Weird ac problem..

[painterdave72]

whenever i turn my ac on it make the car idle go nuts.... i have dpi's cams in my car along with the uk stainless exhaust and it is running open loop at the moment.. the car runs fantastically except when i turn on the ac.. then it idles like going thru warmup phase... could it be an adjustment on the microswitch screw??? or maybe need to clean the idle speed motor?? It does sometimes eventually settle itself into a stable idle with the ac on but for the most part it just fluctuates.. thanks all!! Dave..

[David T]

When you have "hotter" cams (which I suspect they are if they are custom) they generally have greater overlap which means a lower manifold vacuum at idle. This results in a diminished idle quality which is generally overcome by increasing the idle speed. That is not easy to do with the stock idle system. Your best bet is to talk to DPI and ask them how to handle the idle problem. The exhaust and running open loop is not going to have as much to do with the idle as the cams. On most production cars with carbs they had a solenoid that pushed against the throttle to increase the idle speed when the A/C was turned on. The other thing you can do is make sure the mechanical and vacuum advance systems are operating properly. And the CPR. That might be hard if the vacuum is very low at idle. That's one reason to bump up the idle speed.
David Teitelbaum




whenever i turn my ac on it make the car idle go nuts.... i have dpi's cams in my car along with the uk stainless exhaust and it is running open loop at the moment.. the car runs fantastically except when i turn on the ac.. then it idles like going thru warmup phase... could it be an adjustment on the microswitch screw??? or maybe need to clean the idle speed motor?? It does sometimes eventually settle itself into a stable idle with the ac on but for the most part it just fluctuates.. thanks all!! Dave..

[Spittybug]

The stock setup is such that at idle the throttle switch is activating the solenoid that blocks the vacuum from reaching the vacuum advance unit on the distributor. Sounds like (as David T. pointed out) that with the extra load at idle with the A/C on, your RPMs are dropping and you start hunting. Electronic control would advance your spark a bit to compensate for this, but our stock setup doesn't read the RPM, just the throttle position. Perhaps one of the smarter electronics guys on the board could suggest a circuit that would sense the A/C load (tap the compressor 12v line) and "override" the solenoid, thus allowing the vacuum to reach the dizzy and advance the timing nearly instantly.

Megasquirt has these kind of features. Maybe some of them can be back implemented to stock setups?

[Bitsyncmaster]

I assume you still have the stock constant idle speed system connected. With the changing load on the engine, the idle speed motor would need to change it's position to hold the 775 RPM it's set for. So it would be my guess you have a dirty or faulty idle speed motor that is causing problem at the larger open position. First thing would be to clean that idle motor. If you still have problems, change the setting of your curb idle. That will cause the idle speed motor to hold at a different value.

Only other thing common to idle and AC is the increased load on the alternator. AC clutch, blower fan, and cooling fans really load the alternator at idle speed.

[painterdave72]

to clean the idle speed motor.. it is new but now is a few years old.. also i think i already have the idle screw bumped up a bit.. will have to double check on that..

[DMCMW Dave]

to clean the idle speed motor.. it is new but now is a few years old.. also i think i already have the idle screw bumped up a bit.. will have to double check on that..

Also make sure you have the base idle (the small stop screw+the microswitch screw) open as far as you can with the engine warm and maintain the 775. This means the idle motor will be almost closed at the "best idle" condition and will have more range to open under load/cold conditions.

[dmc6960]

Also make sure you have the base idle (the small stop screw+the microswitch screw) open as far as you can with the engine warm and maintain the 775. This means the idle motor will be almost closed at the "best idle" condition and will have more range to open under load/cold conditions.

This is a VERY good adjustment to do, for everybody!

[Bitsyncmaster]

This is a VERY good adjustment to do, for everybody!

+1
I have found the OEM idle motor to have dead spots in every motor I've tested so far. That's one reason I changed to the Volvo idle motor which is very linear. I should have had a demo set up at the show. My new idle ECU has a test mode that slowly runs the idle motor back and forth.

[painterdave72]

Left a message for josh at dpi.. hes hard to get ahold of!! lol