Gas mileage problem

92TBurnSC

Registered User
The Super has been giving me some lousy mileage numbers. The last couple times I filled it up, I calculated 13.xxmpg. She's bone stock with 146k miles. Anyone got an idea?

Oh, and one other thing...my instrument cluster quit lighting up. I'm going to be bold and assume it's a fuse...but the manual doesn't say which fuse is the right one. And I know it's not the bulb etc burnt out because the rest of the instruments (i.e. on-board computer, radio, climate control, etc) don't dim like they usually do when you switch on the light.

Any thoughts for either of these?
 
How stock is your car, and how many times a day do you make boost? I've found that if i keep my car in vaccuum(making no boost) then I get excellent gas mileage. However, if i start making 5lbs+ of boost then I can kiss my gas goodbye. I've found that, when my car is filled up, if I only get on my car to merge onto the freeway(which would be twice a day) and if I keep my boost in vac. while driving around in town, I can go about a week and a half before needing to fill up again. I would calculate mpg, but my trip/odometer quite working a LOOOOOONG time ago....I suppose one of these days I'll have some ambition to get it fixed :p
 
If you have recently done a tune up and your plugs, wires and fuel filter are in good condition .............then I say check your O2 sensors.
 
If you have recently done a tune up and your plugs, wires and fuel filter are in good condition .............then I say check your O2 sensors.
And if you've checked your 02 sensors as well... look for a hole in the bottom of your gas tank.
 
Have you just switched to.....

....oxygenated fuel in your area? Here in Upstate NY they are going to switch to the "winterized" fuel that has 1 or 2% oxygenate in it for wintertime emmissions reasons.

This of course only fools the O2 sensors into thinking "engine is running leaner". Guess what, the car fattens up the curve a touch and combined with cold start programs running longer on cold days and longer delays before cat light off, we end up burning more fuel and emitting more HCx into the atmosphere.

At any rate, the O2 sensors, like previously mentioned might not be developing a good signal to the computer for optimum mileage and power. Sure the signal is "good enough" to keep from flagging a code but it is far from optimal.

If you don't have $120.00 right now to replace with 2 new Bosch O2 sensors, you might want to try and remove the originals, get a wire brush and throttle body cleaner and spray soak and scrub the sensor probes free of built up carbon. Use a propane torch to burn off the carbon/throttle body cleaner and re-lube the threads with anti-seize grease. Throw them back in and see if you notice an improvement with concern to mileage. If not, then you will need new O2 sensors for sure. Run a good quality fuel injection cleaner too. And of course, this assumes you have good plugs and wires too in your car. Check your air filter for excessive dirt as it can choke off the engine. An exhaust restriction could also cut into your mileage. Check the tire pressure and make sure your are running the maximum cold tire pressure stated on the car's door jamb and NOT the sidewall of the tire. Ditch excessive weight, if you've been running around with 300lbs of stereo equipment in the trunk, you've found the answer.
 
When I say stock I mean STOCK. Even stock stereo (which is on its way out). As far as plugs and wires go, I can't really say. I had a tune-up done about a year and a half ago and that's all I can really say. I am at school (Illinois State) and don't have the facilities to fool with the Super.

As far as making boost...I never really thought of that. But even at that, I never really make more than 3lbs at any given normal moment. And that's when I'm late for work. Heh. But I was afraid it was tune-up time...
 
Like stated earlier in this thread, check your 02's. I have found that bad 02's don't always cause a code to set in the ECM. (I troubleshoot cars). They tend to slow down in response to the air ratio changes, or also called crosscounts.

Good news is there is a simple test. Get a good volt meter that reads millivolts. ground one side out and backprobe the 02's connector till you find the signal wire. Don't worry, if you probe the wrong one it won't hurt it. A good working 02 should range from .200 to .800 millvolts, with .500 dead center. A crosscount of 6-10 times a sec @ 2000 rpm is good. A crosscount is when the volts drops or rise's past .500 millivolts.

Bad new is, not very many voltmeters can read fast enough to count the crosscounts accurtaly.
 
Keep an eye on your usage. Check for the smell of gas, a sign of it being overly rich.

Fuel Pressure regulators typically fail by a rupture of the diaphram inside the regulator. This can be a small hole, or a tear. Which-ever happens, this causes fuel to travel through the vaccum line attached to it, dumping raw fuel into the vaccum port in the intake manifold.

Typically easy to detect by simply pulling off the vaccum line and looking for fuel, on the SC, the regulator is buried so it's not that easy.

But keep that in mind if you find fuel economy getting worse and worse, or notice a strong gas smell with no sign of gas leaking anywhere.
 
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