2000 Metro rough idle with air on

I am having a problem with my 2000 Metro 1.3, with 130K miles, that I cannot track down. With the air on it idles very rough and almost dies but only in Drive with my foot on the brake. If it put it in Park or Neutral the idle smooths right out. If I apply the parking brake and take my foot off the brake pedal it also smooths out (when in Drive). When accelerating, with the air on or off, it does miss or load up a bit but not always and only at very low speed.

All the plugs seem to be burning nice and clean and I did a drop test trying to locate a specific cylinder but that did not help. Gas mileage is normal and it runs fine at low speed and high speeds (10 mph and above).

Can anyone offer any ideas?

When you say air do you mean the a/c. The a/c compressor when switched on is a reasonably heavier load on the engine. To counteract that the ECM issues an "idle up" signal (I think it is done via a module on the compressor). The ECM adjusts the IAC to increase the idle speed and prevent stalling. If that signal is not present due to a fault the load may be too much for the engine and it may stall when at idle. When in gear of course the engine is also loaded by the drive to the wheels and will make matters worse.

No idea why it gets better when you use the parking brake but this is the best explanation I can think of.

Thanks for the info hot_sd.

Yes, I did mean the A/C. Are you suggesting the ECM might be faulty? Do you know if there is a way to test the ECM?

I think I will go ahead and replace the spark plugs and plug wires (that work is due any way) in case that is a contributing factor. One of the plug wires has a sloppy connector on one end. But, as I said, the drop test did not single out any one cylinder so I think this is a long shot.

The ECM may not necessarily be faulty. It could just be the wiring. I would check the wiring to the a/c compressor from the ECM and make sure it is secure. Also check continuity with a multimeter and look at the connections. If this does not work check the idle up signal (I believe it is a blue wire) and make sure it has 0V when the compressor is on and system voltage when off. The clutch module inside the compressor could also be faulty. Not sure if that is the case whther you have to replace the whole comrpessor as I've never had this problem but I suspect that may be the case.

If you can get a holod of an OBD2 scan tool that would be ideal as that would tell you right away whether you have a problem or not. Not just a code reader but a proper scan tool.

But, yes anything that could cause the engine not to run well could contribute. I believe you are supposed to change the spark plug wires every 50K miles - not sure if you have been doing that or not. I also have a 2000 metro 1.3L with a/c.

Look at the IAC which sits on the firewall inside the engine compartment. It has a spring loaded adjustment screw on a good size vacuum hose. The IAC only kicks in when the A/C is on. Check the vacuum hoses to/from it for leaks and replace the hoses if there is any question. Turn the A/C on and you can adjust the idle screw on the IAC if necessary.

There is no idle adjustment on the 2000 - it is purely controlled by the ECM.The ECM is always controlling the IAC by sending it PWM pulses and the idle speed varies depending on things like engine temp etc. It is not just used for a/c idle up.

Sorry about that, my mistake. I had a similar problem on my '93 and did not realize the 2000 was different.

I've only ever done work on the 2000 so I'm not entirely clear about the differences between the older and newer cars. I believe there are differences in the body. I think the 1.3L engine after '96 is also different. The auto tranny mechanically is pretty much the same. However there are a lot of differences in the controls between the older and the new cars - both for the engine and also the auto tranny.