AFM Troubleshooting

[jetta caratene]

I started running diagnostics on my car this weekend to determine the cause of the intermittent "bucking" of the engine between 2000-4000 rpm after the engine is at operating temperature. So far I have checked:

CPS- OK
Fuel pump- installed a brand new Walbro pump. I was almost certain this was the problem.
TPS-OK
Fuel pump relay was swapped with the O2 relay with no improvement.
Ign. Coil- out of range but only by 800Ω on the secondary.
Spark plugs- replaced with fresh NGK's
Plug wires- good
cap & rotor- no visible damage
Inspected for any intake leaks, I found none.
AFM- resistance values normal,steadily increasing as flap opens

After putting everything back together the CEL came on and threw a code 1215 for air flow measurement. Now the engine will not start with the AFM connected. I opened the AFM and inspected for bad connections. The potentiometer looks good, no binding, all carbon in place, solder joints acceptable. The plug to the AFM is good. IAT resistance fluctuated by adding heat.

My question is, are there any other tests I can run on the AFM to determine if it is functioning correctly? Are there any other conditions which will throw a code 1215? I checked again for intake leaks and found none. I'd like to get back to the bucking issue. Hopefully the AFM is the root cause of all of this.

[jlevie]

I think at this point that the next thing to do is to swap in another (known good) AFM.

[DmcL]

u certain the CPS is good?

[jetta caratene]

CPS resistance values are normal at 590Ω.

I'm trying to find a known good AFM to swap into place of the suspected faulty one. The only trouble is, the AFM tests good per the Bentley. It does give the 1215 code and the car won't start anymore with it connected. I'm still not 100% sure that it's the problem.

[straight6pwr]

CPS resistance values are normal at 590Ω.

I'm trying to find a known good AFM to swap into place of the suspected faulty one. The only trouble is, the AFM tests good per the Bentley. It does give the 1215 code and the car won't start anymore with it connected. I'm still not 100% sure that it's the problem.

well since the car would start before, what did you change and/or disconnect then reconnect right before you started getting the no-start? something unplugged somewhere else? i am chasing the same bug, and when i replaced injectors one of the vent hoses cracked when i put it back on (unknown at the time) and i started having a hard time with bad starts. chased it back to my mistake since I knew the car was starting fine before.

and same as you, from testing it appeared my AFM was fine, but I bought a known working used one (in the mail right now) to swap just to be sure.

[Ryann]

Wiring from the AFM to computer could be a problem. Use a diagram to find the corresponding pins at the ECM connector and measure for the same AFM values.

[jetta caratene]

well since the car would start before, what did you change and/or disconnect then reconnect right before you started getting the no-start? something unplugged somewhere else? i am chasing the same bug, and when i replaced injectors one of the vent hoses cracked when i put it back on (unknown at the time) and i started having a hard time with bad starts. chased it back to my mistake since I knew the car was starting fine before.

and same as you, from testing it appeared my AFM was fine, but I bought a known working used one (in the mail right now) to swap just to be sure.

I cured the no-start. buy doubling back over my connectors, rechecking every thing I checked before. I found the CTS connector not fully connected. It rests underneath the fuel rail and is kind of a pain to connect/disconnect.
My mistake for not being thorough.

Wiring from the AFM to computer could be a problem. Use a diagram to find the corresponding pins at the ECM connector and measure for the same AFM values.

After checking the connections again and finding the CTS improperly connected, I started the car and left the AFM unplugged. I plugged it back in and the engine smoothed out as expected but the CEL stayed illuminated.
I gave the connector to the AFM slightly more force and it extinguished.
The problem may lie in the connector.

I drove the car to the parts store for some heater hose, but I don't think I drove it long enough for the bucking to start again. I'm going to drive it more today and see if the problem is still present.

Thanks for the help and suggestions too.

[jetta caratene]

I repaired it, I think.

I decided to go drive the car until it started with the bucking. I went home and checked resistance on the CPS. It had 680Ω resistance, with the engine at operating temperature. It was out of range. I took it out and found a small bit of insulation stripped off. It apparently rubbed on the fan clutch when it kicked on after the engine got warm enough. This was odd because the last 10 or so times I checked the CPS, the sensor resistance was in range. I just never checked it warm. The increase in heat and the fan spinning faster upped the resistance enough to send a weak signal.

The inside wiring was still intact, so I repaired the harness and rerouted it. I then drove the car until it got warm. Result: No more bucking.

Thanks for the help:D

[Ryann]

Nice work!