2003 Ml500 backfires, stalls. CKP replaced already. - Mercedes-Benz Forum
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post #1 of 26 (permalink) Old 06-23-2019, 05:03 PM Thread Starter
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2003 Ml500 backfires, stalls. CKP replaced already.

I hate to start another of these threads but I'm running out of ideas.

I've had this car a couple months now. Shaking the bugs out of it for the wife to use. Got the traction control issues dealt with, thought I had the stalling problem fixed but it keeps rearing its head.

This NEVER happens when I'm driving. Car apparently bogs, lights on the dash blink like vegas, stalls and wont start again. Leave it for a few minutes and it will start again and drive like new.

One time when this happened it backfired so loud it sounded like a gunshot in the cab. The wife is hesitant to drive it now, and I pretty much have the other ml sold... so I had better get this sorted.

I replaced the ckp with a new Bosch a week ago, apparently no change. Spent today cleaning up all the battery connections, changed out the MAF with a spare I had kicking around, opened up the electronics nightmare in the engine bay and wiggled all the wires and engine harness while it was running but not even a stutter.

Fuel pressure is fairly steady a few PSI short of 60, doesn't fluctuate for more than a few milliseconds if the throttle is stomped.

I was told the pump had been changed, but not 100% sure of that fact now that I have pulled it to inspect. The access cover HAS, previous to today, been removed and installed with a non-OEM sealant.

Aside from this fairly extreme fault, the car runs awesome. Especially when I drive it.

There are no codes that show themselves when this fault occurs, however I usually have a pair of catalyst inadequate efficiency codes because the PO gutted the cats.

I also have a persistent P2039-8 oil level sensor code. On occasion the low oil light comes on in the dash, but I am 100% positive the oil level is adequate.

I caused a litany of codes today unplugging and plugging things in, cleared out the codes, waiting for Amanda to drive it to see if it faults again.

I have found threads where the ignition switch is suspected, so I spent some time wiggling it around, beating on the dash and steering column with my hands but not even a hiccup.

Sort of lost for direction at this point... any input is welcome.

Thanks for looking.
Steve.
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post #2 of 26 (permalink) Old 06-23-2019, 05:11 PM
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Stick a volt meter on it. Or connect scanner and monitor voltage while driving.

Check oil level with dipstick.

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post #3 of 26 (permalink) Old 06-23-2019, 05:19 PM Thread Starter
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Not sure if the oil level sensor can somehow cut the motor if it is tripping out hard enough, but I do generally use the dipstick to verify the levels. Currently full of Mobil 1 with about 1000km on the oil change.

Voltmeter for intermittent shorts? The battery stays totally full, and is about a year old. I guess I could have an intermittent dead short somewhere, but I would expect more smoke/fire smell. I will slap a voltmeter in there and see what happens.
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post #4 of 26 (permalink) Old 06-23-2019, 06:02 PM
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Originally Posted by thebruce View Post
Not sure if the oil level sensor can somehow cut the motor if it is tripping out hard enough, but I do generally use the dipstick to verify the levels. Currently full of Mobil 1 with about 1000km on the oil change.

Voltmeter for intermittent shorts? The battery stays totally full, and is about a year old. I guess I could have an intermittent dead short somewhere, but I would expect more smoke/fire smell. I will slap a voltmeter in there and see what happens.
Volt meter for freaked out alternator. Over voltage will cause selective control module shut down.

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post #5 of 26 (permalink) Old 06-23-2019, 06:17 PM Thread Starter
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Understood. Many thanks for the details.
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post #6 of 26 (permalink) Old 06-23-2019, 08:38 PM
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I’m not sure how true it may be since I don’t remember it being addressed here before BUT my exhaust guy I use and trust completely told me these things won’t run right with no back pressure from catalytic converters. He mentioned having no catalytic converters can cause unburnt fuel to combust in the exhaust causing backfiring. Maybe someone else with more knowledge can weigh in on this.

Edit: after writing the above, I decided to do some research and indeed, it appears the absence of the honeycomb in your cats could be causing the backfiring. Based on this info, I would recommend you get a couple of good aftermarket universal cats and replace the primaries before throwing any more dough at it.

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post #7 of 26 (permalink) Old 06-23-2019, 09:37 PM
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They ran very well without any CATS. In fact much healthier. No backfires.

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post #8 of 26 (permalink) Old 06-24-2019, 02:10 AM
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See if you can check the fuel pressure while driving. Revving in neutral is never a good indication of consistent fuel pressure under load.
Could be bad fuel pump and or filter.


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post #9 of 26 (permalink) Old 06-24-2019, 06:41 AM
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They ran very well without any CATS. In fact much healthier. No backfires.
But, did you 'retune' the engine to make it work? I am having another conversation at this moment with another mechanic. He said the same thing. Replace primary cats with aftermarket magnaflow universals, don't worry about the secondarys and it is likely his backfiring issue will go away. He said computer is trying to compensate for the excessive hydrocarbons detected in the exhaust which could be his stalling problem. Again, I know little about this except what I'm being told by experienced exhaust guys. It appears you have done this so I want to pick your brain a bit to see how you pulled it off. As for me, if it were my ML, I'd spend the $160 and replace primary cats.

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post #10 of 26 (permalink) Old 06-24-2019, 06:48 AM Thread Starter
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Witek_M I went out this morning and put my handheld voltmeter on the battery, 14.5V dead steady, idle through whatever the limiter is. I'll try to scare up a gauge and wire it in to monitor while driving.

cmitch, thank you for your response, but I don't think the lack of back pressure is in this case the issue. It is more a symptom than the disease.

I am aware that a lack of back pressure will cause small backfires sometimes. This was like cutting the ignition on a carbureted vehicle then turning it back on after loading up the exhaust system. Kaboom. Only happened the one time, 99.9% of the time it runs flawlessly.

I want to agree with art_arev that it runs even better than with cats. I still have the ml430 to do side by side comparisons and there is NO comparison, provided this glitch isn't presenting itself.

mattml430 I had a fuel pressure gauge on it yesterday but I didn't have enough hose to route the gauge back to where I could see it while driving. I will try to find something to extend it as I completely agree that no-load fuel pressure isn't a good indicator of fuel pump health.
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