Is my logic correct? M20 Motronic 1.1 WOT

[hwy84]

My M20b25 makes extremely poor power until 5500 RPM, (like, won't climb hills, and almost dies below 2000 RPM) and has a low idle when warm (500 RPM). I get no CEL. I have separate thread on the troubleshooting here: http://www.r3vlimited.com/board/show....php?p=4641418

I am trying to diagnose the issue, and have replaced/tested lots of parts. However, I am hoping my logic below is correct, to aid sanity and help guide me further. Can anyone verify?

The car runs badly at all throttle amounts, but for this excercise, I only want to consider WOT.

Does the following make sense?

• On a warm engine, at WOT, the ECU only uses RPM and engine temp to determine fueling.
  
• I have just replaced the coolant temp sensor, and the crank position sensor, and made sure the toothed wheel is tight and has no play.
  
• Therefore, the ECU throws no codes, because it THINKS it's doing everything right. It sees a given RPM, and a given temp, and it fuels appropriately based on its map.
  
• But the mixture is actually wrong, due to something else. It makes almost NO power at 4,500, a little bit at 5,000, and finally starts making reasonable, but not great power at 5500-6500 RPM.
  
• ASSUMPTION: The higher the RPM, the more fuel the ECU calls for at WOT. (yes?)

If so, it seems that either:

• 1) Below 5500 RPM, there is too much fuel, and it only starts getting enough air at higher RPM to match that fuel amount.

OR

• 2) Below 5500 RPM that there is a huge vacuum leak, and there is only enough fuel at higher RPM to make a clean burn.

Is my logic correct here?

If so, 1) would point to bad/leaky injectors (I think) and 2 would point to vacuum leak.

What say you all?

[jlevie]

My understanding is that if the DME sees the WOT signal from the TPS it stops using O2 sensor data and determines fuel from the built in maps as modified by learned adaptation. Below about 4500 the DME uses data from the AFM, coolant temp sensor, intake air temp, and rpm. At about 4500rpm the vane in the AFM is fully open and that data is ignored and the built in maps as modified by learned adaption is used.

Things to look at include making sure the DME is seeing the WOT switch close, check rail fuel pressure, and have the injectors cleaned and flow tested.

[hwy84]

Thanks for validating, jlevie. And I'm totally with you - just realized about an hour ago that I only tested the TPS at the switch itself - I should check the pins at the ECU connector as well to make sure the harness is okay.

The FPR seems okay, as I tested it and was just under and over 3 bar based on throttle opening.

I am thinking the injectors could be bad, but I'm surprised I don't have a no-start, or a ton of cranking to start the car.

All thoughts are welcome at this point - I want to solve this thing!

[TobyB]

If it was mine, I'd measure the AFM voltages at the ECU.
The AFM is a weak link in an otherwise very bulletproof system,
and it's super- easy to rule out:
meter it at the ECU, get someone to push the vane open slowly,
and confirm that voltage steadily and evenly climbs...

hth
t

[Kershaw]

Have you tried swapping in a different set of injectors?

[hwy84]

If it was mine, I'd measure the AFM voltages at the ECU.
The AFM is a weak link in an otherwise very bulletproof system,
and it's super- easy to rule out:
meter it at the ECU, get someone to push the vane open slowly,
and confirm that voltage steadily and evenly climbs...

hth
t

Thanks - I have swapped in a known good AFM from a working (crazy quick and smooth, actually) 325is, and I saw no change.

However, if injectors also make no change, and timing is correct, I will start testing every sensor, including the AFM, at the ECU connector to start looking for shorts/faults in the wiring harness.

Thanks for the tip!

[hwy84]

Have you tried swapping in a different set of injectors?

Next on my list...