Hot Starting Issue - GTA V6 Turbo

Hi all, my GTA seems to have inherited a very strange issue when up to temperature. The car will not idle and misfires when on boost (3K rpm onwards), below that the car drives fine on a light throttle applications, but as soon as the car stalls it will not restart until cold. The temperature sensor has a range of c270ohms when cold & 380ohms when warm - I assume this is correct? Fuel pressure seems consistent at 2-2.5bar.

It just seems odd how the car will start and run fine from cold, but after 3-4 miles will not boost, stalls (you can keep it going if you heel and toe) but will not idle and then will not restart from hot??

What am I missing here? Fuel pumps have been replaced and filters too!

Any thoughts would be much appreciated and thanks for reading!

Matthew.

There are a few things that can do this but check the fuel pressure regulators, if the diaphragms have perforated courses all sorts of problems when hot.
If fuel is coming out on the vacuum side they have had it.

WHS.

Fuel pressure is way too low at 2 to 2.5. Should be 2.8/3.0. It's under-fueling on boost massively.

Custard/Clee thanks for your help. Apologies the fuel pressure (only one regulator fitted) was actually much higher than first thought and was 2.5bar on idle and close to 3bar on an 'open' throttle (3K rpm). These figures were static and not on the road. From the above we assume the fuel supply to be OK, but the symptoms seem to be that when the car is cold it runs fine and will start repeatedly on the button. As soon as its up to temperature it will simply not restart....

Apart from the temperature sensor which is measuring OK at the sensor end, what else is the ECU looking to change its running dynamics when up to temperature?

Another symptom - if you can keep the car running and not letting it stall) is that it will cut either the fuel/ignition when on boost - it's like the ECU is saying we don't need any more fuel - could this be a MAP sensor issue?

Why will run OK when cold and not hot!?!?!

As always your help is much appreciated and thanks for reading!

Have you checked the air bypass valve is working when the engine is ticking over, basic stuff first once you start on electric stuff I’m out call the auto sparks, doubt it will be the Ecu.

[stephendell]

That’s the idle valve I showed you and remember to check for splits and cracks in all the vacuum hoses particularly to the map sensor.

Coil pack can behave like this or TDC sensor. Both break down with age and heat and will misbehave a lot more when hot.

Coil pack is cheap and easy to replace. TDC isn't too expensive but a pain to get to.

The problem with the TDC is due to de-magnetising over heat cycles. Coil pack is similar but just gives a weak spark and again tends to die once hot, but can work again when cold.

[stephendell]

Also check all your wiring and the earth to the ECU.

By coincidence a club member kelvin called me today. He was having trouble with his car not running properly and changed the ECU, both fuel pumps, cleaned the injectors, cleaned the idle valve, new coil, new ignition leads, map sensor, adjustable pressure regulator, cleaned throttle position switch, new dizzy cap and arm and all the time is was just a poor earth to the ECU.

Don’t you just love electric, it’s supposed to make things better but unfortunately only for a while, then it becomes a nightmare. Just think about the new stuff single strand wire, will that still be working in 30 years

Wow.....thanks all. We have looked at the idle control valve and when forced either open/closed or 50% between the two it will still not start when hot. We have a spark at the plugs.......

Will the TDC sensor pick-up/read OK when cold then fail when hot? Re: ECU earth - this we have not checked, does the ECU have a common ground point or individual ground points per sensor/signal inputs? I have an R25 turbo wiring dia to work from but this is not in my possession (but my trusted mechanic who after last weeks investigation has taken it away for further reading).

D Day will be Saturday 16th February where the car will be in a professional workshop, ramps/lifts and all tooling available - but no parts.

Mr Dell, I'll be in touch regarding some other parts so we can cover all bases!

Again, thanks all - much appreciated and will keep you posted!

Re: ECU earth - this we have not checked, does the ECU have a common ground point or individual ground points per sensor/signal inputs?

These two pics may help you. The first shows the pins on the ECU connector (item 120) .... there are two pins used for the earth .... which go to a common Earth point ... M8.
The second pic (lower one) shows .... not too clearly .... where M8 is situated on the engine. As I do not have a Turbo, the exact position of the earth is unknown to me, but from the pic, I suspect it is around the rocker cover on the right hand bank of the "V".
I am sure some other member can give you the exact point of this earth.
Click on pics to expand.

Turbo Ign.jpg

Turbo ign Earth.jpg

TDC will usually work when cold and won't when hot. Eventually it would fail altogether but obviously most won't get to that stage.

Search on here and you'll find multiple threads discussing it.

It will stop you getting a spark though so that's a clue, but then are you getting a strong regular spark?

Likewise coil pack failure. You'll still get a spark, but it'll be too weak to run without a misfire.

Do you know if either have ever been changed out? If you do change the TDC then watch out as they are wired in reverse... red to black, black to red. Again careful though as I'm not sure if Simon Auto versions have reversed the reversal...

[stephendell]

Also if fitting a new TDC sensor as well as the back to front wiring make sure it has heat proof sleeving as otherwise it will get cooked by the turbo. Some of the cheaper ones don’t have this but easy to retro fit.

Apologies for the delay in the reply - but a big thank you for all your help so far.

John & Darren thanks again - on Saturday I secured some working parts from Mr Dell over at his lock-up, these consist of:-
1 x Ignition module
1 x map sensor
1 x tdc sensor
1 x ECU. Stephen I know you'll read this and the ECU will only be installed as the last resort providing the above test OK, big thanks for Saturday & NICE biscuits.

This Saturday will be day and will keep you all posted.

Hi Matt - I should add, I'm assuming the basics are in good order, spark plugs, HT leads etc?

Hopefully yes. If so then if possible take the old school science experiment approach and change one variable at a time and then re-test.

I'd start with the coil pack as it's relatively simple along with the MAP sensor - plug and play plus hose! You can just balance the coil pack on the shelf or similar, at least when static for testing purposes. Otherwise it's then a tight squeeze to then bolt it up.

ECU are very reliable, so more likely to be wiring to the ECU than the unit itself. But a doddle to swap.

TDC is a very likely / most likely culprit but the most troublesome to change out and you can't just temporarily fit it. Stephen or Lee may have some hints on changing.

Good luck.