hello again

you may remember me from such threads as this one: http://www.briskoda....bia-mk1-14-16v/

still having the same problem.

quick recap:

1.4 16v (100hp) manual 2005 fabia elegance

ongoing problem - rough idle constantly usually runs fine at speed. Every now and then the car starts misfiring badly (generally happens when sat still in traffic for a while or when the car has recently been driven and is turned back on again - e.g. after coming out of a supermarket after a 20 minute shop and turning the engine back on again.). The engine warning light/P0303 code only show up when the bad misfire happens.

Now, i'v done a bit more investigation into the problem (with the help of my boss who is pretty car savvy.) and was wondering if anyone had any more ideas. here's what we did:

- Tried resetting ECU as per Fabravdav's post in my other thread ^^

- Started running on 97 Ron fuel - had no effect (other than making my bank balance decrease rapido!)

- Coil paks and spark plugs swapped (seperately) - misfire is still showing on cylinder 3 (where it's always been)

- Engine flushed and oil changed in case it was a tappet/follower problem - had no effect

- Compression test - showed up lower than normal compression on cylinder 3. (just under 11 bar, where all the other cylinders were just above 12 bar)

- Put some oil in the cylinder and did another compression test to see if piston rings were the problem, - still low compression, suggesting the rings were not at fault.

- Little camera put into cylinder 3 to see if there were any visible faults - couldn't find any.

- When the car was misfiring badly, we took the electrical connectors off each fuel injector in turn, when we took 1, 2 and 4 off it ran even worse, when we took 3 off there was no difference. suggesting the misfire was being caused by the fuel injector being turned off.

- We also found that when the car was misfiring badly, if we plugged a diagnostics machine in and cleared the codes (P0303), the misfire stopped and the car would run fine (although it would still have the rough idle)

I/We came to conclude then, that the low compression in cylinder 3 was causing the constant rough idle, and that the fuel injector for cylinder 3 was being turned off every now and then causing the more serious misfire...

my questions:

- why would this only happen every now and then, and when the car is just started/sat stationary in traffic as i mentioned above?

- could it be the injector at fault and could the low compression have nothing to do with it?

- could the low compression cause the ECU to turn the injector off?

- could it be a fault with the ECU?

- could there be a dodgy sensor somewhere causing the injectors to be turned off?

Cheers!

Edited 17 March, 201313 yr by broongerboy

Swap injector 3 over and see if the fault follows. That'll rule out the injector if it doesn't.

seems sensible. thank you!

never changed an injector before. I'v looked at some general guides on the web, but they don't really explain the first step very well. i.e relieving the fuel system pressure/getting the residual fuel out of the fuel line.

any tips??

Quick update in case anyone googles the problem and stumbles across this thread:

I've still got the car and never fully got the bottom of what was causing the misfire. I did however, find a way to stop/manage it - I realised after a while that the misfire only occured when I was coasting or idling. I therefore stopped letting let the car idle. I manually keep the revs around at least 2000 at all times. Doing this has stopped the car misfiring for about a year now (doing 60 miles a day usually). Not ideal that I still don't know what caused the misfire, but it was a decent enough fix for me!