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Mallone
Trainee
Joined: 10 Feb 2017
Posts: 57
Location: London
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 Posted: Tue Oct 01, 2019 7:14 pm Post subject: Cold start stumble
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Slowly but surely working through the list of items 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 2000rpm for a half second and then stumbles straight back down to about 500rpm for a second or so before something (the ecu?) kicks in and it jumps back up to a rock solid 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 etc.
Fires up without this issue when warm. This little 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.
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 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 a 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?
PS. And because all threads are crap without pictures, here's one of the car after a cross country thrash as an intro to the forum 
EDIT: Edited to include a bit more detail about the car for diagnosis.
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Previously: 2000 996 C2, 2004 986 Boxster S
Current: 1999 996 C2
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Peter B
Nürburgring
Joined: 18 Aug 2003
Posts: 476
Location: United Kingdom
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| Posted: Wed Oct 02, 2019 9:05 am Post subject: stumble
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That's how they are programmed to start!
Blast up to 2k then fall back to tick-over.
Air con on adds a few hundred rpms....
HTH
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very late 996 C4S Cab
M3 gone 
996 Turbo tip now gone, too :-(
993C4 now gone
996.1 long gone
986 RIP
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Mallone
Trainee
Joined: 10 Feb 2017
Posts: 57
Location: London
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| Posted: Wed Oct 02, 2019 11:48 am Post subject:
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Yeah, I know what you mean about the normal starting procedure but this isn't quite right. It's nearly stalling every time on a cold start.
Neither my previous 996 or my 986 have done this...
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Previously: 2000 996 C2, 2004 986 Boxster S
Current: 1999 996 C2
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Alex
Le Mans
Joined: 06 Mar 2014
Posts: 17349
Location: The Ribble Valley, Lancashire
2000 Porsche 996 Carrera 4
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| Posted: Wed Oct 02, 2019 12:25 pm Post subject:
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Deffo something wrong there and not normal. MAF disconnected puts the ecu on a standard map. Have you tried with that disconnected?
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2000 Manual 996 C4 Arctic Silver Convertible
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Mallone
Trainee
Joined: 10 Feb 2017
Posts: 57
Location: London
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| Posted: Wed Oct 02, 2019 12:40 pm Post subject:
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| Alex wrote: | | Deffo something wrong there and not normal. MAF disconnected puts the ecu on a standard map. Have you tried with that disconnected? |
No, Haven't tried that. Will give it a go this evening to see if that makes a difference - I was lead to believe the MAF isn't used during cold start/at idle due to the cats/lambdas not being hot enough?
I've also been doing a bit more reading on the ICV too and I may not have done enough of a thorough job in cleaning it out as others describe it moving fairly freely and mine was still quite stiff, even after chucking in loads of carb cleaner. Figured that's just how it was. Will remove that and give it another once over too.
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Previously: 2000 996 C2, 2004 986 Boxster S
Current: 1999 996 C2
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deMort
Dijon
Joined: 21 Mar 2015
Posts: 7848
Location: Brighton
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| Posted: Wed Oct 02, 2019 7:16 pm Post subject:
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Idle valve is a common fault .. its best to replace it before moving on .
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My Daughter's Crowdfunding has hit the target .
Thank you all so Very much .
She's not going until july 2020 though .
7pm - 9pm
Indy Mechanic .
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Mallone
Trainee
Joined: 10 Feb 2017
Posts: 57
Location: London
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| Posted: Thu Nov 07, 2019 6:12 pm Post subject:
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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?
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Previously: 2000 996 C2, 2004 986 Boxster S
Current: 1999 996 C2
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Robertb
Dijon
Joined: 01 Sep 2003
Posts: 7587
Location: South Oxfordshire
2002 Porsche 996 Carrera 4S
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| Posted: Thu Nov 07, 2019 8:09 pm Post subject:
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My 3.6 behaved quite oddly for a bit... then the AOS failed.
I forget how they work, but they do produce issues with idling when they are on the way out... though mine played up when warm.
If you are clutching at straws maybe worth checking... I think you can do this by checking vac pressure at the oil filler.
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"911- the guilt-free supercar"
Current: 2003 C4S Coupe, seal/black
Ex: '02 C4S Coupe, '96 993 Targa, '88 Carrera Sport Coupe
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deMort
Dijon
Joined: 21 Mar 2015
Posts: 7848
Location: Brighton
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| Posted: Thu Nov 07, 2019 8:39 pm Post subject:
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You need to look at fuel adaptations ... this will say if there is an air leak or the car is running rich due to a fault ..
After that actual values of the sensors to see what is actually going on .
What i would call basic checks to get your head around the fault .
Some faults are not that obvious and a garage well versed in these cars may be able to diagnose the fault .. lets face it .. i along with everyone here can only guess at the fault ..
As a mechanic i diagnose a car i have in front of me .. on here its hard .
_________________
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My Daughter's Crowdfunding has hit the target .
Thank you all so Very much .
She's not going until july 2020 though .
7pm - 9pm
Indy Mechanic .
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Mallone
Trainee
Joined: 10 Feb 2017
Posts: 57
Location: London
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| Posted: Thu Nov 07, 2019 9:04 pm Post subject:
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Think the AOS is fine. No smoke, no oil in throttle body, no excessive vacuum from the oil filler cap when running (though removing it does make the engine chug a little)
I've got a wifi OBD2 reader and a couple of apps I can use to monitor values.
Is there a link to a good guide or thread on how to read the fuel adaptations? Not sure what I'm looking for....
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Previously: 2000 996 C2, 2004 986 Boxster S
Current: 1999 996 C2
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Martin996RSR
Nürburgring
Joined: 08 Dec 2016
Posts: 472
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| Posted: Fri Nov 08, 2019 12:44 pm Post subject:
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I had a similar situation with my 98 c2. It turned out to be the AOS, though I did have the oil in the inlet as a further indicator, which I note you do not.
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Robert SausageTrousers
Monza
Joined: 02 Apr 2018
Posts: 246
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| Posted: Fri Nov 08, 2019 12:49 pm Post subject:
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Can't offer any help I'm afraid, but just wondering whether you bought the car from Cornwall?
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Mallone
Trainee
Joined: 10 Feb 2017
Posts: 57
Location: London
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| Posted: Fri Nov 08, 2019 1:24 pm Post subject:
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| Quote: | | Can't offer any help I'm afraid, but just wondering whether you bought the car from Cornwall? |
No, bought from a guy near Reading about 6 months ago. Reminds me I need to do a proper thread on it as it looked quite different when I first got it - Fuch replica wheels, horrible exhaust, nasty tints etc. It's been getting a lot of TLC to bring it up to my kind of OEM+ spec. Been inspired by the work Marky911 did on his 996 C2.
| Quote: | | I had a similar situation with my 98 c2. It turned out to be the AOS, though I did have the oil in the inlet as a further indicator, which I note you do not. |
Yeah, I'm starting to lean towards something funky going on with the AOS and/or vacuum leaks etc. Just this morning booked into AMS so they can have a proper look at it. At this point it's not sensible to throw any further parts at it in the hope it fixes stuff.
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Previously: 2000 996 C2, 2004 986 Boxster S
Current: 1999 996 C2
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fot0
Newbie
Joined: 03 Apr 2018
Posts: 47
Location: Thames Valley
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| Posted: Fri Nov 08, 2019 8:45 pm Post subject:
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As mentioned AOS could be culprit. On bank II you will see one of the AOS pipes top of head with flexi bend - the connector starts to weap when it cokes up and it stains oil at the rear of engine and under air-con pump. Possibley a sign it needs replacing at next major service.
Flat6innovations have a youtube demo of checking vacuum "Does your AOS suck?". You might be able to rig up a tube to meaure pressure drop to confirm an issue....just a thought. AOS was replaced on mine; cold start it will rev then drop to high idle and has improved general smoothness of the engine.
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deMort
Dijon
Joined: 21 Mar 2015
Posts: 7848
Location: Brighton
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| Posted: Sat Nov 09, 2019 8:53 pm Post subject:
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For the AOS to be at fault then it would be a split bellows pipe allowing air to enter which as its after the maf would cause issues .. its an unmetered air leak basically ... this depends on the year though .. some have a ridgid pipe that won't leak .
This will show up on the adaptations and indeed you will have a low maf reading of say 12 kg/h or possibly less .
This will tend to give a poor idle but also bring on an EML which you don't have
I often sit in a car looking at figures before coming to a possible conclusion at that point i actually start looking at components .
The car is booked in to be looked at so i'll leave it there as anything more is pure guesswork im afraid .
_________________
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My Daughter's Crowdfunding has hit the target .
Thank you all so Very much .
She's not going until july 2020 though .
7pm - 9pm
Indy Mechanic .
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Mallone
Trainee
Joined: 10 Feb 2017
Posts: 57
Location: London
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| Posted: Sun Dec 15, 2019 10:31 am Post subject:
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Thought I’d update this for anyone who has similar issues in future.
Car went into AMS for a few days for them to diagnose. When Angus plugged it into PIWIS It was showing fault codes for O2 sensors which wouldn’t have affected the cold start but I elected to have them replaced anyway. Judging by the pictures he sent through it looked like they had fun changing them!
Over a few days he was able to replicate the problem reliably and after checking all the basics he was able to find a tiny vacuum leak from somewhere at the back of the inlet manifold. Clearly too small to throw up a CEL but wouldn’t have been helping. Behaviour was improved but still not perfect.
I’d been getting a a bit suspect about the new (non Bosch) ICV I’d purchased as even when hot I was now getting some weird idle behaviours. Revs ‘hanging’ at around 1500rpm when coming to a stop, weird fluctuations with revs sometimes ‘bouncing’ up and down etc.
Bit the bullet and splashed out on a proper Bosch unit and all the problems have completely vanished. My guess is the tired old Bosch ICV (and the cheap Chinese replacement) just couldn’t get their act together fast enough at cold start to prevent the revs from dropping after that initial burst of revs. Car now cold starting like a champ and idle behaviour is much, much better all round. Result!
PS. First time dealing with AMS Porsche and a big thumbs up from me. Angus was a top guy to deal with, great comms and super flexible. Recommended.
_________________
Previously: 2000 996 C2, 2004 986 Boxster S
Current: 1999 996 C2
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