Crazy MPG gauge
 

My MPG gauge has become very erratic. I seem to recall reading somewhere that this was indicative of vac leaks. True?

could be...
but if your idle isn't affected, then i would start looking at the connections behind the cluster...

when i took my gagues out, i had the same problem.. turns out it was just one of the wires back there that wasn't exactly perfect.

with that said, check both :p

Gauge has no connection to any vacuum. It's run off of the input from the hall sensor on the back of the speedo, the tach signal and another signal from the DME that is derived from the time the injectors are open. If it's erratic then there is a problem with the wiring or the fore mentiioned inputs.

socal, a vac leak could affect the tach, which in turn could affect the mpg gague, correct?

is your idle erratic, slam?

Vacuum has no connection to any of the electronic derived gauges. There is no vacuum sensor. Well I guess the AFS could be construed as a vacuum gauge but has no direct connection with the tach.

Indirectly a vacuum leak at a certain point in the intake can change the rpm of the engine.

Thanks, gents. I'll check wiring. If there's a ground fault it would read to the extreme left, yes?

All of the faults here seem to be from the extreme left.

Right you are.

Are these gauges very accurate? Mine stopped working completely. It's pegged all the way to the right.

Wow. That is some milage!

Most likely the little pc board that the hall effect mounts on is broken. Have to epoxy it back together then solder the traces closed.

mine gets pegged sometimes...
I tap the dash above it and it jars it loose...

when i had the vac leak on the car, the engine would drop revs, showing a lower reading on the tach... just thinking if the mpg gague gets it's info from the tach, that it may (indirectly) be affected by a vac leak.

Oh dear, where to start....

it doesn't do that anymore... just thinking if A affects B, and B affects C, then A affects C... but then again, i've been wrong many a time before ;):P

Theoretically, anything that affects injector on-time would reflect in the MPG gauge, providing it's working properly. Question is, is it a symptom or an ailment?

The vac leak cause actual lower RPM's, it wasn't affecting the tach's reading, it was directly affecting the engine's speed.

i realize that, but because slam didn't say whether or not his tach was screwey (unless i misread), i simply provided an option.

You're still missing it. A vacuum leak cannot cause the issue.

A screwy tach can't really cause it either. The mpg guage gets a reading from the DME's tach signal, the signal will be the same regardless of whether the tach is there or not.

The DME itself would still have to be screwing up the signal to the tach, which would probably have other, more severe, symptoms since it would most likely also require that one of the readings the DME takes to calculate the tach signal was wrong, which would cause engine management issues.

Tach's fine. Engine management's fine. MPG gauge is wonky. Frankly, I just plain haven';t felt like fixing it, but thanks to you guys I have a good idea where to look.