W639 low power, no obvious problem

Page may contain affiliate links. Please see terms for details.


New Member
Mar 19, 2017
vito 111
Detailed long winded post ahead, sorry! I'm usually good at solving my own problems but I'm getting to the end of my chain trying to figure out what's wrong with my vito. It's '07 w639 111 with OM646 engine.

It wasn't a gradual failure, just suddenly decided to stop making boost, max speed now about 60mph and it struggles to do 30mph up hill. In 7 years of ownership I've had similar happen twice, once due to EGR failure and once the turbo actuator. Both caused check engine light and relevant fault codes so were easily sorted. Other than the severe loss of power it drives normally.

In this case the check engine light didn't come on. However it did produce a code for MAF sensor not functioning (more on that soon). A new MAF sensor didn't solve the problem.

I have an icarsoft scanner with MB software. I checked values of all sensor readings I could interrogate:
Ambient and charge air temperatures consistent with what I'd expect.
Fuel, coolant and oil temperatures as expected.
Fuel pressure rises a lot with rpm as expected.
O2 sensor voltage does change with load but I don't know what values to expect.
A few others I can't remember off top of my head but all seemed normal apart from:

Two didn't look correct though in the MB mode on the scanner:
-MAF value doesn't change other than slight fluctuation however the units are mg/stroke (around 425mg/str) and since airflow increases with RPM, I would expect a fairly constant number regardless of load, however not entirely constant. It remains constant even when unplugged which straight away looks dodgy.
-Charge air pressure measures at 800hpa all the time whether plugged in or not (I assume they mean the sensor with 3 pin plug on lower intercooler pipe).
However if I leave the MB mode and use generic OBD mode both the MAF and charge air pressure sensors do work and give plausible readings:
-MAF about 16g/s at idle. 70g/s at full speed. 600g/s when I blow the air compressor over the sensor. This DID stop reading when unplugged as it should.
-Charge air pressure sensor tested between 100kpa and 250kpa using syringe and hose to pressurise it. Shows only about 120kpa in the van when driven at full speed.
So the MAF and charge pressure sensors work correctly as far as the OBD system works. Whether the engine module is seeing them correctly I'm not sure of.

I took van to a dealer to scan with Star tool. They said the ECU doesn't match and is from a 109 vito with a different VIN number. Star system would not proceed beyond that. They are probably correct and likely a previous owner has swapped or cloned the ECU perhaps after an immobiliser failure. However it's been no issue if the 6 years I've owned it so that's not a new problem. They were still happy to take £125 off me for plugging it in.

I suspected a boost leak and pressure tested the pipes and intercooler. I did find two small holes in the intercooler but they were not big enough to cause this level of power loss and I've plugged them for now until I replace intercooler. Subsequent test shows no leaks @ 2 bar.

Turbo actuator moves through full range as commanded with the scanner tool. However when actually driving it stays at the no boost position except under deceleration. I disconnected the actuator from the arm and wired the arm in the full boost position. Now it makes boost (as shown on the charge pressure readout on OBD, 205kpa and a little bit more turbo noise), however it still doesn't make power - I guess it's only injecting the same amount of fuel in as it was before. At least this shows turbo is good.

So this makes me think its all ok mechanically but actively doesn't want to make power due to whatever condition it thinks its experiencing. it would be nice to think the pedal position sensor is telling it I'm only giving partial throttle. Strangely my scanner give me three different pedal position readings in 3 different modes:
On the cruise control test mode it shows 0-100% throttle in a nice linear way like it should
In the engine control module mode goes to 84% when fully depressed.
In generic OBD mode it only goes up to 64% fully depressed.
Saying that if I get up to speed on a downhill, put the cruise control on and leave it on for the next uphill I still run out of steam so I think problem is independent of the pedal sensor.

At this point I'm running out of ideas, I swapped the EGR with a known good one I had lying around. New air and fuel filters of course. I'm not sure how to test the O2 sensor but it's probably worth swapping that to eliminate possibility (although I would expect a code if it were faulty). I deleted DPF years ago so no issues there.

Other than that all I can think of is the ECU itself, however unless it's a common failure point on these vehicles I'd be extremely surprised to find that was the fault.

I bet the actual solution ends up being simple and obvious - does anyone have any ideas?
Now solved. The MAF needed recalibration - despite there being nothing wrong with it. It sensed a calibration fault and went into limp mode. This particular fault can only be cleared with proprietary STAR software but was a 5 minute job once I found someone to do it.
Now solved. The MAF needed recalibration - despite there being nothing wrong with it. It sensed a calibration fault and went into limp mode. This particular fault can only be cleared with proprietary STAR software but was a 5 minute job once I found someone to do it.
Pleased you've resolved it Luke, and thanks for posting up the outcome so that others can benefit.
Good shout

Users who are viewing this thread

Top Bottom