Omega Owners Forum
Chat Area => General Car Chat => Topic started by: al brown on 20 September 2017, 08:33:01
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Good morning all,
Car is a 2006 r class 3.5 petrol, it was missing badly when cold and when warned up the misfire stopped but it would change revs by itself at idle. My mate was advised to unplug the maf and instantly it ran fine so he fitted a new one. Yesterday it ran fine, but this morning it's missing badly again.
What other difference is there with the maf unplugged other than its using default settings for air and temp.
He's asked if I will go look at it with him tonight but before I go stripping it to component form is like to understand what other faults could be masked by unplugging the maf. I was thinking post maf air leak, but not too sure past that.
It is missing on both banks but only when cold, no misfire when warm. No misfire with the maf unplugged.
Cheers for any pointers,
Al
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Does it have a separate air intake temperature sensor?
Unplugging the maf might switch this off as a side effect :-\
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Was the new Maff a genuine part or after market ?
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Iat sensor is part of the maf.
Replacement one is made by Bosch and came from the local autofactors, oem was also Bosch
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Had a quick look at it tonight a bit of live data but didn't have much battery so didn't look for long.
Anyway, from cold it misfires randomly across both banks, once it starts to warm up the misfire goes away but it will sometimes hold the revs high after you press the throttle. Codes are for the engine running lean on bank 1 and a pending one for the same on back 2, didn't misfire enough to trip a misfire code. If you are gentle on the throttle and open it an extra degree from idle it will stay open and the revs stay up around 1100 but as soon as you unplug the maf the idle drops back down to normal.
It's not a sticky pedal but I guess it could be a sticky throttle but to me it seems the fuel mixture is out and causing an odd idle/misfire as if the cold start side of things isn't happy.
I can see why he thought it was the maf though as the faults go away as soon as you unplug it.
Going to spend some more time with him on the weekend, but I'm a little stumped as to why a modern petrol engine can rev by itself.
Any ideas are welcome.
Al
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I'm still suspicious of MAF. Are readings from MAF viable?
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My old school rules of thumb of g/s at idle equaling capacity is slightly out, 2.8 g/s is what we got and it's 3.5l engine, does kinda fit as that would cause the ecu to trim fuel, causing the lambda to show lean and try and richen the mixture and eventually trip the lean code. I am thinking that from cold the lambda isn't used which is why it misfires from cold as it's too lean.
Thing is, both mafs give the same reading at idle although the new one reads slightly higher under load and is nearer what I'd expect for the power of the engine.
Could a post maf air leak cause this ?
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My old school rules of thumb of g/s at idle equaling capacity is slightly out, 2.8 g/s is what we got and it's 3.5l engine, does kinda fit as that would cause the ecu to trim fuel, causing the lambda to show lean and try and richen the mixture and eventually trip the lean code. I am thinking that from cold the lambda isn't used which is why it misfires from cold as it's too lean.
Thing is, both mafs give the same reading at idle although the new one reads slightly higher under load and is nearer what I'd expect for the power of the engine.
Could a post maf air leak cause this ?
Try to ascertain what it should be at idle ;)
Little bro chased a lean fault for ages, turned out to be loom to maf had a high resistance.
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Cheers TB, I have been looking for set of values for the maf for that engine but I can't find any Merc ones. I always worked off idle = engine size, so it's a bit low.
I was hoping someone on here knew what other engine sensors the ecu effectively ignores(if any) when the maf if unplugged. Other than that I'm thinking post maf air leak, I know some of the bigger engines Mercedes make are prone to the inlet gaskets leaking a bit, but I can't find anything on an R-class as they didn't sell many I guess!
The odd thing though I'd the fact it holds the throttle open a bit sometimes, only an extra 1 or 2 degrees but it shuts it back to idle setting as soon as you unplug the maf.
I am open to the idea that he's bought a duff maf, even if it was over £200. Especially as the faults all disappear when you unplug it, however it runs almost identically (live data also) on both maf's.
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Not many R Class and most of them are R320 diseasal like mine
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Not many R Class and most of them are R320 diseasal like mine
Yep, apparently when he went into the autofactors the bloke has never heard of one and had no record of ever ordering any parts for one.