MPG Gauge in 84 NA

user426 · 07-08-2004, 06:14 AM

What does this really tell me......?????

Brando · Brando's Garage

Your DME works??!!

I dunno. was never accurate for me...

Stan944 · 07-08-2004, 07:00 AM

I once thought it was a direct measure of fuel consumption. Somebody explained to me that it actually measures the air flow to the engine, so it can be off if the air/fuel ratio is off.

user426 · 07-08-2004, 07:33 AM

But.....the air/fuel ratio is set in the DME....correct??????

nynor · 07-08-2004, 07:39 AM

mine worked up until recently. basically, as far as i can tell, it shows your fuel consumption in MPG at a given moment.

i tested this on a long trip to modesto, about 800 miles. while cruising, it sat at about 33 MPG, which turned out to be within 2 MPG when compared with the miles driven and the amount of fuel consumed by the vehicle at the pump.

but don't quote me: i am probably talking out my arse...

Stan944 · 07-08-2004, 07:44 AM

Quote:

Originally posted by user426
But.....the air/fuel ratio is set in the DME....correct??????

yes, but the DME sets the amount of fuel INTENDED. This may not be the actual amount of fuel if the fuel system is not perfectly OK. Also the air flow is measured right after the air filter. If there is an air leak after the air-flow meter, more air will mean different amount of miles per gal of fuel!

But if everything works fine, I think it should be accurate.

TCMdocs944 · 07-08-2004, 09:29 AM

The owners manual says..

under 2000 rpms it shows mpg (bottom # ?)

over 2000 rpms it shows gallons per hour (top # ?)

I just found that out after the PO sent me the owners manual that came with the car. (1.5years after purchase)

Don 944 LA · 07-08-2004, 08:12 PM

Oh, you mean that thing that just flips back and forth...
I thought it was bouncing to the misic..

Does it actually do something ??

Don

singpilot · 07-09-2004, 06:14 AM

Quote:

Originally posted by Don 944 LA
Oh, you mean that thing that just flips back and forth...
I thought it was bouncing to the misic..

Does it actually do something ??

Don

This from the guy who has everyone flipping back and forth every time we see his ID.

AaronM · 07-09-2004, 06:32 AM

Quote:

Originally posted by TCMdocs944
The owners manual says..

under 2000 rpms it shows mpg (bottom # ?)

over 2000 rpms it shows gallons per hour (top # ?)

I just found that out after the PO sent me the owners manual that came with the car. (1.5years after purchase)

Uh, it should be that under 20kph it show gallons per hour and over 20 kph it shows miles per gallon. GPH are the lighter numbers.

That's from an '87 924S, but the 924S uses the same guage cluster as the early 944.

The way the guage works: It reads the injector fuel delivery rate from the DME and uses that data directly for the GPH readout below 20kph. The guage also receives input from a hall-effect sensor in the speedometer that will tell it how fast the car is going (this is why there is an electrical connection between the speedometer and the tach). The speed data from the hall-effect sensor is coupled with the DME's injector delivery rate information to compute miles per gallon when the car is above 20kph. If the hall-effect sensor on the speedometer starts to go flaky (fairly common) the the guage will pick up a speed signal only sporatically and will therefore jump between MPG and GPH modes when the car is at speed, which manifests itself as the needle jumping around or just reading really low if there is no signal at all from the hall-effect sensor.

Aaron