Still Another CIS Troubleshooting Question

[radcon]

I know that CIS troubleshooting has been beaten to death multiple times, but I've had a problem for several months and I just can't chase down the cause. The car runs perfectly when warmed up, but when cold(I live in Hawaii, by cold I mean 70-75 degrees) I runs like crap - resistant to rev, backfires when trying to accelerate. I have checked the fuel pressures and timing multiple times, they are both good. What could be the cause? Would it be possible for a fuel pump or WUR to be the cause even though the pressures are good and it runs fine after about 3-5 minutes? I want to say that this problem is fuel related, but maybe I'm justing going down the wrong path.

Thanks in advance.

[mb911]

What is your timing set at? What are your pressures at? Have you any other syptoms? Fuel filter replaced? Do you have the trouble shooting CIS book? Lots of possibiltys give us some more info if you could as to what has been done where all your pressures are etc. Then we can better help

Ben
77 911 3.0
75 914-6 2.7

[Sunroof]

Cold Start is WUR if everything runs well at warm up. Your getting good starting so your cold start valve is okay. You should check the mixture setting as well. Take a look at the CIS Trouble Shooting Chart in the Tech. Article Section. That usually will narrow it down to a few times to go after. I had the same problem and changed my WUR and re-adjusted pressure and mixture. After that all was fine.

Bob
73.5T Ist CIS

[grant lyon]

Are you quite quite sure the electrical side is correct? Defective ignition wires (spark-plug wires) can cause weirdly erratic performance. On a dark night, run your engine and look at the wires. Any Christmas lights?

I didn't believe good advice to check wires, chased mixure etc. with similar symptoms with my '76 2.7. Between points (bad news) and ign wires my cold running problems were solved. Note moisture on ign wires disappears as the engine heats things up.

[azasadny]

My 1974 911 with CIS did the exact same thing and it turned out to be the spark plug gap. I gapped the plugs according to the PermaTune CD instructions (.060), but it was too wide. I regapped the plugs to .030 and replaced the plug wires and no more cold start problems! I hope this helps...

[targa80]

The 1980 SC is the first year with the K-Jetronics with lambda.

Recently I posted info on the sequence of the cold start to warm engine theory.

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.

[radcon]

Thanks for the suggestions. Looks like the thermoswitch might be my next place to look. I have been going through all the possible causes I can think of - troubleshooting them with the help of my Bentley.

[hoosierdaddy]

I've been fighting the same problem for a few weeks now and by accident I found it tonite , my pop off valve had lost it's seal around the base. I reset the valve and the car runs great again. Might worth a try.

Jim

[stlrj]

It sounds like a WUR problem. If you don't want to spend the bucks for a new one then try lowering the control pressure on your existing one with a punch and light tap with a hammer on the adjustment pin on your regulator.

I'd do it for you, just send the plane ticket to my home address, I could use a little R&R.

Cheers,

Joe

[Sean Hamilton]

I'd ditto Joe - "the WUR" ...... Been there, done that.

btw, Plug n' seal the AAV and say good riddance to the cold start surging and the need for a PUV.