Random misfire/ Fuel trim - Mercedes-Benz Forum
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post #1 of 62 (permalink) Old 06-25-2019, 03:42 PM Thread Starter
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Random misfire/ Fuel trim

I am consistently getting the following fault codes under acceleration in my 98 ML320: P0170, P0173, P0301,P0304, P0302, P0300. I have fairly recently (January of 2019) replaced the spark plugs, coil packs, both the fuel and air filter, and the fuel pump. What else could cause these fault codes? I have very little power above 4000 RPM and virtually none above 5000 RPM. Around 62 MPH with my foot to the floor, but not on the kickdown, it misfires and the check engine light starts to flash.
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post #2 of 62 (permalink) Old 06-25-2019, 04:29 PM
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Disconnected MAF to see if it will run better.

Also check all your vacuum lines for leaks.
Check your cats too.
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post #3 of 62 (permalink) Old 06-25-2019, 04:36 PM Thread Starter
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Originally Posted by khomer2 View Post
Disconnected MAF to see if it will run better.

Also check all your vacuum lines for leaks.
Check your cats too.
How would I check the vacuum lines and the cats?
Next time I take the car I will disconnect the MAF sensor and see if it still acts up
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post #4 of 62 (permalink) Old 06-25-2019, 06:28 PM
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How would I check the vacuum lines and the cats?
Next time I take the car I will disconnect the MAF sensor and see if it still acts up
With the vacuum lines, very carefully inspecting them visually for breakage/cracks OR do a smoke test.

For cats, use a mallet for a quick rattle test. Use the search feature to look for further testing of a bad cat(s).
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post #5 of 62 (permalink) Old 06-25-2019, 06:59 PM Thread Starter
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Quote:
Originally Posted by leeadrian75 View Post
How would I check the vacuum lines and the cats?
Next time I take the car I will disconnect the MAF sensor and see if it still acts up
With the vacuum lines, very carefully inspecting them visually for breakage/cracks OR do a smoke test.

For cats, use a mallet for a quick rattle test. Use the search feature to look for further testing of a bad cat(s).
Upon inspection, I did see that the hose clamp after the MAF sensor wasn't tightened fully, I tightened it and unplugged the MAF sensor. I haven't had a chance to road test because it's about to storm near my house, I will try tomorrow.
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post #6 of 62 (permalink) Old 06-26-2019, 01:21 PM Thread Starter
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Originally Posted by khomer2 View Post
Disconnected MAF to see if it will run better.

Also check all your vacuum lines for leaks.
Check your cats too.
After driving without the MAF sensor connected I did get more power once I was at speed, around 50 MPH. But before then I still have the same issue where I have no power above 4000. It still misfired though and I got the following codes the first time I had my foot down around 60 MPH after that it was fine: P0110, P0100, P0304, P0305, P0301, P0300.
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post #7 of 62 (permalink) Old 06-26-2019, 01:32 PM
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Looks like clogged cats.
The clogged cat is giving codes: P0300, P0304, P0305
Causes increased exhaust back-pressure. You can test this back-pressure by using a gauge at the upstream O2 sensor port.

The p0100 p0110 is your MAF disconnected.

Plug it back in, erase the codes.

And re-test drive.
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post #8 of 62 (permalink) Old 06-26-2019, 01:58 PM Thread Starter
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Originally Posted by khomer2 View Post
Looks like clogged cats.
The clogged cat is giving codes: P0300, P0304, P0305
Causes increased exhaust back-pressure. You can test this back-pressure by using a gauge at the upstream O2 sensor port.

The p0100 p0110 is your MAF disconnected.

Plug it back in, erase the codes.

And re-test drive.
I will plug it back in, erase the codes, and retest. Is there any actual way to unclog the cats? If it's any help the car sat without being started for 6 to 7 years.
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post #9 of 62 (permalink) Old 06-26-2019, 02:58 PM Thread Starter
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I will plug it back in, erase the codes, and retest. Is there any actual way to unclog the cats? If it's any help the car sat without being started for 6 to 7 years.
I took the car out for a test drive, still gave me fault codes, this time: P0304, P0302, P0301, P0300.
This time I took a video of it: https://drive.google.com/open?id=1--...D5QgGbHlIRvSny

The only reason the truck shifted is because I took my foot off the gas when it started losing power, however after 60 it wouldn't upshift so you can really see what it's doing.
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post #10 of 62 (permalink) Old 06-26-2019, 03:58 PM
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Originally Posted by leeadrian75 View Post
I took the car out for a test drive, still gave me fault codes, this time: P0304, P0302, P0301, P0300.
This time I took a video of it: https://drive.google.com/open?id=1--...D5QgGbHlIRvSny

The only reason the truck shifted is because I took my foot off the gas when it started losing power, however after 60 it wouldn't upshift so you can really see what it's doing.
Those codes should have associated freeze frame data. Retrieve it and post.

1972 280SEL 4.5
1999 ML320
2000 E320
2003 ML350
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