Cold Start Issue - Need some advice please

[HeitzWelle]

I have tried to upload a video but it looks like that is not possible. This is my issue: When cold the engine fires up fine but when rpm's drop to idle after a few seconds, she sputters, backfires & stalls unless I play with the throttle to keep RPM's up. When she warms up, which takes a few minutes, she runs fine.

It has been the situation since I bought the car (I'm sure the seller had her warmed up before my arrival). So far, I have "serviced" the WUR ("90") with new seals and tested the heating elements (measured OK). I have also replaced the fuel filter, fuel accumulator and fuel pump.

I have re-tested and found the following fuel pressures:

System Pressure: 5.0
Control Pressure Cold: 2.2 @ 24 C
Control Pressure Warm: 3.6 (a little higher than spec)
Leak Test: 1.64 @ 10 min and 1.57 @ 20 min

When pulling a vacuum (with hand pump) on the top of the WUR the fuel pressure dropped rather than increased. According to the book this is not normal... (?)

I also checked the TTS and got the following resistances:

Cold: Ground-G: 24 / Ground-W: ~0 / G-W 24
Hot: Ground-G: 59 / Ground-W: 98 / G-W 39

These all seem to be on the low end but not sure if this would materially affect the functioning of the CSV.

My next move would be to test the CSV and the Auxiliary Air Valve but wanted to ask if I'm on the right path or missing something obvious.

Finally, I found a loose connector (see picture) that seems to have no destination that I can find. Perhaps I am missing a part removed by PO? What would that be?

I'm new to the 911 community so any feedback would be great.

Frank

[timmy2]

That connector was only used on 78/79 on US engines. Used for all ROW years.
Goes to the vacuum delay unit on those motors. Deleted for Lambda engines.

You should tell us what year your car is to get help. Can only guess otherwise.

[HeitzWelle]

It's a 1981 Targa. Thought that was in the profile box but no. Sorry for that.

[pmax]

Originally Posted by HeitzWelle

My next move would be to test the CSV and the Auxiliary Air Valve but wanted to ask if I'm on the right path or missing something obvious.

Once it fires up, the CSV/AAV job's done. During the warmup cycle which the engine is having trouble with, the CIS gizmo called the AAR not to be confused with the flying saucer looking thing called AAV above it on the passenger side between 5 and 6 intake runners provides extra air. It should be open cold and slowly closing during that cycle.

About the "vacuum" port, this says it's just a vent which I tend to agree with given this is a lambda engine.
https://forums.pelicanparts.com/porsche-911-technical-forum/473520-wur-vacuum-82-911-sc-what-does-do.html

[HeitzWelle]

Thank you pmax & timmy2. I will test the AAR next. Sounds like the logical next step.

[boyt911sc]

Originally Posted by HeitzWelle

It's a 1981 Targa. Thought that was in the profile box but no. Sorry for that.

Frank,

What type ‘81 SC Model do you have? Is it a US Spec or ROW? They differ in CIS. Do you know the ID # of the WUR? Why are you pulling vacuum?

Tony

[HeitzWelle]

Hi Tony,

The car is a US spec and the WUR # is 0438-140-090

I just looked at the 1981 SC WUR test spec sheet I used again and it was for a different WUR# (089). My mistake... Went back to the Bentley book and indeed no difference for atmospheric vs vacuum for the 090. Thank you for clearing that up for me.

Based on my pressure tests I conclude that the WUR is functioning properly.

Planning to test AAR next.

Appreciate the comments. Really enjoying the learning process.

Frank

[mysocal911]

Originally Posted by HeitzWelle

Hi Tony,

The car is a US spec and the WUR # is 0438-140-090

I just looked at the 1981 SC WUR test spec sheet I used again and it was for a different WUR# (089). My mistake... Went back to the Bentley book and indeed no difference for atmospheric vs vacuum for the 090. Thank you for clearing that up for me.

Based on my pressure tests I conclude that the WUR is functioning properly.

Planning to test AAR next.

Appreciate the comments. Really enjoying the learning process.

Frank

But it's leaning the mixture too quickly, i.e. you state "it runs find when warm".

[boyt911sc]

Originally Posted by HeitzWelle

Hi Tony,

The car is a US spec and the WUR # is 0438-140-090

I just looked at the 1981 SC WUR test spec sheet I used again and it was for a different WUR# (089). My mistake... Went back to the Bentley book and indeed no difference for atmospheric vs vacuum for the 090. Thank you for clearing that up for me.

Based on my pressure tests I conclude that the WUR is functioning properly.

Planning to test AAR next.

Appreciate the comments. Really enjoying the learning process.

Frank

Frank,

Since you rebuilt your WUR-090 and checked the heater resistance, what resistance (Ohms) value did you get @ room temperature (cold motor)? You need to re-calibrate the WUR.

[HeitzWelle]

I get 9.5 ohms with cold motor. I measured almost exactly the resistances that were printed on the two elements inside when it was opened up. What should the resistance be?

Recalibration is likely beyond my skillset. Maybe a professionally rebuilt unit is the better move at this point.

Also read about an electronic unit but opinions seem to vary.

[HeitzWelle]

Originally Posted by mysocal911

But it's leaning the mixture too quickly, i.e. you state "it runs find when warm".

So theoretically, an AAR that is stuck closed would cause the mixture to be too lean during warm up correct?

[boyt911sc]

Frank,

I could lend you a WUR-090 that you could install in your car and experience having a good working WUR. No obligation to buy it. Send it back to me if you don’t want to keep it. No questioned asked. I have provided hundreds of WUR’s to fellow Pelicanparts members with amazing success.

PM me if you are interested. Thanks.

Tony

[mike sampsel]

Originally Posted by HeitzWelle

So theoretically, an AAR that is stuck closed would cause the mixture to be too lean during warm up correct?

I'm not sure about that, the AAR should allow more air (note this is metered air) to enter when the engine is cold to allow the idle speed to increase (more air, higher idle, it is similar to letting the idle screw increase idle, but this does not change the mixture!). As the AAR warms up, it closes somewhat (not entirely) and allows less air to enter the system, lower idle, from less metered air in. I do not think the AAR leans or richens anything, it's the fuel pressure from the WUR which handles the change in mixture richness, the AAR acts as an idle screw to up the idle when cold to aid in idle at warm up. Now air allowed in that is not metered is a different thing, and of course the mixture adjustment screw changes the mixture too.

I'm not an expert and might have it wrong. If I do this group is not shy and will let me know the error.

Cheers

[pmax]

Originally Posted by HeitzWelle

So theoretically, an AAR that is stuck closed would cause the mixture to be too lean during warm up correct?

Yes, the line of reasoning behind the comment by mr socal and the question from mr boyt is that the WUR and AAR must march in lockstep so to speak from cold to warm hence the heater resistance being a point of investigation to gauge if the WUR's busted that way so ... either the WUR or the AAR's too fast or slow or both are.

[HeitzWelle]

I measured 9.5 Ohms at the WUR terminals. When I had it opened up the two elements measured exactly what was printed on them. Is the 9.5 ohms within spec?

As far as I can gather, the only adjustments are moving the mounting bolt in or out and the tiny screw in the picture. Without some kind of test setup that seems like a lot of trial and error.

Taking Tony up on his offer to provide a properly calibrated WUR to try. Will also remove and test AAR.

Frank

[David Inc.]

AAR changes air flow only, not mix. All air through the AAR is metered.

[pmax]

Originally Posted by HeitzWelle

I measured 9.5 Ohms at the WUR terminals. When I had it opened up the two elements measured exactly what was printed on them. Is the 9.5 ohms within spec?

The resistance value determines the heating power and hence the control pressure rate of change. Your measurement as reported above is missing the time interval from CCP to WCP which will give you the 1st order rate of change.

[HeitzWelle]

I think I understand that... The change from CCP tp WCP was faster than what I have seen in a video about WUR. But still a lot slower than when the engine starts idling low and stalling.