snb13

It seems I may be using more fuel on the highway than around town, even when traveling at consistent speeds. It looks like my gauge likes to dip at those times, leading me to this belief.

If this is the case, what might this symptom indicate? I do a lot of highway driving. Currently my mileage is sitting right around 18 MPG.

SloMo228

I get about 25-27 mpg on the freeway with my 944S. If you're only getting 18, I'd think something is probably wrong.

The most likely culprit is the O2 sensor. After that, check for vacuum leaks, functioning temp sensor, and proper AFM operation.

snb13

Right. So in the spirit of full disclosure.

'83 944, unknown miles (prob less than 200k), NA Tune, FQS set at 0, new O2 sensor, new FPR & damper, new plugs & wires, cap and rotor only have a few thousand miles on them, air filter is within it's normal life/mileage, new tires (1,300 miles on them.) Haven't recently replaced the DME temp sensor, but there's a new fan switch. I believe vacuum lines are OK.

Nothing seems to make a difference.

snb13

My next step is to reseal the throttle body and probably replace the TPS. I don't mind replacing or refurbing all the old parts, I consider it normal maintenance on a 30+ year old car, but it would be nice if it made a difference.

V2Rocket

are you actually calculating mileage driven or are you going off the MPG gauge on the tach?
the function behind that is a calculation of engine load and injector firing time.
load is something the DME figures based on the AFM signal...i think that was one of the things that needed messing with when converting to MAF, so maybe the gauge's calibration is off now.

snb13

No vacuum gauge. Converted to 924s gauges. Actual calculations.

snb13

I also plan to install a WBO2 set-up.

odonnell

Mine worked perfectly with NA tune. Also have an 83, I saw easily 30 on the highway when I drove 75. Bear in mind it's 103 kPa (sea level) here and I have slightly larger wheels than anything that was available on a 83 944.

snb13

We're about 1,100' above sea level. I have 205/55/16 on the front & 225/50/16 on the rear. I saw no change from my stock 215/60/15.

snb13

Also forgot to add the head is shaved .020", with turbo springs and a 2 angle valve grind. Along with the NA Tune, I run the MSD HVC II coil. The resistor is there and the tune accounts for the hotter coil.
I also know my cat is not plugged.

bonus12

snb13, do you experience a loss of power?

I'm not even joking but seafoam is a great product. Try a full can of it through the intake (ie brake booster line) 1/3 can at a time.

snb13

Thanks. No loss of power. Seafoamed regularly.

SloMo228

I'd check the vac lines and re-seal the throttle. I had a similar fuel consumption issue when I first got my S, I was seeing about 18 mpg on average, 20 if I really babied it. I Seafoamed the heck out of it, replaced the O2 sensor, changed the fuel and air filters, spark plugs, cap and rotor, and saw no improvement in mileage.

Later, I replaced the intake manifold, during which time I resealed the throttle body, replaced all the vac lines with silicone hoses, and re-sealed the AOS. I also opened the AFM and re-set the wiper arm to ride on a new track, rebuilt the fuel injectors, and replaced the DME temp sensor at the same time. Now I am seeing the 25-27 mpg I mentioned earlier. I can't point to one particular item as the solution, but I strongly suspect multiple small vacuum leaks were the biggest problem.

bonus12

I, too, noticed an improvement in mileage and power when I had the injectors professionally cleaned. I think it was 1-3 mpg. O2 sensor helped also, separately.

944Ross

Are you running a K&N type (oiled gauze) air filter? If so you might check to see if the AFM is gummed up on the vane seals. I had that happen on a VW, the added weight of oil and the dust it collected threw things way off.

I have a bone stock car and at 70, at mile high altitudes, A/C on, with lots of big hills, I get 27. On the plains I regularly see 30+ at the same speed. Can you put the stock tune back in and see if it improves?