Cold start problem on 911 SC
 

Dear 911 Gurus,

by googling and reading the recent post on “Idling speed versus outside temperature" I have been motivated to describe the following issue on my 911 SC 3.0 (built June 1978). The car is from Orange County (CA) with new home in Germany and is in close-to perfect shape. After acquiring the car in summer 2008, we (3 Porsche enthusiasts) have replaced most of the “commodities” (oil/air/fuel filters, spark plugs etc.) and also aligned valves, ignition timing, etc. such that the engine runs extremely well and smooth. Power, fuel consumption, transition behaviour is all working perfect.

Now here comes the issue: when starting the engine after 1 or more nights in the garage, the following pattern occurs:

1) Engine starts immediately on 1st key turn

2) Rough engine performance, with idle speed going down to below 500 RPM for about 5-8 seconds, often accompanied by 1-2 backfiring events (independent on outside air temperature)

3) Sometimes, engine does not recover from this mode, stops and requires 2nd trial on start up

4) In case engine recovers after 5-8 seconds, idle speed goes up to about 2000 RPM where it stays for about 2-3 minutes before the CIS regulates idle speed down to 950 RPM (I regard this as normal operation of the CIS as described in the "Idling speed versus outside temperature" thread)

Does anybody have an idea on this start-up pattern?

PS: I came over a great site I would like to share with you :) CIS Primer for the Porsche 911

Best regards,
Merlin62 (Germany)

As the experts here will tell you, you must check/confirm your CIS fuel pressures, both control and system pressure first, to accurately diagnose the cold start issue.

Your second symptom might be the result of a fuel pump check valve or fuel pump going south on you, or something as simple as adjusting the A/F mixture setting .

Dear friend,
thanks for your advise which I will try to follow. Please advise on fastest way to measure CIS pressure levels -- visit Porsche garage (expensive) versus do it on my own (by getting pressure gauges etc.)?

I forget to mention: car has brand new fuel pump (BOSCH) with built-in check valve installed -- I replaced old one (which was a re-build version) due to high noise level. However, replacing the pump/ check valve did not effect the cold start behaviour.

Best, Merlin62

check your CO %

This may help

http://forums.pelicanparts.com/porsche-911-technical-forum/232089-cis-idle-speed-mixture-setting-without-analyzer.html

Welcome Merlin62.

Follow hbkramer's advice and check your fuel pressures first. If you are going to work on your own CIS car, you must have a set of fuel gauges so you should start now--you might as well invest now. Testing is easy and is the first step in tracking down cold start problems, IMO, before you go into more complex testing or replacing parts.

CIS troubleshooting.......
 

Merlin,

Follow the above suggestions. For troubleshooting fuel injection system/s first and foremost is to check the fuel pressures. Followed by a vacuum check. If you could determine that the fuel pressures are within spec and no vacuum leak (unmetered air) in the system, the rest of the troubleshooting would be simple and easy. Not being able to locate a vacuum leak source is not the same as confirming the integrity of the vacuum of the system. Adjusting the mixture at this point to make the engine start or run is not an appropriate remedy. Mixture adjustment should only be done when the engine is fully warm and for calibrating the correct mixture.

CIS troubleshooting is simple and easy!!!! Stop the quess work and you'll find the problem/s in your injection system. You could check and measure the following: ignition, fuel pressures, combustion mixture, compression, vacuum leak, and every single CIS components on the engine could be bench tested (DIY).

Tony

I had exactly the same issue and it was driving me mad. After lots of checks and tinkering I found the small wire gauze on the input line of the Warm up regulator was not very clean. I cleaned it with carb cleaner and carefully poked 3 little needle holes in the gauze. Since then - perfect! Hope this helps

merlin62:

Cold-start problem sounds like WUR and/or AAR.
Check fuel pressures like Tony says and consider making the WUR adjustable.
Correct WUR for '78-'79 = Bosch 0 438 140 045
WUR can be cleaned and made adjustable:

http://forums.pelicanparts.com/uploa...1264694471.jpg

AAR-check:

Check the AAR located on the right side of the engine between 5-6 intake runner.
It's the item that has a small electric plug and a large vacuum hose connected to it.
Open the clamp holding the outside-hose.
Pry it off with the screwdriver just enough to get it out of the way so you can see into the opening.
Don’t worry about the Alu pipe; it’ll move enough.
With the help of a small mirror and a light, peek inside the AAR.
When COLD, you should see an opening in the slide shaped like a half-moon.
When HOT, the opening was completely closed on mine. (Summertime air temps)
If it isn't closed, the AAR is either kaput or doesn't get any power.
You can spray some WD-40 into the valve in case it is just sticky.
To check the power, carefully open the tiny clip on the plug with a very small screwdriver and pull it off.
The wire clip is tricky but necessary; don't yank it off.
Start the engine and check the plug for 12 V.
Just ignition ON will not get power to AAR or WUR; engine has to run.
(One wire is power, the other (brown) is ground)
With power to the AAR, it should close after about 5 minutes.
If all is well, replace the hose and clamp securely.

After WUR and AAR work correctly, check for vacuum leak and reset the mixture.

Dear All,

many thanks for your various inputs and your advice from which I have built a checklist on problem solution. Once the winter season in GErmany has ended and the road salt has gone, I will work on it. Will keep you posted!

Best, Merlin :)