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air mass meter -- hot wire[200/1989] posted by Geoff on
Sunday, 26 March 2000, at 6:10 p.m.
In trying to track down recent poor gas mileage, I've tested the air mass meter. Resistance accross the heated wire is within range but, when testing for the burn-off voltage upon engine shut-down, there is no voltage spike as specified. I see no indication of a relay between the air mass meter and the ECU -- cabling checks out ok.
I'd rather not have to purchase another ECU. Can anyone shed some light on how critical burn-off is to the air mass function (and my mileage concerns)?
Thanks,
Geoff
--geoff
Re: air mass meter -- hot wire[200/1989] posted by Don Foster on
Sunday, 26 March 2000, at 8:53 p.m.
Geoff --
"Burnoff" burns off any foreign materials that might coat the wire (as I'm sure you understand).
Foreign crap coating the wire will insulate it from the cooling effect of air, thus telling the ecu that less air is rushing through the AMM than actually is. The ecu will adjust the fuel flow downward, thinking the engine is inhaling less air—so I'd expect a dirty wire to result in a leaner mixture.
As I understand it, the ecu controls the "burnoff" current to the AMM for a short interval after shutdown—maybe 10 seconds, or so. I think this is well explained in the Bently book (Probst tutorial).
If your gas mileage is dropping, I'd be suspicious of a bad temperature sensor (or wiring or connections, etc.). An increase in resistance tells the ecu that the engine's colder, so it will increase fuel flow.
--Sunday, 26 March 2000, at 9:35 p.m.
There are conditions that have to be met before the ECU sends burnoff current to AMM.
The eng temp must be warmed up and the eng speed must have exceeded 2500 +/- RPM.