Dismal fuel economy - 19 mpg average on freeway

I have 3:73 gears, cobra intake, exhaust, tune, and a heavy right foot and I still see 24 to 25 on the highway and 18 or so mixed on my 15 mile drive to work every day.
Ever clean the maf?
One of the front O2's could still be bad.
Fuel filter should be changed.
What does the instant mileage say if you reset it while you're on the highway?
 
97MarkJA said:
I have 3:73 gears, cobra intake, exhaust, tune, and a heavy right foot and I still see 24 to 25 on the highway and 18 or so mixed on my 15 mile drive to work every day.
Ever clean the maf?
One of the front O2's could still be bad.
Fuel filter should be changed.
What does the instant mileage say if you reset it while you're on the highway?
Click to expand...


The O2 sensors were replaced several days ago. Are you suggesting the new O2's could be bad?

Instant mileage hovers mainly below 20 mpg at 70 mph. If I reset the the average and drive at 70 mph for 35 miles on cruise control, it'll get 19 mpg. If I instantly lift up on the throttle, the instant mileage will go up to only 48 mpg. Shouldn't it go higher than this?
 
You def got a problem. I got 25ish@70-75mph fully loaded a month back.

I think it's time for a new engine.
 
sbcaprice305 said:
You def got a problem. I got 25ish@70-75mph fully loaded a month back.

I think it's time for a new engine.
Click to expand...

The engine runs fine and has less than 75k on the clock. There are no driveability issues and no check engine lights present. The issue is almost certainly the fuel management system. What else should I check for? I've done the following.

1. Replaced O2 sensors - Didn't notice any difference
2. Cleaned MAF sensor
3. Cleaned throttle blades
4. Replaced spark plugs
5. Cleaned air filter
6. Pressurized tires to 41 psi
 
sigh....Put the correct air into the tire. Better yet try helium. Instant mpg's...Also try fuel injectors

I still think a brand new engine would fix the excesive gas consumption.
 
So far, here are some of the more plausible suggestions I have received for fixing the poor fuel economy issue.

1. Replace injectors
2. Install new fuel filter
3. Check for clogged CAT
4. Replace coils

Are the injectors due for replacement at 75k miles?
 
My 1st Gen Mark had 209k on the clock when I lost the trans and only think I ever did to the injectors what run cleaner in a tank of gas threw them.

Also both my 1st and 2nd Gens got well over 60mpg if I let off on the highway. Oh and on the cleaned filter...just remove it and take a quick trip to see if it makes a noticeable difference? If it does easy to fix and you know it was contributing to the problem.
 
Ed, arent you the one that runs 87 octane?



Along with other unique and very well thought-out money saving ideas for your Mark....:shifty:
 
This might be a stupid question but are you positive that your trans is going into overdrive on the HWY? You should try #4 from the list and I would seriously throw in some 764's and no, the injectors shouldn't need to be replaced at 75K

I would run some injector cleaner in a tank of good gas. That's another thing I meant to ask was what type of gas are you using? Something could also be going on with the computer itself. The message center isn't always right either. They have been known to be a little off sometimes but not by too much. If possible, use 93 octane gas. It might cost more but in the end, you will have better fuel mileage and the extra 10 cents a gallon really isn't extra like most think it is. Be on the lookout for another computer at your local JY. I just sold one to someone on here because their car was acting up and it fixed it right up. Theirs was a 98 I believe.
 
sbcaprice305 said:
Ed, arent you the one that runs 87 octane?
Click to expand...

After seeing this post I will throw this in too. I had a 87' 305 GMC with a Quadrajet and if I ran 91 octane instead of 87 it would get 20 to 30 miles more out of a tank of gas.

Yeah doesn't seem like much in Mark terms but in 12mpg truck terms it was noticeable :confused:
 
Time for some seafoam in the tank and crank! What oil are you using? If it's the cheap crap it can break down to fast causing excessive heat and heat is no good for mileage. Just because there are no CEL's doesn't mean that there is no problem with the electronics of the system. Components can be at the end of the acceptable operating range still be "good" and cause driveabity problems. You can't always wait for the car to tell you it's sick before you give it a good check up. Get a good scanner and look at the numbers of the sensors while the car is running then get a good manual that gives the proper voltages for said sensors and compare the two. It is very possable that the trans is not going into OD especially after pulling the load you did with it. That's not a bash at you for doing that just an option.
 
I was just out on the highway and I was seeing aywhere from 10-24 mpg depending on how I drove and thats with 4.10's, TC and a supercharger.
 
Fuel grade??? A proper test to rule out computer is fill tank note exact gallons at each fill, count your Milage per tank the. Do the math, but it's always best to run a few tanks and the average your results
 
possibly pinched fuel line.
bad wheel bearing or brake caliper causing excessive drag.
:q:q:q:qty gas.
no overdrive. (although, you should probably still do over 19 without od.) what are your rpms at 70.
pull a plug or two to see if you are running stupid rich.
throw some scanner on it and watch the o2s flip flop. the new ones might be junk. happened before.
 
sbcaprice305 said:
Ed, arent you the one that runs 87 octane?



Along with other unique and very well thought-out money saving ideas for your Mark....:shifty:
Click to expand...

I switch back to high octane to see if gas mileage would improve but it didn't make a difference.
 
NoLimit95 said:
This might be a stupid question but are you positive that your trans is going into overdrive on the HWY? You should try #4 from the list and I would seriously throw in some 764's and no, the injectors shouldn't need to be replaced at 75K

I would run some injector cleaner in a tank of good gas. That's another thing I meant to ask was what type of gas are you using? Something could also be going on with the computer itself. The message center isn't always right either. They have been known to be a little off sometimes but not by too much. If possible, use 93 octane gas. It might cost more but in the end, you will have better fuel mileage and the extra 10 cents a gallon really isn't extra like most think it is. Be on the lookout for another computer at your local JY. I just sold one to someone on here because their car was acting up and it fixed it right up. Theirs was a 98 I believe.
Click to expand...

I am positive the transmission is shifting into overdrive and the torque converter is locking up. The RPM's on the tachometer read around 2000 rpm's on the freeway. The RPM's would be a lot higher than that if it were not shifting into overdrive.
 
keatonsdad said:
Time for some seafoam in the tank and crank! What oil are you using? If it's the cheap crap it can break down to fast causing excessive heat and heat is no good for mileage. Just because there are no CEL's doesn't mean that there is no problem with the electronics of the system. Components can be at the end of the acceptable operating range still be "good" and cause driveabity problems. You can't always wait for the car to tell you it's sick before you give it a good check up. Get a good scanner and look at the numbers of the sensors while the car is running then get a good manual that gives the proper voltages for said sensors and compare the two. It is very possable that the trans is not going into OD especially after pulling the load you did with it. That's not a bash at you for doing that just an option.
Click to expand...

I have not tried using Seafoam but at this point, it may be worth the try.

I'm using Chevron 5/30 oil and the oil was last changed around 1000 miles ago. For the next oil change, I'm going to use synthetic.

Getting a good scanner might be what I have to do to nail down this problem. Right now, I've been throwing parts at it and not getting any results.

Without any doubt, the transmission is shifting into overdrive and the torque converter is locking up and not slipping. I've had this happen on a 700R4 transmission so I know exactly how it feels.
 
chris2523 said:
possibly pinched fuel line.
bad wheel bearing or brake caliper causing excessive drag.
:q:q:q:qty gas.
no overdrive. (although, you should probably still do over 19 without od.) what are your rpms at 70.
pull a plug or two to see if you are running stupid rich.
throw some scanner on it and watch the o2s flip flop. the new ones might be junk. happened before.
Click to expand...

Should I be changing the spark plug boot covers? I'm still running the originals.

I don't think I've got a bad wheel bearing or stuck brake caliper. A bad wheel bearing would be accompanied with some noise and a stuck brake caliper would cause pulling and hot brake smell. I have neither symptom.

The scanner idea is an excellent idea. I'll have to do some research on what to buy.
 
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