Horrible (IMO) gas mileage

HelluvaEngineer

Registered User
I've put a couple of tanks of gas through my SC now. I only drive it to work, and it's all back roads with a couple of stop signs. No real stop n' go city driving.

I'm averaging about 16mpg. This seems horrible to me for a bone stock car.

Any suggestions on problems to check for? I've noticed it only makes about 9lbs of boost, so perhaps a vacuum leak? The ignition system seems to be in decent shape - good idle and no misses. No bogging or signs of a fuel problem either.
 
Stay out of boost if you want gas mileage.

Don't worry about gas mileage if you're boosting.

Check your exhaust and free that up first - see the threads for what's worked best for others here.

RwP
 
I don't get above 45 mph on my drive to work and I keep off the boost. Only reason I know about the 9psi issue is from playing with it when I first got the car. This think absolutely chugs gas.
 
I don't get above 45 mph on my drive to work and I keep off the boost. Only reason I know about the 9psi issue is from playing with it when I first got the car. This think absolutely chugs gas.

The 45MPH thing is probably the issue. I would classify that as city driving. Low speeds, stops on the way, 4Klbs car. That seems about right on the gas mileage.
 
Seems about right. Plus the gas nowadays with the ethanol is less potent so the engine burns more of it to burn Stoich. And our engines weren't designed to run the e10 efficiently.
 
Run the codes, check the air filter, air up the tires. But yeah, 16 around town (not open highway same speed) seems pretty close to expected.
 
Thanks for all the input. I'll check the codes, but I suspect everything is functioning within specs. I run high tire pressures anyway, so I guess I'll have to live with it.

Sucks that my pickup gets better mileage, and runs 87 octane :-(
 
I don't get above 45 mph on my drive to work and I keep off the boost. Only reason I know about the 9psi issue is from playing with it when I first got the car. This think absolutely chugs gas.

Not really chugging that much gas if your getting 16 mpg. Is this an automatic or manual trans. With an auto at 45 mph you are really lugging the motor in OD.

David
 
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16MPG is not normal, 1st thing to do is hunt for vac. leaks. seal off/block the throttle body, use compressed air & nozzle to pressurize the intake system via any of the hoses going to the intake. listen for leaks, the big ones will be easy to hear & find, next spray soapy water on suspect areas & seal/fix one at a time. See how long it takes for your Vac/boost gage to drop to 0 when you turn off the engine. mine takes 6 seconds. If your good on vac leaks then check the EGR valve, I found mine was commanded to far open when driving, it ran fine but vac was around 10 @ 60 & I know that is not good. I disabled the EGR when I unplugged the vac solenoid & plugged the vac line to it. What is your Vac/boost read at 60-70 MPH?
 
What is your Vac/boost read at 60-70 MPH?
Thanks. These are really useful suggestions.
I'll have to check the 60mph reading soon. My tach is dead so it's hard for me to give much useful info, but the highest vac reading I've noticed during cruising is 22 (in hg?) and the highest boost I've seen is 9 psi. This seemed low to me.

I'm sure the EGR has never been replaced.
 
If it makes you feel better i only get like 12mpg on my sc with just driving it around here on nice days. When i drove it back from dave dalke ohio to chicago area, i was tailgating my dad on the express way and got 21mpg. So there is a big range possible from city driving to highway driving, for me 16 mpg would be what i get for mixed city/hwy.
 
I don't know how much modifications you have all done to your cars, but my 1990 is mildly modified from stock and gets about 21 mpg in my daily commute. That's about 60% highway, 40% city. I check the mileage at each fillup with a quick mental calculation. (Just out to 1 decimal place, to keep my math skills sharp.)

I think some of these mileage numbers sound awful. Might be worth looking into.

I should also say that my mileage seemed to drop 1 or 2 mpg when the 10% ethanol gas came out. Yech.
 
90/95

on a trip about 2000 miles round trip in the 90 i got 26 on the hwy no cats or resonator kn panel filter only
on a recent trip of 1000 miles round trip in the 95 20-21 on hwy all stock except cold air.. the 95 avgs 20 no matter so i guess that good for these cars.
 
If it makes you feel better i only get like 12mpg on my sc with just driving it around here on nice days. When i drove it back from dave dalke ohio to chicago area, i was tailgating my dad on the express way and got 21mpg. So there is a big range possible from city driving to highway driving, for me 16 mpg would be what i get for mixed city/hwy.

Mine is the same way except for the fact that on hwy mine seems to be the same as city driving I only drive a 30 mile round trip to work about 70% hwy 30% in town and average 220 total miles with full tank and limited boost usage but mine is a 5 speed with 308 gears out back
 
My gas mileage performance

I have a 92 5 speed with some exhaust modifications. It has performance cats dropped down to take out the 90 bend, performance resonator, and performance mufflers with 2.5 in pipes all the way back. I also have high flow air filter and I get 12 mpg on a 5 mile trip one way to work with several lights.

I thought this was bad so I did search on this topic. I have a question about the vapor recovery system does this affect gas mileage if it was not working correctly? I do smell gas coming from the front passenger side by the charcoal canister would this affect my gas mileage? I did check the solenoid valve was working with 12 vdc applied but I am not sure if it's getting the signal to open during normal operation?

I haven't check for any vacuum leaks should I just replace the canister? Any help would be appreciated.
 
Before I tore down my '93 5spd.(also before e10 was everywhere) i routinely got 19 around town (keeping out of boost) and 31-34 @70 on the freeway.
Just driving 'cuz I likes my manual---11-15 up to 11 psi boost.

My truck absolutely hates e10! It takes 10-15% right off the top of my milage. I get 7.5-9.2 off the freeway maybe 12 on the freeway(keeping it under 70).
But it's a 6.8l v10 AOD 4wd cc sb w/4.30 gears and 31" tires. It has a Banks Trans-command but, it dosen't seem to do any thing 'til you put a load on it.
Working on the 'Bird, getting the engine and trans joined up for install. I really need to get her done for summer. Really need to save some $ on gas this year.
MikeH
 
I have a 92 5 speed with some exhaust modifications. It has performance cats dropped down to take out the 90 bend, performance resonator, and performance mufflers with 2.5 in pipes all the way back. I also have high flow air filter and I get 12 mpg on a 5 mile trip one way to work with several lights.

I thought this was bad so I did search on this topic. I have a question about the vapor recovery system does this affect gas mileage if it was not working correctly? I do smell gas coming from the front passenger side by the charcoal canister would this affect my gas mileage? I did check the solenoid valve was working with 12 vdc applied but I am not sure if it's getting the signal to open during normal operation?

I haven't check for any vacuum leaks should I just replace the canister? Any help would be appreciated.

If you are smelling gas from that area, I would guess you do have a problem with the EVAP system. However, I think the part that is most likely broken is one of the soft rubber hoses that connect the hard plastic lines to the solenoid and canister. On my car, they were all disintegrated. My solenoid seemed to be bad as well, so I changed it at the same time. The canister seemed to be fine.

I think that could contribute to bad gas mileage, but yours is REALLY bad. Have you been through an emissions test lately? Those results can help figure things out.
 
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