Cause of Misfire?

[57jam89]

Hi all,
My Megane 2004 1.6 16v had a full engine service about 6 mths ago (all filters, plugs, oil etc..)and has been running fine up to 6 weeks ago then it started misfiring. After a few days the engine light came on with cylinder 3 misfire. so as the plugs had been changed and the coils all looked old I ordered a new set of coils from ebay. All 4 fitted and was running fine again (fault codes cleared).

Now its booked in for a mot tomorrow and today engine light started flashing and engine was misfiring. So checked codes and 'misfire cylinder 3'
So swap coils from cylinder 3 to no 4 (cleared code) and again 'misfire cylinder 3, then removed and checked spark plug and swapped with cylinder 4.
Also did a compression test on cylinders 3 and 4 and both were 12/13 psi.

And again engine light flashing and 'misfire cylinder 3'!

Any ides on what else can cause this?

Thanks.

[q292u]

If compression ok and spark ok, then it has to be fuel. Check the injector..

[57jam89]

Update- swapped petrol injector from cylinder 3 to 4 and checked the injector loom (the bit from plug which clips into the lifting eye to each and very injection ) and stIill have ' misfire cylinder 3' !

So far all new coils, swapped coil, spark plug and fuel injecter from cylinder 3 to 4, checked loom beside injectors and checked compression. And still misfire cylinder 3!

Any ideas?

[grandadbobby]

Hi,
l don't have an answer for you and l take it that the engine runs ok. lt would only make sense if it was the head gasket.
l would give 3 a proper misfire, disconnect the injector and spark plug and run for a few seconds. Reconnect and see what comes up.
Then disconnect 2, see if fault on both occur. Then erase codes, see if 3 remains.
l know you said that you tested compression on 3 and 4, but was that just turning engine over or running.
l would do a compression test on all the cylinders one by one when engine running. (lbs per sq in)would be more accurate.
Good luck and if you find the cause please let us know.
Bob.

[57jam89]

Hi, yes I will give it a try. Thanks

But surely it can't be the head gasket because if I clear the fault codes and then start the engine, the engine light flashing and 'misfire cylinder 3' comes back straight away which makes me think it must a electrical fault for the ecu to see it immediately.

[fredcamp2]

Misfiring covers a multitude of problems. Imbalance between cylinders. Sticking valves. Wiring from no 1 coil to no 4 straight through to correstp0nding ecu pin no's. the same check at coil 2 to coil 3 to corresponding ecu pin no's. A manifold vacuum test will tell you a lot. What I would recommend you to do is a compression test when engine is cold & another test when warm

[grandadbobby]

Hi again,
a thought went through my head and l have to run it by you.
When you check injector 3, are you sure you have them numbered correctly.
l coded injectors on a clio not long ago and found that renault number them 1234 from the flywheel end.
l don't know about petrols but it was just a thought.
Bob.

[57jam89]

Hi again,
a thought went through my head and l have to run it by you.
When you check injector 3, are you sure you have them numbered correctly.
l coded injectors on a clio not long ago and found that renault number them 1234 from the flywheel end.
l don't know about petrols but it was just a thought.
Bob.

Hi Bob, yes I do know that this megane is numbered from the flywheel end of engine.

I will be doing some more compression test later.

[57jam89]

Update-
Just done a compression test with a hot engine and every cylinder is 11/12 bar. So all fine inside engine!

The misfire is at its worse with a hot engine.
Have noticed about 20% of the time it's fine no misfire then it's like someone throwing a switch the misfire starts. Then it's at its worse I have 'check injection', flashing engine light and flashing service light with fault code 'misfire cylinder 3'. If I turn the engine off then it's at its worse and restart it's fine for maybe a minute or 5 minutes.

So just to sum up - I've swapped coil, spark plug and petrol injector with cylinder 1 which has made no difference.

Anyone know I can get a wiring diagram of engine management?

[grandadbobby]

Hi again,
worse when warm points me in the direction of a dry joint on a connector, least you know it's on 3 somewhere.
l am not conversant with what you've got, but check the resistance between the actual connectors and the wire they are soldered to.
lntermittent faults are hard to find but maybe with a multimeter and the engine warm you might find a different reading to a good cylinder.
Good luck, would love to hear you solved this.
Bob.

[fredcamp2]

Owing to the high energy coils fitted to these Renaults it's imperative that the proper recommended spark plugs are fitted. Do not lightly dismiss the the the wiring from coil 1 & coil 2 straight through to the ecu corresponding pin no's. Can you post up your engine code starts with k4m ??? what are the 3 no's that follow the k4m

[57jam89]

Sorted - turned out to be 2 faulty coils (all 4 were only 6 weeks old)!
I swapped the coil from cylinder 3 to no 4 and still had the 'misfire cylinder 3' all because no 4 was also faulty!

Do not know why ecu didn't pick up a misfire in cylinder 4! And why I had 'check injector' on the dash.

Replaced the 2 coils and it's running sweet again after about 15 hours trying to sort it!

Thanks everyone for their help!

[grandadbobby]

Well done, just shows how diagnostics can mislead, would have cost a fortune in a garage because they encounter the same problems.
Process of elimination can bring you right back to where you started.
Bob.

[57jam89]

Well done, just shows how diagnostics can mislead, would have cost a fortune in a garage because they encounter the same problems.
Process of elimination can bring you right back to where you started.
Bob.

Cheers, just my luck that I swapped a faulty coil with a faulty coil. Still got it sorted in the end!

[57jam89]

Just don't believe this- it's back to misfiring, flashing engine light, flashing service light and 'check injection'!