I have a 94 Impreza with what we think is an early sti engine fitted with 440cc injectors, front mount intercooler, power fc, avcr etc whilst mapping it the other week it highlighted a problem that has stumped me. Over 1.4 bar of boost it coughs and splutters occasionally and at idle you can feel and hear that it's not running on all 4 cylinders, on boost it does tend to cough and go though and I can get it to fly without an issue

We have disconnected the coil packs at idle and noticed that the offside bank don't really notice too much i you disconnect a coil, the nearside makes a huge difference

far we have swapped the following but got nowhere:

Spark plugs
coil packs
ignition amplifier
cam sensor
crank sensor
ecu

It's always the same.

We then checked the compression & cambelt timing and both were good

Last night I tried to demonstrate to a mate how bad it is at idle but when the engine is cold it makes a huge difference disconnecting the coil packs but once it's warm it hardly notices if you disconnect coil number 3

What on earth could be causing it? I'm at my wits end now and I just want to break the bloody thing because it's pissing me off and I can't be bothered anymore

 

Anyone got any thoughts?

 

Isn't #3 the one that tends to run lean because it's furthest from the fuel pump? Maybe that's on its way out, or the fuel filter is blocked, and you're not getting enough fuel to that cylinder?

 

Have you changed the leads? Is the engine earth to body strap tight and have a good connection?

Failing either of them it could be an ecu fault or broken wire/loose dirty connection somewhere?

 

Tried a different maf?

 

Andy

It's got a walboro pump on there and I also wouldn't expect fuel pressure to be an issue at idle once warm

Blu

I've tried a different ecu and because it's an early car it has individual coils and I've changed them. I haven't checked the engine earth but it's a good thought

V4

It's got a Z32 on there at the moment, could that cause a misfire?

Any other ideas for me to try please?

Thanks

 

I have a spare MAF but it's for a later model - not sure if it can be made to fit

 

MAF can cause rough running/misfires. You could try the fault code reading to see what that thows up.

 

I'll try my sister's Z32 tomorrow then to see if that makes a difference. Other than wiring we're running out of ideas.

Thanks

 

Is it possible for, say, a scored bore or damaged valve to pass a compression test but still leak significantly under combustion loads?

Was the plug you took from that cylinder any different to the others? Might be a clue?

 

It was ever so slightly different but it had been misfiring on that cylinder so it doesn't give any clues. I wish I knew what was causing it.

 

did u find out your prob mate

 

No I gave up with it and I'm going to break the car, I can't be bothered with it now so I might just buy another after I've broken it

 

Ive just cured my slight miss on boost from changing my leads, magnacor upgrade.l

 

Mine is on individual coilpacks. We think the wiring to either the coil packs or injectors gets warm and break down because it runs perfectly when it's cold.

 

so its fine when its cold but once warm you have a issue?
Could it be the coolant temp sensor ?

 

Coolant temp sensor wouldn't cause it to misfire on one bank though I know what you are thinking though.

 

The cause of the misfire has been found but not until I'd broken the car for parts One of the cam pulleys was loose and it must have been wobbling around and throwing the timing out. It was that loose it had actually damaged the cam and taken a chunk out of the locating lug

 

Holy sweet lord mate. Lucky mate.

Steve