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M635uk

More motronic woes, leans out at cruise and idle 535

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So my car runs great, except at idle and cruise.  M30 Alpina spec 535 engine.

 

i have fitted an AEM air fuel ratio and this is what happens.

 

when warmed up the idle will start at slightly rich -13.0 afr and the drift slowly to 16 or 17.  I know it is leaning out as the engine tone changes significantly and it’s starts to struggle, coming off idle it bogs down but once the throttle is open say above above 1/4 all returns to normal and engine runs good in the 13’s.  WOT gives strong power with an afr in the 12-13 range.

 

the same happens at cruising speeds, afr drifts upwards beyond 16-17 over about a 10 second period  Very odd.

 

i have checked fuel pressure and static (Vacuum pipe removed) it runs at 3bar, dynamic it runs at 2.4 bar at idle and stays rock solid at this when the afr starts to drift.  So it’s not a fuel pressure problem.   I have also checked the air flow meter reference voltage at idle and again it stays rock solid at about 1.51 volts as the afr drifts,, so there isn’t additional metered air being requested.  There are no vacuum leaks that I can find and apart from the drifting afr the car runs great.

 

so it appears the ecu is reducing the injector pulse width (could it be compensating for something)at idle and cruise, or is it trying to run in closed loop?  What is confusing me is how the afr can drift so significantly,  I can understand a rich or lean condition, but drifting when everything else is stable seems odd.

 

any thoughts would be welcomed.

 

i may be able to get a video clip to show the problem.

 

Thanks

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14 hours ago, Bumbaclut said:

throttle switch problem?

 

Thanks, dont believe so though, changed it out for a new one and it passes all the necessary tests I.e. clicks on opening and has continuity for idle and WOT conditions.

 

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Is this motronic 1 or 1.3? ie e28 or e34 based?

 

On the former its a pretty basic look up system (TPS is idle/ load/ WoT ie on off on) and i think 1.3 simply has different crank sensing and a variable TPS on later versions so for a value to drift i would suggest you would need to be getting a erroneous signal so say a temperature sensor varying its out put or a failing component's resistance changing with heat.

 

You say the AFM is giving a stable voltage?

 

What about the ICV could that be closing and changing the air post AFM?

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19 hours ago, duncan-uk said:

Is this motronic 1 or 1.3? ie e28 or e34 based?

 

On the former its a pretty basic look up system (TPS is idle/ load/ WoT ie on off on) and i think 1.3 simply has different crank sensing and a variable TPS on later versions so for a value to drift i would suggest you would need to be getting a erroneous signal so say a temperature sensor varying its out put or a failing component's resistance changing with heat.

 

You say the AFM is giving a stable voltage?

 

What about the ICV could that be closing and changing the air post AFM?

Hi, It’s Motronic 1.3 e34 535 with Alpina Chip.

TPS is simple 3 wire not variable

 

i’ll check for drift on Coolant Temp Sensor, this is new, and I know it has a influence on fuelling.

I’ll also check Air Temp sensor drift, but I understand this has minimal impact on injector pulse.

 

AIr Flow Meter output signal was measured at idle as the problem manifests itself, it stays rock steady at 1.51v.   It varies as it should for higher air flows, this indicates that additional metered air is not being measured as the fuelling goes from rich to lean.  Hence with a steady measured fuel pressure at idle also, it strongly suggests the ECU is changing the injector pulse width in a controlled manner due to an erroneous input as you suggest.  I could be getting unmetered air into the system but the change from rich to lean seems very “controlled” at idle for cruise.

 

ICV is a proper Bosch item, not the useless copy ones - that was my previous problem.  It is working correctly and with the steady AFM voltage output it would not appear that the ICV is being pulsed open to cause the leaning out as I would see this as rising metered air.

 

i have done the Stomp test to get the error codes out of the ecu, after first fitting a bulb to the check engine light!  It’s an 1988 car which I believe did not have the bulb fitted as it’s constantly lit when the engine is running.  The error code is always the same 1222.  - Lambda probe failure!  No surprise there as it’s not a CAT equipped car (but the chip might be from a CAT equipped Alpina?  Did Alpins have CAT’s?  I assume they must have post 1992.)

 

Anyway next steps are to check for drift on CTS and ATS, I will also replace the “Alpina” chip for the standard chip which should give me more info.  Alpina chip is a direct copy of an original I was told.  I will also take a video clip at the weekend of the problem and see if I can upload that.

 

using this useful link to get more understanding of the motronic operation.

https://www.bimmerforums.com/forum/showthread.php?1430281-Bosch-Motronic-1-1-1-3-BMW

 

 

cheers for now.

Edited by M635uk

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Ok so 1.3 seems to have adaptions via feedback which 1 doesn't (the bulk of my knowledge) but as you say no Lambda.

 

Could it be that the Alpina chip is correcting as its not seeing this value? i'm thinking that at a steady state Idle/ cruise its assuming its rich and leaning out to compensate?

 

Trying a standard chip seems a start

 

It also says coolant temp is the main correction factor so as before maybe this is drifting somehow?

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Well did some further testing today.  Checked CTS resistance hot and cold and it’s very stable if a little high at 400 ohms when hot, 3k+ ohms when stone cold,  so I don’t think it is that.   Changed the chip for a non Alpina but “unrated chip” and car ran very well for 15 mins and about 10 miles.  Returned home and then it started leaning out again at idle...

so next step is to fit the original Bmw chip and give it a long run to see if the same problem occurs.  I am actually hoping it does so I can rule out the chip being the issue.

 

seems either time or temperature related though.  More digging is needed.

Edited by M635uk

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