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Engine stubles and bogs down

flagreen on Wed September 22, 2010 9:15 PM User is offline

Year: 1985
Make: Audi
Model: 4000S Quattro
Engine Size: 2.2L
Refrigerant Type: R134A

When I turn the A\C on the engine in my Audi stumbles and then boggs down. If I pump the gas pedal it sometimes recovers. But most often it won't recover unless I turn the A\C back off. Any ideas as to what's wrong?

TRB on Wed September 22, 2010 9:24 PM User is offlineView users profile

Maybe time for a tune up. Compressor pressure readings might help give us an idea what's going on.


When considering your next auto A/C purchase, please consider the site that supports you:

Cussboy on Wed September 22, 2010 10:01 PM User is offline

Or there's something wrong with the compressor, and when it's switched on it produces too much drag on the engine...

Rick-l on Thu September 23, 2010 12:36 PM User is offline

how do they boost the idle on those old Bosch CIS cars to compensate for AC?

NickD on Thu September 23, 2010 5:44 PM User is offline

Engine idle or idle control problems can be a myriad of problems. But since this is an AC board, what happens if you are driving at a steady 65 mph, then switch on the AC?

If the engine stalls then, then you really know if you have AC problems, you never mentioned how well it cools.

flagreen on Thu September 23, 2010 9:52 PM User is offline

Thank you to all who replied. I have ordered a set of gauges and should have them in a day or two. I'll post back after testing. The engine was tuned up not too long ago. By the way the system cools just fine. The stumbling and bogging down intermittently occurs when accelerating with the A\C on. It has happened at highway speeds too. Other times there is no problem.

It occurred to me it might just be water in my gas because thinking back I believe the problem started after my last fill up. I bought some dry gas today and we'll see if that cures it.

Thanks again.

fasto on Fri September 24, 2010 10:56 AM User is offline

Originally posted by: Rick-l
how do they boost the idle on those old Bosch CIS cars to compensate for AC?

If it's K-Jetronic like my '82 Scirocco it uses the "frequency valve" throttle bypass that's used for warm up. The Scirocco also used an inline doodad called a DIS (Digital Idle Stabilizer) that plugged in between the ECU and the car's wiring harness.

Edit, now I'm not certain that I'm not confusing this with the Motronic control on my '88 Jetta. Both cars are long gone so I can't check!

Edited: Fri September 24, 2010 at 11:01 AM by fasto

flagreen on Sat September 25, 2010 3:32 PM User is offline

Ok I ran the dynamic test with the engine fully warmed up and the A\C on high. The engine was not stumbling or bogging down when I took these readings.

With A/C on max cool; Low side 20psig and High side 225psig

With the A/C and engine shut off; Low side 105psi and High side 100psig

Ambient temp was 83F and the Humidity was 68%

Then the stumbling occured just as I was finishing. The engine RPM fell off about 200rpm. The low side stayed at 20psig. The high side dropped to about 200psig.

Any thoughts?

P.S. I'm still unsure about water being in the gas. I need to find a hose and syphon some gas out of it.

Edited: Sat September 25, 2010 at 9:41 PM by flagreen

1stbscout on Sun September 26, 2010 1:34 AM User is offlineView users profile

It sounds to me like your idle is set wrong and/or the controller is "dropping out". Most likely the reason you notice it more with the a/c on is that the amount of power needed to run the engine at idle without the a/c is minimal. If the idle was set, with the throttle screw or bypass higher than the computer wants it the idle controller would just close down completely. When you turn the A/C on it could be pulling it under the computers desired idle setting and as it is closed completely it would stumble until the computer compensated for the extra load.
There are several other possibilities as well. One thing to check before doing anything else is all the "body" grounds going from the battery to the body and from the engine to the chassis. If I am not mistaken that one should have several wires attacked to the battery cables. If there is a poor connection at any of those wires, either positive or negative, it can and will effect the way the engine runs.

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