C531XHO

Hi Folks

So away on holiday with my GT having driven it ~1200 miles from cold Scotland to hot South of France (~34 degC) where it ran pretty faultlessly at 80kmph all the way down the autoroute. Had a couple of non starts once I arrived but swapping FP relay seems to have solved that but I am left with a cutting out problem which only occurs when coming to a standstill. Doesn't happen every time and always restarts thank goodness. If I slow down and dip the clutch at the last minute the engine will continue to run but if I dip it early it will sometimes cut out. I have been told there is a problem with the resonator flap not working whilst cranking, but I though this was a cold start idle problem, not hot start. I have read various posts but not really sure where to start the hunt...EZK, LH module. The fact it runs faultlessly at speed inclines me to think the fuel pump is OK. Thoughts anyone?

David

SwayBar

Sounds like your idle control valve is misbehaving.

Mrmerlin

how much fuel do you have in the tank when this happens?
Are you running the AC, shoot the tank with an IR gun to see how hot it is when it shuts off same for the relay panel.
Do you have the in tank fuel pump?
If no Internal pump it could be running out of fuel pressure when its hot ,
or if you do, the internal pump hose could have split.
Could be a vapor lock type of issue.

Could ber a relay, one of the 4 running relays ,
X bus,
the fuel pump,
the LH,
the EZK
Replace these one at a time.
Also clean the battery terminals/ connections all of them inspect the ground strap for internal corrosion,
use deoxit on any of the these connections.

Also inspect the fuse for the fuel pump and replace it with a new fuse,
dont use the Chinese fuses

FredR

First thing I would eliminate would be actuation of the idle switch contact. If the throttle cable adjustment is marginal it just may be that the contact is not making/breaking as needed and when this happens the ISV does not kick in. You may also have a situation where the switch itself is on the verge of failure [very common] - the solder in the joint starts to fail and thus feasible that it may cause intermittent failure of the switch to make contact.

Next time this happens, if circumstances permit, check the circuit to see whether the contact is open or closed.

C531XHO

Thanks everyone, I will do some digging and report back. I am travelling light re tools - stupid really as I hadn't appreciated how easy it is when travelling abroad with ones own car.

Mr Merlin, I am familiar with FP, EZK, LH but not sure what you mean by X bus?

Sway bar, Fred - is the ICV the same thing as the ISV - item 2 in the WSM picture?


For the record
- FP relay is brand new
- I think I have an external FP, but now you come to mention it I don't know..

Regards

D

FredR

Item 2 is the Idle Stabiliser Valve to give it its correct Porsche name. Some folks do refer to them as Idle Control Valves.

They are a known failure item over time but your description does not sound like an ISV failure to me. However stranger things have happened!

If you have a GT it should have an in tank pump unless someone deliberately deleted it [as some do- unwisely in my humble opinion]. If this pump fails or the in tank transfer hose cracks you would have much more problems than an odd idle to worry about or so I would expect but always keep an open mind about problems until the root cause has been nailed.

C531XHO

Thanks Fred - I will have a look. Most of my previous experience is with an S and 2 x S2s. This car/928 type is new to me so I am still on the steep bit of the learning (and expenditure) curve...

D

Mrmerlin

Roman numeral X = the X relay

FredR

Don't worry- the curve never flattens especially on expenditure- with many years of ownership one tends to be that bit more "aware" when the next problem lands.

GregBBRD

Generally, when GT's stall when returning to idle, the problem is with the mixture being too rich or the incorrect LH chip. If your car has an O2 sensor, monitor the voltage with a volt meter to eliminate the mixture as a possibilty. If it is a non cat car and has the mixture pot....lean the mixture down.

The other problem I've seen quite a bit (so common, it amazes me) is the incorrect LH computer chip. Most all of the LH computers have been replaced by now....and it is super common to get a computer with the incorrect chip for the GT model. (The GT model has a higher idle setting to compensate for the more aggressive camshafts.)

If someone ended up getting an S4 chip, a GT will stall 50% of the time, when the A/C is on. (I've fixed 4 cars with this issue in the past 6 months......and each one of these cars had been to other shops to fix the stalling problem....and had a small fortune spent on them.)

Red Flash

The MAF could also just be failing. I have had exactly this failure mode on other L-jetronic cars, at least. Though not on 928s.

SwayBar

Well, those folks are clearly wrong!

GregBBRD

Watching the O2 sensor output will tell volumes and can be used to diagnosis bad MAF's.

jon928se

Intermittent failure of the idle switch on the throttle body will cause random failures to keep idling especially when the revs drop from way up (when you dip the clutch early) versus revs at or about idle speed when you dip the clutch. You can readily determine if the Idle switch is a possible culprit by watching the fuel consumption when off the gas but in gear above about 1500 RPM. Should go to 90MPG when the idle switch is activated i.e. foot of the gas. If it doesn't the idle switch is the first point of investigation.