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MAF Sensor Fault Diagnosis

E55BOF

Hardcore MB Enthusiast
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CLS63 SB, ML63, CLK350 'Vert, E350 CGI 'Vert, Triumph Sprint (Bike not Dolly...),
Surfing the net regarding MAF sensor cleaning or replacement to get rid of my CEL, I gather there are numerous possible causes of my P101 generic fault code, as well as the MAF sensor itself. The engine runs well, except for a lumpy idle, and slightly odd throttle response at lower revs.

I also came across a suggestion that if you simply unplug the MAF sensor, leaving just the map and the throttle to control the revs, if the lumpy idle is then cured, the fault definitely lies with the MAF sensor itself. In general terms this might seem plausible, but would it work on the n/a M113 in my ML500, or would it simply not run at all?

I also found info (still looking...) that if you unplug the MAF sensor and the engine stops, this means the MAF is definitely faulty!

They can't both be correct. Anybody know which is?
 
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The engine will still run; whether it clears the lumpy idle is another matter. Accidentally leaving the MAF disconnected on my CLK500 (same engine) resulted in a check engine light and slightly reduced performance, but no noticeable change to the idle (but then it was smooth in the first place).
Not having the engine cover firmly in place gives a lumpy idle, once the engine is hot.
Ian.
 
Thank you for that; it's the confirmation I wanted.

The lumpy idle is there all the time, but it's not like a misfire; there's no discernible misfire in the exhaust, and very little vibration in the engine itself, but I can feel it in the driver's seat. If it weren't for the CEL and the occasional slightly odd throttle response, I'd be inclined to think it might be an engine mounting.
 
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I can't see how a loose engine cover could give a lumpy idle on the W163, because it is entirely separate from the induction system; the air cleaner is remotely mounted on the inner wing, not on top of the engine. The iCarsoft in MB mode finds no fault, which surprises me, but there it is; in OBD mode it does.

The car runs perfectly at higher throttle openings- plentiful power and torque, and it revs out smoothly and happily at full throttle - and if it were not for the slightly lumpy idle and the slightly odd throttle response (it's almost as though the MAF was taking too long to respond at small throttle openings; perhaps that's a clue...) I'd not worry about it, except that I don't want to drive with the CEL on, lest something more serious goes wrong. If it is duff, it's perhaps just starting to play up, but if so, it won't fix itself. I'll try unplugging it and see what happens, and that should show if it's doing anything. If not, I'll try cleaning it first.

New Bosch units seem to be about £105 and up, so it's cheap enough to replace. Reading an eBay ad for the sensor assembly only, without the housing, although it states it's UK-made I suspect it's no such thing, and I've read plenty about Chinese pattern parts, nearly all of it negative, so a genuine new complete unit will be the way to go.
 
Unplugging it won't kill the engine. It will revert to default parameters, so this can give a clue.

MAFs use 0 to 5v dc, hence they can be 'pinned out' with a DVM over the normal rev range.
 
Surfing the net regarding MAF sensor cleaning or replacement to get rid of my CEL, I gather there are numerous possible causes of my P101 generic fault code, as well as the MAF sensor itself. The engine runs well, except for a lumpy idle, and slightly odd throttle response at lower revs.

I also came across a suggestion that if you simply unplug the MAF sensor, leaving just the map and the throttle to control the revs, if the lumpy idle is then cured, the fault definitely lies with the MAF sensor itself. In general terms this might seem plausible, but would it work on the n/a M113 in my ML500, or would it simply not run at all?

I also found info (still looking...) that if you unplug the MAF sensor and the engine stops, this means the MAF is definitely faulty!

They can't both be correct. Anybody know which is?

have you explored the possibility of a vacuum leak? Not sure what engine you have but it’s a common cause of dodgy idling on m271 engine and throws all sorts of fault codes.
 
Surfing the net regarding MAF sensor cleaning or replacement to get rid of my CEL, I gather there are numerous possible causes of my P101 generic fault code, as well as the MAF sensor itself. The engine runs well, except for a lumpy idle, and slightly odd throttle response at lower revs.

I also came across a suggestion that if you simply unplug the MAF sensor, leaving just the map and the throttle to control the revs, if the lumpy idle is then cured, the fault definitely lies with the MAF sensor itself. In general terms this might seem plausible, but would it work on the n/a M113 in my ML500, or would it simply not run at all?

I also found info (still looking...) that if you unplug the MAF sensor and the engine stops, this means the MAF is definitely faulty!

They can't both be correct. Anybody know which is?

have you explored the possibility of a vacuum leak? Not sure with the engine you have but it’s a common cause of dodgy idling on m271 engine and throws all sorts of fault codes.
 
I have the M113. I don't think it's a vacuum leak, because the engine doesn't hunt at all; the idle is steady, with just the slight irregular vibration and no discernible misfire. There is no problem starting, hot or cold. If it weren't for the CEL, the problem is so minor I'd not worry about it, but with the P101 fault code, I'll have to sort it to get rid of the CEL.

I think I've worked out why the iCarsoft in MB mode didn't pick it up; it just checks MB-specific equipment in that mode. Note to self: run a basic OBD check before using MB mode. I had a feeling it would be operator error...

I've gone ahead and ordered a new MAF; they're cheap enough, and with 123K miles on the car, it deserves a little love and care...
 
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"I can't see how a loose engine cover could give a lumpy idle on the W163, because it is entirely separate from the induction system; the air cleaner is remotely mounted on the inner wing, not on top of the engine."
Got it - in the rather cramped CLK bay, the loose cover permits hot air to enter the induction system, something to which the M113 seems sensitive.

Best to buy the complete Bosch unit:
(1) less chance of getting a fake
(2) the other part is little more than a short drainpipe with an arrow on it
(3) the sensor is held in place with a pair of 5 point security screws. Awkward if you don't have the tool.

I've come to the conclusion that these plastic film MAFs are a service item with a lifetime of around 90K miles. I'm on my third. The metal dual filament in my Audi 90 was still working fine at 250K.

Ian.
 
I have a set of security driver bits, fortunately, which includes that type. I took the sensor unit out of the housing to clean it (a good blast of contact cleaner), and now I have TWO fault codes - the generic P101, and a specific intake air sensor code. Doh! I'm hoping that the spray of cleaner has just pushed it over the edge, where it was already teetering...

Still weird about the fans coming full on, though; I can't imagine how that would be linked to a MAF sensor fault.

The new unit should be here by Monday, and it is a genuine Bosch. Fingers crossed, that will solve the problem. If not, off to Terry Gates, but I hope not; every time I take a new acquisition there, he finds plenty that needs doing...
 
Fitted the new sensor, and the EML went out on starting up, and there are no fault codes after a twenty-mile drive. It's fixed...:D
 

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