Can someone give me an opionion of this fuel pressure reading

redcoupe86 · 04-26-2006, 06:01 PM

I have an '86 Carrera with a running fuel pressure (vacuume lines attached) of 32 psi.

Im trying to diagnose a rough cold start issue.

A synopsis of a cold start: I turn the key, engine fires up withing 3 turns. Runs real rough for a few seconds (between 700-900 rpm), then all of a sudden the rpm jumps to 1500 or so and is very smooth an steady. If I cut the engine off right after its starts running smooth and start the car again (engine is still cold remember) the car fires right up and picks up at the 1500 rpm - smooth.

The Bentley says Im suppose to be running 29 psi.

Could a faulty fuel press. regulator be to blame?

tonypeoni · 04-26-2006, 06:13 PM

that reading is with in specs have you check your idle switch?

tonypeoni · 04-26-2006, 06:18 PM

or idle speed control valve?

redcoupe86 · 04-26-2006, 06:33 PM

I've checked the idle speed switch and I just installed new ICV.

Quote:

Originally posted by tonypeoni
or idle speed control valve?

tonypeoni · 04-26-2006, 06:40 PM

does it drive fine once its warmed up do you have hesitation?

redcoupe86 · 04-26-2006, 06:49 PM

Yes. After the car revs to 1500 rpm (thats about 6-10 seconds after i start it cold) its fine.

Hot starts are no prob either.

Quote:

Originally posted by tonypeoni
does it drive fine once its warmed up do you have hesitation?

tonypeoni · 04-26-2006, 06:54 PM

Then your problem is definitely not fuel related and diagnosing that problem yourself could end up cost you more in parts than if you took it to a official porsche center.

redcoupe86 · 04-26-2006, 07:10 PM

i thought it might be a leaky injector or a small leak in a fuel line that takes all night to loose pressure so only on cold start would there be any symptoms.

I've done multi meter tests on most everything else so I don't know what else to look at. Unfortunately I dont live near any Porsche specialist so I've been trying the DIY route.

tonypeoni · 04-26-2006, 07:20 PM

I'm not a bosch tech but i understand the basics of electronic fuel injection. Went to school to be a tech but got out there and realized it wasn't for me lol. Anyways with electronic fuel injection the pump builds instantly as soon as you turn the key. So even you had leaky injector it would not cause this.

redcoupe86 · 04-27-2006, 05:01 AM

I guess I need someone to point me in the right direction. If I have to take my car in to somebody I want to do as much dianogtics myself before I hand it over.

These are the things I know, checked or changed:

New O2

Have updated CHT

AFM temps sensor's resistance checks out and I moved the pot. to a new track though it didn't look like it really needed it.

Fly wheel sensors check out

New ICV

Set CO mix and idle speed.

Checked that little mirco switch at the throttle body that makes that little click (forgot the name of that thing) and checked its resistance - and it seems ok.

Replaced DME

Running fuel pressure = 32 psi

Car drives fine no problems. I just can figure out what would cause such a rough start!!

Bill in OKC · 04-27-2006, 06:37 AM

I didn't realize my car had a rough start - it is my first Porsche but... working around the engine compartment I found a disconnected hose that should have been connected to the Ambient Air Valve. The cold start got noticeably smoother after I connected the hose back up. It might be worth a look in that area to see if all the components are working properly. Good luck.

redcoupe86 · 04-27-2006, 06:49 AM

Quote:

Originally posted by Bill in OKC
I didn't realize my car had a rough start - it is my first Porsche but... working around the engine compartment I found a disconnected hose that should have been connected to the Ambient Air Valve. The cold start got noticeably smoother after I connected the hose back up. It might be worth a look in that area to see if all the components are working properly. Good luck.

interesting, where is this "amient air valve" ?

Bill in OKC · 04-27-2006, 07:25 AM

It is mounted high behind the oil filler neck. A black plastic saucer shaped thing with three hoses. Pelican sells them as ambient air valves. There is a PCV type valve also involved in this circuit. If any of the hoses have leaks then the engine could have a vacuum leak when it is cold. I ended up replacing the vacuum hose from the back of the throttle body to this valve because it was cracked around the ends.