MY 93 WRX with ScoobyECU, Turbosmart MBC @ 15.5psi, Scoobysport exhaust, HKS SPF induction kit and Bosch coils.

Car is running great below 4000rpm, but anything above half throttle from 4000 up is dropping me onto 3 cylinders.

Igniter unit possibly or knackered plug? fitted NGK 7B's 7000 miles ago.

 

Rev

I had that on mine- lift off slightly and boosts again - Right?

if so then the knacked coil pack or plug is sparking away from the
contact due to the increase in pressure, as you back off the
pressure drops , the spark makes contact again, and car goes again.

I cleaned mine first which lasted a bit then changed the plugs and a pack
I thought was knackered, again lasted a while.
Ended up changing all. touch wood never had prob since.

WW
WRX MY 93

 

Strangely enough, it's boosting fine, but it's definately only firing on 3 until I back off to below 50% throttle where it will pull full boost quite happily while the AFR shows rich

Definately have to get the plugs out and take a look at the state of them.

 

Now it's cleared completely after redlining it in second (with a bit of a splutter). Weird.

 

Rev,

Now mate.

I have had similar misfires in the past as you described. I also have the bosch coils fitted from your group buy.

I have always put it down to a bit of damp getting in to one of the leads or plug due to the plug bores not being sealed as per the original coil pack design.

I blast mine regularly now with wd40 to keep them free from moisture.

I think the genuine leads for the 99 impreza had seals to bung the plug bores. The Halfords/Bosch leads I got are more conventional and dont have the seal to stop moisture getting in.

When we thrash the car to stop the misfire, speed and temperature build up could be clearing out the moisture and sorting the misfire.

Similar to a dizzy getting water on it from older cars etc. My Mini from years ago used to be a real tw@ and a damn good thraping and around 12k rpm always sorted it out

Thats my theory anyway.

Works for me

Ste.

 

Ste, I think you're right mate, either that or water finding it's way into the rubber boot at the coil end (2 of my coils sit under the bonnet vents and I don't have the covers fitted)

 

Rev,

That could be the main reason if youve got the coils drenched in water coming through the open vents.

There used to be Holts spray that sealed the dizzy, plugs and leads that I used to used years ago but I cant remeber whatit was called. It worked well at stopping moisture getting under each plug boot.

Failing that I smear of silicone sealant never hurts either

Ste.