996 3.4 - Cylinder 4 misfire.

First post and new owner in the world of Porsche. Bought a nice example with great history and hoping to get plenty of use out of it however there is a problem to over come first. Isn't there always!! Started with a flashing CEL which would become a solid CEL then would disappear. This would repeat over the last few days. Clear misfire and showed to be cylinder 4 from a simple diagnostic machine. Changed the spark plugs and coils but not fixed the problem, so I'm hoping for some advise on the next steps please. I could send the car off to a specialist of course but am fortunate to have plenty of space a ramp and every tool required to carry out the jobs. Plus I enjoy playing with cars. I would like to carry out a compression test but do any of you knowledgable people have any suggestions of things to look at or test first before the more intrusive investigations? Any help appreciated, thanks.

What codes did it throw up?

I think from memory is was something like P0304 or similar. I will be able to double check when back from work.

Just to add, when we removed the spark plug from cylinder 4, offside and furthest back to rear of car, you could smell petrol on the end of it and there is a smell of unburnt fuel from the exhaust on that side.

Given you have replaced plugs and coils we would assume the spark is OK, if you smell fuel on it it's probably getting fuel OK, injector faults are relatively rare on the 996 AFAIK, which leaves compression. So I would suggest a cylinder leak test to see if compression OK and borescope it to check for borescore/cracks/valves ok.

Fault code is for a cyl 4 misfire .. you had the eml then it went so im assumeing its an intermitant fault .. that kinda rules out compression as something like a burnt valve would be a permanent fault .. but still worth checking . Ok so spark , fuel and compression is needed .. you have changed the plug / coil but is it actually sparking :?: thinking wiring fault here .. Quickest check .. use a spare plug/coil or the old one , plug it into the loom then use a jumper from the plug to the engine to earth it .. pull the fuel pump fuse and get someone to crank it whilst you watch .. is it sparking. Downside is if its intermitant then you need the fault to be there other wise it will just look fine . Moveing onto injector . i think i would swop it with another cyl to see if the misfire moves cyl,s . If both ok you are into a compression check .

Thank you for the quick replies. I haven't driven a lot since plug change but couple of steady drives and CEL light does appears to go just between flashing and solid now, turned off only once for a short time. So compression test .... is it a fairly easy strip down to carry out the test?

I would also check what i first mentioned but for a compression check then remove all the plugs , pull the fuel pump fuse then do the check , check every cyl and compaire , cyl 4 should be much lower than the others if its a compression fault ... a failed tappet is possible but i would expect a rattle / knock noise if it was that and you havent mentioned that so far . Access is not brilliant without a ramp but can be done .

I had a corrado vr6 once with a failed stuck open injector. Worth swapping them round to see if the fault follows (I have no idea how easy this task is). Will be a good chance to inspect your fuel rail for corrosion.

Demort made a good point, we have assumed spark OK but no proof: pretty simple to get a visual confirmation before you borescope or compression test. Swopping injector to see if fault moves cylinders Would prove injector OK as again we have only assumed not proven, although I have no idea how difficult that is to do Either..