RPM oscilation during warmup leads to stalling. What to do?
 

Hello,

I have a problem on my 1980 3.0 911 SC.

when it's cold, and the engine is cold, when I start the car, it is ok for the first couple of second (startup), but after that, as it warms up (ie: at the begining of the warmup time), the RPM starts oscilating more and more around the Idle until the ampliture is such that it stalls...

Any idea what to do to fix this?

Regards, Cyrille

Check your WUR. Fuel pressures and mixture settings, It is probably a little rich.

hello,

What is the WUR, where is it located and what does it do?

I wikk try to lean a little and see what happends...

cyrille

WUR=warm up regulator and controls cold control pressure. It is not intended to be adjustable, but you can make it adjustable with some modifications. Since it starts OK and becomes worse during warm up suggests the heating element in the WUR might not be working to make the control pressure to go up to avoid the rich condition. Unplug the connector to the WUR and check that you are getting voltage there, then check the resistance across the WUR terminals to make sure the heating element is working. You should get ~20 ohms. Once you check the heating element is OK, then you can think about changing the cold control pressure with the modification (lots of info if you search on that), but I would check the heater first.

Yes, your symptoms appear to be a too rich cold start mixture as jwakil has said.

Your WUR is located on the left side of the engine by the #2 intake.

Can you give us a bit of background? Did this just begin to happen? Is the car new to you? Have you had any work done recently on the engine? Answers to these questions can help narrow down what's causing the problem however, you must begin with a fuel pressure reading. If you don't have a fuel gauge set, you need to get one and take pressure readings, especially of your cold controlled pressure. That will be the most important first step in helping track down your problem and we can go on from there.

Your initial start up is due to the cold start injector but the idle is controlled by the WUR which adjusts fuel pressure. If the pressure is too low it causes an overly rich fuel/air mixture which leads to an oscillating idle as you describe. This is why the fuel pressure gauge set is necessary. Don't do any adjusting until you take pressure readings of your system, cold controlled pressure, warm controlled pressure, and residual pressure. Lots of threads on here on how to do that. Don't be afraid to ask.

If you choose to diagnose the heating element, understand you won't get voltage readings at the connector unless the fuel pump is running, and on your car, it will not run unless the fuel pump relay is jumped or the plunger in the intake is lifted. For CIS work however, you don't want to lift the plunger so do a bit of searching on how to jump the relay terminals to test not only the heating element, but to perform your fuel pressure tests as well.

Also could be the Distributer getting a little sticky with the mechanical advance or the vacuum advance unit. The vacuum line could be getting a little leaky too. Sounds like a hunting Dizzy to me that gets better when warm.

Make sure you add a few drops of oil down the shaft under the rotor too keep things lubed up. I do it every oil change.