slant911

Well I've found out something interesting today on my issue. When I did the tests before on turning the A/C on it was with the engine warmed up already. Today I thought I would try it out from cold. Surprising enough even on the 4 setting of the fan it revs, idles, etc, perfectly for the first 5 minutes of running. After 5 mintues it ran like sh** again with the fan set on 4. Could not even touch the throttle again or it would *cough* *cough* and die. The temp guage hadn't even moved from the very bottom yet, but, had been idling for around 5 minutes in the driveway. In fact for the first 5 minutes it ran better than it has ever ran since I've owned it. I did NOT drive it so no load on the engine, but, just idling and revving in neutral ran wonderful.

Any ideas now???

slant911

BTW - Perhaps we should seperate this into a couple of different posts as it is confusing me and I'm sure others.

964Cab

Slant:

Are you suggesting that during your recent test that if you had NOT turned your A/C on that you could have driven the car with no problems ? Am I to assume that your problem is only when the A/C is on ? Did this problem happen before the non standard fly wheel was installed ?

I ask because my 1990 C4 Cab has a similiar problem only the locus of the problem remains a mystery. At least you have id'd the A/C (I think) as the source of your problem.

The CCU (Climate Control Unit) feeds the DMe brain and when the A/C is ON the CCU tells the brain that your A/C is on and that's supposed to cause the DMe brain to adj your throttle somehow (right Adrian ?). I assume a non-standrad flywheel would confuse the DMe brain with regards to the mapping of the engine and if the mapping is confused then the engine will run bad; perhaps the confused DMe brain is going into "emergency limp home" mode when the A/C is ON as the DMe now has an additional load on the engine to deal with (A/C) coupled with that non-standard flywheel (might be too much to ask of the Dme brain ?).

In my situation I have two conditions. One is just like yours in that my car can at times run superb and idle perfect and then for some unkown reason the car can run terrible and become undrivable. For example, today a Porsche Tech placed my DMe brain in a known working donor car (same year but a C2 and my car is a C4, both 5spds though). My DMe brain worked perfectly in the donor car. Then I got home and placed my DMe in my car and the car ran terrible. Then I went to lunch and came back and the car ran perfect and now, just a few minutes ago I tried to start the car and it won't even start, turns over strong but no start.

BTW: My car's DMe brain is a #911----04 and the above noted donor car's DMe brain was #964---02 but yet the two cars were both 1990 (USA) models.

Adrian:

I jumpered the fuel pump as a test and it hums away nicely.

I'm on my 3rd DME relay.

I put in a new 02 sensor 3 weeks ago.

I put in a factory rebuilt twin dist 2 weeks ago.

I disconnected, for a test, the connector to the Mass Air Sensor to force the DMe to go into an "emergency limp home" mode and the problem persisted. The "flap door" in the Mass Air Sensor is smooth and the unit yields proper Ohms values at the DMe connector.

I'm tempted to just replace the two ignition coils (they both measure proper primary and secondary windings resistance in Ohms). Also I may replace the two ignition "ignitors".

To rule out the alt I took the belt off and the problem persisted. HOWEVER, for some unknown reason the XMAS Tree effect has gone away but the airbag and seatbelt warning stay lite up as that's a different problem and as you noted can only be cleared via a "Hammer".

Thanks,

Patrick

MikeF

I haven't read through all of this thread but just a simple question...when was the last time you replaced your fuel filter? I believe Adrian already suggested this possibility but I did not see an answer.

slant911

Yes. You are correct. The symptoms are as follows:

Car runs fine with A/C off. Hot or cold.
Car always starts fine. Hot or cold.
When engine is cold (first 5 minutes of running)and you turn the A/C on any setting everything is fine.
After 5 minutes warm up time and you turn the A/C on that's when things go bad. On fan speed settings 0,1,2 everything is O.K. On fan setting 3 things start to go ugly. On fan setting 4 you can not even touch the throttle or the engine dies. It will idle all day long, but, ZERO throttle response. If you touch the pedal the engine coughs and dies. It will however start right back up.

I have not replaced the fuel filter.
I have on my agenda this weekend to check out my part number of Temp control unit, check cylinder head temp sensor, check O2 sensor. I will add to that a good look at the fuel filter.

BTW - does anyone know a good source (cheaper) for the instrument idiot lights? The dealer wants almost $5 for one of those. It couldn't cost more than .25 to make. I want to add the Check engine light as it appears to work, but, can't find anywhere that has them.

Adrian

Dear Slant,
If the problem is purely when the airconditioning is on and the fan is selected to position 3 or 4 you are looking at a problem output (or lack thereof)from the controller to the DME or the fan itself sucking down the power. Is this 100% correct. The problem is only aircon on, fan selected 3 or above. No other times.
Ciao,
Adrian
911C4

slant911

As far as I can tell at this point. YES.

The odd part I thought was that when the engine was stone cold it ran great no matter what the fan was selected at 0,1,2,3,4. Didn't matter. As I said though after idling/revving for 5 minutes then it took a poop. It's like once the cold start richening went off then things got bad. Just my theory. No supporting info for that opinion.

964Cab

Slant: That is odd that when the car is cold the A/C can be on and the engine runs fine but when your car gets hot the DME fails to keep the car running properly. So when the engine temp sensor sends the lower resistance value to the DME (engine hot) the DME fails as a result. What might work (Adrain what do you think) is a fixed value of resistance that the DME would "see" instead of the varying resistance values that are coming out of a stock engine temp sensor ??? Find out what value resistance (at the DME connector pins) the engine operates fine at and then maybe install a fixed resistor of said value in place of the engine temp sensor wires. That way the DME will never "see" a hot engine and the problems your having will cease. However, that may mess up your oil cooler fan operation. remember that the hotter the engine gets the lower the resistance value that the engine sensor is sending to the DMe brain, once the "brain" sees that critical lower resistance value the DMe fails to cope with the lightweight flywheel load and the additional load from the A/C.

FYI: Those lamps for the instruments cost $5 as they are special made and have their own onboard "carrier" that screws right into the open bore of the instrument.

Regarding my problem: I FIXED IT !!!
I ran a long wire from the + battery terminal to one of the two ignition coils and then ran a short wire from the latter ignition coil to the remaining ignition coil. I took off the two stock Porsche wires that were already on the two igntion coils. The car started and ran perfect and no more sudden loss of high voltage to the dist and no more XMAS Tree effect !!!!!!!!

However, I don't know why but the original wiring provided + volts to the ignition coils that was NOT stable. The voltage would flucuate as seen on my voltmeter. Don't know why it wasn't stable but when the voltage to the coils would flucuate the car would suffer and loose high volts to the twin dist. Randall noted that that coil + volts first goes through a lot of other circuits such as the "chimes" alarm. I think it also travels through the Cab top control unit as well.

I'm not worried about where the problem is as I have my mind set on installing a "kill switch" that will provide stable + volts to the two ignition coils. I just have to remember that in an accident the coils won't lose + volts as the crash sensor will lack the ability to kill the juice to the coils. The fuel pump will stop though in the event of a crash so maybe I''ll be ok ???

Any idea where I could put a kill switch for the igntion coils ? There is an un-used connector in my '90 Cab's engine fuse panel area that I could use but I can't find said connector on my wiring sheets. Is that maybe the electric rear defogger for a Cpe or Targa ?

Thanks,

Patrick

Adrian

Dear Slant,
There is no specific cold start programming in the 964 series. Intake air temp is compensated for by a temp sensor.
There are two modes of operation. Open loop (without O2 sensor) and closed loop (with O2 sensor) which occurs at around 90 seconds after start.
There is also emergency or limp home mode which is basically open loop when the O2 sensor or other sensors fail.
Your problem seems to be the input from the controller is wiping out the DME. Or as I said the fan electrical load through the controller is causing the problem.
Does the problem occur if you switch the LH fan to 4 then switch on the RH or defrost fan. This fan overrides the LH fan and switches it off. I would certainly be checking the wiring to the O2 sensor and the connector. Maybe a mechanical problem related to the airconditioning system. Something rubbing. Hooking wiring,
Ciao,
Adrian
911C4

slant911

As stated above I have those items on by "to-do" list this weekend. I tried it again today at lunch and found that the car ran like sh** at about 7 1/2 minutes after start from cold. Again, it revved fine with air on or off. As soon as the car started to get warm things went bad.

Bill Wagner

Slant:

This is a long post, so maybe I've missed this, but when you turn on the AC, what do you have the cabin temperature set at? Is it set on "maximum cold", or is it set at an actual temperature (like 21 deg C)?

If you've already stated this, forgive me. I'm very old

Bill Wagner

Adrian

Dear Slant,
The fault, or your posting of the fault seems to chop around a bit making it a little difficult to follow.
Are you now saying that your 964 runs okay with our without air for around 7.5 minutes and then runs badly, with or without air.
If this is the case I would suspect the O2 sensor breaking down. Remove the Left hand rear wheel and the sensor will be right there. Follow the wiring and then disconnect the O2 sensor. Try your experiment again.
I would also check to make sure that your compressor clutch is actually engaging and disengaging. This is very easy to check.
However I have to add. The 964 system is heavily integrated. Almost everything in some way affects the DME and its job.
Ciao,
Adrian
911C4

slant911

No. I have been saying and will restate as I agree the post has gotten WAY TOO long.

The car runs fine hot or cold with A/C off.
If you start from stone cold and turn the A/C on any setting it runs fine for about 7 minutes. After 7 mintues of warm up the car will not run with A/C on setting 4 on the dial. This is with temp set all the way cold. Full A/C. If you turn the A/C system off (push button). The car will run O.K. again.

Gary C2

Slant,
The O2 sensor connector is located in the left side of the engine compartment, just above and to the left of the top spark plug of the #3 engine cylinder. It is a round connector approximately 3/8" in diameter. No need to remove the wheel.

I would also try "stomping" on the accelerator from idle (out of gear) when the problem exists with and without the A/C on and note the results.

Gary

Adrian

Dear Slant,
Based on your last post. BTW I said on the first page things were going off track I believe your problem is the interface between the DME and the controller. I suspect the fault is the fan itself. The driver circuit is inside the controller but the blower final drive unit is a separate assembly. I suggest the LH fan in position 4 (or the blower final stage unit) is causing a controller overheat and then the controller feeds bad data to the DME.
This is obviously aircon system related and the only direct link to your problem is the controller to DME interface. Maybe you could consider swapping the RH and LH blower fans over and seeing what happens. Crazy idea but it might work, they are the same fans.
The only other culprit maybe an overheating condenser fan. I would certainly be having a look at the condenser resistor. This area is known for corrosion damage,
Ciao,
Adrian
911C4