Rough Cold Start

[riparian]

I purchased a 1986 Carrera in May of '21 and have been working my way through what it was in need of (including a complete suspension restoration). One thing that has been eluding me is the behavior on cold start. I've scoured this forum, cleaned the cold idle valve and checked the vacuum (16-17 hg, steady at normal idle, good?).

On cold start (Bosch MFI, not touching accelerator), it starts right up, but runs rough at around 500-600 RMP for about 15 seconds at which point it smooths out quickly and revs up to about 1100. Over the next 45 seconds it settles to 1000, and then over the next 10 seconds gently drops to a normal idle around 800 RPM. Does this every time on cold start.

Once the car gets about 15 seconds in, past the rough spot, I can drop it in gear and all is good, runs fine. I usually wait the minute or so for it to settle to a normal idle before I get going, though. It runs great otherwise, and starts without the rough spot when warm.

Here's the cold start video for my car showing the RPM and you can also hear the motor sound quite well: https://youtu.be/MEli2LTVzqM

Is this normal behavior? Some guy has a video posted on Youtube called "Porsche 911 3.2 correct normal cold start idle" that mirrors the behavior of mine. Unfortunately there are no useful comments on whether this is correct or not. Here is his video: https://www.youtube.com/watch?v=1XBg5PeARAY

Thank you.

[917_Langheck]

That initial rough idle sounds a lot like the warm idle roughness when the oil cap is removed.

Where is your air mixture screw set, that is, how many turns out from fully closed? (not to be confused with the CO2 mixture screw on the underside of the air flow meter).

The prior owner of my car had it too open and it would cause the ICV to hunt for an idle setting on start up; I wonder if yours is too closed. I reset mine from the 4 turns out it was set at to 2.5 and no more cold or hot start hunting.

There are any number of how to assess threads on PP. Another treatment for the visual learner can be seen here, though I suppose you have seen this: https://youtu.be/MRZG-s_55CU.

[Schulisco]

Some threads here as well in other forums available:
https://www.google.com/search?q=911+carrera+low+rpms+cold+start

Also quite useful: https://bit.ly/3IGI6Mr

Heidi&Franny restored a Carrera some time ago and documented it on Youtube: https://www.youtube.com/playlist?list=PLESd2KXwHSVKKl3FxYSFB3rGKZTsfWpQp, there is at least one video working on a bad idle:
https://www.youtube.com/watch?v=MRZG-s_55CU
https://www.youtube.com/watch?v=u512ogk5trA

Not quite the same issue as yours, but it gives you a good overview.

Thomas

[riparian]

Originally Posted by 917_Langheck

That initial rough idle sounds a lot like the warm idle roughness when the oil cap is removed.

Where is your air mixture screw set, that is, how many turns out from fully closed? (not to be confused with the CO2 mixture screw on the underside of the air flow meter).

The prior owner of my car had it too open and it would cause the ICV to hunt for an idle setting on start up; I wonder if yours is too closed. I reset mine from the 4 turns out it was set at to 2.5 and no more cold or hot start hunting.

There are any number of how to assess threads on PP. Another treatment for the visual learner can be seen here, though I suppose you have seen this: https://youtu.be/MRZG-s_55CU.

Thanks very much. I did watch the Franny video (she's great!), which lead me to cleaning the ICV. So far, I've avoided messing with any mixture settings as I don't want to make things worse and have not researched mixture settings yet. Thanks for the tip. I will at least check, and give it a try, although as noted, mine does not hunt, it just has that roughness for 15 secs.

[riparian]

Originally Posted by Schulisco

Some threads here as well in other forums available:
https://www.google.com/search?q=911+carrera+low+rpms+cold+start

Also quite useful: https://bit.ly/3IGI6Mr

Heidi&Franny restored a Carrera some time ago and documented it on Youtube: https://www.youtube.com/playlist?list=PLESd2KXwHSVKKl3FxYSFB3rGKZTsfWpQp, there is at least one video working on a bad idle:
https://www.youtube.com/watch?v=MRZG-s_55CU
https://www.youtube.com/watch?v=u512ogk5trA

Not quite the same issue as yours, but it gives you a good overview.

Thomas

Thanks for the great link to the manuals, and also the google tip on searching across forums for the problem. Those Franny videos are excellent, and I've tested vacuum and cleaned the ICV. I even bought a replacement ICV but it did not resolve the issue. I am going to check the mixture setting as mentioned above, and also review the other forums and will report back.

I found it interesting that the behavior in that other video I posted had the same issue as mine, even though he says it's normal. I know it's not normal because I have listened to plenty of 3.2 cold start videos on BaT in which the motor starts directly to 1100 RPM with no roughness.

[Schulisco]

Originally Posted by riparian

Thanks for the great link to the manuals, and also the google tip on searching across forums for the problem. Those Franny videos are excellent, and I've tested vacuum and cleaned the ICV. I even bought a replacement ICV but it did not resolve the issue. I am going to check the mixture setting as mentioned above, and also review the other forums and will report back.

I found it interesting that the behavior in that other video I posted had the same issue as mine, even though he says it's normal. I know it's not normal because I have listened to plenty of 3.2 cold start videos on BaT in which the motor starts directly to 1100 RPM with no roughness.

Welcome

I'm not into the Carreras, I own a SC only...but the roughness is from my point of view also not normal. What I read about this issue might be one of the following:
1. Temp sensor
2. Mass Airflow Sensor misadjusted/worn (there is a resistor sensor inside, could be worn)
3. DME relay (although with hot engine everything works fine...)

With the manuals you have all the necessary information to check all values and electrics/electronics. Only a step-by-step troubleshoot will solve the problem.

Good luck and keep us posted!

Thomas

[Mark's 86]

My 86 3.2 behaves the same way when started cold. Always has since I have owned it. I have owned the car for 12 years. It has 86000 original miles. I have replaced the ICV, the base idle and CO are set as per factory specs. I replaced the DME relay with a solid state unit from Focus 9 Technology with the pump prime feature thinking it may be an initial low fuel pressure issue. Other than the initial brief 600rpm cold start idle the car runs excellent.

Mark

[DIANA]

From experience, I would start at the beginning: check the fuel pressure.

[Schulisco]

As mentioned I'm not into the 3.2, but I did some reading because of personal interest. It might be a problem of the residual pressure of the fuel system. I would also check all fuel pressures of the car as given by Porsche, as DIANA already recommended. See the workshop manual for the values.

The 3.2 Carrera has the same fuel delivery system as the SC's. And the fuel pump has also a check valve like the SCs. On my SC somebody removed the ball and the spring from that valve - don't know why. This leads to an insufficient filling of the fuel lines, especially when the car was sitting for a while. And it takes time to refill all fuel lines and recreating the normal fuel pressure and entire filling of the fuel lines. This could be the reason on your 1st time start problem, I had the slightly same problem (near many others...another story...). Either the valve hangs or the ball from inside the valve is missing also.
The other thing to check is the fuel accumulator next to the fuel filter. This keeps the fuel pressure on to the residual pressure of 2,5bar for at least 1/2 hour to prevent hot start issues. But this won't work perfectly if the check valve behind the fuel pump is missing...

Thomas

[wazzz]

There's no fuel accumulator on 3.2.

[Schulisco]

Originally Posted by wazzz

There's no fuel accumulator on 3.2.

You're right, thanks, I was also wondering if the 3.2 should also being equipped like the SC on that, but I was on the wrong page of the parts catalogue - for the Turbo

Thomas

[riparian]

So, like Mark I bought the solid state DME relay (good idea regardless) with the pump prime feature just in case, but I still get the same behavior.

I'm taking the car to Gator Crawl this coming week, so will stay away from working on this problem (i.e., making it worse!) until I get back. But as advised above, I will be testing fuel pressure and fuel system voltages, etc., as next steps.

Thanks for all the advice, so far!

[Mark's 86]

Any updates on troubleshooting the cold rough start?

Mark

[mikedsilva]

Originally Posted by riparian

I purchased a 1986 Carrera in May of '21 and have been working my way through what it was in need of (including a complete suspension restoration). One thing that has been eluding me is the behavior on cold start. I've scoured this forum, cleaned the cold idle valve and checked the vacuum (16-17 hg, steady at normal idle, good?).

On cold start (Bosch MFI, not touching accelerator), it starts right up, but runs rough at around 500-600 RMP for about 15 seconds at which point it smooths out quickly and revs up to about 1100. Over the next 45 seconds it settles to 1000, and then over the next 10 seconds gently drops to a normal idle around 800 RPM. Does this every time on cold start.

Once the car gets about 15 seconds in, past the rough spot, I can drop it in gear and all is good, runs fine. I usually wait the minute or so for it to settle to a normal idle before I get going, though. It runs great otherwise, and starts without the rough spot when warm.

Here's the cold start video for my car showing the RPM and you can also hear the motor sound quite well: https://youtu.be/MEli2LTVzqM

Is this normal behavior? Some guy has a video posted on Youtube called "Porsche 911 3.2 correct normal cold start idle" that mirrors the behavior of mine. Unfortunately there are no useful comments on whether this is correct or not. Here is his video: https://www.youtube.com/watch?v=1XBg5PeARAY

Thank you.

I don't know if it's normal, but I worked on a friends car that responds exactly the same. When cold, it runs rough for maybe 5-10 secs.. after that it's as if the ECU has woken up and takes full control. Fast idle that slowly settles down to normal after a minute or so... All sensors are working, and responding. Again, it's almost as if the ecu is "still booting up" !!

[Mark's 86]

I agree. It seems as though it takes 5-10 seconds for the Motronic to regulate the idle speed. If I start it cold The engine immediately responds to gas pedal inputs. It makes me sceptical that it is an initial low fuel pressure issue. The car runs good otherwise. I am hesitant to start disturbing 36 year old electronics. Interested to see if the original poster is able to figure this seemingly common issue out on our old Motronic cars.

[Schulisco]

Here's also a discussion on the same issue - spoiler: The ICV was the cuplrit.

http://forums.pelicanparts.com/porsche-911-technical-forum/914292-cold-start-idle-issue-87-carrera-3.html#post9156339

[riparian]

Just back from my road trip. Brake lights decided to short during the trip, so will be tracing that problem first. Schulisco, thanks for the link. As mentioned in my initial post, I did test, remove, clean and reinstall the ICV valve, but that had no impact on the issue. However, it looks like there is more detail in the post you linked, and I will definitely go through it.

[Porsche 935]

I've had a lot of issues with my fuel system but it starts up immediately and idle is 12-1400 on cold start, no low idle. But mine is an 84. It did take me a long time and many tests to get the engine back to where it should be but lots of help from knowledgeable people here.

[Schulisco]

Originally Posted by riparian

Just back from my road trip. Brake lights decided to short during the trip, so will be tracing that problem first. Schulisco, thanks for the link. As mentioned in my initial post, I did test, remove, clean and reinstall the ICV valve, but that had no impact on the issue. However, it looks like there is more detail in the post you linked, and I will definitely go through it.

Welcome

Did you already checked out the videos of Heidi&Frannys I linked above?
There's a 3rd video as well:
https://www.youtube.com/watch?v=tRn_nstudAQ

Thomas

[riparian]

Originally Posted by Schulisco

Welcome

Did you already checked out the videos of Heidi&Frannys I linked above?

Thomas

Yes, Franny's video was one of the first things I found on this issue. I have not watched the third one, but will certainly check it out!