VW Golf wants to stall after A/C repair. Fixable?
September 8, 2007 6:10 AM   Subscribe

After having the A/C compressor replaced in our VW Golf, the engine idle speed drops way down to near-stalling whenever the clutch is released while driving. It pops right back up, but it's a bit unnerving.

Last week the A/C stopped blowing cold air. I took the car to our local mechanic who suggested replacing the expansion valve first, then the compressor if that didn't fix it. After the operation, I have a new expansion valve and compressor.

When we first got the car I noticed the idle speed was a little low and jumpy, but it wasn't anything unnerving. Now, though, whenever you cut the A/C on the engine idles extremely low. The especially disturbing part is when driving around and pressing the clutch to shift. When you press the clutch, the engine drops down in speed, of course, but instead of hovering around 1200, it drops all the way toward zero, then bounces back up to jump around 1000.

All of this goes away when the A/C is turned off. What's the problem? Can I have it adjusted or fixed? What can I suggest to the mechanic on Monday?
posted by odinsdream to Travel & Transportation (9 answers total)
In most AC equipped vehicles, there is an idle adjustment device that kicks the vehicle idle up about 200 to 400 rpms, to supply power for the AC when the vehicle is idling. So, if your car is idling, and you turn your AC on, your idle speed should go up a little bit, not down. There may be a sensor or position switch that is not being activated when the AC is turned on, preventing this.

A car should not materially idle differently in neutral with the clutch pushed in, or left out, regardless of whether the AC is on, or not. The clutch throwout bearing is likely going bad if you can change the idle speed, just by depressing the clutch.
posted by paulsc at 6:20 AM on September 8, 2007

paulsc, maybe I'm not describing the clutch issue correctly. Here's exactly what happens:

When driving in, say, second gear, and you're about to switch to third. The engine is going to be at about 2800-3000, and you'll press the clutch in while releasing the throttle. Before the repair, the RPMs would slide easily down to around 1000, you'd shift, and tap the throttle to bring everything back up, and release the clutch.

After the repair, when you press the clutch and release the throttle, the RPMs begin their slide down to 1000, but they go past 1000 dipping way down towards 0 for a couple of seconds, after which the engine then spins back up to around 1000 by itself.
posted by odinsdream at 6:42 AM on September 8, 2007

The problem probably lies with the idle adjustment device (known as an Idle Stabilizer Valve in WW parlance, I believe).

It's worse because--now that there's refrigerant in your system-- the A/C compressor is drawing more of a load from the engine when it is running.

If your car has more than 60k miles, the valve itself may well be kaput. They get dirty and die.
posted by Kwantsar at 9:52 AM on September 8, 2007

Here's what the part might look like. I can't say for certain that that's what is broken, and "shotgunning" parts is a bad policy.
That being said, however, if you can verify that you have no vacuum leaks, and you can get to the part yourself without doing a bunch of rearranging, you'll save a few bucks if you comparison shop for the part and install it yourself.
posted by Kwantsar at 9:58 AM on September 8, 2007

Oh, yeah, and while I'm at it.

If you can find this valve, and you can unplug it, do so. Then drive around. Is anything different? If the ISV is working, things should be different.

If things aren't different, your valve is probably broken. It's possible that something else is wrong (vacuum leak, wiring fault, of bypass screws way out of adjustment), but the valve is the likely culprit.
posted by Kwantsar at 10:06 AM on September 8, 2007

Thanks very much everyone! I'll check out these suggestions tomorrow when I get a chance to dig around in the car. The idle stabilizer sounds promising.
posted by odinsdream at 9:55 PM on September 8, 2007

It's lonely in this thread, odinsdream. Let me know if my suggestion works, or email me with any questions. I once had a month of joy nursing an ill Corrado back to health. It's a great time, and I wish you luck.
posted by Kwantsar at 10:34 PM on September 8, 2007

Kwantsar; I've taken a look through the service manual for the ISV, but I wasn't able to find it. Here's what I've discovered so far, though:

The engine idle speed is controlled electronically by the Motronic ECM, which is a little black box up near the windshield with about 40 wires going to it. The service manual runs through several tests you can run by hooking a multimeter up to the harness after disconnecting it from the ECM. I've done some of these, and discovered one faulty ground, and, more importantly, that the "A/C running?" test isn't working. That is, where I should be reading battery-level voltage while the A/C is running, I read zero volts.

Perhaps, then, the ECM isn't even aware that the A/C is on. Where this state information comes from is still a mystery, as it isn't really explained in the service manual aside from being buried deep within the wiring diagrams in the back. This is where I am now.

Any suggestions on where I can look for the ISV? Does this car even have one? It's a 1997 Golf Jazz.
posted by odinsdream at 2:02 PM on September 9, 2007

Looks like the problem was a dirty throttle plate. After a $30 cleaning, everything works normally.

Thanks everyone!
posted by odinsdream at 11:30 AM on September 12, 2007

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