2.7 CIS cold start issue

[allaircooled]

I have had this issue for a few years now and I just learned to live with it. Previous job was 100% travel and with only a week or two home at a time I just didn't have much time to figure it out. New job and more time now.

A little history on the car, a few years back I rebuilt the CIS with all new seals, new airbox, rebuilt WUR, rebuilt fuel distributor, injectors, fuel lines and I moved the fuel pump to the front of the car. I have done several fuel pressure test and smoke tests for leaks in the past couple of years and pressures are ok. I can repeat the test to have more recent results if needed. All air leaks I could find have been fixed.

During a cold start, the car will fire right up but idle will dip down way too low for a few seconds until it finally "catches". I made a video of this which is way better than my explanation. Engine is a 77 2.7 in my 75.

[Tony V]

Wish my ‘74 cold started as well as yours!
Hesitate to ask but are you pulling up on the “cold start” lever to the left of the hand brake? If so, it doesn’t sound like it’s working. There a plastic piece that’s prone to breaking, that raises the idle when raised.
Tony

[allaircooled]

Originally Posted by Tony V

Wish my ‘74 cold started as well as yours!
Hesitate to ask but are you pulling up on the “cold start” lever to the left of the hand brake? If so, it doesn’t sound like it’s working. There a plastic piece that’s prone to breaking, that raises the idle when raised.
Tony

Hey Tony, engine is a 77 so the hand throttle is no longer used as this engine has a AAR. Thanks for reply.

[al lkosmal]

Yes,
I would suspect that the AuX Air Regulator is not functioning properly. Your engine seems to fire up fine when you apply the pedal...once you take your foot off of the pedal the RPMs go too low.....you should not need to use the pedal for cold starts as the combo of Warm Up Regulator, Cold Start Valve and Aux Air regulator ., if working properly, provide the air/fuel mix for starts...based on temp.

regards,
al

[allaircooled]

Originally Posted by al lkosmal

Yes,
I would suspect that the AuX Air Regulator is not functioning properly. Your engine seems to fire up fine when you apply the pedal...once you take your foot off of the pedal the RPMs go too low.....you should not need to use the pedal for cold starts as the combo of Warm Up Regulator, Cold Start Valve and Aux Air regulator ., if working properly, provide the air/fuel mix for starts...based on temp.

regards,
al

Hi Al,

I didn’t use the pedal at all to start. I simply just turned the key and this is how my engine performs naturally. If I try to give it pedal it will most likely stall.

[Schulisco]

Originally Posted by allaircooled

Hi Al,

I didn’t use the pedal at all to start. I simply just turned the key and this is how my engine performs naturally. If I try to give it pedal it will most likely stall.

Sounds to me like a too rich mixture during cold start phase. Check out if the control pressure is in spec. Also check the AAR if it opens as it should on ambient temp. If one or both are not in spec the mixture get's too rich and the additional fuel from throttle will let the engine stall which it shouldn't obviously.

Are you sure that the amount of air from the AAR get's into the bottom of the airbox? Does the car already has a cold start valve? The flange at the bottom backside of the airbox combines the CSV with the air coming from the AAR.
There were two versions of the flange, one has a closed AAR input, one is open.

More details here:
https://forums.pelicanparts.com/porsche-911-technical-forum/1020014-csv-flange-911-110-264-00-alert.html

Thomas

[al lkosmal]

Originally Posted by allaircooled

Hi Al,

I didn’t use the pedal at all to start. I simply just turned the key and this is how my engine performs naturally. If I try to give it pedal it will most likely stall.

My mistake....the revving made it appear to be from using the pedal to start and keep it running.
However, it is likely that the cold start valve fuel gets it initially fired up, but that either the AAR or WUR are not providing the proper fuel pressures/AFRs to keep it running...do you have a test kit to test the system fuel pressures etc..?

https://www.pelicanparts.com/search/?q=cis+pressure+tester

A search will show many posts regarding the testing of the system, but here is a good troubleshooting chart....



regards,
al

[allaircooled]

Like I mentioned in my first post, I have tested the fuel pressures several times in the past couple of years when I originally was trying to figure out the problem. Last time I checked pressures my results were as follows:

Temp 88 F
System 5 bar
Cold 2.7 bar
Warm 3.6 bar
Shut engine off after 10 min 1.6 bar
After 20 min 1.5 bar

I can check these pressures again it’s just that this problem existed since the last time I checked them.

AAR does work, I have pictures of it open and then closes after supplying 12 v and waiting a certain amount of time, I don’t have that in my notes but it was within spec.

I will check fuel pressures again maybe this weekend just to get some fresh numbers.

[allaircooled]

Originally Posted by Schulisco

Sounds to me like a too rich mixture during cold start phase. Check out if the control pressure is in spec. Also check the AAR if it opens as it should on ambient temp. If one or both are not in spec the mixture get's too rich and the additional fuel from throttle will let the engine stall which it shouldn't obviously.

Are you sure that the amount of air from the AAR get's into the bottom of the airbox? Does the car already has a cold start valve? The flange at the bottom backside of the airbox combines the CSV with the air coming from the AAR.
There were two versions of the flange, one has a closed AAR input, one is open.

More details here:
https://forums.pelicanparts.com/porsche-911-technical-forum/1020014-csv-flange-911-110-264-00-alert.html

Thomas

I was thinking it was too rich at startup as well but didn’t know how. AAR is working, fuel pressure were in spec last I checked. I’m going to check them again soon. And the flange you mention is the correct one. Thanks for your help I appreciate it.

[allaircooled]

I went ahead and did the fuel pressure tests again. This is what I got:

Temp 88F/31C
System Pressure 5.2 bar
Cold Pressure 2.8 bar
Warm Pressure 3.7 bar
Heater element 22.9 ohms

It has been awhile since I did this test and realized I didn't time how long it takes to get to warm pressure. This 3.7 bar I got was almost immediately. I will redo the test tomorrow but what would happen if the WUR pressure increased too fast to the spec pressure? Too lean at cold idle?

[SkiVT]

Does your 77 have the number 4 in this pic? I saw similar behavior when either thermo was broken OR the vacuum lines were on the wrong ports/backwards. In either case it doesn’t hold off vacuum in this first bit (45 seconds or so) of time after starting.

[boyt911sc]

Chris,

What is your WCP (with and w/o vacuum)? Thanks.

Tony

[pmax]

Originally Posted by allaircooled

I went ahead and did the fuel pressure tests again. This is what I got:

Temp 88F/31C
System Pressure 5.2 bar
Cold Pressure 2.8 bar
Warm Pressure 3.7 bar
Heater element 22.9 ohms

It has been awhile since I did this test and realized I didn't time how long it takes to get to warm pressure. This 3.7 bar I got was almost immediately. I will redo the test tomorrow but what would happen if the WUR pressure increased too fast to the spec pressure? Too lean at cold idle?

Check fuel pressure vs time, more than one electromechanicalpneumatichydraulic doodad at play here as SkiVT's screenshot shows above.

What does the spec say for the exact WUR in the engine ? See if you can find it here in this thread, don't mind the name ... https://forums.pelicanparts.com/porsche-911-technical-forum/758788-cis-troubleshooting-dummies-5.html#post9681535

[allaircooled]

Originally Posted by boyt911sc

Chris,

What is your WCP (with and w/o vacuum)? Thanks.

Tony

I re ran the WCP test Tony with and with out vacuum. With vacuum it is 3.7 bar. Without vacuum it is just about 3.0 bar.

One note on the time for WCP to reach 3.7 bar, I didn't pay close enough attention the first time and thought it was instant, but this time it took around 20-30 seconds for the thermo valve to open. Once the WUR received vacuum it went straight to 3.7 bar. I must have missed that the first test.

Thank you all, I hoping to figure this out once and for all.

[boyt911sc]

Originally Posted by allaircooled

I re ran the WCP test Tony with and with out vacuum. With vacuum it is 3.7 bar. Without vacuum it is just about 3.0 bar.

One note on the time for WCP to reach 3.7 bar, I didn't pay close enough attention the first time and thought it was instant, but this time it took around 20-30 seconds for the thermo valve to open. Once the WUR received vacuum it went straight to 3.7 bar. I must have missed that the first test.

Thank you all, I hoping to figure this out once and for all.

Chris,

There could be multiple problems you have in your CIS. Your WUR has NO cold control pressure (CCP). Only WCP (warm control pressure). The transition time from a cold start to WCP is too quick. The WUR is already in warm state (20~30 sec.) but the engine is still too cold. The WUR has to wait for the motor to warm up.

Test for the operation of the other CIS components like:

• AAR
• AAV
• Decel valve
• Fuel injector spray pattern
• Unmetered air.
• Fuel mixture.
• Etc.

Tony

[allaircooled]

Originally Posted by boyt911sc

Chris,

There could be multiple problems you have in your CIS. Your WUR has NO cold control pressure (CCP). Only WCP (warm control pressure). The transition time from a cold start to WCP is too quick. The WUR is already in warm state (20~30 sec.) but the engine is still too cold. The WUR has to wait for the motor to warm up.

Test for the operation of the other CIS components like:

• AAR
• AAV
• Decel valve
• Fuel injector spray pattern
• Unmetered air.
• Fuel mixture.
• Etc.

Tony

I tested all those when I was first trying to diagnose the issue and found no issues. Any air leaks I found have been fixed. I’m willing to retest everything though as I just want to figure this problem out finally.

What can cause the wur to transition too quickly to wcp? I’m scratching my head think about this.

[boyt911sc]

Originally Posted by allaircooled

I tested all those when I was first trying to diagnose the issue and found no issues. Any air leaks I found have been fixed. I’m willing to retest everything though as I just want to figure this problem out finally.

What can cause the wur to transition too quickly to wcp? I’m scratching my head think about this.

Chris,

The WUR-033 #848 arrived yesterday afternoon and had a chance to test it this morning. The CCP is a little bit high at 34 psi. (69°F). Will need minor adjustment. The WUR is NOT the culprit! You have to test for unmetered air getting into the system. The problem is people are not doing it correctly. You need to isolate the air box from atmosphere to do an effective investigation. Keep us posted.

Tony

[allaircooled]

Originally Posted by boyt911sc

Chris,

The WUR-033 #848 arrived yesterday afternoon and had a chance to test it this morning. The CCP is a little bit high at 34 psi. (69°F). Will need minor adjustment. The WUR is NOT the culprit! You have to test for unmetered air getting into the system. The problem is people are not doing it correctly. You need to isolate the air box from atmosphere to do an effective investigation. Keep us posted.

Tony

Hey Tony,
When I do my smoke test I cover the throttle body with a glove and zip tie it closed so no smoke comes out. I also cover the pipe that connects to the boot with a glove. My last smoke check I had no leaks.

I did find something not correct and I will look into that once my wur gets back to me. I will post my what I find and whether or not I fixed it after I try starting it up. Thanks again for verifying my wur for me.

[Funracer]

Originally Posted by allaircooled

Hey Tony,
When I do my smoke test I cover the throttle body with a glove and zip tie it closed so no smoke comes out. I also cover the pipe that connects to the boot with a glove. My last smoke check I had no leaks.

I did find something not correct and I will look into that once my wur gets back to me. I will post my what I find and whether or not I fixed it after I try starting it up. Thanks again for verifying my wur for me.

Did you find the problem?