and noticed that the ecu is supposed to get a signal from the ac and boost fuel/air or something to keep the idle at 750!? Is this true, or does the idle supposed to drop?

If i hold the brake and raise the idle to 750 or higher the vibration goes away.

What do you guys think needs fixing?

1) ac not sending signal to ecu
2) engine mount
3) balance belt
4) compression
5) iac valve
6) other

I know as a last resort would be to raise the idle but rather fix the problem no mask it.

Thanks in advane...

Jay

 

I have the same issue with mine. I never run the AC because of the theft of power and the lame idle. I think it is totally normal though. If it isn't causing a stall, it shouldn't be too much of a concern. Any time the AC is turned on, the AC compressor switches on and robs power from the crank, so this seems normal. I would get it checked out by a mechanic to make sure it is normal, though.

 

Just a follow up:

I figured it'd be #1 for sure but looked at #2, engine mounts, and the front mount is cracked. Didn't find the condition on the rear one. Will change it out and see if the vibration goes away...

 

Couple thoughts...

Stock tachometers are not very accurate down below 1000. Maybe it's not really 750...

AC (among other things) signals the ECU, like you said. Then the ECU tells the IAC valve to open in order to hold the idle. That's another signal path to check. Besides, maybe the IACV is dirty/sticky. So I'd move the IACV up higher on the list.

 

Good point(s) JimBlake. I just bought a DriveRigh CarChip over ebay. Hopefully it comes in soon. I'll double check the rpm w/ it. I never messed around with the IACV, will poke at that a little, does carb cleaner take care of the sticky? Also the front engine mount should come in today, will hopefully get a chance to put it in this week.

 

just a follow-up:

1) replaced front engine mount - helped a little, not as much as i hoped
2) cleaned fully clogged screen on iacv, didn't even know it was a screen till i poked at it and a chunk of carbon crusted off - helped a little
3) tweaked the idle set screw.

Turns out it's not the AC compressor on by itself, but the combination of: Drivie, AC on, fan medium, headlights on, and the radiator fans kicking on causes the vibration. It's much better now. Shouldn't the ECU signal the IACV to allow more air during this condition via the ELD, AC signal, A/T signal, and such? How reliable is the DTC for IACV, w/ my fairly (if not fully) clogged screen it didn't set a code!? Could this be an alternator thing? Under thoses loaded conditions the voltage at the battery were 12.9vdc, unloaded it's 14.1.

I'm not sure about the tweaking of the idle screw. I must have turned it 2 complete turns open. The rpms do a funky blip to 1.5k (almost) every time I release the gas to slow down for a light. But I want to say it's happening less - is the computer still learning? . I may try putting that back to the desired 550rpm (w/ IACV disconnected) if the wierd blip thing continues. I don't think I let the compter relearn long enough. Is 60 sec of idle enought (engine warm)?

Will read my Helms manual some more but it's 80% better now. Let me know if you guys have other ideas.

 

If you had one bad motor mount mostlikly the rest are bad or weak.

 

i've seen all the motor mounts except the rear. Maybe I'll get under the car, don't like risking my life going under... how much to replace typically? How difficult? I have the right tools execpt lift. Having the impact wrench made the front a piece of cake.