SC Idle hesitation problem CDI or O2 Sensor?

[Rodney Nelson]

I have a 1982 911 SC:

Here is the Problem:

1. Rough Idle
2. Hard to Start
3. Hesitation in low RPM ranges (Runs fine 3000 RPMs and Higher)

Here is what I did to the car 2 months before this problem started:

1. New Plugs
2. New Wires
3. New Rotor
4. New Cap

Then the problem arose and I did the following:

1. New Fuel Filter
2. New K&N Air Filter
3. Checked all of the Plugs
4. Checked the Box under the front seat (forgot the name of it), when I disconnect it the Idle gets worse, so I ruled this out as a problem.

Then I gave up and took it to the Mechanic:

1. He told me that the Green Wire harness to the Distributor had been spliced, and that he believed that it needed a new Wire Harness and Distributor. I told him that I doubt that it needed both, I asked him to just change out the Harness he said that would do nothing? He did the following:
a. Changed partial wire harness (wires from the CDI to the Distributor.
b. Changed the Distributor
c. Then he said that it needed a new coil, my paper work said that it was only 1 year old from PO. ?

Worst part was that he charged me full retail price for everything.

When I got the car back it worked!!!!!!.............for 2 weeks then the problem came back. I don't want to take it back to him, he did not know what he was doing.

So here is where I stand now:

1. The CDI unit does hum when the key is in the on position
2. However, it gets extremely hot when only Idleing for a couple of minutes.
3. Checked the fuel pump relay and fuses seem to be good.

What do I do next? I don't think the O2 sensor has been changed in a while? Wayne's book says every 30K miles, so I will do that ASAP.

How can I tell for sure if the CDI is working correctly? I found a place in phoenix that will sell me a rebuilt one for $395.00.

What about Grounding? I keep reading about grounding, which ground straps should I inspect and where are they located?

Thanks for any input.

[Lorenfb]

Borrow another CDI to use as a tester before buying another.

Good luck
Loren
Systemsc.com

[emcon5]

You can eliminate the O2 sensor as a source fairly simply, just unplug it. If the problem goes away, you are done.

It the frequency valve buzzing? You should be able to feel it vibrate when the engine is running.

Tom

[Rodney Nelson]

Tom,

Where is the Frequency Valve located? I hear a humming which I am pretty sure is coming from the CDI.

Rodney

[Rodney Nelson]

Loren,

That is a good Idea, but I don't have any friends that are Porsche guys so I can't really get my hands on one to Barrow, Maybe I can ask 20th street auto if I could try one of their units out before I buy?

Rodney

[emcon5]

It is hard to see, but it is part of a thick fuel hose behind the throttle body, with a electrical plug on top.

This is a pic of the photo from the Benteley SC manual. To give you an idea of where this is, the little half circle cut off at the top of the pic is the idle adjustment screw. According to the book, the test is to put your finger on it to see if it is vibrating. If it isn't, pull off the electrical connection, and check the resistance of the winding, it is supposed to be 2 to 3 ohms.



If it isn't buzzing, and the resistance is OK, check the power relay under the passenger seat, next to the Jetronic "brain".

Hope this helps,

Tom

[Rodney Nelson]

Thanks a lot Tom,

As soon as I get off work I will check this out. What about the CDI do you know if they are supose to get really hot? Too hot to leave your hand on?

[Lorenfb]

CD units should not get real hot. Ask Larry at 20th if you can
borrow a CD (with a deposit). He's a nice guy and I just shipped him one a week ago.

Good luck
Loren
Systemsc.com

[Rodney Nelson]

So I got home and I re-installed the CDI unit before I did I heard a rattle inside and I opened it and found a small ball of solder rolling around inside the unit, I removed it and re-installed the unit.

I then Started up the 911 and as always it was hard to start then once I got it going I disconnected the O2 sensor and it did not make a difference in the way it was running so I reconnected it.

The frequency valve was definately vibrating when I started the car up so that seems to be working.

Then I turned it off again and restarted it and it was running fine, I then took it out around the block and the problem seemed to disappear.

I drove it to work this morning and it was still fine. Then I went to go to lunch and it would barely start, the problem reappeared.

So, I definately have a intermitent problem here.

Can CDI units give you an intermitent problem?

What else could this be?

Could this be a grounding problem of some sort?

I will call up 20th street auto to see if I can test out a CDI unit.

Thanks for all your help,

Rodney

[Rodney Nelson]

Loren,

I just called Larry at 20th street auto, He wasn't not even a little bit interested in letting me try it out. He said I could buy it and if I wanted to return it, He would give me credit.

So that Idea is out the window.

There has to be some way to test these darn things.

[jason2guy]

doesnt sound like an ignition issue. i would say fuel instead. i just replaced all of my fuel injectors and i cant believe the change it has made. i had a bunch of surging, hot start issues and already replaced all the normal stuff-accumulator, WUR, AAV, then i started to think about the injectors and i figured why not, ive already done everything else.
its all smoth as silk now. those things are 20+ yrs old throw em away and get new ones. youll thank me

you should be aware of your basic idle mixture setting as well.
this setting affects the way the car runs, in a big way

[Rodney Nelson]

Jason,

Can injectors be an intermitent problem?

I am not opposed to changing them, How much do they run?

[jason2guy]

probably not. either they are clogged or leaky or both.
ok, if it worked AFTER the mechanic repaired it, the problem most likely exists in one of the areas he fixed. why dont you tell him what has happened and see what he says. if not i would go over his repair work and verify that its still intact and functional

[GMR911]

My car is doing the exact same thing. Was running fine then all of a sudden it is stumbling slightly below 3000 rpm. Above 3000 rpm seems to run fine. I thought I fouled a plug so I changed them. Still does the same thing. I am thinking a vacuum or air leak? Maybe timing. Right now I am just guessing. It is also backfiring and poping slightly at the low rpms? Jerry

[Rodney Nelson]

Jerry,

That is exactly what mine is doing. I will tell you this much don't let some mechanic change out the distributor, wire harness, and coil to fix the problem!!!

I just was reading wayne's book again, and uder the oxygen sensor section, He describes what may be happening exactly. I orded it just in case and it should be here on Friday so I will have it in by Monday.

I will post my results and move forward with more trouble shooting after that.

Yes, I will be looking at vacuum leaks too.

Rodney

[Lorenfb]

Bosch CDs can be intermittent, but the SC type is rarely
intermittent. They either work or they don't. Some, though,
have a hot start problem or cause back-firing when hot.

You may have done unnecessary parts replacement in the
dist. area. The problem sounds to be fuel related. You don't need
to replace the O2 sensor for testing, just leave it disconnected.

You need to isolate the problem to fuel or ignition, and don't
do the shotgun approach. Take your time. The next time you
have a starting problem, check for a good spark.

Check the website Systemsc.com (Diagnostics) for some
troubleshooting hints.

Good luck
Loren
Systemsc.com

[Lorenfb]

One other thing! Make sure that you don't have a silver Bosch coil.
My customers and Porsche dealers have had problems with these types.

Good luck
Loren
Systemsc.com

[targa80]

Here is the theory of the Kjetronics with lambda operation from cold start to warm operation. It might help you in figuring out your problem.

The Lambda (O2) sensor requires high temperature to operate properly. With the O2 sensor connected and the engine is cold the Kjetronics with Lambda system operates in the open-loop mode. The lambda control unit does not have a valid signal from the O2 sensor at cold start To correct drivability problems at cold start a lambda thermoswitch is installed to give the lambda control unit an additional input and insure proper fuel metering during warm-up. When the engine is cold, the thermoswitch is closed. the lambda control unit sends a fixed, slightly-rich duty cycle signal of 60% to the lamba control valve (frequency valve). when the thermoswitch warms enough to open, the control unit sends a fixed middle signal, 50% duty cycle to the frequency valve. When the lamba (O2) sensor reaches operating temperature so its signals are valid, the control unit switches to closed-loop operation. In closed-loop oeration the duty cycle signals are constantly changing cycling back and forth from 45% and 55%. so the air-fuel ratio is maintained near stoichiometric; lambda=1.

So disconnecting the O2 sensor at a cold start would do nothing to effect the running of the engine. A bad thermoswitch would effect the running of a cold engine since it would not provide the necessary signal for the control unit to run at a 60% duty cycle. Instead the control unit would put out a 50% duty cycle.

When the engine is warm and the O2 sensor is disconnected (open-loop) the contol unit would put out a constant 50% duty cycle.

The thermoswitch is something you can check for proper operation with a meter.

[Rodney Nelson]

Loren,

I had one of those silver Bosch coils and then the mechanic replaced it with another one.

How can I check to see if it is working properly?

Thanks,

Rodney

[Rodney Nelson]

Pat,

You were right, the Oxygen sensor had no affect. I knew it wouldn't but I figured that it needed to be changed anyway.

So I guess I want to check the thermo switch like you said and the signal that the control unit is sending out. So how can I get started?

Where exactly is the thermo switch and how can I test it?

As far as the control unit, this is the unit under the passanger seat right? How can I measure the signal to make sure that it is correct?

Thanks for your help,

Rodney