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Discussion Starter #1
Working on my neighbor's 2005 E350 again. The complaint was an intermittent "rough running", which I was eventually able to experience myself - took the car for a test drive, ran totally fine initially, only to have it develop a stumble mid-way through the trip with the CEL coming on. Came home and pulled codes - Cyl 1 mis-fire. Rather than throwing parts at it, I swapped the cyl 1 and 2 coil-pack and spark plug (they all looked fine visually). Car ran fine again until the same complaint came back, now with a bunch of codes...
P0300 Random mis-fire
P0303 Cyl 3 Mis-fire
P0174 Left Cyl Too Lean
P0171 Right Cyl Too Lean

So now I've determined it's not a cylinder-specific issue, but rather an overall lean issue. As well as the engine runs when I step into it, I believe it's not a fueling issue. Instead, much more likely a vacuum issue, which this video would seem to confirm...

In fact, I wouldn't be at all surprised if it's pretty much the same issue as depicted in this video, with one or more of the seals for the intake actuators failing. I was shocked to see that the "fix" for that was replacing the whole intake manifold assembly!

But I'm getting ahead of myself, since I've not yet gone searching for the vacuum leak. Since I don't have a smoke machine, and other suggestions on places to check?
 

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Discussion Starter #3
I thought 2005 was the E320 M112 V6?
This is a late 2005 which seems to be a mixed bag overall. It's definitely a E350 which has the intake manifold as depicted in the video above.

I took a quick look at the car yesterday and found the access to the intake actuators was very limited without taking other stuff off. Thoughts on verifying the intake leak without needing to take a bunch of stuff off and no smoke machine?

At this point I have NOT gotten a P2006 code. If I did, I'd just move ahead with replacing the intake manifold.
 

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You would need to have live O2 sensor data at minimum. You could use propane tank with hose to pipe un-lit propane in vicinity of leaks, but with that volume in the 'melon-sized' intake plenum, there is a latency in seeing the effects that makes this method more sketchy. Dan's video shows this latency at 2000+ rpm. At idle it's going to be significantly slower.

I had small intake leak on 2003 M112 that has similar intake and tried the flammable spray trouble shooting method and results/conclusions were sketchy. Tried smoke machine and found the leak instantly. My leak was the intake manif gasket at a back corner which should have had a much lower latency than the leaks shown in Dan's case.

R&I of the intake manifold is not a real easy job so it's best to know as precisely as possible what the problem really is before taking things apart.
 

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99 ML430, 00 ML320, 05 E500 4matic Wagon
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What is the build date on the VIN plate in the driver's door frame. If it is 07/2005 or later it is a 2006.
 

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Discussion Starter #7
R&I of the intake manifold is not a real easy job so it's best to know as precisely as possible what the problem really is before taking things apart.
I see it's rated at 4 hours. This video doesn't make it look too bad...

Although I've been green-lighted to go ahead with the job, as you suggest, I'd like to know exactly what the issue is before proceeding. The flip side is that with the car at 195K miles, it wouldn't be a terrible idea to replace the manifold and gaskets as a preventative maintenance. I presume it's just three gaskets that need to be replaced, yes? The Throttle body and the two runners.
 

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Working on my neighbor's 2005 E350 again. The complaint was an intermittent "rough running", which I was eventually able to experience myself - took the car for a test drive, ran totally fine initially, only to have it develop a stumble mid-way through the trip with the CEL coming on. Came home and pulled codes - Cyl 1 mis-fire. Rather than throwing parts at it, I swapped the cyl 1 and 2 coil-pack and spark plug (they all looked fine visually). Car ran fine again until the same complaint came back, now with a bunch of codes...
P0300 Random mis-fire
P0303 Cyl 3 Mis-fire
P0174 Left Cyl Too Lean
P0171 Right Cyl Too Lean
This has all the signs of (possibly) bad MAF sensor.

What tools do you have to look at ME module actual values… Air Mass @ idle speed, 2000rpm, full throttle under load? Check fuel trim values? Check Intake manifold pressure?

Do you have good known MAF? To swap and see if that cures the problem?
 

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Discussion Starter #9
This has all the signs of (possibly) bad MAF sensor.

What tools do you have to look at ME module actual values… Air Mass @ idle speed, 2000rpm, full throttle under load? Check fuel trim values? Check Intake manifold pressure?

Do you have good known MAF? To swap and see if that cures the problem?
Thanks very much for the input. The one thing I was struggling with was the intermittent nature of the symptom. Wouldn't a vacuum leak likely show it's symptoms consistently? Whereas a failing MAF could more likely be intermittent.

I have a Launch Creader VII+ which does provide some live data feeds. Youtube video linked below shows it in action...

So hooking this up and watching air mass would certainly be a good idea. I may go junkyarding in the next week, so if I could come across a spare MAF that wouldn't be a bad part to have as a swap-out diagnostic step given that it's a short time investment to swap. Looks like Bosch p/n 0 280 218 190 is used across all Mercedes from 2005 - 2015.
 

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With both Misfire and Fuel Trim codes present, MAF sensor is a strong possibility but certainly not the only possibility. Weak fuel pump, bad fuel filter, vacuum leaks are among others.

If you don't want to just throw the money at the problem…

Inspect the air filter. It must be clean and sealed correctly in the housing.

Test the fuel pressure at the injector rail. Engine Running it should be 55-60 PSI. Push the accelerator pedal to the floor briefly several times, reading should not drop below 54 PSI.

Connect MB compatible scanner or Star Diagnostic System (SDS) and read data on the Mass Air Flow (MAF) sensor. At idle, the reading should be approximately 3.8-4.8 G/S = 14-18 Kg/H. Rev the engine to 2000 RPM in P/N, it should go to 14-25 G/S = 30-45 Kg/H. Drive the car at 40 MPH and then floor accelerator completely to Wide Open Throttle (I'd advice to have an assistant for this). The reading should immediately go to 130 G/S = 480-580 Kg/H

Check the Fuel Trim values: idle speed and partial load self-adaptations…

Keep in mind that at idle… Adaptations and Air Mass would be effected be vacuum leaks and underperforming MAF… so smoke testing vacuum leaks would be necessary for complete diagnostics.

You could pay somebody to do smoke test or you can build smoke machine for ~ $20.
 

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Discussion Starter #11 (Edited)
Connect MB compatible scanner or Star Diagnostic System (SDS) and read data on the Mass Air Flow (MAF) sensor. At idle, the reading should be approximately 3.8-4.8 G/S = 14-18 Kg/H. Rev the engine to 2000 RPM in P/N, it should go to 14-25 G/S = 30-45 Kg/H. Drive the car at 40 MPH and then floor accelerator completely to Wide Open Throttle (I'd advice to have an assistant for this). The reading should immediately go to 130 G/S = 480-580 Kg/H
Just back from a ~20 mile test drive with my scanner displaying live data. The MAF output was reasonably consistent with what you have indicated above. Fuel trim was typically +/- 3%, occasionally blipping up to as much as 6% when I was on and off the throttle.

Vacuum reading at idle was reasonably steady at 32k P/A. It increased smoothly as I got on the throttle.

CEL never came on and the engine behaved totally normal the whole time. Fun times chasing intermittent issues.... <sigh>
 
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