'99 C2 Cold Start Stumble

[Mallone]

Hi all - First post!

 

I'm slowly but surely working through the list of fixes on my new (to me) 996 and this one has me a little stumped so far. It's a 1999 C2, cable throttle.

 

Every time I start the car from cold I have an issue where it fires immediately up to the normal 1800/2000rpm for a split second and then stumbles badly straight back down to about 400/500rpm for a second or so before something (the ecu?) kicks in and it jumps back up to a rock solid cold idle at around 1100/1200rpm.

 

When it's warmed up it stays at around 6/700rpm with AC off, 900rpm or so with it on. Again, rock solid. Never had it stall, never had it run rough, pulls hard etc.

 

Fires up immediately to a nice steady low idle without this issue when warm. This stumble only appears on cold starts, but it doesn't sound too happy about it and I'd like to try and fix it. It's not normal behaviour based on my previous 996 and 986.

 

It's got a new air filter and I've cleaned out the ICV. It recently had a new water pump (and the bearing was really grumbling in that so wondered if that was 'dragging' the RPM's down on a cold start until the ECU could compensate) but nothing has cured it.

 

I'd planned to do plugs and coil packs soon, as well as sort out a leaking spark plug tube, but not sure what else to try.

 

The only thing I had considered was that when I picked the car up it had an old K&N kit on it that I removed and replaced with the OEM airbox. I reused the same MAF sensor (after a thorough cleaning with electrical contact cleaner) but didn't think the MAF was used in a cold start (might be wrong here...)

 

Any ideas?

Edited October 1, 2019 by Mallone

[Loren]

 

Could be the solenoid in your ICV is gotten weak.

After all it is 20 years old.

 

Yes, the MAF does nothing at idle. It comes into play above 4000 RPM.

[Mallone]

👋

 

Would a failing/old ICV not exhibit other poor idle issues? It’s absolutely rock solid the rest of the time.

 

Or are you suggesting the solenoid is just slow to respond when first fired up/cold?

 

Any way to definitively test it’s working correctly?

[DBJoe996]

You can test the ICV by removing it from the throttle  body and cycling it with a 9V battery.  It should move smoothly and not bind.  Make sure to clean all the gunk and carbon from the rotating valve body inside the ICV.

Since you replaced the air filter with a stock box, reset the DME/ECU by disconnecting the negative battery lead for 30 minutes, then  reconnect and go for a 20+ minute drive to allow the DME to re-establish sensor values.

[Mallone]

Thought I'd give an update on this as I'm still struggling to diagnose/solve. 

Tried with the MAF disconnected. No change. Issue remains. 

Replaced the ICV with a new one and also pulled the throttle body and gave the entire thing a thorough cleaning. Idles cleaner generally, but still the same cold start problem. 

Did a major service and gave the car a set of new plugs and coils. Issue still there. 

I'm now running out of ideas  

The car sounds quite 'hissy' when it's first started so starting to wonder if there might be a vacuum leak somewhere that's causing the issue that the ECU is compensating for? 

The 'hissing' goes away when warm so could just be a cold start thing. No SAI on this car, if that makes a difference.

Also get quite a bit of chain rattle on this car, would the tensioners coming up to pressure have anything to do with it? (doubt it, but worth asking!) 

Any other smart ideas before I start to think about taking it to a garage?

[Loren]

Have you checked for faults codes?

The hissing does sound like a vacuum leak.

[Mallone]

Only using a cheap wifi OBD2 scanner and an iOS app, but not seeing any fault codes or getting a CEL