Misfires clear with restart? P0302 - Mercedes-Benz Forum
 
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#1 (permalink) Old 12-24-2011, 03:51 PM
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Misfires clear with restart? P0302

OK, I get misfires and they clear as soon as I restart the car.
I pull over, ignition off, restart, runs fine.

>Only get them on the freeeway, especially on downside of hill
>Usually just P0302, and that can be felt in the idle when I pull over.
>One occasion P0300,P0302,P0304,P0306 and the car barely ran..

In the past I got the CEL once on a 40 mile trip, if at all. Today 6 times, so the gig is up.

I have changed the fuel filter.
I think I have cat rattle.

Given the severity that clears with just an ignition reset, it seems like something that puts the engine computer 'off'
-MAF?
-Crank sensor?
- ???

I can't see how it can be coils/wires/plugs

Runs just perfect inbetween faults.

Thanks

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#2 (permalink) Old 12-24-2011, 04:27 PM
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It could be lots of things.

The misfires are in all probability happening all the time. That's why the cel is on.

When they are bad enough the ecu shuts down the cylinders to protect cats, etc. That resets when you restart the car.

My first guess, if you want one, is that your maf is failing. That often will throw myriad and random misfires.

But it could be many other things. If you have been deferring maintenance, you might need plugs and O2 sensors, for example. Or since they are all on the same bank, perhaps a vacuum leak, perhaps a clogged cat.

Some live data would be useful, but that is more diagnostics. That said, I'd probably start by checking for vacuum leaks and be sure I had a good air filter in place, plus taking care of any deferred maintenance.

Good luck.

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#3 (permalink) Old 12-27-2011, 12:45 PM
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Originally Posted by Check Codes View Post
It could be lots of things.

The misfires are in all probability happening all the time. That's why the cel is on.

When they are bad enough the ecu shuts down the cylinders to protect cats, etc. That resets when you restart the car.

My first guess, if you want one, is that your maf is failing. That often will throw myriad and random misfires.

But it could be many other things. If you have been deferring maintenance, you might need plugs and O2 sensors, for example. Or since they are all on the same bank, perhaps a vacuum leak, perhaps a clogged cat.

Some live data would be useful, but that is more diagnostics. That said, I'd probably start by checking for vacuum leaks and be sure I had a good air filter in place, plus taking care of any deferred maintenance.

Good luck.

Thanks Check Codes
>OK makes sense that the cylinder is being shut down, then clears.

>Reading elsewhere I now understand your point about CEL. I thought it came on automatically with codes, but others have lots of codes and no CEL.

>However, if I check for codes, I don't find any, until the CEL comes on and I am down a cylinder

>CEL flashed for the first time last time out, so I guess that is something extra bad

>So, I think I am getting some event the both triggers the P0302 (or more) and shuts down cylinder(s)

-Bad wire short on #2?
-I'm 120k and don't know the plugs were changed.
I overpaid on the car from a widow based on all the care they supposedly took. Body and interior nice, but the breathers and front were covered with oil. Bad sign when superb care doesn't include the oil blank off.

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Last edited by Benzer aus Mehlem; 12-27-2011 at 05:25 PM.
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#4 (permalink) Old 12-31-2011, 12:35 AM
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I found this post, and this IS my problem

http://www.benzworld.org/forums/w210...m-misfire.html

But I read to the end and there is no concluding post !!

Notice he wrote: "However, yesterday morning I was about 20-miles into a 150 mile trip when the CEL again illuminated and the same old symptoms had returned. I restarted, clearing the rough engine, and turned around for home not wanting to risk further damage. While the engine ran fine on the way home, I did have the CEL flash about 10-times while coasting down a hill with no throttle input,"

This CEL on the long trips when coasting, increased vacuum, has got to be a clue.
Cars like to backfire under this condition, and that has me thinking cats.
I am changing plugs this weekend, and since I live in the socialist republic of California, am looking into 50 state cats...

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#5 (permalink) Old 12-31-2011, 07:48 PM
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I had misfires on all cylinders on one bank. Thread here. It was a clogged cat. Had an aftermarket cat welded in, codes and misfires went away.

Regards,

doug

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#6 (permalink) Old 01-12-2013, 10:54 PM
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The PLUGS!

Late update, but the car had 120M and was still on original plugs. Put in the NGKs and no problem since, more than 6 months.

It had gotten so bad I parked the car for 6 months, again, only ever happened on the freeway.

Also, driving with the scanner I could see that there were no codes until the moment the misfire came on the respective cylinder shut down on the spot

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#7 (permalink) Old 01-13-2013, 07:04 AM
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Bad coil? You could swap coils and see if the code moves to a different cylinder.

Regards,

doug

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