Is this repair estimate for misfiring and fuel trim on a '98 Mercedes reasonable?

Question

Just got a '98 Benz ml320 and two days later it cut off on me, twice, within about 5 minutes. I took it for a diagnostic. I'm wondering how overpriced this estimate is and possible ideas of best course of action at this point. To that end, no I don't have a warranty, and I'm not yet sure if return is a possibility (currently in discussion).

The returned codes:

• 170 - Fuel trim malfunction (bank1)
• 173 - Fuel trim malfunction (bank2)
• 460 - Fuel level sensor malfunction
• 300 - rand misfire of cylinders
• 301 - misfire cyl 1
• 303 - misfire cyl 3
• 305 - misfire cyl 5
• 306 - misfire cyl 6

The estimate: [US Dollars $]

1. engine assembly & parts
   • timing chain: 414.99
   • slides: 92.99
   • labor: 544.00
2. engine sensors
   • manifold absolute pressure sensor: 319.99
   • labor: 68.00
3. engine sensors
   • electronic control unit 1600.00
   • labor: 68.00

What I'm being told:

The timing chain is the main issue, but just replacing it isn't enough. if the ECU is bad, then it will continue to misfire and lead to more problems.

What I've researched on my own:

I'm not sure i'm looking up exactly the correct one, as I've yet to find exactly where, on the vehicle, the "ECU" in question is, but the used ones I've found online all cost less than 200. I "think I" know from having worked on car electronics before, it may still need to be synchronized with the vehicle, but I know this labor isn't hard to find on YouTube. I'm mainly short on tools.

I've also checked the codes and forums, as well as here, for information about there errors and so far it looks like it could be as simple as replacing spark plugs, to as complicated as this estimate or something as simple as replacing gaskets (which i still don't have tools for, unfortunately).

So my problem here is, is this my best estimate, or could this be fixed for much cheaper. How much am I being ripped off? And being as I'm short on tools, even if I figure out how to fix the problem myself, can any of you mechanics tell me how I might convince an auto shop to do only the repairs i request and not try and screw me into more costly repairs?

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Update!

5/22/14 [_{2 days since estimate}]

I'll try to include as much detail as possible and keep everything in order.

• I live in FL and yesterday the average temp was about 80°F
• I had worked about 9 hours, left work around 6
• My work is about an 1/8mi from the interstate
• I left work and got on the Interstate, all seemed fine
• About 5mi down the interstate, my speed starte dropping from 75mph to about 65
• I couldn't get more than 65
  • If I pressed the gas peddle more
    1. at first, it felt like it was in the wrong gear, a couple times, the RPMs even went up, but no increase in speed
    2. most of the time, pressing the peddle to the floor resulted in both a loss of speed and a loss of "power" (RPMs dropped out)
    3. keeping the gas about half pushed in kept me at around 65, but I couldn't go faster
    4. I finally pulled over and sat about 30 seconds, the vehicle sputtered like it would cut off and sat around 500RPMs, but would not cut off, so i turned it off
• After about a 30 seconds or so wait, I crunk it back up. It sounded fine.
• Started to go and everything felt fine, till i got up to 70
• @ about 70+mph, I got a complete repeat of the numbered steps listed before
• Again, I pulled over and cut it off. This time I let it sit about 2 minutes, and tried again ... worked fine all the way home (about a 30 minute drive)

This morning, I got up, turned it on, took son to school and came to work, 0 issues. Could heat be impacting the issue somehow? If so, what about my estimate maybe misleading and what should I FIRST look at to fix this problem?

ALSO! I should mention, this vehicle has a "Low Range" option, that I guess is meant for "off-road". The light comes on every time I turn it on and I usually have to push the button a couple times to make it go off. Could it be possible this is being "stuck on" even tho the light is off? The "wrong gear feeling" and "loss of power" is why I ask. Is it possibly related?

Oh and I had refilled the gas tank yesterday morning. I never let any vehicle I drive drop below half a tank unless it's an emergency or a long road trip.

UPDATE

Zaid's Answer below was very helpful! In the end, simply replacing the MAF sensor was all that was needed to kill the initial problem!

Answer 1

I'm a little concerned by the proposed fixes. While I don't know the specifics of your Merc, I find it odd that the timing chain would be responsible for fuel trim codes. The ECU swap seems to be a concerted attempt at what I like to call 'parts roulette': possible, but not probable.

If you have codes for fuel trims and misfires, the principal components that impact the fuel management system should be the first port of call. By the sound of things you have a lean AFR (air-to-fuel ratio). This could be due to:

• underestimation of the air mass flow

  This could be due to an under-reading MAF sensor. These sensors tend to foul over time.

  Remedy: clean or replace the MAF's.

• insufficient fuel delivery

  A lack of fuel flow could be down to a failing fuel pump, clogged fuel filter or clogged fuel injectors. Further inspection and tests could be conducted to help pinpoint the root cause in this case.

  Remedy: Replace if fuel pump or fuel filter. Clogged injectors could be treated with fuel injection cleaner and/or ultrasonic cleaning.

• underestimation of the AFR

  If the oxygen/lambda probes are falsely sensing a rich AFR, the fuel timing will be reduced, resulting in a lean AFR.

  Remedy: Replace the offending oxygen sensor(s).