Short irregular idle period on cold start

[Niels911SC]

Everytime I cold start my '78 SC it seems to take a while before the mechanisms that regulate the warm up of the engine kick in.
The car idles irregularly and low for about 5 tot 10 seconds (and when parked outside during a cold night even stalls the first time) before the idle goes up the engine starts running normally.
When I press the gas pedal during these first 5 to 10 seconds it'll backfire and sometimes stall.

I made a video to make my explanation clearer:


This problem (not dramatic but still) has been present ever since the car sat for about a year while getting resprayed.
I'm not really a k-jet expert, so any ideas on what it could be? Perhaps the WUR?

[tirwin]

With a '78, and I'm assuming a RoW car, you don't have K-jet. Is your car RoW or a US import?

When I bought my '83 4 years ago, it started VERY rough on cold starts. I couldn't keep it running without giving it a little gas and most of the time that caused a backfire. I could hear the pop-off valve opening.

Long story short, I had all of the classic problems. Vac leaks, fuel pressures out of spec and poor/weak spark.

If you develop a plan to systematically troubleshoot and resolve any issues in those areas you will be able to have the car run as it should.

In order to test the WUR you need a set of fuel pressure gauges. Do you have those?

A lot of what you need to know is summarized here:
CIS Troubleshooting for Dummies

EDIT:
Here is the link to the video of my '83 when I first got it. Look familiar?

https://www.youtube.com/watch?v=uB_gt06ZPSM

[boyt911sc]

Niel,

Could you get the engine to start cold without help from the gas pedal? Without knowing your fuel pressures (cold, warm, and system), we would be doing a lot of guess work to determine the culprit/s. Have you tested your vacuum for unmetered air?

Tony

[Niels911SC]

Tony, the car starts without the gas pedal. The whole video is shot without touching it.
I have not measured for unmetered air. How do I do that?

Tirwin; my car does eventually transition to cold running; I don't need to help it with the gas pedal. If I do; it'll backfire.
I just read your post on how measure the fuel pressures so I will be doing that next friday (don't have any time during the week sadly).

[boyt911sc]

Originally Posted by Niels911SC

Tony, the car starts without the gas pedal. The whole video is shot without touching it.
I have not measured for unmetered air. How do I do that?

Tirwin; my car does eventually transition to cold running; I don't need to help it with the gas pedal. If I do; it'll backfire.
I just read your post on how measure the fuel pressures so I will be doing that next friday (don't have any time during the week sadly).

Niel,

'Unmetered air' refers to air leak (unwanted air) that gets into the system. You don't measure it. Either you have a leak or not. A smoke machine or pressurized air test could tell you the status. They all leak. It is a matter of how much air is going in to affect the fuel mixture. You might not have any leak but you don't that for a fact. And you are hoping to be lucky.

Tony

[Paulporsche]

As Tim and Tony say, check for vac leaks and then get fuel gauges so you can measure your fuel and control pressures.

I see from your video that the engine is slightly warm when you start it. Right at start the revs are a little low, and then they go higher. Do they then gradually go down after a few minutes to the correct idle speed? If so, then it looks like your control pressure could be slightly low at the temp you did your video, which caused the mixture to be just slightly rich at first.

Does the same thing happen at a colder engine temp and/or a lower ambient temp?

Does the backfiring you mentioned occur through the airbox or the exhaust? Usually airbox indicates a too lean and exhaust, too rich.

What I just mentioned are just some typical symptoms. Only by testing the fuel pressures with the gauge you can actually check to see if the control pressures are right for a particular temperature. You can then adjust the WUR to get the correct figures and/or adjust your fuel mixture to suit.

Let us know what number is stamped on your WUR. Each one has a chart that specifies the curve of control pressures for a particular temperature range.

Before you do this testing make sure all your ignition components are functioning correctly.

[T77911S]

i would look at air leaks, control pressures andn aux air reg.

[Niels911SC]

It's been quite a while but I finally had time to measure the control pressures. Sorry for having you guys post helpful replies but not following up Now I'm ready to tackle the problem!

I followed tirwin's tutorial and the results are as follows:

1. Year of engine: 1978
2. US or RoW (Rest of World): US
3. WUR model number: -045 (edited: found thanks to Tony)
4. Ambient temperature at time of test (in degrees C): 15.5C
5. WUR Resistance (in Ohms): 26 Ohm
6. System Pressure (in bars): 4.95 bar
7. Cold Control Pressure (in bars): 1.85 bar
8. Warm Control Pressure (in bars): 2.85 bar
9. Time delta for Cold -> Warm Control Pressure (in minutes & seconds): 2min26sec
10. Residual Pressure @ 5 min (in bars): 2.05 bar
11. Residual Pressure @ 15 min (in bars): 1.45 bar
12. Residual Pressure @ 30 min (in bars): 1.25 bar
13. Residual Pressure @ 60 min (in bars): 0.90 bar

In the mean time the cold start problem has gotten a little bit worse, it stalls at least 2 or 3 times before it can keep itself running. As far as my vacuum goes; if the engine is running and warm I can take off the oil cap and it changes the idle (starts to go irregular) so I suppose if I have an air leak it is not significant.

So the question is, what is the culprit? Is the starting resistance of the WUR too high? Or is it the mysterious AAV or AAR? Or perhaps the Cold start valve?
I hope you guys can help me!

EDIT: picture of the setup

[boyt911sc]

Originally Posted by Niels911SC

It's been quite a while but I finally had time to measure the control pressures. Sorry for having you guys post helpful replies but not following up Now I'm ready to tackle the problem!

I followed tirwin's tutorial and the results are as follows:

1. Year of engine: 1978
2. US or RoW (Rest of World): US
3. WUR model number: 731(Not sure if that's correct but it had 731 stamped on the top and that was the only number I could see)
4. Ambient temperature at time of test (in degrees C): 15.5C
5. WUR Resistance (in Ohms): 26 Ohm
6. System Pressure (in bars): 4.95 bar
7. Cold Control Pressure (in bars): 1.85 bar
8. Warm Control Pressure (in bars): 2.85 bar
9. Time delta for Cold -> Warm Control Pressure (in minutes & seconds): 2min26sec
10. Residual Pressure @ 5 min (in bars): 2.05 bar
11. Residual Pressure @ 15 min (in bars): 1.45 bar
12. Residual Pressure @ 30 min (in bars): 1.25 bar
13. Residual Pressure @ 60 min (in bars): 0.90 bar

In the mean time the cold start problem has gotten a little bit worse, it stalls at least 2 or 3 times before it can keep itself running. As far as my vacuum goes; if the engine is running and warm I can take off the oil cap and it changes the idle (starts to go irregular) so I suppose if I have an air leak it is not significant.

So the question is, what is the culprit? Is the starting resistance of the WUR too high? Or is it the mysterious AAV or AAR? Or perhaps the Cold start valve?
I hope you guys can help me!

EDIT: picture of the setup

Neil,

Look very closely at your WUR. You should have on top of the WUR these numbers: 0 438 140 xxx. The last three (3) digits would identify which one you have. My guess is -045 or -069. The 731 is the serial number.

The warm control fuel pressure of 2.85 bar is too low if this was measured with just the FP running with engine running. Was the engine off? What is the system fuel pressure? What was the warm idle speed change when you removed the oil cap? Not sure what you meant by irregular. Give us numbers in RPM. Please keep us posted. Thanks.

Tony

[Niels911SC]

I took a closer look and it indeed has another number hidden behind the vacuum connection, which ends in -045.
Engine was off during these tests. System fuel pressure is in the list (4.95 bar).
When I remove the oil filler cap the idle speed drops by a 100RPM or so (from 950 to 850) and starts to fluctuate up and down a bit (maybe 50RPM), which I think is how it should be?

EDIT:

Troubleshooting would propose the following reasons for hard cold start:

- Sensor plate / plunger not moving freely
- AAV does not open
- FP not operating This is not it since the car does run warm
- CSV defective
- WUR defective
- Cold pressure outside tolerance I believe this is correct too looking at the attached graph
- Injector leaking, opening pressure too low Injectors were all renewed 3 years ago
- Injected fuel quantities unequal Injectors were bench tested last year after the car stood still for 9 months, correct&even spray pattern
- Basic idle setting incorrect Idles well at 950RPM when warm

Cold control graph:


Where do I find the correct WUR resistance? And how do I check CSV / AAV?

[Bob Kontak]

"CSV defective" can be eliminated from list if car starts when stone cold. It only sprays when your starter is turning.

[tirwin]

Tony would know the resistance for sure but as I recall 26 Ohms is spot on.

[boyt911sc]

Originally Posted by tirwin

Tony would know the resistance for sure but as I recall 26 Ohms is spot on.

The heater resistance value is no where to be found in any maintenance shop manuals I've come across. This is probably a Bosch proprietary trade secret that they keep for their authorized shops. Over the years, I collected the different heater resistance values used for CIS Porsche only. A second check is reading the stamped resistance value on the heater itself when you opened it up. Between 25 - 26 Ohms would be ideal for -045.

Tony

[Niels911SC]

Updated list leaves only 2 options:

Troubleshooting would propose the following reasons for hard cold start:

- Sensor plate / plunger not moving freely I can push the thing up and it seems to fall back freely so unless there's more to it; not it
- AAV/R not open
- FP not operating This is not it since the car does run warm
- CSV defective It does start/fire on the first try but then dies, so with Bob's info this is also not it
- WUR defective If 26 ohms is correct then also not it I guess?
- Cold pressure outside tolerance I believe this is correct too looking at the attached graph
- Injector leaking, opening pressure too low Injectors were all renewed 3 years ago
- Injected fuel quantities unequal Injectors were bench tested last year after the car stood still for 9 months, correct&even spray pattern
- Basic idle setting incorrect Idles well at 950RPM when warm

Is there any way to check for correct AAV/AAR operation on the car or do I have to remove them?
The funny thing is, it doesn't take long for the engine to 'pick up' its normal warm up routine, if the AAR were defective it would idle bad throughout the cold up right? And the AAV only provides initial air until the vacuum is built up, so it's not something that would kick in after 10s, only shut off.
So what could make my car stall on the first 2 tries, then let it idle rough/low for 10-15 seconds and then finally kick in to bring the rpm's up to normal cold running @1250rpm ?

[tirwin]

In the CIS Troubleshooting thread there are links to test procedures for both the AAR and AAV. You will have to remove the AAR. Maybe you could test the AAV in place but it would be easier to test if removed.

The AAR has an electrical connector just like the WUR is on the same circuit as the WUR. An easy test is just to make sure it is getting 12v too.

The purpose of the AAR is to allow more air on a cold start and then close once the cold start process completes. If it were stuck open it would be adding extra air past the air metering plate all the time. It works on the same principle as the WUR so if there isn't 12v it would only close once it was heated by the engine. It gets power and then it heats the bimetallic strip which causes the internal valve to close. This should be happening in roughly the same time span as the WUR cold -> warm control pressure rise. Basically on a cold start the WUR is adding more fuel and the AAR is adding more air at the same time. Once the engine is warmed up they are both essentially "off" as they have done their cold start jobs. I would just pull the electrical connector to the AAR and make sure it has 12v when the WUR is energized as a first step before going any further on the AAR troubleshooting.

Tony has posted his vacuum cleaner method for searching out vac leaks. Have you checked the intake runner boots and places like that?

[Bob Kontak]

Originally Posted by tirwin

You will have to remove the AAR.

For Niel's scenario, I would only pull the hose off the snoot when stone cold and make sure the 1/4 - 1/3 moon shaped hole is present by using a mirror looking into the snoot.

[tirwin]

Originally Posted by Bob Kontak

For Niel's scenario, I would only pull the hose off the snoot when stone cold and make sure the 1/4 - 1/3 moon shaped hole is present by using a mirror looking into the snoot.

Ok Iwould have to take it off.

[Bob Kontak]

Originally Posted by Niels911SC

And the AAV only provides initial air until the vacuum is built up, so it's not something that would kick in after 10s, only shut off.

Good job on walking through this step by step.

AAV - Vacuum is instantaneous when the engine fires cold. That said, I don't know what "the vacuum is built up means". Is it engine vacuum with the AAR open or is it engine vacuum after the AAR closes? This one needs to be tested. It may be a culprit.

One thing that sticks in my mind is you mentioned the idle changes when you pull the oil cap off when warm. Idle should drop a hundred or more RPM's but not really become irregular.

When you pull the oil cap, you drop the vacuum inside the engine. I wonder if the AAV is allowing air to bypass into the engine when this vacuum drops. Also, the air restriction thingie in the oil hose is around the size of the AAR opening when cold.

Maybe the AAV is getting tired and allows air to bypass at a lesser vacuum than it should?

Just thinking out loud. Getting interesting.

[Bob Kontak]

Originally Posted by tirwin

Ok Iwould have to take it off.

I hear you. Tony walked me through the full bench test including putting it in an oven. It can be tested if you can deal with those hard to access allen head maching screws.

I think Niel is getting too much air at cold start but I don't think there is an AAR issue causing this.

In the last post, I am speculating about the AAV. Don't fully understand it's function as noted. Need to test when it closes off.

May be a reason yours and mine work better with the vacuum retard sealed off? You probably tested yours, though. I never did.

[Niels911SC]

No worries guys, I got them both out without too much of a hassle:


I'll bench test them this week with the help of the two threads in the CIS troubleshooting
AAV: How to test Auxiliary Air Valve
AAR: Is This AAR Good or Bad

It'll be a busy week so I hope to be able to test them one of the evenings, but it could take a little time. I'll post a report when I do.