bebbetufs

In the last session of track running this weekend I experienced a strange occurrence. After a day of problem free running, when gunning down the straight and pushing hard in 4th gear when at 5000 RPM the came two loud "pops" one immediately after the other and the car lost all power.

I immediately pressed the clutch and coasted, then the engine lights came on and the engine stopped. I could not make the corner as the lack of power steering caught me by surprise so I ended up in the runoff zone. I worried someone would hit me so I decided to risk restarting the engine. It fired up straight away and everything seemed normal for the rest of the lap, so fine in fact that I decided to go for another lap, only to have the exact same thing happen again.

The car started up fine and ran fine for the rest of the lap, and the 600km home trip.

This has never happened before, but this was my first track day and I've never pusshed the car this hard before.
What may be causing this?

It is a 1990 Turbo by the way!

M758

could be a loose hose from the turbo to the intercooler. These can pop at high boost when loose. By Pop I mean vent all turbo pressure at one of the clamps. Then at lower boost say connected.

sydneyman

i also had a leak in the actual boot so make sure you take off the j-boot and inspect it for wear. had the same kind of symptoms as your car except mine were every time you floored it, not just in extreme conditions. i imagine my leak was just worse...

bebbetufs

This was my first thought as well so I did precursory check of the intake system, and found nothing. I guess I'll have to be more thorough. On the way home from the track I reached 1.8-1.9 BAR in third with no symptoms. Must be a very specific kind of leak if it only pops open under extreme pressure in 4th (I hardly ever reach fourth on the road, so hard to test). Don't you think it strange that the first time this happened there were two pops in a row before the engine died?
I guess I need a more accurate way of measuring and looging boost.

One mechanic at the track suggested faulty fuel delivery, I don't like the thought of that. Woluld not the car stay in safe mode if heavy knocking was detected?

JohnKoaWood

Knocking will send it into ignition retard, but shouldn't cause the engine to die.. restarting also cycles power (as you have to turn the ignition off then back on..).

bebbetufs

If this was knocking, would it not have caused noticeable engine damage? The popping noise was quite loud and the engine did switch off.

Eric_Oz_S2

Overheated/worn coil or ignition ignitor module?

bebbetufs

Possible, but would that not have happened in other places on the track as well?
I have no service records so the coil might be 21 years/63K miles old. Should I replace it regardless of whether it is the culprit or not?

PorscheDoc

Is the car 100% stock?

bebbetufs

Perhaps I should mention that I had done several sessions without this problem, and that this only happened late in the day when the tank was about 1/2 empty.

No. It has the following mods:
- 3" exhaust with test pipe.
- Lindsey dual port waste gate with manual boost controller set to absolute minimum boost to make it work as much like a single port WG as possible.

I also installed a brand new cycling valve for safety as the original was brittle.
The fuel filter is new.
I've had some rough idling problems and some vibrations when accelerating from low down in the RPM range, like when pulling out from a car park. I've put this down to bad (new) engine mounts, and or a bad ground. I've checked all grounding points except the ones on the engine. I will do them when I do some major overhauls later.

Rogue_Ant

It sounds like you may be hitting the stock overboost protection. What is your boost set at?

dillon410021

yeah, maybe boost creep in fourth gear

bebbetufs

Thanks guys.
I don't know to be honest. The stock gauge is a little off. That's why I've set the MBC to absolutely minimum boost. I know that the spring in the dual port WG is stiffer than stock, so I assume I have a little higher than stock boost.

If this is the overboost protection why would it make a loud blam? and why would the engine switch off? Should I not get fuel cut off first, like hitting the limiter?

This reply to another thread I have going sounds interesting:

dillon410021

bebbetufs

Thanks.
Had a look at the video. It did not look exactly the same. Mine made two sudden and loud popping noises in a row, nothing like fuel cut-off or ignition retard. Then the engine died.

Will the KLR cut ignition? If so the popping could be raw fuel igniting in the headers?