Warm start makes the system run lean for about 10 mins

ZV8888 · 11-16-2010, 07:40 AM

My 3.0l Euro SC '79 is gving me some grief on warm starts. The AFR is up at 17:1 for a few mins then drops down to the usual 13.5:1 to 14.5:1 after say 10 mins of running. This would suggest to me that I am getting some unwanted air in the CIS system. My suspicion is pointing me in the AAR direction. Maybe the AAR is opening too quickly when the voltage is removed. It would seem that it takes 15 mins to close completely from cold in the first place. I will check the resistance of the AAR (should be 25 - 30 ohms I believe) before I start to take it apart and bend bits.

Any comments / ideas?

P.S. I am pretty confident that the fuel side of the equation is in spec.

Thanks

Bob Kontak · Bob Kontak's Garage

You may be correct. Here is a description of the AAR's function from the CIS primer:

"Provides extra starting air (bypassing air around the throttle body) with a cold engine. The valve slowly closes with heat, provided by an internal resistor, and to a lesser degree, from heat from the engine itself. For part number 0 280 140 200 (from a 1976), the cold resistance of the heater is about 17 ohms. This device was added in '76.

Failure mode: May remain partially open in cold ambient conditions due to open resistance element. Idle speed stays high after warm up. If it stays closed due to mechanical malfunction, the car may be hard to start."

CIS Primer for the Porsche 911

I put my AAR in the a 200 degree oven a month ago and it took 10-11 minutes to close. Maybe your 15 minutes from cold includes the extra few minutes to warm the engine exterior up.

Only thing I don't get is that if "all that is wrong" is an open circuit, the re-opening (after warm) should still take quite a while.

ZV8888 · 11-16-2010, 08:37 AM

Fair point on the re-opening. I suppose the only thing to do it take it off and go through the tests .... There is another Saturday afternoon ......

Thanks

ZV8888 · 11-16-2010, 08:37 AM

Fair point on the re-opening. I suppose the only thing to do it take it off and go through the tests .... There is another Saturday afternoon ......

Thanks

Bob Kontak · Bob Kontak's Garage

That is definitely a crap job. Two allen bolts and a boat-load of swearing. I did not do this on my AAR removal, but if you have air conditioning it may be time efficient to move aside the compressor and bracket for easier access.

If you have an extra female wire harness laying around you could just put that on and check the resistance. Or you can buy the harness from NAPA. If you are interested I will post their part number. It's $15.

ZV8888 · 11-16-2010, 09:05 AM

Thanks Bob, But I'll figure something out to test the electrics.

Air Con .... what's that? We probably had 10 days last year above 65F and last summer was regarded as a good one. I have to drive at 100mph+ for at least 15 mins to get the heat exchangers warm enough to defrost the window these days. Maybe I should retune the engine for very lean - 18:1 so that is generates enough heat in 5 mins to defrost ... now there's an idea.

psalt · 11-16-2010, 09:42 AM

What you are measuring is probably normal and not an indication of a faulty AAR. How much it is open should be compensated by air flow sensor height. The lean mixture is probably due to a combination of other factors, temps, control pressure, leaking injector seals and sleeve O rings. The AAR is a crude cold idle device calibrated for the lower control pressure provided by a cold WUR, not on a warm start. Many SC's have an elevated idle on a warm start, but if it goes away quickly the AAR is OK.

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by ZV8888

My 3.0l Euro SC '79 is gving me some grief on warm starts. The AFR is up at 17:1 for a few mins then drops down to the usual 13.5:1 to 14.5:1 after say 10 mins of running. This would suggest to me that I am getting some unwanted air in the CIS system. My suspicion is pointing me in the AAR direction. Maybe the AAR is opening too quickly when the voltage is removed. It would seem that it takes 15 mins to close completely from cold in the first place. I will check the resistance of the AAR (should be 25 - 30 ohms I believe) before I start to take it apart and bend bits.

Any comments / ideas?

P.S. I am pretty confident that the fuel side of the equation is in spec.

Thanks

ZV8888,

The air passing thru the AAR is metered air including the by-passed air during cold start!!!!! Unless you have an air leak at the hoses connected to the AAR. Plus AAR should be partially or totally closed during warm start ups. There are tons of write-ups how to bench test an AAR.

Before you go serious about AFR, make sure you don't have a vacuum leak in the system. The most common culprit with the mixture going LEAN is unmetered air (vacuum leak). Test it and stop guessing.

As long as the FP is running, the following CIS components ( AAR, WUR, & TV) are also getting power. Even if you unplugged the wires from a warm AAR, it would not close quickly as you believe. It is controlled by the bi-metallic spring that depends on temperature change. HTH.

Tony

ZV8888 · 11-16-2010, 10:28 AM

Why would an vacuum leak disappear after 10 mins of running on a warm engine or not appear on an engine that started cold and warmed up nicely? Surely it has to be down to a component of the CIS that changes characteristics with temperature? But Hey, I am open to all ideas .....

psalt · 11-16-2010, 12:49 PM

A leak in the pre AAR hose that lets unmetered air in when the AAR is open, but is irrelevent after the heater closes the AAR after 8 minutes. The AAR/AAV/DECEL pipes and hoses are some of the usual suspects.

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by ZV8888

Why would an vacuum leak disappear after 10 mins of running on a warm engine or not appear on an engine that started cold and warmed up nicely? Surely it has to be down to a component of the CIS that changes characteristics with temperature? But Hey, I am open to all ideas .....

ZV8888,

Have you checked your fuel cold/warm control pressures versus time? You don't even need to run the motor to do this test. What this test shows is how the WUR performs from cold to warm condition. Keep us posted.

Tony

ZV8888 · 11-17-2010, 02:35 AM

Tony,

Cold / Warm pressures with time .... an interesting point is that it takes my WUR about 10 - 12 mins to go from cold to warm - all within spec though. Can't figure the relationship to my problem, but maybe I'm missing something?

T77911S · 11-17-2010, 04:27 AM

if any of the hoses for the AAR and AAV are leaking, the air leak is there ALL the time.
its NOT the AAR. i have said that and now tony has said that. also, on a hot engine, the AAR will not open in 10min.

check the power to the WUR and AAR if your warm up times are long. the AAR closes pretty quick under normal operation. the test the guy did in the oven was without the 12v, so it would take a long time to close.
one of the design problems a lot of guys have is the warm up cylce is too short, not too long.

could it just be a function of the LM2 not reading the AFR properly at first. i know mine has acted strange at times. try using the sensor that goes on the tail pipe. wait a minute or two after you start it, then put it in. i never start the car with the sensor in, but i use the tailpipe sensor.

how many miles on the engine?
does it RUN bad when started warm or do you suspect suspect something is wrong because of the AFR's?
what heat range plugs are you using? if it is missing when you start it, the plugs could be fouled, due to either oil, fuel too cold a plug. i dont know if that would show lean or rich. my car will sometimes run on 5 cylinders when starting hot, but it clears up pretty quick. but my rings are shot.
what about the last time cap, rotor and plugs were changed?

is it really 10 minutes or just 2 or 3? could be residual fuel pressure or maybe even the check valve. will the car start after 30-60 minutes of sitting? (hot)

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by ZV8888

Tony,

Cold / Warm pressures with time .... an interesting point is that it takes my WUR about 10 - 12 mins to go from cold to warm - all within spec though. Can't figure the relationship to my problem, but maybe I'm missing something?

ZV8888,

How did you know? If you don't understand what I suggested, you have no concept of the fuel pressure change over time by the WUR after cold start. If the numbers are within spec, what are they? Give us the numbers so we could make sense of what you are trying to solve.

You are not missing anything. You are simply hearing what you wanted to hear!!!!!If you know what you are doing, you won't be here asking for help.

Tony

T77911S · 11-17-2010, 10:49 AM

check your residual pressure just before you restart it (hot), then check the control pressure and primary pressure while it is in the lean condition. post these along with what tony asked for.

Bob Kontak · Bob Kontak's Garage

T77 - When you say primary pressure are you referring to system pressure?

ZV8 - Do you have a fuel pressure tester?

ZV8888 · 11-17-2010, 01:23 PM

Hi Guys, Thanks for your help. I will get the pressure gauges hooked up again at the weekend and see what's what.

T77911S · 11-18-2010, 03:59 AM

yes. he needs to verify there is not a drop in system pressure and/or control pressure WHEN it is lean.

ZV8888 · 12-14-2010, 03:53 AM

Got it figured out. WUR bimetalic strip / heater is faulty. When I thought it was fully warm (like after 15 mins driving) the strip wasn't fully lifting away from / releasing the pin. It only opened fully after 45 mins of driving OR 15 mins of driving and left standing for an hour or so (I presume heat radiation from the engine keeps heating it up). So the warm start component of the WUR was running the car too lean when it had unrestricted access to control pressure tit thing in the WUR. Once re-calibrated and mixture adjusted it is pretty happy again. I don't think the solution makes complete sense as the original Lean warm start would come back into spec after running the engine for a few mins. But such is the idiosyncrasy of the CIS system.