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Marty
Marty, Mercedes Technician
Category: Mercedes
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Experience:  Bosch Certified Master Tech. Over 35 years experience.
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1999 ML320: Check engine light for fuel trim. checked maf..MAP

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1999 ML320 - Runs very poor with no power. Check engine light for fuel trim. Checked MAF and MAP function and both seem to be reading correctly. Engine is running very lean, if I spray carb cleaner into the intake it will run much better.

eurotec :

Hi, I'd smoke test the intake system for vacuum leaks and perform a fuel pressure and volume test and take a fuel sample. You might have a plugged fuel filter. Plugged catalytic converters are another possibility. If all checks good I'd try a new MAF.

Customer:

Sorry I forgot to mention that we did check fuel pressure and back pressure on the cats both checked out good. We also unplugged the MAF to see if it would run in MAP mode and it made no difference. I then checked the MAF reading using a scan tool and the MAF readings were changing with RPM and throttle position. I don't know if they were correct but it was responding to different inputs.

eurotec :

Ok, make sure you have good fuel volume also and that the fuel is good and has no water in it. You need to smoke test the intake system for leaks next. If all is good I'd still try a new MAF.

Customer:

We will check teh fuel for water but right now it is 5fdegrees F outside and the fuel is still running do I don't think that is it. The fuel pressure stays constant at all throttle and RPM, if volume was a problem we sould see a drop in fuel pressure. But we will check it anyhow. Also I don't think it is an intake leak as I would expect it to run decent at WOT if it was a leak but again we will smoke it. It will be a while to complete testing.

Customer:

I will get back to you soon.

Customer:

Thanks - Mike

eurotec :

Ok Mike, it might just be easier to substitute a new MAF. I'll be around if you need anything.

Customer:

Ordered MAF and will try this late today. I'll let you know how it works.

eurotec :

Ok

Customer:

MAF it is!!!

Customer:

Thanks!

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