Head scratcher... Need help on cold start.

[xtreme997]

Ok here goes

2001 Boxster S approx 75k miles

Cold start runs rough and misfires after about 5-10 sec then misfires for about the next 45 sec or so, clears up and runs great.

No faults stored (UGH!)

check MAF with PST2, reads approx 55 kg/h (<=problem but don't know why) during this cold start period then goes back to normal readings

also checked all the temp readings, ambient temp, intake temp, oil, temp, coolant temp, all is spot on correct.

also checked plugs which looked good, and ignition coils have no cracks (plus the car runs good when warm)

Replaced MAF sensor and air filter - still not fixed (I know, I know, just throwing parts at it. but need to start somewhere)

Checked fuel pressure (check's ok, although It seems to loose pressure over night and the pre charge when you first turn the key doesn't always build pressure, weird I know, but doesn't seem to affect the misfire weather you start it at 20 psi or 50 psi pre charge It won't misfire until it after it gets to the 44 psi running pressure.)

Injectors are NOT leaking, fuel pressure must be bleeding back to the tank, Don't think it's pump issue (once warm car won't come off operating pressure even running on hard accel. to red line)

also checked for intake vacuum leak with smoke tester - checks great.

just to rule it out I replaced the original AOS with new

also thought maybe the servo motor in the throttle body was acting up so I cleaned it with no luck then I swapped with another known good part with no change, performed many TB adaptions during the whole process.

even checked FRA's, and RKAT's (even though cold start enrichment is precalculated time based operation) and they checked within spec.

Any more idea's would be helpful. Thanks

[Loren]

:welcome:

So you are saying once it's a bit warm it runs fine?

If so, then you likely have one or more bad connections (including battery ground) that are causing the rough running.

If your code reader will show you pending faults then you likely have one of more at startup that never go into full faults because the problem clears (when the wires get warm).

That's what it sounds like to me.

[timbo]

This might sound odd, but do you have any coolant loss? I say this because I once had a problem (non Boxter) where coolant leaked into a cylinder overnight, caused a misfire but once warm was ok, seems strange that the problem clears after 45 seconds,

How is enrichment achieved? this is what I would look at or what lauren says, have you tried changing a coil pack, maybe start it in the dark and look for any tracking.

[am993]

Sounds like carbon build up. Mine had similar symptoms, but only if the car had been sitting for a few days. I ran a full tank of gas with a bottle of Techron Concentrate Plus, followed by an oil change and voila, problem solved.