Valafaar

Hey guys I’ve had this car for roughly a year and it’s been like this , even after I changed the transmission oil and filter, basically almost any time I let go of the gas the revs would drop by about 100-200 rpm . Sometimes when in 2 or 3 gear and going downhill if I let go of the gas pedal when in low rpm’s they would drop to idle and jump back up to normal and a jolt will be felt. And if I hard accelerate and I let off the gas and the car shifts the revs would fall slightly bellow normal and come back up .This mostly happens in city driving. So in effect I’m in 4-th gear and if I pulse the throttle basically step on and off continuously the revs would go up and down. There is no shake, judder, vibration whatsoever. Is this normal?

TimC300

I would scan the car with a Mercedes specific obd scanner. See if any faults are present.

Valafaar

No faults come up when scanning the car with Xentry.

TimC300

I'd just be guessing at this point on what the issue could be.

I'm trying to think how my car acts when i let off the gas. I dont think the rpms fluctuate, its more of a slow engine breaking.

When was the last time you did a service or tune-up to the car, air filter, spark plugs, check for vacuum leaks, clean maf?

Valafaar

Thank you for the reply , I highly appreciate it because nobody has any clues what it could be and I’m tilting towards the gearbox being faulty at this point.

so to respond to your questions , it’s a diesel, filters were changed 4000 km ago as was the oil, the MAF was not cleaned. Also is like to add that it only does this when fully warmed up and after I accelerate med/hard . I’ll definitely have my MAF sensor looked at and reply. I’m also going to do a gearbox flush and change the oil + additive , but was advised that if the oil was never changed this could cause the gearbox to not work at all

Valafaar

Bump.

I’ll be changing the transmission oil and am wondering if I should do a flush as well and if I should let the mechanic use a pump to pump out the old fluid . Ill as the mechanic to also check the throttle body , and check for any leaks . The widest thing is that this only happens when fully warmed up and when I accelerate hard once.

bv9900

There's no way you could replicate it in neutral?Do maintenance on the gearbox, if it dies, fix it, if you don't, it will just come back soon.If you do maintenance and everything is fine, in 500km do a reset on the gearbox

Valafaar

Update !!

changed gearbox oil , old one was flushed, new filter , etc , no re-adaptation performed . Gearbox works the same as before if not a slight bit stranger , hopefully this goes away after I drive it around a little bit.

worst thing was was that the old oil was orange (the old type of oil) and it had been with this oil for 15k km + .

issue remains , when cold works normally , when letting go of gas the revs initially drop by about 50-100 rpm and continue falling down.

when fully warmed up revs drop down and come back up after letting go of gas pedal.

what else could it be ??

TimC300

I know basically nothing about diesel engines, which is why I would never get one, they look hard to work on.

When I search around for the symptoms you describe im seeing alot of different ideas. EGR related, throttle valve, Y94 Quantity Sensor whatever that is. i stumbled upon a site where they were scanning theres with Xentry and showing all types of readings and graphs for exhaust back pressure, injection quantity, boost pressure etc. All over my head not understanding since I dont know what "normal" readings would be.