(Video) Pls help diagnose wild idle swings on cold start

[Johnny Graham]

Hey folks, this is my 79 Euro SC Targa starting from cold - my wife taking it to work today. In the video she is not touching the throttle AT ALL, until she backs up (and it stalls on her.)

I am hoping that there arent that many things that can cause this, presumably it is something in the CIS. It only happens from cold, and here is no problem after this 1 minute warm up period. What do you think? WUR?

BTW the car looks rough (it has sunburned original paint which I like) but is mechanically great, motor and transmission rebuilt by one of the good guys, on bilsteins and wevo mounts, just back from faultlessly driving 2000 hard miles on alpine passes in Switzerland/Italy. It is also surprisingly economical, so isnt running rich - I made The Alsace to Manchester on a single tank. It sounds a little rough on the iPhone too, but honestly in real life it is sweet! Cheers.

[DRACO A5OG]

Sounds like a lean situation. Check for vacuum leaks, if no leaks found check base idle and/or fuel mixture

[tirwin]

We could speculate that it is the WUR. Of course, it is speculating and not knowing. So remove the guesswork from the equation.

Get a set of fuel pressure gauges and test system, warm control and cold control pressures. That’s where I would start but I wouldn’t stop there. Check for vacuum leaks. Check the AAR and AAV functionality. The difference between how the car drives between a properly functioning CIS and one that isn’t can be astonishing.

If you need help with what tests to do, how to do them, etc. let us know so we know how to help you.

[proporsche]

I would say your auxiliary air valve regulator could be a little sticking. I would not really worry about it....

The car start fine, runs fine don`t mess with it...Once you get time have a look at the regulator.
Ivan

this one

[Paulporsche]

That usually is a sign of a too rich mixture. Your cold control pressure could be too low, which richens the mix. You should check your CCP and verify it against the available charts for your year. It could be the WUR, the AAR, as was mentioned, a faultyO2 sensor (if you have one) or your mix might just be set too rich.

Did this just start recently? How was it during the warmer months?

Post the number on your WUR and check your control pressures.

Always make sure your ignition components are functioning correctly and there are no vac leaks.

[jpnovak]

CIS lean. adjust the mixture for fall/winter cooler air.

[pmax]

It's rich.

Check your CCP.

[proporsche]

the OP said
"It is also surprisingly economical, so isnt running rich - I made The Alsace to Manchester on a single tank. "

[AndrewCologne]

Yes but surging idle is a tipical symptom of an actual too rich mixture "in that moment".
So as already stated it could be a WUR problem if the engine runs economical when warm/hot.

1. Check the base CO mixture
2. Check fuel control pressure bewteen WUR and FD and compare with the values referring to your model/year

[proporsche]

i have seen plenty of these with the same symptoms...and once the person starts to mess with it was hard to get it running the way he wanted.
Of course installing fuel press.gauges would be a good start..again why, the car runs perfect and fuel consumption is perfect too.
And this is on cold start, that means cold start valve is adding fuel.Nothing in hot start so fuel mix probably not the problem

ivan

[boyt911sc]

Originally Posted by jpnovak

CIS lean. adjust the mixture for fall/winter cooler air.

You don’t adjust the fuel mixture seasonally. If you are doing this adjustment, there is something inherently wrong with your CIS system and doing a band-aid fix. You said it was LEAN but the next guy says RICH.

I own a ‘78 SC and have not touched or tinkered the fuel mixture since 1990. I use a fuel pressure gauge for my fuel pressures and pressurize the system to locate unmetered air. There are several CIS components that could contribute to this problem and all could be bench tested by any DIYer with ease.

Tony

[Nick Triesch]

Tighten down the 12 intake runners bolts to18 foot pounds. I bet they are loose !mine were and my 3.2 idle went back to 800!!! Really! Get the torque wrench at harbor freight for cheap!!

[Johnny Graham]

Thanks for the discussion. I like the last suggestion, that is more at my level

I also like the idea of leaving it alone Ivan, for the sake of giving it 30 seconds to level out in the morning - as it goes so well otherwise. Unless 400+ miles to a tank of gas suggests an issue.

In the fleet I have a 78 928 and an 83 Mercedes 2.8 litre. So I realise I probably should invest in some k-jetronic compatible diagnostic equipment..cheers

[hughc]

Love the sound of the exhaust on that Targa.

It'll be interesting to know how the erratic idle problem gets resolved.

[pmax]

Originally Posted by Johnny Graham

Thanks for the discussion. I like the last suggestion, that is more at my level

I also like the idea of leaving it alone Ivan, for the sake of giving it 30 seconds to level out in the morning - as it goes so well otherwise. Unless 400+ miles to a tank of gas suggests an issue.

Why not ?

Idling to warm up an engine is not recommended.

Stalling on a cold engine is unacceptable either, even more so on the road instead of the driveway.

In the fleet I have a 78 928 and an 83 Mercedes 2.8 litre. So I realise I probably should invest in some k-jetronic compatible diagnostic equipment..cheers

Yes, the FP kit can be used across all these Bosch k-jetronic cars.

[proporsche]

Pmax..30 seconds on idle could never be a problem for 911..remember it is not California and up there it gets cold ..it is very normal to start the 911 get a coffee and then take of.
His wife probably just stalled.When you see the video you can tell the idle is just about be normal before she backed up...again quiet
normal in cold climates....
Ivan

-correction -quite-

[pmax]

Originally Posted by proporsche

Pmax..30 seconds on idle could never be a problem for 911..remember it is not California and up there it gets cold ..it is very normal to start the 911 get a coffee and then take of.
His wife probably just stalled.When you see the video you can tell the idle is just about be normal before she backed up...again quiet normal in cold climates....
Ivan

Sure, just quoting what's recommended, if I'm not mistaken, in the owner's manual itself.

The warmup issue can only get worse, meaning longer idling times, as it gets colder.

[proporsche]

funny,i have never read the owners manual ;-) so i believe you ..i just work on them since 1980`s...

Ivan

[dyount]

Once it's verified that the pressures are ok and that it isn't too lean why not just give it a little higher idle? My SC runs a bit rich and will hunt an idle until it dies at times. Moving it from 950 to 1100 makes all the difference for me.

[pmax]

Originally Posted by proporsche

funny,i have never read the owners manual ;-) so i believe you ..i just work on them since 1980`s...

Ivan

Don't shoot the messenger

Isn't extended idling for warmup not a good thing in general, running too rich as is the case here for instance ? Car will warmup faster when driven.