Got me some mis-fires! - Mercedes-Benz Forum
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post #1 of 33 (permalink) Old 08-11-2019, 01:08 PM Thread Starter
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Got me some mis-fires!

Okay everybody...have had cylinder miss P0303 on for a while pop up usually with spirited driving which always resets by cycling ignition.

However yesterday, I was set on cruise around 79 mph on the highway going up a steep hill section into Franconia Notch. So she downshifted as normal and WHAM..flashing light and no power!
Pulled off and read the codes and came back with P0300, 303, 304, 305, 306, 307.
Cleared those and all was good.

I get home and check fuel pressure as it is an older unknown age pump and its 50lb KOEOff and 55lb running. Filter and hoses recall was done with Mahle 6 months or so ago.

Was thinking possible cats plugged, however cyl. 4,6 are pass side and 3,5,7 are driver side...so doesnt seem likely.

I was also thinking vacuum leaks, but visually all looks good and I dont hear any hissing.

I do hear a pop,pop,pop sound from the airbox/ passenger side especially when cold. It seems to match the lope in idle.

MAF...? Dont know..no codes for it

No O2 codes either.

Plugs done 10k miles ago, wires ohmed within spec.

Thanks in advance!

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post #2 of 33 (permalink) Old 08-11-2019, 01:21 PM
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Have you tested the back pressure at the upstream o2 sensor port?
What about the wires and ign coils?
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post #3 of 33 (permalink) Old 08-11-2019, 02:26 PM Thread Starter
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Originally Posted by khomer2 View Post
Have you tested the back pressure at the upstream o2 sensor port?
What about the wires and ign coils?
I haven't tested the pressure. Is that something Advamce or AutoZone etc have for rent?

The wires all ohmed ok when I replaced the plugs. Not sure how to check the coils.

Could so many be faulty at the same time though?

Egr was also replaced with new tube.



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post #4 of 33 (permalink) Old 08-11-2019, 08:16 PM
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Carl:

Do you have all 4 cats on that ML ?

Rear clogged cats are a huge possibility on it, esp given how it happened high exhaus flow & temps on a hill !

I pulled my ML500 rear cats last year, & our e320 rear cats last month.

Massive increase in power, performance & engine smoothness.

The backpressure test is good if you can find correct gauge AND get a perhaps stubborn o2 sensor loose.

I found it simpler to use a non-contact Laser Thermometer to discover that mine were clogged.

I just did a large thread on all the details & also shared them on a couple others.

If you check my profile you can find them.

I even did YouTubes on them.

Cheers- David in Texas

Last edited by 2000 E 320 White Wagon; 08-11-2019 at 08:18 PM.
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post #5 of 33 (permalink) Old 08-12-2019, 09:40 AM
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Quote:
Originally Posted by khomer2 View Post
Have you tested the back pressure at the upstream o2 sensor port?
What about the wires and ign coils?
I haven't tested the pressure. Is that something Advamce or AutoZone etc have for rent?

The wires all ohmed ok when I replaced the plugs. Not sure how to check the coils.

Could so many be faulty at the same time though?

Egr was also replaced with new tube.

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Carl:

Here in small town East Texas I couldn't find backpressure gauge for rent. Reviews for cheaper DIY BP gauges were mixed & didnt feel like throwing $100+ on Pro Model.

Which is why the Harbor Freight Basic $25 IR Thermometer was chosen, and it showed my drivers side rear cat as very plugged & pass side as partly plugged.

The huge clue in your described condition is all of that happening at once.

Really could only be the cats.

You need to keep the front cats for engine management / smog reduction but the rears are optional.

As our MBs age and conditions deteriorate the rears gradually plug up but so slowly we hardly notice.

Your hammering it 80mph on the grade triggered that, is my guess.

There are a ton of YouTubes showing how to test Cats using IR Thermometer.

On my prior extensive posts I cover exactly how I used this test method & removed rear cats & benefits.

See here: https://www.benzworld.org/forums/w16...muffler-3.html

YouTube is linked in post # 6

This happens to the rears as they have no o2 sensors & thus no way to alert us as to their clogging up.

Check your state, (New Hampshire ?) to see if your state inspectors care if rear cats are gone, so long as Front Cats remain & are working properly.

IF your state requires rears to be in place, then replacing them with decent universal cats should clear up your issues.

My anecdotal research shows many states don't care if rear cats are gone.

Texas is that way.

California, not so much !

Cheers - David in East Texas

Last edited by 2000 E 320 White Wagon; 08-12-2019 at 10:08 AM.
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post #6 of 33 (permalink) Old 08-12-2019, 10:16 AM
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To be 100% accurate, I don't show or discuss using the IR thermometer method on the ML Cat delete / muffler upgrade.

[The exhuast shop said the rears were clogged I took their word; they were right.]

I did the IR Thermometer method on my wife's e320 Wagon, to discover (suprise, suprise) that the rear cats were plugging - on her m112 engine.

I had noted a decrease in mpg & didn't sound quite right but testing confirmed it.

The solution was simple: "Rear Cat Delete" - straight pipes welded in their place.

So, I cover the e320 using IR Thermometer & cat delete plus exhaust system overview here:
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post #7 of 33 (permalink) Old 08-12-2019, 01:27 PM Thread Starter
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Hey Dave!
Thanks for the extensive and informative answer!
I had read your previous and lengthy post of your muffler and cats and all that went into it with pics, videos. I will reference back to that as I do have an IR gauge already and NH does not care about rear cats.

Thanks again and I'll update once I do the IR test!

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post #8 of 33 (permalink) Old 08-12-2019, 02:09 PM
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Originally Posted by Carl Burr View Post
Hey Dave!
Thanks for the extensive and informative answer!
I had read your previous and lengthy post of your muffler and cats and all that went into it with pics, videos. I will reference back to that as I do have an IR gauge already and NH does not care about rear cats.

Thanks again and I'll update once I do the IR test!

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Roger that Sir !

As I recall, the proud motto of New Hampshire is "Live Free or Die" - how fitting that y'all reserve the right to kill your Secondary Cats when needed.

Sorry, couldn't resist - love your part of the world.

So much incredible History in New England !

Best Regards - David

Last edited by 2000 E 320 White Wagon; 08-12-2019 at 02:31 PM.
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post #9 of 33 (permalink) Old 08-12-2019, 04:48 PM
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Originally Posted by Carl Burr View Post
Was thinking possible cats plugged, however cyl. 4,6 are pass side and 3,5,7 are driver side...so doesnt seem likely.
On your LHD ML500 engine, cylinders 3 & 4 are passenger side, 5, 6 & 7 are driverís side.

Apart from old spark plugs and clogged cats, I have seen multiple cylinder misfires caused by bad engine mount(s).

Last edited by DrX; 08-12-2019 at 04:56 PM.
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post #10 of 33 (permalink) Old 08-12-2019, 05:29 PM Thread Starter
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On your LHD ML500 engine, cylinders 3 & 4 are passenger side, 5, 6 & 7 are driverís side.



Apart from old spark plugs and clogged cats, I have seen multiple cylinder misfires caused by bad engine mount(s).
Hello good DrX,

This is the diagram I was going from, is it wrong?

Motor and tranny mounts done a year ago, wasnt as bad as I thought it would be. Idle smoothed out a bit but is still feels lopey like its got a mild cam in it.

I am going to check the rear cats with the IR thermometer and see it they are clogged. As the plugs are recent and the wires all ohmed with in spec.

Thanks!

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