Strange AFM problems. My wrench called me today and said he got it all put together (had a bad injector) and W/O the AFM plugged in, the car starts up and idles fine. As SOON as he plugs in the AFM it dies. So he went over to Scott's (sh944) and borrowed the AFM off of the yellow race car. Same thing happens. Any ideas/experiences???

 

put a meter on the afm harness and see if you have 5v coming from the dme

 

its an early car, 11.7volt book says 12v shouldnt be a problem.

 

SUPER bump...

 

Check the TPS.

 

Is it an early AFM, the early and late are two different meters. That would cause a problem.

 

Something is WAY wrong. With the AFM disconnected the engine should start and then die immediately.

During the start sequence the DME ignores the AFM. Once the engine RPM reaches a certain value, the DME "polls" the AFM for an output. If the DME does not see the AFM, the DME "assumes" the engine did not start. at which point the DME turns off the injectors and stops firing the plugs.

If your wrench was replacing a bad injector, why was the AFM disconnected in the first place?
Also, I take it the engine will not start with the AFM connected?

 

there's no reason the afm is necessary at idle as it isn't monitoring anything. without any throttle, the throttle body door is closed and the engine isn't taking in air through the afm. our cars should idle without the afm, and stall as soon as the gas pedal is pressed

 

I think he's meaning that it's not on the car at all. whereas StupEd05 means he just pulled the wires off, he didn't pull it completely off the car.
StupEd05, please clarify what you did exactly...

 

My wrench did not remove the AFM. Once all back together it would start and die immediately, unless you unplug the AFM (as in just pull the connector, not pull the AFM completely off of the car). So he figure the AFM was bad, borrowed an AFM from a known good car, slapped it on there and same thing. I will call my wrench to verify though, but im 99% sure thats exactly what he was saying.

EDIT: And yes, the borrowed AFM was also from an early car.

 

Wrong.

If the AFM was not being monitored by the DME when the engine is at idle, then the Idle Stabilizer on late cars and the Auxiliary Air Valve on early cars would be UPSTREAM of the AFM instead instead of being down stream.

Not only that but if both the throttle plate and the AFM "Barn Door" are closed at idle, where pray tell, is the engine getting air from?

In fact, the DME is an ENGINE LOAD-based management system, in that it is designed to calculate engine load using intake air flow rate and engine RPM.

Once engine load is calculated, the DME reads the fuel and ignition map data and uses that information to compute injector duty cycle and ignition timing.

To get a better undstanding of how a Bosch Motronic DME actually works, try reading "Motronic Engine Management," Edition 94/95.

I stand by what I posted previously.

End of lesson.

 

So in other words, there is no way the engine should idle without the AFM plugged in. Unless my DME is retarded and cuts off the spark/injectors WHEN an AFM IS detected istead or when its not... (that would be weird)

I have always thought my car was possessed. Maybe I should call an exorcist instead of a mechanic.

 

Am I the only one that thinks it is weird that he is talking with a wrench? Sorry I had to say it...

 

I'm lonely... It was the only thing around...

 

Its not weird unless the wrench answers.