MyE28.com

My MPG gauge swing around wildly and I don't really know where to start to diagnose. When I search the topic, all the answers seem to be about gauges that don't work at all. If I'm touching the gas (at all) its a zero, but every time I let off or let off to shift, it swings all the way to the left. Its like an on off switch.

Thanks for any help.

... and when you're cruising down the highway is it steady?

Is it a new problem? What's the last thing you did if it is? Is the coding plug in place?

The MPG gauge appears to get inputs from the speedo, the ECU and the OBC, but I think it computes it with those inputs. It does tend to make wild swings, but yours sounds extreme. Check the OBC and it's MPG average, and while driving reset it often, if you do it often enough it's almost like an instantaneous reading, or at least for the past second or so.

1st 5er wrote: Sep 06, 2022 3:24 PM ... and when you're cruising down the highway is it steady?

It’s steady at zero. If I’m on the gas, even the slightest bit it’s zero. If I’m off throttle, it’s pegged the other way. Never anywhere on the gauge except extreme left or extreme right.

Dale3783 wrote: Sep 06, 2022 6:53 PM

1st 5er wrote: Sep 06, 2022 3:24 PM ... and when you're cruising down the highway is it steady?

It’s steady at zero. If I’m on the gas, even the slightest bit it’s zero. If I’m off throttle, it’s pegged the other way. Never anywhere on the gauge except extreme left or extreme right.

I'm gonna be no help other than cluster si board issue guess.

Wild fluctuations are normal between on / off throttle but it should hold steady with steady throttle.

Dale3783 wrote: Sep 06, 2022 3:12 PM My MPG gauge swing around wildly and I don't really know where to start to diagnose. When I search the topic, all the answers seem to be about gauges that don't work at all. If I'm touching the gas (at all) its a zero, but every time I let off or let off to shift, it swings all the way to the left. Its like an on off switch.
Thanks for any help.

On the E30 the gauge uses the electrical speed sensor signal, and injector signal from the ECU.
The ECU is likely cutting fuel when you let off so it would show best mileage on the gauge.

Does the gauge behaviour change at all with speed vs stationary.
I suspect the wheel speed signal is not making it to the SI board. I'm assuming your speedometer works.

I would start with a new SI battery and cleaning the SI board connections. Failing that a new SI board.

with Ford displays its also a fuel signal from the ECM and speed signal. On those when you have no speed signal it shows 0 mpg. If yours pegs on decel but 0 otherwise I'd agree it sounds like no speed signal, and your ECM has a decel fuel shut-off function. You'd have to look at the wiring diagrams to see where it gets that signal from.

Did you ever figure this out? I have the same issue. It's either at one end or the other, even with the slightest touch of throttle. The car is new-ish to me, so I've never seen it work. And for what it's worth, my OBC has never seemed to work that great for anything other than the timer...