what scan tool [odd misfire/lack of power help needed]

2011 M3 VVT

Hey all, getting an odd misfire, occasionally when cold or in the first 15-20 minutes of driving. Feels like it's running rough and has very little power. Comes on very suddenly and can last about 10 seconds when it's all good again. May not happen again for 100 miles. 

SZ viewer isn't showing any fault codes and nor am I throwing a CEL. 

Is it worth buying one of those Delphi clones off eBay to help diagnose the issue. 

Hi,

For a misfire to be registered as a code/CEL you have to have 2 (or more, I don't know the exact number) to occur in a row or over a short time period.

If you just get one misfire or less than the number it doesn't generate a code and therefore doesn't light the CEL.

For misfires, if you get a code logged, you will get a CEL, so no CEL, no code (at least for misfires).

Not all codes set the CEL, but the important ones like misfire do.

I am not sure about SZ viewer, but if you have a dongle download Torque Pro, this shows misfire codes when you get a CEL, at least it does on my other car.

Robin

Forgot to add....

My other car (not a Jimny) started with this back in June, but it did give a CEL when the misfire occurred and then a random cylinder misfire code, sometimes cyl 1, but 2, 4 and 6 were all recorded at some time (sorry it's 6 cylinder).

It only got a misfire on decelration, when the revs were below 1500 rpm (top or 4th gear) and then when you feed the power back in.

I have tired all sorts, O2 sensors (which were worn) throttle body, air filter clean, MAF sensor checked, vacuum, but it has finally turned out to be fine cracks in the coils, a weak point on this model.

The misfire was more evident when the battery was low, once fully charged or keep the revs up a bit it never did it, so I should have suspected coils earlier.

Might be worth you having a look at the plugs and coils, it's quite a quick job on a Jimny.

wrote:

Hi,

For a misfire to be registered as a code/CEL you have to have 2 (or more, I don't know the exact number) to occur in a row or over a short time period.

If you just get one misfire or less than the number it doesn't generate a code and therefore doesn't light the CEL.

For misfires, if you get a code logged, you will get a CEL, so no CEL, no code (at least for misfires).

Not all codes set the CEL, but the important ones like misfire do.

I am not sure about SZ viewer, but if you have a dongle download Torque Pro, this shows misfire codes when you get a CEL, at least it does on my other car.

 

Hiya Robin. 

Is that two misfire events or or a cylinder misfiring twice? Because if it's going on for 10 seconds it's definitely more than two. 

If SZviewer is not showing it then no others will either

Hi,

I am not 100%, but I think it is two in a row from the same cylinder.

The usual way of detecting a misfire is to look at the crankshaft sensor, the ECU expects to see a small kick in the rotational speed each time a cylinder fires, so if it doesn't see this twice in a row it will generate a code and light the CEL.

I suppose if you had a number of misfires on several cylinders the crank's rotational speed might be all over the place and this might fool the 'two in a row'. I would then expect a general misfire code (P0300) to be registered.

This is quite a good article
www.nyvip.org/PublicSite/OBDII/misfire-detection-monitor.html

So no CEL, no P03xx code will be present.

My other car is not misfiring now and no CEL, but until I change the coils and plugs (this weekend) it is still running rough and feels as though it is about to misfire at times.

Robin