odonnell

I'm getting odd readings from my gas mileage gauge. I know it's not sticking, and the problem may be transmission related. I just noticed it today... nothing else is unusual; this just started happening seemingly out of nowhere.

I'm getting about half of what I usually see... cruising at 40 in 4th usually lands me at 30 mpg, now I'm getting 15. Going to 5th stretches that to 20 mpg.

This is true in all gears.

On a warm engine, my "mileage" (fuel flow at injectors according to gauge) is 30-32 mpg at idle (in neutral), and this is still true.

I've eliminated any possibility of e-brake dragging or any calipers dragging. Car coasts fine (in neutral) and engine braking feels the same.

FWIW the dash was unplugged/taken apart last weekend. The fact that I'm getting readings at all tells me that it's not a connection problem, but I thought I would mention it. Any pointers?

Butters944

I'm thought the MPG gauge was vacuum driven? I think I remember reading an old 80s text explaning how it works. Just an idea.

Dougs951S

It is vacuum driven with 60 mpg corresponding roughly to 18 in HG. There should be no reason the gauge is acting up unless there is a vacuum leak, and any leak large enough to cause those types of readings and not substantially effect the running of the car would most likely be located at the gauge its self.

V2Rocket

check the wires again, it is an output of the calculation from the DME based on afm/throttle/load

odonnell

Will do. That must explain why it reads accurately at idle, but when there's a higher pressure, it acts weird.

odonnell

Just wanted to update this thread with my findings, and what worked to fix it.

Cleaning all the electrical connections on the gauges and reseating them is step one, and cleaning the ground responsible for them never hurts.

For me, I think my broken speedo cable contributed - I betcha some of the wires that run from the speedo pod to the tach pod are for the mileage gauge. I finally got around to getting my speedo working this afternoon, and voila! It was fine. I also cleaned and tended to a lot of the wires/connections, so I can't guarantee that was the problem, but there you have it.

(oh, and there aren't any vacuum lines whatsoever)

Tl;dr if your readings are off at high speeds, but are accurate at low speeds or at idle, check the connection from speedo to tach. Hopefully this helps anyone who has this problem in the future.

Dougs951S

Thanks for filling us all in michael, I had forgotten that the gauge was not vacuum powered, but I figured you had gotten it squared away by the time I realized my mistake. Glad all is well again. How's that adjusted clutch pedal been treating you?

944Ross

It's in the FSM, it absolutely needs the speedo to be working. (It's the Economy Conrol)

odonnell

Thanks guys.

Doug, it's holding up fine, thanks again! I "assumed the position" under the dash today (as us early 944 guys are too familiar with) and inspected it - no problems at all

william_b_noble

one more hint - if it seems per odometer that your actual mileage is dropping off, it may well be the odometer gear (inside the gauge) that has failed - when it drops one or two teeth, the odometer still moves, but not correctly. I got tricked by this once (gave the "worn out" 944 to my brother and bought a 993 - the 993 is gone - stolen, but the 944 returned home)