Firing on 3 cylinders
 

I have a 1996 Legacy GT-B (JDM Twin turbo) with a problem that once it gets hot (it's not overheating though) it loses spark to no4 cylinder (I think it's 4 - rear passenger side). It's fine cold but as it gets warm it gradually gets worse until eventually it won't rev past half revs or so (may be even running on 2, haven't checked drivers side for spark yet) and can hear the unburnt fuel popping in the exhaust.

I changed plugs, inlet manifold with all wiring on it, tried 3 ECUs, MAF, coil packs (old but working on other cars), added earthing kit, changed O2 sensor twice. changed cam and crank sensors (cam sensor showed signs of contact with whatever it senses, not sure if that would cause problems..)

What could be the cause of lost spark on one cylinder only? changing coils around it still stays on no4.

I had this engine in another car originally and this fault developed after I changed the head gasket and clutch. I then bought another car which turned out had also Failed HG so swapped the engine over but this fault has come with it, so it's not the wiring loom on the car side as it's happened in two cars, can't be the wiring on the manifold as have tried two. What else could it be?

any help or thoughts/ideas would be great.

:(

Have you checked the injectors, with particular attention to the connector from the wiring loom?

no but have changed the whole inlet manifold which includes injectors and all that wiring and the problem remains.

no 4 is getting weak or no spark, can't see how that would be affected by the injectors.

It could be an electrical issue caused by a poor contact or a trapped wire to either a coil pack or injector which would cause a resistance when the metal/wire has heated up and expanded and only exhibits itself under load when the resistance would restrict the current supplied and the circuit would break down.
Therefore it's worth checking that all contacts are good and clean and there are no trapped or suspect wires.

Just read your original post again and it may not be this but well worth checking as it very much sounds electrical.

yeah good point. However what's odd is that most of the electrical contacts and wiring have been changed or swapped, either by replacing it or in the process of moving the engine to another car. the only things that haven't changed are the knock sensor, water temp sensors, crank sprocket and thing under it that fires the cam sensor, and the cam wheels, and the block itself and heads. Everything else has been changed and it still does it.

Will have to wait until I get home with all my tools in a couple of weeks to investigate farther but I'm getting to the end of my tether with this.

thanks anyway for your thoughts :)

Are you certain that there is no spark?
Otherwise, it is possible that the valves are expanding and sticking as the engine heats up. Possibly caused by restricted oilways?

JOhn is onto something...doesn't that cylinder "share" oil with the turbo?

well the first time I checked the spark it definitely seemed like there was none on that cylinder under load, but I checked again and it was a bit weaker than the others but was there.

I certainly would not be surprised if it was a sticky valve, and to be honest that's what I'm leaning towards there seems to be few other possibilities left.

sorted this finally, thanks for all your suggestions. Turns out cam wheel was not straight, was hitting cam sensor on one side and gap was too large on the other. straightened it out and it's running sweet as a nut now :)

Woo hoo good on ya to no and nice to no u got there in the end.

Cheers Kev :)