944 Turbo cuts out and stalls when hot

[Spongebob]

So it's started a new game! When I sit in traffic and the car gets hot the engine will stall and refuse to restart. Let it cool it fires as if nothing ever happened and drives beautifully. Done it twice now, once on the M4 and more recently in a jam on the M25. Turns on the key, fires and then just dies. Wait 10 minutes with bonnet up it fires and runs as if nothing was ever wrong.
Am i seeing early signs of a DME failure?

 

[PSH]

Hmm.. I was reading an article yesterday and came across something that I hadn't seen before, the article was talking about the 924 and hot starting issues but I guess it could be the same. The symptoms were that the car would start and run fine while cold, would be fine once up to temp but would not restart now it had warmed up. The problem was tracked down to worn rings/bores...when cold the engine produced enough compression to start, once warmed up and the metal had expended the car couldn't restart due to not enough compression? IIRC this was detailed by 'Hartech', I read a lot yesterday so can't swear to where I saw this. I guess a way of checking this would be to do compression tests both cold and hot, you'd have to be pretty quick doing the hot test but if the car isn't starting you know that if this is the problem the test will show you what's going on.
Hope it's something more simple than this but as this was new to me thought that I would share, Hartech have a good reputation for diagnosing engine issues. LOoking at other possibilities, yes DME, also head temp sensor and crank sensor, I'm sure there are others that can cause this but can't think right now..got a headache...lol

Pete

 

[IanK]

Whilst mine (S2) hasn't been stalling and failing to start, its been running and idling rougher when it got warm. Whilst I swapped a fair few parts out, it was the crank sensor and 02 sensor that I changed last and its run perfectly ever since.

I did also read that DME relays tend to fail when hot, before they fail completely (I already had a new one in).

Based on my experience, in your shoes i would test/swap in the following order:
DME Relay
DME Temp sensor
Crank sensor

First 2 are circa £20-£25 but the latter £100 to renew.

Ian

 

[bmnelsc]

944 hot start issues (by 2 questioners) were covered recently in our PCNA monthly mag. These guys had no stalling while driving issues but problems re-starting when hot. The expert's opinion was "vapor lock" due to failing check-valve in the fuel pump AND/OR bad pressure regulator AND/OR leaking injectors which failed to hold pressure in the fuel system when the car was stopped allowing fuel to vaporize in the fuel rail. Once this occurs it takes a couple of minutes of running (usually very poorly) to get the fuel pressure back up and clear the vapors. Both of the questioners had already replaced relays, temp sensors, fuel regulator and fuel pump (the 944 check valve is integral with the fuel pump),and one had replaced injectors - with no success and the expert seriously doubted a failure of the DME control unit. So even tho you are not shutting the engine off perhaps you are getting sufficient under-hood heat to generate a vapor lock while sitting in a jam.
The expert recommended connecting a pressure gauge to the test port on the fuel rail and checking for fuel pressure drop when the engine was shut off. If found, the fuel pump/check valve, pressure regulator, etc were suspect although he specifically recommended pulling the fuel rail with injectors still connected - placing paper towel under each injector and checking for a leaking injector which would bleed pressure off the rail even if was producing just a few drops. All this with the engine stopped of course.
In your case I wouldn't think injectors are a problem as you are still running when it quits but if you have low fuel pressure from a failing pressure regulator or a failing check valve in the fuel pump you might not be maintaining enough pressure to prevent vapor lock when you are sitting in traffic. Worth a look IMHO.

 

[Spongebob]

Thanks for the replies. I should add that 99.9% of the time it runs and starts perfectly hot or cold. It is literally only when its sat in traffic for an extended period of time going no where. The engine was fully rebuilt approx 6k miles ago and included replacement injectors. I fitted a new Bosch fuel pump about 18 months ago and a new O2 sensor to get it through its MOT 2 years ago as emissions were off. Under normal operating conditions it starts, runs and idles perfectly. When it died on M25 last week after it had cooled a bit it started first crank and drove the 70 miles home faultlessly. Before that it had taken 45 minutes to travel the bit between J14 and J15.
I'm off to OPC Swindon this morning to pick up a new DME (they hold stock) but they are £52 with our discount! Much cheaper on eBay but never quite sure what you are getting.
I'll try that first. I'm off to Bicester tomorrow morning and i am expecting to have to sit in traffic so that will test the theory. If you are going and are held up by a dead silver 944 Turbo please wave and say hello.....

 

[robwright]

I have seen coils fail in such a manner before now. They break down when hot then return to normal when cooled. Could be a number of things though as already said above. I think I have known FPRs and dampers do this before now.

 

[pse_SC]

I'm off to Bicester tomorrow morning and i am expecting to have to sit in traffic so that will test the theory. If you are going and are held up by a dead silver 944 Turbo please wave and say hello.....

Very frustrating - really hope that doesn’t happen - try going a bit earlier and miss the traffic []?
We’ll be there early tomorrow too in my Kalahari 944 Turbo []... see you there - will be good to chat about this and other things.
Paul

 

[Spongebob]

I have seen coils fail in such a manner before now. They break down when hot then return to normal when cooled. Could be a number of things though as already said above. I think I have known FPRs and dampers do this before now.

Yes, coils on the way out do die when hot but they tend to give up the ghost under load rather than at idle.

 

[Spongebob]

Yes, aiming to be there around 8.00am. It's only 30 minutes up the road from me. Zermatt Silver with a 944 number plate. I'm taking my friends son as he's asked to go. Someone else to push if it lets me down!

 

[Indi9xx]

My first bet would be on the DME relay.. But be careful, there are some seriously dodgy ones sold by national chains and on the internet, get a genuine one from a Porsche main dealer.

Take a foot long length of wire and cut it into three short lengths.

Take one end from each strand and twist them together to make a tripod.

Either solder the joined ends together and insulate it, or use a crimp if you have to.

Then crimp or ideally solder three flat blade terminals onto the three ends you have left.

When the car next does this, remove the relay and install your special tripod test wire into where the three large terminals on the DME relay go into the fuse box and try to start it, if it then runs fine, this is the cause.

This tripod test wire will be useful for all kinds of tests as it takes a live from the fuse box and distributes it to power up the DME-ECU as well as the fuel pump, injectors and everything else in the engine bay, without things like injectors spraying or the coil firing, so you do not need the engine running to perform lots of tests, such as fuel pressure tests, voltage tests to injectors and a whole host of things.

In the workshop I build an electronic break out box to do all the functions of a DME relay along with led indicators of each circuit and switches to over ride things, but before I had that, the three wire tripod for the DME relay bypass and a single wire jumper for the idle adaption port, were two of my most used items in my tool box when 944's were the main cars arriving here.

 

[Indi9xx]

If it is not the DME relay, it could be a whole host of different things that are potentially heat soaking.

But "stuck in traffic" "Ran fine again when allowed to cool" or "Ran fine once the RAC man got here" are the two tell tale signs which are 99% likely to be the DME relay, and even a new one from the wrong sources can be iffy.

 

[robwright]

Agreed with Jon. Every 944 owner should have the three wire jumper in the glovebox. I don't even carry a spare DME. What's the point when the wire will get you home if the DME fails? If it works you then you know you need a DME. There is a words and pictures guide on www.clarks-garage.com

 

[Spongebob]

Update for you all. New DME from OPC fitted. Ran the car to Bicester this morning and sat in a little traffic as we drove in to the site. Literally pulled in to the parking area for the Porsches and it died again. Not sure if that was the same issue or pure coincidence as it started straight back up again. Drove home no issues and left it to run on the drive. It sat there idling for about 5 minutes and did not stall although revs would occasionally drop and then pick up again. I left the car with the engine off and then went back out to start it after about 5 minutes so still hot. Fired then died. Did this a number of times. I then opened the bonnet and tried again. Car fired and ran perfectly so it would appear that something doesn't like getting hot. Fuel, electric???
Thanks for all the advice. The car will probably go in to hibernation end of this month and intention is to book it to JMG after Christmas and get them to give it a really good going over as about time it got some professional love and attention rather than me fiddling and fettling with minor services bits.
BTW I had seen the 3 wire thing on Clarks and will be making myself one of those to go in the glove box with a spare DME.

 

[Indi9xx]

Could be the coil overheating due to age/oil loss and getting heat soaked when in traffic.

I have also seen a bad fuel pressure regulator cause these kinds of problems. When they fail high they cause the fuel pump to work harder against a brick wall of pressure and very little fuel flow at low rpm and no load, which causes the fuel pump to start cutting out.. When it gets this chronic, you can often hear the fuel pump revving up and down if you listen by the back wheel.. This extra load also strains the DME relay.

Another problem could be injector load, if one of them is playing up it can cause it to draw more amps through the power transistor in the DME/ECU, which then causes the transistor to overheat... These cool down really quickly and almost have a thermal overload side to them. When I have suspected this fault if the car starts and runs ok (although missing) when you disconnect one injector, but then cuts out when you try to run it on all four, then it could either be weak injector driver transistor/s in the DME, or a faulty injector etc. This is a specific 944 Turbo issue, not something that tends to be a problem on the 944S2 DME/ECU... An especially big problem if the car has cheap uprated fuel injectors.

Could be hundreds of other potential causes, such as even a wiring fault, aged crank sensors, all kinds of things, but often the problem can be found quite quickly, one of the things I love about 944's is that they almost tell you what is wrong, the clues are usually there somewhere, unlike the 997 onwards Porsche models, where a control module on one place slightly playing up can cause havoc anywhere else!

 

[Spongebob]

Could be the coil overheating due to age/oil loss and getting heat soaked when in traffic.

I have also seen a bad fuel pressure regulator cause these kinds of problems. When they fail high they cause the fuel pump to work harder against a brick wall of pressure and very little fuel flow at low rpm and no load, which causes the fuel pump to start cutting out.. When it gets this chronic, you can often hear the fuel pump revving up and down if you listen by the back wheel.. This extra load also strains the DME relay.

Another problem could be injector load, if one of them is playing up it can cause it to draw more amps through the power transistor in the DME/ECU, which then causes the transistor to overheat... These cool down really quickly and almost have a thermal overload side to them. When I have suspected this fault if the car starts and runs ok (although missing) when you disconnect one injector, but then cuts out when you try to run it on all four, then it could either be weak injector driver transistor/s in the DME, or a faulty injector etc. This is a specific 944 Turbo issue, not something that tends to be a problem on the 944S2 DME/ECU... An especially big problem if the car has cheap uprated fuel injectors.

Could be hundreds of other potential causes, such as even a wiring fault, aged crank sensors, all kinds of things, but often the problem can be found quite quickly, one of the things I love about 944's is that they almost tell you what is wrong, the clues are usually there somewhere, unlike the 997 onwards Porsche models, where a control module on one place slightly playing up can cause havoc anywhere else!

Thank you Jon. As mentioned in the post I do intend to book the car with you over the winter for some TLC so no doubt I'll be on the phone to sort in the coming weeks.