[Elliott19864]

Hi everyone I am hoping you can help me in diagnosing a fault on my 2012 320i.

I gave the car a service with new plugs, oil, coolant and air filter.

Immediately after this the car misfired underload quite badly.

I gave the car to a specialist who did all the relevant testing. More new plugs and coils etc.

They also tested vacuum all ok.

Compression all ok.

Leak test all ok.

Voltage test all ok on wiring.

They changed the injector on cylinder 2 which has improved the misfire. It now only misfires when cold, the idle is rough for maybe 30 seconds.

When warm the car is good on idle and doesn't seem to misfire underload.

The car is displaying a fault on cylinder 2 misfire.

The garage is now recommending to strip down the engine to diagnose the fault. Which I am reluctant to have done.

With the garage only replacing the one injector, is it recommended to change all 4 on these engines?

We are at a loss as to what to do as at times the car drives fine.

Any recommendation? Car is a 2012 with 30k on the clock.

[thescouselander]

Perhaps you damaged/contaminated the air mass flow sensor when you did the air filter - this would cause a lean fuel mixture and missfires. Its easy to test - just unplug it and the ECU will estimate the appropriate value; if the sensor is the culprit the symptoms will go away.

[Eddamoo]

I know you say everything has been changed, but when diagnosing a misfire it's easiest to swap a known bad configuration to a known good. So my next step would be to swap the coil, injector and spark plug on 2 to another cylinder. Clear the codes and see if the code moves.

Make sure you're not losing coolant since the service. Some cars have a very thin water jacket on the cylinder head so can crack and leak with improper torquing

If the compression test and leak test came out good, if love to know their reasoning for the strip.

[Elliott19864]

Originally Posted by Eddamoo

I know you say everything has been changed, but when diagnosing a misfire it's easiest to swap a known bad configuration to a known good. So my next step would be to swap the coil, injector and spark plug on 2 to another cylinder. Clear the codes and see if the code moves.

Make sure you're not losing coolant since the service. Some cars have a very thin water jacket on the cylinder head so can crack and leak with improper torquing

If the compression test and leak test came out good, if love to know their reasoning for the strip.

Exactly my reasons for not going ahead with this. They did show me an old piston from another car where the shoulder of the piston had came away and was still in the bore causing a misfire but passed all tests.

They have swapped everything around and still the misfire stays on cylinder 2.

Also removed the inlet manifold and checked inlet ports for carbon which are pretty clean.

Appreciate any possible solutions.

[thescouselander]

Originally Posted by Elliott19864

Exactly my reasons for not going ahead with this. They did show me an old piston from another car where the shoulder of the piston had came away and was still in the bore causing a misfire but passed all tests.

They have swapped everything around and still the misfire stays on cylinder 2.

Also removed the inlet manifold and checked inlet ports for carbon which are pretty clean.

Appreciate any possible solutions.

Has anyone actually checked the air mass flow meter? It's easily overlooked and could well cause the symptoms described.

[Elliott19864]

Originally Posted by thescouselander

Has anyone actually checked the air mass flow meter? It's easily overlooked and could well cause the symptoms described.

I will try that tomorrow thanks. I would have thought that would throw its own separate code?

Car is back with me now.

[thescouselander]

Originally Posted by Elliott19864

I will try that tomorrow thanks. I would have thought that would throw its own separate code?

Car is back with me now.

No, they dont throw codes unless they're really off. Just thought you should check as its a 5 min test and you'd be kicking yourself if it turns out to be that after doing major engine surgery (I've seen it happen).

[Elliott19864]

Ok I tried it with the MAF disconnected and the idle was still rough for the first 30 seconds.

Have been pottering about in the car all day and haven't had one misfire.

Just rough on idle when cold :

[thescouselander]

Originally Posted by Elliott19864

Ok I tried it with the MAF disconnected and the idle was still rough for the first 30 seconds.

Have been pottering about in the car all day and haven't had one misfire.

Just rough on idle when cold :

When you were pottering about was the MAF still disconnected? If so the lack of misfire could point towards the MAF sensor (although it still seems inconclusive).

I think its worth taking the MAF sensor out and inspecting it. If you can see the hot wire you could carefully spray some contact cleaner (the no residue sort) on it to clean it up - be very careful though as its delecate and expensive it you break it. Also have a good look to check for debris and that the hot wire is still in tact.

[Elliott19864]

Originally Posted by thescouselander

When you were pottering about was the MAF still disconnected? If so the lack of misfire could point towards the MAF sensor (although it still seems inconclusive).

I think its worth taking the MAF sensor out and inspecting it. If you can see the hot wire you could carefully spray some contact cleaner (the no residue sort) on it to clean it up - be very careful though as its delecate and expensive it you break it. Also have a good look to check for debris and that the hot wire is still in tact.

Initial drive in the morning.

MAF disconnected. Rough idle, no misfire when driving.

Drive in the afternoon. MAF connected. Rough idle when cold but ok when warm (after maybe 10 seconds) no misfires when driving.