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Turbo misfire

944turboS

944turboS

New member
Hi Guys,
Hope someone can help me here. My 1989 944 Turbo has a nasty misfire when I am getting around the 100mph mark (never on public roads of coarse) it happens in both 4th & top, it feels like the overboost protection is cutting in then resetting itself instead of having to kill the ignition. The car has a BBR chipset but apart from that its stock. I have an aftermarket boost guage that I've put in the console over the winter (sorry I'm a poor downtrodden employee & can't afford to run the Turbo as an everyday car) I will be able to see how much boost I'm getting early next month when I re Tax her. All help is greatly appreciated as I want to go on a couple of track days this year to see what it will realy do.
[:(]
 
Jeez, I don't know; only been here a day and nicked my avatar already...[8|] [:D]

My Turbo used to reset its overboost protection when it kicked in if I backed off for 20 seconds or so, so they don't all need the ignition off and on trick. I used to run a boost enhancer set high enough that it would do that on 95RON but it was OK on Optimax so it was the knock sensor that was tripping mine I guess.
 
Hi Fen,
[:eek:] [&o] Sorry got my avitar from e-bay originally & I confess it is borrowed. I will change mine as soon as I find something suitable.
There hows that?
[:)]
 
No problem, I was only joking, use whatever you want. Possibly it's time for me to change mine anyway.
 
Check the plug gaps. Depending on how aggressive Ken Brittan set the timing in your chip & how much boost you're running, you could be getting flame out.

HTH
 
Thanks Rick,
I do have a trouble shooting sheet from BBR that suggests the problem may be the plugs so have alredy checked & replaced them.
 
Another possibility I have come across is insufficient current to the coil. If you've got an immobiliser, quite often they interrupt the live feed to the coil. If the relay or connections start to go bad, they create a high resistance, limiting peak current available to the coil when it's under load.

Slightly harder to detect without a logging DVM (digital volt meter), or with a crypton type setup and a rolling road. The quick way to check is to bypass the live feed to the coil by linking from the power post located near the vacuum resovoir under the bonnet. Exercise extreme care because of the way the DME is configured.

Substituting the coil for a known good one is also a suggestion ...

HTH
 
Thanks again Rick,
It seems if Andrews fuel pressure regulator fix doesn't work then I will be searching around for a good rolling road, probably a trip to Promax to see Andrew.
Regards
Graeme
 
Guys,
I realy need some help here, took the car out for a run today Before the snow came, the "overboost" symptoms happen when the in dash guage is reading a full 2Bar (I know its not relible) I have checked the vacuum lines as suggested by Andrew, put a meter across the coil feed (on tickover) both these seem ok. Andrew says he can't realy look at my car till about the 20th any more ideas or how do I turn down my boost, does anyone know another turbo guru in the midlands?
HELP !!!!!!!!
 
You could bypass the cycling valve temporarily to reduce boost right down.

It would allow you to test the ignition through the full rev range running only about 6/8 psi. A sort of diagnostic ...

I sent the package today, should be with you in the morning [;)]
 
Alasdair

If the car is running slightly more than standard boost and is ~ 20 years old, the chances are it could be the fuel supply = misfiring/cutting out under heavy boost. This is usually (in my personal experience) a sign of a failing fuel pump.

Best bet, drain the tank, remove pump and purolator in tank filter. Clean and replace filter and reseal install a new pump. This worked for my car following issues at Silverstone on a GP track evening.

Yours

Chris
 
Fuel filter is outside tank visible in offside rear wheel arch

Assume all fittings are rusty and pipes and filter will need replacement

Also could be spark plugs or coil breaking down and rotor arm is also a key suspect.

They also have a soft cut if over revved.
 
If it has standard chips then it will see 1 bar as overboost. If it has a manual boost controller fitted the the cycling valve has probably been bypassed (however the ecu will still reduce the ignition advance). Without seeing the plumbing its hard to diagnose but I'd start by reducing the boost to 0.7 & try it again.
 
Paul

sorry to publicly disagree with you, but....

On the Turbo model (probably all models but can only speak from personal experience) there are TWO filters. The conical purolator that screws into the plastic fuel tank and the visible external aluminium cannister filter.

When I replaced the fuel pump I had the whole system apart including the internal fuel filter.

Yours

Chris
 
All the usual basic ignition and fuelling stuf needs to be checked as sound, but apart from that I share Bertelli's view that it sounds like a funny configuration.
Two questions spring to mind:
1) why is it running 1 bar of boost if that's all that's been modified?
2) what chips does it have in it?


 
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