Clues needed: engine cuts out with out warning

[944 turbo cabrio]

Hi,

I have a 1991 Turbo that Friday started cutting out while at a Track Day. It cuts out quite harshly without warning primarily at long wot accelerations but also in midrange high gear accelerations overtaking on the highway. The error is random - doesn't happen everytime I do a heavy acceleration - maybe every second time.
As I couldn't find any loose hoses, all plugs seemed in very good shape and the tank was full (ergo lots of fuel available) I ended up driving home without much track time assuming I needed a new fuel filter.

I went to the garage yesterday to collect a new filter, changed it and everything appeared to be ok when I took a short test drive but unfortunately I have today experienced the same problems again :-(

I am not the greatest mechanic (unfortunately) and time is short in the garage before my next trackday which is Friday this week. I would very much like to get this problem solved as I will not dare participating with a car I don't know when will cut out and possible swing the tail out on me []
Anybody having experienced this type of problem - or have a good idea what to check besides the fuel pressure and perhaps checking the alternator more thorough? When time and garagetiem is limited it is always nice to start by checking the most suspected parts

I have 3 weeks ago added a Lindsey Boost Enhancer as my wastegate is very weak - it has driven since no problem but could this be source of trouble?

Hoping for replies I wish a pleasant week with hopefully lots of motoring for you all!

Niels

 

[Lowtimer]

Could be lots of things, I suppose, but what I would first suspect is an air leak somewhere, which is causing the knock sensor to chop the power. I had much the same thing some time ago.

 

[alwigley]

Can you give a little more info - can you restart immediately after cutting out, or do you have to wait 10/15 mins before you can restart. Is this a total loss of power, or effectively dropping off to idle ? Has it cut out on you anywhere other than the track, or are you able to drive it home OK ? Have known HT leads/coil leads to develop hairline cracks in them that can cause loss of power when hot, but as soon as engine cools slightly will restart then repeat the cycle 10 minutes later, so could be a damaged lead........ hence the questions above.

Hope you manage to get her reliable again for Friday []

 

[chrisg]

One thing it could be with the Turbo if it's only happening at WOT, it could be a fuel pressure problem. This would accentuated by the boost enhancer (holding higher boost and therefore using more fuel).

I had a similar thing years ago at Silverstone, but only when hard on boost and at WOT = needed a new fuel pump.

A word of warning though, if this is the case, don't run it for too long as it will lean out the mixture and can lead to burnt exhaust valve (as happened to mine....)

 

[GPF]

Hi Niels,

It sounds to me like you are hitting the DME's overboost cutoff, particularly as you have just recently fitted a Boost Enhancer.

The Boost Enhancers are not going to give very consistant boost levels and changes in temperature will affect them.

I suspect that if you turn it down a little your problem will go away.

Do you also have aftermarket chips, or are you still on the stock ones?

 

[homesea]

As Graham says it sounds like the overboost cut off, cheeck all hoses my old old one did just that and it was the hose to ISV had split (hard to see as its under the manifold). The car will build boost to fast and cause the cut off to trip.

 

[944 turbo cabrio]

Thanks for good and fast response!

More (and clearer) info:

As soon as I get off the throttle the engine returns to normal and I can restart the acceleration - only I do step more slowly on the accelerator - and everything is fine again.

I could drive all 200 miles back with an average of 95 mph without a problem - as long as I don't jump to full boost and keep a steady pace. I have never experienced the KLR/DME cutting in before so I don't know how violently it is operating, but I would for sure say that the way this happens is a bit dangerous on a track if it happens at the wrong time and place corner wise :-(

I am on stock chips, I ran 95 octane as 98 octane is sometimes hard to get by
I haev been searching for leaks but not under the intake - what is "ITV"?

Thanks again for your help and readiful answers!

Niels

 

[GPF]

ORIGINAL: 944 turbo cabrio
As soon as I get off the throttle the engine returns to normal and I can restart the acceleration - only I do step more slowly on the accelerator - and everything is fine again.

I have never experienced the KLR/DME cutting in before so I don't know how violently it is operating, but I would for sure say that the way this happens is a bit dangerous on a track if it happens at the wrong time and place corner wise :-(

Yes, this is just how the DME overboost behaves. It's nothing to do with the KLR - that does something completely different.

ORIGINAL: 944 turbo cabrio
I am on stock chips, I ran 95 octane as 98 octane is sometimes hard to get by

Turn the boost enhancer down.

 

[944Turbo]

Yes, overboost protection its the BE needing backing off a turn,
Tony

PS what colour is your cab?

 

[944 turbo cabrio]

I will turn down the BE and then I think I will add some Octane booster - I guess that should help a lot also!?

My car is Panterschwarz metallic with black leather Clubsport seats. I am actually considering to upgrade my own seat to a racing seat as I still am fighting to stay in the seat despite of the big bolsters on the side
Also my car has the very nice Techart rear spoiler that so subtly alters the look of the cars back to something very beautiful (well at least in my view)

 

[Diver944]

ORIGINAL: 944 turbo cabrio

I will turn down the BE

Try just one full turn, that should do it.

You may find in the winter you have to turn it down another one, as all that lovely cold dense air will increase the boost a little and you'll trigger the overboost again

 

[GPF]

Hi Niels,

The octane booster will not help at all - your problem is not knock related.

All you need to do is stop the car overboosting so the DME is happy.

 

[Lowtimer]

I may have got this wrong but I thought the overboost protection worked by sensing knock?

I would advise the original poster to use better petrol in the first place rather than adding octane boster to the cheap stuff. It has to work out cheaper.

 

[Fat Albert]

I have had the same issue on one of my SAABs and was the Overboost sensor when my Automatic Pressure Control was playing up, so my specialist fitted a Boost enhancer and that sorted it!

 

[GPF]

ORIGINAL: Lowtimer

I may have got this wrong but I thought the overboost protection worked by sensing knock?

I would advise the original poster to use better petrol in the first place rather than adding octane boster to the cheap stuff. It has to work out cheaper.

There are 2 types of protection.

You are thinking of the KLR knock protection (which activates the limp mode that keeps boost right down until it is reset), but the one in play here is the DME overboost protection which is triggered by the afm voltage, which instantly resets when airflow reduces again.

 

[DivineE]

What they said. Overboost cut. Triggered by 2secs of over set max (11psi I think). I had this on my first car when I fitted a boost enhancer with no chip. Remove Lr boost enhancer until chipped.

 

[Hilux]

ORIGINAL: Diver944

ORIGINAL: 944 turbo cabrio

I will turn down the BE

Try just one full turn, that should do it.

You may find in the winter you have to turn it down another one, as all that lovely cold dense air will increase the boost a little and you'll trigger the overboost again

I will turn down the BE and then I think I will add some Octane booster

Dont add octane booster

The turbo will overboost in cooler weather as Paul states as the air molecules trap more petrol in a denser mix. Simply tweak the BE.

 

[TTM]

Niels, whereabouts are you in DK?
Do you know the guy(s) in Hornbaek with 928s?

 

[944 turbo cabrio]

Dear all,

thank you for your help!
I turned the booster down (quite a few rounds!) and now everything is good again
I had a nice tiem at the track in the beautiful weather but know now that I need wider tires at the front (and also a little bit in the rear) and also I need suspension upgrades - whos has some elephant, racers edge or similar upgrades that they want to sell?

Dear Thom,
I now a few 944's in Denmark but I don't know any 928 owners unfortunately.
I live in Glostrup - suburb to Copenhagen.

Nice weekend to you all

/Niels