Stalls when I run the AC - what the H@LL???

[Jcon]

I had been having a cold start problem which appears to be the result a very lean mixture in my 81 SC. I took the car into Dougherty Automotive Services (DAS )here in West Chester last week to have new front struts installed. I asked them to check out the cold start issue while they had the car. They spent 2 hours checking out the ignition system, checking CIS system pressure and looking for vacuum leaks. All checked out fine. Exhaust gas showed very lean at 0.2, they adjusted to 2.0 The mixture adjustment seems to have done the trick.


Except....On the drive home if I turn on the AC the car stalls when I come to a stop. As I come to a stop, I push in the clutch and I watch the RPMs just drop to Zero and it stalls. No sputtering, No Drama, just dead. It starts right back up and runs fine as long as the AC is switched off.

Any thoughts... I do most of my own work, but CIS work I find to be extremely frustrating. Any help would be appreciated

[kstylianos]

Jeff,

Where does the engine idle w/o the AC? If its low to begin with (~600rpm), the AC compresser could very well bring the engine to its knees when it turns on.

[ruf-porsche]

could be your compressor is going. Is it still the original York compressor? Should consider upgrading to a sanden or Seiko-Seiki rotary compressor. It will make a different in not only the idle but in the actual cooling of the a/c system

[Guy Chiattello]

My car was doing the same until I increased the ideal speed.

Guy Chiattello
82SC Black Coupe

[ZOA NOM]

It's a long shot, but the DME relay's contacts may be on the ragged edge, and starting the A/C may be just enough draw to make it fail. Your symptoms are identical to my intermittent DME Relay.

Good Luck

By the way, it's located under the driver's seat.

[ZOA NOM]

I didn't pay attention to the year of your car. I don't think they started using the DME until the Carrera in 84. My bad.

[Milu]

I get the same problem on a 3.2 sometimes. Cause mixture is too lean.

[Sean Hamilton]

Had exactly the same problem last summer. I compensated by tweaking the idle speed but only to mask the real problem - a crook WUR running too lean.
I replaced the WUR and all is fixed.

[Jcon]

I've still got soo many questions.


1) I don't understand how a lean mixture would cause stalling under the load of the AC.


2) Is a setting of 2.0 (with the oxygen sensor disconnected ) too lean. It was just set three days ago by a very reputable shop. If it is too lean where should it be.

3)How does the mixture change on it's own. Once it's set why would it change. Remember, when I had it checked 3 days ago it was very lean at 0.2. I haven't touched the adjustment since it was set at a Porsche shop last year.



What's odd is that just idling in the driveway at 950 RPM I turn on the the AC - no issue. RPMs drop very slightly.

At running temp while drivingwith AC it runs great except that it stalls at each stoplight as soon as I push in the clutch. I restart it with the AC on and it starts and idles fine. I would expect the WUR, but mines new ( 1 year old).

Where do I start...I'd like to cure it myself. At $85 an hour CIS problems get very expensive, very quickly. This is a 60,000 mile 1981 car in top condition. Two years ago I had the engine removed by a Porche Dealership (I got a great deal ) and gone over with a fine tooth comb. They had it for three weeks while the alternator and transmission we're brought back tp perfect and found it all to be very tight so I'm guessing my problem will not be an obvious one.


Thanks in advance

Jeff C

[turbo6bar]

Jeff, my '78SC does the same thing. It has an aftermarket A/C setup. If I keep the clutch engaged and roll nearly to a stop (rpms around 1000-1500), and THEN push in the clutch, the engine won't stall. However, if I push in the clutch when rpms are still up, the engine will stall. I have noticed that rpms drop to around 400-500 rpm.

I have noticed that even with the A/C off, decelerating, clutch in, the rpms will drop to around 500 rpm, but then it will climb back up to 950 rpm. I think the load from the A/C is just enough to stall the engine.

It is annoying, but I haven't made the time to diagnose. Let me know if you find anything. I will do the same.

Jurgen

[Sean Hamilton]

Originally posted by Jcon
........ At running temp while drivingwith AC it runs great except that it stalls at each stoplight as soon as I push in the clutch. I restart it with the AC on and it starts and idles fine. I would expect the WUR, but mines new ( 1 year old)........

Hmm, well THAT's the "opposite" to what I had going on - mine would stall when I came to a stop and depressed the clutch in.
The load of the A/C would pull the RPM's way down to less than 300 and the engine would stall. Turn the A/C off and all was okay.
Still, whilst your's is correct at 950 I'd try tweaking the RPM's up to 1100 and see what happens. I can only think that the mixture is too lean for the start-up load of the (after-market?) compressor.

Also, check the following:

quote from Hladun: AAV and Decel Valve
3. The decel valve also prevents the engine from stalling when you take your foot off. If your car stalls you need to reconnect the decel valve.

[turbo6bar]

Alright, I'm having the same problem as Jeff C. When I'm running without A/C, and I lift the gas, the rpms drop to around 500, but the car never stalls. When I have A/C on, AND I lift the gas pedal, the rpms drop to 500 or even lower, and it almost always stalls.

I searched the forum, and many pointed to the mixture. I hooked up the wideband O2 unit, and I got around 12.9-13.1 AFR at idle. That looks pretty good to me. I then ran the engine up manually to 4000 rpm, and the AFR stuck at 13:1 for the entire range. Very nice. OK, then I slammed the throttle shut and the AFR jumped to 17.5-20:1. Whoa, that's really lean. When I closed the throttle, the engine would idle down really low, but never stall. It would hit 500 rpm, and then recover to 900-1000 rpm.

The engine runs really lean when the throttle plate is close quickly. My car is a '78SC Euro coupe, but I do believe it has the decel valve (maybe part of federalization).

Can anyone give me some hints as to where I should look next? The ignition timing is spot on at 5 degrees BTDC with vacuum advance disconnected. I think the really lean mixture upon run-on is my problem, but I'm not sure how to fix it. Hmmm, Megasquirt is sounding good right now.

Jurgen

[ruf-porsche]

If you have a copy of the Bentley Service Manual for 911SC on page 240-3 it shows the location of the decel vacuum valve. It appears to be operated by vacuum, and if you have a vacuum leak, or the vacuum line is not connected the valve will not operated as designed.

The decel valve is to the left of the rubber boot and behind the boot. It is round and there is a vacuum line connected to it. Hope this help everyone with engine stalling problem.

[Sean Hamilton]

Originally posted by turbo6bar
.......The engine runs really lean when the throttle plate is close quickly.......where I should look next?.........

As Ruf said - Vacuum leak and/or WUR and/or Decel Valve. Other than a too lean mixture setting there isn't anything else I can think of. As I said, I fixed mine by replacing the WUR, but I validated the Decel valve's operation and checked for vacuum leaks first.

[turbo6bar]

TTT with update:
I verified the decel valve operation. It does work. I spent half an hour in contortionist positions trying to work with that sucker. Hey, what's that? Looks like the idle screw. Well, I turned it clockwise, thinking it would open the throttle plate more. Wrong! I ended up backing it out counter-clockwise. What can I say? I'm too cheap to buy a service manual. Idle is now at 1100, and the car does not stall with the A/C on. Whoochy Mamma, I am a happy man now. When it doubt do not assume it's the most expensive part. Check the easy stuff like mixture and idle speed. Oh, I love this car!!!

Jürgen, who is chilled by A/C in muggy 90 degree heat

[tsuter]

Revs dropping/bouncing to nothing on a throttle lift off and then bouncing back to normal (if it didn't die altogether) indicate a lean condition. Add a 1/4 turn or less to the AF with motor hot and at stable idle.

[turbo6bar]

Originally posted by tsuter
Revs dropping/bouncing to nothing on a throttle lift off and then bouncing back to normal (if it didn't die altogether) indicate a lean condition. Add a 1/4 turn or less to the AF with motor hot and at stable idle.

On wideband O2, the engine was reading 13:1 air/fuel ratio from idle to 5000 rpm. Do you really think that's too lean? I don't have a CO meter, but I think 13:1 is over 3% CO. I thought 2% was considered a nice fat mixture.

My engine does exactly what you describe, with or without A/C, except with A/C it will stall if idle is set under 1000 rpm.

I will try to adjust the mixture. That screw is in such a tight spot.

Jurgen

PS> Thanks a lot for the advice. This forum is superb.

[autobonrun]

My 79SC had done the exact same thing for years. When I would go in quickly on the clutch with the A/C on, it would stall. I could prevent this somewhat by slowing almost to a stop before going in on the clutch.

I'm now running rich and had the idle adjusted with the car fully warmed up and the A/C on. The problem is that if I don't have the A/C on, the idle is up around 1500 RPM after it heats up. However, it does not stall anymore. I just live with the higher RPM's at idle since I'm seldom there for very long. The decel valve replacement was the first thing I tried and it did not fix the issue.

[spinner]

Raising the idle speed to prevent stalling is a bandaid. It seems to me that the A/C clutch has failed. It's supposed to raise the idle speed to compensate for the A/C compressor load when engaged (preventing stalling). I had the same problem many moons ago with my long gone Jetta.

[turbo6bar]

OK, dumb question but what does the A/C clutch have to do with raising idle speed? I know on newer cars (in particular, cars with EFI) there is an idle air solenoid that raises idle when the A/C is turned on, but I did not know it existed on the SCs.

With decel valve connected, the car will stall. With the valve connected, it will open with xx inches vacuum. On my valve, it opened when vacuum was around 22 inches. I do not know if my gage is reading properly. When I slam the throttle plate shut (lift throttle), the rpms will drop to 800 rpm or sometimes even 500 rpm, but then I will notice it hesitate and then regain rpms. This hesitation and recovery coincides with the valve opening and bypassing air around the plate. Either my valve is slow to act or it needs to be adjusted to open at lower vacuum. Maybe it needs to be adjusted.

Well, I guess I better order than Bentley manual. At least I don't have to buy a new decel valve at $145.

It sounds like the system could use a tiny bleed valve actuated electrically. When the A/C clutch is activated, the valve would open and raise the idle. When the clutch is disengaged, the valve would be closed, and you'd have a normal idle at 900-950. However, I'd rather figure out why it is not working just right and fix that problem.

Jurgen