Folks, Please advise on possible causes for this...

Every cold start, if I sit and let the choke shut off I get CEL indicating misfire on 1 2 4 5 6.
I can reproduce this every time.

Coil packs and plugs changed a month ago.
I have cleaned the MAF and have new air filter on.
Tried different gas stations (always 93)

If I drive away before the choke cuts out, no CEL.
This only occurs if I let car sit and warm up for a minute.

Any thoughts are much appreciated.

 

Your problem isn't exactly like mine as mine was only on 1 bank but the rest was as you describe: i could drive away before it dropped to low idle from cold, for about 1/2 mile, no CEL. If I had to come to a stop before 1/2 mile it would throw a CEL.

My problem ended up being one of the hydraulic valve solenoids. I replaced both preventatively.

 

Thanks for the reply.

I'll look into getting those replaced.

 

Your odds of both bank's cam position sensors or hydraulic lift solenoids failing at the same time would be pretty uncommon, though I guess if the oil you were running is too thick or too thin on cold start, then they may not pump up properly - what oil are you using.

Aside from that though, I'd look at something common to both banks of the motor before you start just replacing parts. An intake vacuum leak could cause this as you'd have unmetered air entering the motor which would cause the O2 sensors to read out of range compared to the fuel trims the DME is controlling based on the amount of air going past the MAF. Any intake work recently - CAI installed, freshly oiled K&N filters, plenum or throttle body changed or cleaned?

It could also be the MAF itself having gone bad. Even if you cleaned it, if it's not giving proper air mass readings to the DME, then the fueling to the motor on both banks will be improper. Try disconnecting the MAF completely and see what happens. This will make the car use middle of the road settings that are safe for the motor without doing any intake air analysis. If it doesn't happen with the MAF unplugged, that's likely your issue.

 

Thanks Pete, this has been happening for a while now and agree that solenoids on both banks failing at the same time is not the most likely option.

I did clean the MAF but i think I'll replace it anyway as I'm certain that its original to the car *05 997.

 

Unplug it first and do the test. If it does the same thing, it's not the MAF and you'll have spent a few hundred $s unnecessarily.

 

Thanks. I'll do this tomorrow on next cold start.

 

My first thought is either the intake air temperature (probably supplied by the MAF) or coolant temperature is off -- high -- and the engine is not being fueled properly at cold start. The engine is running lean and the cylinders misfire.

With an OBD2 code reader/data viewer/logger you can view both the coolant temperature and intake air temperature in "real time" at cold engine start and after. If either reading is much above ambient that can be the problem.

However, the misfires not appearing if you drive away immediately with no cold idle time points to a problem with the VarioCam Plus system, the low lift feature.

Not all misfiring cylinders have to have bad lifters. One "bad" cylinder (with just one bad intake lifter) can cause adjacent cylinders to misfire.

In the case of the bad cylinder (or cylinders) at least one of the two intake lifters isn't switching to low lift mode at cold start.

In this case the cylinders with one or both intake valves stuck in high lift mode misfire because the large lift valve lets in too much air into the cylinder.

I believe the Porsche diagnostic computer can obtain supplement error code info that indicates the misfires are due to failure to switch to low lift. The supplement error code information indicates something is "below limit"; in the case some lifters can't switch from low lift to high lift, and if the misfires occur at higher engine speeds, the supplemental erorr code information indicates something is "over limit".

When you drive away the DME switches back to high lift mode and of course all lifters work in that mode.

To properly diagnose the possible problem with the VarioCam Plus system requires a Porsche diagnostic computer and at least in the case of my 996 Turbo two techs. One to drive the car while the other uses the diagnostic computer to monitor engine telemetry (O2 sensor values among other things) concurrent with activation of low lift.

 

What ended up being the issue back then?