Engine Tinkering - Running lean at idle? Watch

StriderHort
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Heya, having a bit of trouble troubleshooting something and would appreciate any thoughts

My 02 SLK keeps spitting up an engine light and codes for the fuel mix at idle being too lean, and this matches how it runs at idle in terms of revs. It's potentially moaning about something in cylinder 4 and something seems seriously off with at least 1 lambda sensor, the upstream one's voltage is all over the place between 60-800mV odd, and it's in red text. I don't have much skill with electronics but that doesn't look good.

By contrast the downstream sensor voltage is stable, but it's warning me in red text that heating capacity is reduced...I'm bit vague on this, but the Lambda sensor is the same as the O2 sensor? it's measuring the fuel:air ratio so a likely culprit?

A bit of back story on what i've done so far to the engine, i'll try and be brief..

It had an EML on when i got it, and was assured it was was a sensor (lying £%^$! ) I got a scanner and got motor errors for hot film warning on the Mass Air Sensor and something indicating a misaligned/broken rod in my intake manifold (NOT the code i'm having now) . cleaning the MAS didn't help much, and it also looked suspiciously new as if someone had already been trying this...

Did a fair bit of youtube research and determined a likely culprit was a clogged Positive Crankcase Ventilation system spitting waste oil back into the intake, so it would follow that the MAS keeps getting new faults...this meant taking most of the intake/throttle system and the intake manifold out, along with a variety of for want of a better word, car guts. Cleaned everything out and replaced the whole PCV with OEM parts along with a few other gaskets and vacuum pipes...

Doing this has deffo solved the my two initial codes for the MAS and the Intake Manifold, but it's only THEN this 'running too lean at idle' has cropped up :/

I can't find it atm, but a random forum post tipped me off to an issue with this engine where waste oil gets into the socket of a wiring loom at the front of the crankcase and basically wicks it's way up the wiring towards the ECU, not sure offhand but i think this loom is the one a lot of the sensors run off. I checked that socket and yep, it was soaked, def to the point i'd expect it be causing an issue. Apparently there's enough metal particles to mess up voltage but again, not really my field, could that be throwing my lambda sensor off?

It was a few weeks ago i did this and cleaned it out, I've checked the socket twice since and it's been wet (but less so) both times...I don't believe NEW oil is getting in that quick (I degreased the crankcase to keep an eye out for fresh leaks), so does this mean it's kind of 'draining' it back out the wiring loom? *looks hopeful*

A few articles are tipping me off that the idle fault could be down to the PCV being incorrectly hooked up, but i'm 100% it's right, it wasn't complicated to replace. The fuel pressure or injectors could also be a culprit...and i had those out for cleaning when i did the other stuff...possible that could tally with the fault readings from cylinder 4?

Cheers if you read this far, just curious if anything occurs to anyone
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Kasey00
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#2
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(Original post by StriderHort)
Heya, having a bit of trouble troubleshooting something and would appreciate any thoughts

My 02 SLK keeps spitting up an engine light and codes for the fuel mix at idle being too lean, and this matches how it runs at idle in terms of revs. It's potentially moaning about something in cylinder 4 and something seems seriously off with at least 1 lambda sensor, the upstream one's voltage is all over the place between 60-800mV odd, and it's in red text. I don't have much skill with electronics but that doesn't look good.

By contrast the downstream sensor voltage is stable, but it's warning me in red text that heating capacity is reduced...I'm bit vague on this, but the Lambda sensor is the same as the O2 sensor? it's measuring the fuel:air ratio so a likely culprit?

A bit of back story on what i've done so far to the chatroulette engine, i'll try and be brief..

It had an EML on when i got it, and was assured it was was a sensor (lying £%^$! ) I got a scanner and got motor errors for hot film warning on the Mass Air Sensor cyberghost and something indicating a misaligned/broken rod in my intake manifold (NOT the code i'm having now) . cleaning the MAS didn't help much, and it also looked suspiciously new as if someone had already been trying this...

Did a fair bit of youtube research and determined a likely culprit was a clogged Positive Crankcase Ventilation system spitting waste oil back into the intake, so it would follow that the MAS keeps getting new faults...this meant taking most of the intake/throttle system and the intake manifold out, along with a variety of for want of a better word, car guts. Cleaned everything out and replaced the whole PCV with OEM parts along with a few other gaskets and vacuum pipes...

Doing this has deffo solved the my two initial codes for the MAS and the Intake Manifold, but it's only THEN this 'running too lean at idle' has cropped up :/

I can't find it atm, but a random forum post tipped me off to an issue with this engine where waste oil gets into the socket of a wiring loom at the front of the crankcase and basically wicks it's way up the wiring towards the ECU, not sure offhand but i think this loom is the one a lot of the sensors run off. I checked that socket and yep, it was soaked, def to the point i'd expect it be causing an issue. Apparently there's enough metal particles to mess up voltage but again, not really my field, could that be throwing my lambda sensor off?

It was a few weeks ago i did this and cleaned it out, I've checked the socket twice since and it's been wet (but less so) both times...I don't believe NEW oil is getting in that quick (I degreased the crankcase to keep an eye out for fresh leaks), so does this mean it's kind of 'draining' it back out the wiring loom? *looks hopeful*

A few articles are tipping me off that the idle fault could be down to the PCV being incorrectly hooked up, but i'm 100% it's right, it wasn't complicated to replace. The fuel pressure or injectors could also be a culprit...and i had those out for cleaning when i did the other stuff...possible that could tally with the fault readings from cylinder 4?

Cheers if you read this far, just curious if anything occurs to anyone
Yes it possibly could tally with the fault readings from cylinder 4.
Last edited by Kasey00; 1 month ago
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StriderHort
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#3
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(Original post by Kasey00)
Yes it possibly could tally with the fault readings from cylinder 4.
Yeah I'm thinking that's likely my best lead...I was v suspect of the o2 sensor, but having done some more reading indersatandhow it works a bit better now with the voltage cycling, however mine still seems to be dipping v low...but if that's the sign of it running lean then it surely follows the sensor is reacting right, it would need to be kicking out too high a voltage to throw the mixture lean...I think

The problem didn't crop up till after i had the fuel rail and injectors out anyway , so i'll check the main fuel pressure then start volt/resistance testing the individual injectors.
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StriderHort
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Yeesh, did a live graph of the o2 sensors and it's a juddery mess dipping down to 0.03v, absolutely not an oscillating wave at idle at all, and it DOES settle into a predictable wave under load so i can see the difference (and also know my diagnostics are displaying correctly, it's not lag ect) But still don't really know if it's duff injection throwing off the sensor, or duff sensor throwing off the injection (or both)

I thought maybe sparkplugs...but misfires/incomplete compression would result in the o2 reading too rich, not lean, right? and i'd expect to notice while driving. (or would the car do it's best to compensate?)

So I suppose aside from the o2 sensor i'm needing to investigate whatever injector is linked to cylinder 4 & 3, i'm only getting a generic live code for that, but indicates 'P1263 Cylinder #3 High To Low Side Short or P1264 Cylinder #4 High To Low Side 'Short, I'm warned these particular codes are a bit generic and my diagnostic might be getting the cylinder number wrong. The o2 sensor may well need replaced, but even that shouldn't trigger faults in one cylinder and one only right?

I'm a bit confused why an injector issue would cause different issues at idle and load though, but i'm vaguely aware it uses different ratio management

Well aware i'm largely talking to myself, but it's good for me to have a record as well :P Curious what folk think I should be trying first though.
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StriderHort
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Had a fun day today of messing about

My injectors seem 'ok' for now...code hasn't came back. I got a stethoscope and measured them when running for the wee 'TikTikTik' noise so all 4 are getting power...it's possibly just one I didn't clean right.

I got a new O2 sensor as well, but i'm holding off fitting it tbh, as it's got more than a hint of the AliBaba about it. I suspect if i take off the Bosch one I've got it won't go back on again (some sort of one use thread), so i'll hold off on that.

I found the weird oil leak at the front is my Camshaft adjuster magnet, I've not opened it yet but youtube leads me to believe i will find a fried seal, so i've got o-rings and gasket maker ready for that and a new wiring adapter on the way.

Today's main job was ripping the supercharger out....was easy-ish getting it out, mostly the same tools/techniques as taking off the intake manifold...so that's had it's oil & grease replaced, new gasket/threadlock and the pesky silencer accidentally mislaid. I've basically got it back in the car, but really struggling with some of the bolts (HTF did I get them out?) and I'll need to come back in daylight and work out how the drive belt goes back on. (after some weird spanners have arrived as a last ditch bolt solver)

Name:  sc.jpg
Views: 8
Size:  93.9 KB

Random supercharger pic, I thought about changing out some of the pulleys while i'm doing this to try and coax more power from it, but the various reviews/forums i'm reading make it look a raw/tricky deal so i'll leave them.

Sill curious how many views this thread can get with no one chipping in, no one else seems to be doing any projects or such so fk it
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InfiniteWill
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#6
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#6
(Original post by StriderHort)
Had a fun day today of messing about

My injectors seem 'ok' for now...code hasn't came back. I got a stethoscope and measured them when running for the wee 'TikTikTik' noise so all 4 are getting power...it's possibly just one I didn't clean right.

I got a new O2 sensor as well, but i'm holding off fitting it tbh, as it's got more than a hint of the AliBaba about it. I suspect if i take off the Bosch one I've got it won't go back on again (some sort of one use thread), so i'll hold off on that.

I found the weird oil leak at the front is my Camshaft adjuster magnet, I've not opened it yet but youtube leads me to believe i will find a fried seal, so i've got o-rings and gasket maker ready for that and a new wiring adapter on the way.

Today's main job was ripping the supercharger out....was easy-ish getting it out, mostly the same tools/techniques as taking off the intake manifold...so that's had it's oil & grease replaced, new gasket/threadlock and the pesky silencer accidentally mislaid. I've basically got it back in the car, but really struggling with some of the bolts (HTF did I get them out?) and I'll need to come back in daylight and work out how the drive belt goes back on. (after some weird spanners have arrived as a last ditch bolt solver)

Name:  sc.jpg
Views: 8
Size:  93.9 KB

Random supercharger pic, I thought about changing out some of the pulleys while i'm doing this to try and coax more power from it, but the various reviews/forums i'm reading make it look a raw/tricky deal so i'll leave them.

Sill curious how many views this thread can get with no one chipping in, no one else seems to be doing any projects or such so fk it
how did you learn so much about cars?
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StriderHort
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#7
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(Original post by InfiniteWill)
how did you learn so much about cars?
I KNOW ALMOST NOTHING ABOUT CARS!

I've only been driving a few years, but the last car I got, an 02 SLK, was intended as a learning project and i've honestly been loving it. I got myself a little plug in diagnostic thing and basically just trawled youtube and owner forums for problems other people had, luckily it's an older car so it's well known and there's lots to read. I got in touch with a youtuber who had done a series on the same engine and he was kind enough to talk me through some weird bits.

I'm really just working it out and buying tools as I go along, but loving how much it makes my car feel like MY car.
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InfiniteWill
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#8
Report 3 weeks ago
#8
(Original post by StriderHort)
I KNOW ALMOST NOTHING ABOUT CARS!

I've only been driving a few years, but the last car I got, an 02 SLK, was intended as a learning project and i've honestly been loving it. I got myself a little plug in diagnostic thing and basically just trawled youtube and owner forums for problems other people had, luckily it's an older car so it's well known and there's lots to read. I got in touch with a youtuber who had done a series on the same engine and he was kind enough to talk me through some weird bits.

I'm really just working it out and buying tools as I go along, but loving how much it makes my car feel like MY car.
Sounds fun and something I wanna do. Not right know tho. Got too much
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