Cold Start Issue

[1987 Porsche]

Just finished the rebuild of my 3.0 motor a couple of months ago and I am now trying to tackle a cold start issue. Here it is: On initial cold start (car inside garage to move it out) the car starts up fine, immediately the idle goes up to 1800 rpms then in a second drops to 800 rpm and lingers there for 4 to 5 seconds, then goes back up to 1800 rpms and eventually within a minute settles down to 900 rpms where it will remain. When I attempt to leave the driveway and rev the engine it will rev up but then drop low enough to stall the car. Sometimes it will stall out at a couple stop lights after leaving the house, but once warm the car runs perfectly fine.

What has been done:
Injectors were cleaned by Mr. Injector, new sleeves and o rings were installed during rebuild. New intake boots were also installed. Removing oil cap when engine is running I do get a 200 rpm drop indicating the possibility of no vacuum leaks.

Today I did a fuel pressure test (bypassed the fuel pump relay, car was cold, garage temp 68 degrees F). Here are the results:

Control Fuel Pressure (Valve Open on tester) (car cold) (WUR electrical connector Disconnected): 1.7 bar
System Fuel Pressure (Valve Closed on tester) (car cold) (WUR electrical connector disconnected): 4.9 bar

Control Fuel Pressure (Valve Open on tester) (car cold) (Wur Connected) 4 minute test with fuel pump running:

Initial: 1.7 Bar
2 minutes: 1.7 bar
2:30 (2.1 bar)
3 minutes: (2.4 bar)
3:30 (2.65 bar)
4 minutes (2.75 bar)

Residual Fuel Pressure (Valve open on tester):
Once car was shut down after above test the gauge read: 2.3 bar
After 10 minutes: 1.5 bar
After 35 minutes: 1.2 bar

Whats your thoughts with this rpm drop on initial start up and stalling when leaving the driveway?

Thanks,

Lou

[SkiVT]

What is the year of your engine and the last three numbers (Bosch part number) of your wur? Hard to know if the pressures are ok without knowing the wur number. Also likely need to confirm other cis part numbers as user name doesnt tie to a cis engine year.

[boyt911sc]

Lou,

I received your email regarding your cold start problems. We just got back from an Eastern European trip and still recovering from jet lag. Stay in tuned with SkiVT and he will guide you to your investigation. He has a remarkable understanding of CIS test procedures. Will participate later after I unpacked my luggage and get my pending works in order.

Tony

[T77911S]

it sounds like it is too rich.
when RPMs drop like that after a rev is too rich.
start with fuel pressures.

over the years as air leaks develop people will compensate by making the mixture richer, then when you do a rebuild the air leaks fixed and you are now too rich,

[1987 Porsche]

The car is a 78. The last 3 of the WUR are 005.
I figured I add some pics.

[1987 Porsche]

For what it's worth, the more I look into the last three of the WUR (005) it looks like it could be a BMW wur.

[SkiVT]

Do you have a pic of the top of the wur including the path of fuel and any vacuum lines? Have to find the correct pressure specs if its an 005 wur to see if its in specs as is. Whether those specs would also work in a 78 911 without other modifications would be the obvious next question.

[boyt911sc]

Originally Posted by 1987 Porsche

For what it's worth, the more I look into the last three of the WUR (005) it looks like it could be a BMW wur.

Lou,

Please post a picture of your WUR. You need a vacuum assisted WUR-045 or -069. I am not even sure if -005 is a vacuum assisted WUR. Keep us posted.

Tony

[1987 Porsche]

I'm out right now but here is the best pic I had on my phone.

[1987 Porsche]

[1987 Porsche]

Guys I may have made a mistake. From the pics when I had the engine out the WUR number looks like 045. I checked it twice this morning and only saw 005. When I get home ill confirm.

Thanks

[Bigtoe32067]

Nice looking car.
Tony

[1987 Porsche]

Thanks

[stlrj]

Could be a massive vacuum leak. Pop of valve?

[1987 Porsche]

So it is a 045 WUR. I had to clean it to confirm but the 4 is there but barely visible. WUR is getting 26ohms.

[cmcfaul]

Did you change / lighten the flywheel? Any mods to the engine requiring a chip update?

Chris

[SkiVT]

Is it correct to assume your warming pressures are without vacuum applied? Do your vacuum lines from the wur connect through a ttv (4 in pic)? Did you test your AAR as part of the rebuild?

Thanks

[1987 Porsche]

AAR was checked during rebuild and functioned fine. What do you mean in regards to warming pressure without vacuum applied? I bypassed the fuel relay, hooked up the gauge and turned the key to on without starting the car.

Yes the vacuum lines from the WUR connect through a thermo valve.

Thanks

Lou

[pmax]

Originally Posted by 1987 Porsche

AAR was checked during rebuild and functioned fine. What do you mean in regards to warming pressure without vacuum applied?

Thanks

The WUR operating pressures changes when engine vacuum is applied via the hoses connected to it.

Look up the 045 or Tirwin's dummies thread...

[SkiVT]

Lines connected without engine running points to pressures without vacuum, the lower numbers on the specs. So wur doesnt look materially out of spec. AAR typically takes a couple minutes to close off extra air so rpms stay up much longer than the time you describe. I would have that as a suspect but It tested good so must be another issue. Do you have access to an AFR to check mixture, as suggested could be rich from earlier “tuning”. As Pmax mentions, vacuum raises the pressure which leans the mix so testing the wur pressure with vacuum applied is needed to rule that out.

Great looking ride by the way.