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M50B25 -413 red label AFR surging at high rpm?*solved*

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Joined: 26 Jul 2016
Posts: 83
Location: Finland

PostPosted: Sun Jan 15, 2017 7:54 am    Post subject: M50B25 -413 red label AFR surging at high rpm?*solved* Reply with quote

We have strange problem with M50B25 -413. Rev limit is raised to 7400, and when we run it on ~7000rpm few seconds (for example sliding at long gentle curve on ice track, or long straight) afr goes suddenly very lean, like 17:1 or leaner, and power is lost. Then it goes back to 13:1, when revs was decreased and it pulls normally at moment. It is surging this way in appr. 1 second perioid, and so long time as WOT. We tried to disable lambda (signal wires short, target to 1.03), but it has no effect.
And this is funny - engine works fine for few km's, and then the problem appears.

Fuel maps are stock, and we discovered the DME reads fuelling from PT map with WOT for some reason.. Is the WOT fuelling map only RPM based, or have it effect from air mass meter?

I think that the problem can be caused of DME swithing from PT to WOT map and back. Engine has more better breathing at high revs than stock, so if the WOT map is only RPM based without air mass correction, the engine could run very lean at high revs. We tried to pull TPS sensor plug off, but it has no effect to problem.

Engine is slightly modified: Non vanos 260/260 cams and tubular headers, but rest are from stock vanos engine. We have vanos intake cam trigger wheel which is modified to fit on non vanos camshaft.

EDIT: Information of vanos removal - vanos maps are modified to vanos never switching to "advanced" position. Vanos solenoid is still in place, but it has no oil pressure.

EDIT2: Problem is solved. Reason was missing speed impulse. I cutted pin42 v-signal wire which is going to cluster and hooked it to pin17 cam sensor signal. (9km/h at idle Very Happy) This is temporary solution - original problem is now replaced by idle control problems, but now we can safely remap that car on dyno. There are no trigger wheel fitted in final drive, so the speedometer is not working.

Don't ask why the ECU does what is does, but original surging AFR problem is now solved. Now we know: 413-ecu needs V-signal for correct working. It rans tens of laps on ice track without any time for surging AFR.
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