Idle stumble issue and weirdly quick starts...advice?

[john walker's workshop]

Old single wire head temp sensors do that.

[monkeyodeath]

Originally Posted by john walker's workshop

Old single wire head temp sensors do that.

Alas, I replaced it with a 2-wire as part of this whole process.

[monkeyodeath]

Originally Posted by jlex

Double check that idle speed. Almost sounds like that IAC is acting up.

Settles to about ~700 RPM after a cold start. You think maybe the ICV is glitching open or closed or something?

[cabmandone]

You can bypass the IACV with a jumper. I think this is typically done when setting the base idle mixture as well as setting idle speed.
This thread asks what a jumper is but tells you which pins to jumper
http://forums.pelicanparts.com/porsche-911-technical-forum/188748-what-jumper.html

When you're testing the idle switch you're rotating the throttle by hand and testing it with a meter to make sure that it's opening/closing right?
Don't open up your AFM. The car was running fine and passed a smog test not long ago so I doubt the tracking went to hell that quickly. Plus there's a way to use an ohm meter to test the pins that will tell you what kind of shape the AFM is in.

Normally an idle issue is related to unmetered air but you've replaced the intake gaskets (assuming you also looked at the intake spacers) and you've verified the vacuum side is tight.

You mentioned a stumble at speed I think. That would take the IACV out of the equation. Might be worth having a look at the boards in the DME. The IR thermometer is a good idea for a simple check and they're just handy to have around in general.

Here's a thread that talks about the Jumper and setting the idle speed. Beware of Zao's avatar. It's mesmerizing
http://forums.pelicanparts.com/porsche-911-technical-forum/369102-idle-adjustment-hex-screw-without-jumper.html

Here's a thread by Sal on solder joints. Loads of threads on DME's developing cracks or bad solder joints but this one specifically mentions 3 for the transistor that drives the ignition coil.
http://forums.pelicanparts.com/porsche-911-technical-forum/652375-dme-bad-solder-joint-repair.html

[monkeyodeath]

Thought I'd update this thread as I think I solved my issue. It was the ignition coil. I picked up a good used one and swapped it in. Before the swap, I could barely get the car to idle. 30 minutes later, after swapping in the replacement coil, it was running fine.

Confusing since I'd never really read about the coil failing in this way -- seemed like bum coils mostly made issues at higher revs and not at idle.

But I guess I should have suspected it was ignition related (or at least electrical), given how abrupt the stumble issue was -- like someone switched the engine off, then back on. Problem was extremely intermittent -- sometimes would disappear for days, other times would make the car impossible to start.

There's a slight possibility that the issue was not the coil but the wiring to the coil -- DPO of my car installed an ignition cutoff switch and now there a couple of butt splices in the wiring. It's possible those connections might be the issue.

But after replacing the coil, my car runs again without issue. Certainly threw the kitchen sink at it, and it took months to track down, but ultimately I took care of a ton of deferred engine maintenance as part of my efforts so it's all good.

[RobFrost]

Great thread. Glad you got it fixed.

[Tea Tray]

Did you check the idle micro switch with a meter?
Try disconnecting and plugging the brake booster vacuum line.
Check the multi-pin electric connector for the injectors. Mine was green with guck.