Dismal fuel economy - 19 mpg average on freeway

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Ed Hall

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I'm experiencing poor fuel economy on my 1997 Mark. Fuel economy has been steadily declining and it's down to about 19 mpg on the freeway. No check engine light is present and the torque converter is locking up fine. For comparison, my 1993 Cadillac with the same power rating and 3.71 gears gets 27 mpg with the same driving.

So far, 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. Pressurized tires to 41 psi

This morning when I started the car, the engine ran very rough like it was missing on 1 or 2 cylinders. However, it cleared up after idling for several minutes. It has done this on 3 other occasions that I can remember over a period of 1 year.

Also, the highest I ever see on the instant fuel economy meter is 48 mpg if I completely let off the throttle while going down hill. Shouldn't it be higher than this?
 
Ed Hall said:
I'm experiencing poor fuel economy on my 1997 Mark. Fuel economy has been steadily declining and it's down to about 19 mpg on the freeway. No check engine light is present and the torque converter is locking up fine. For comparison, my 1993 Cadillac with the same power rating and 3.71 gears gets 27 mpg with the same driving.

So far, 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. Pressurized tires to 41 psi

This morning when I started the car, the engine ran very rough like it was missing on 1 or 2 cylinders. However, it cleared up after idling for several minutes. It has done this on 3 other occasions that I can remember over a period of 1 year.

Also, the highest I ever see on the instant fuel economy meter is 48 mpg if I completely let off the throttle while going down hill. Shouldn't it be higher than this?
Click to expand...

You try the air filter yet?
 
2002WRXSTi said:
You try the air filter yet?
Click to expand...

The air filter looks clean after banging out the dust that it had collected. I thought the air filter cleanliness doesn't affect fuel economy on a fuel injected engine but maybe I'm wrong.

I haven't replaced the fuel filter. However, I would think a clogged fuel filter would exhibit driveability issues during WOT.
 
Ed Hall said:
The air filter looks clean after banging out the dust that it had collected. I thought the air filter cleanliness doesn't affect fuel economy on a fuel injected engine but maybe I'm wrong.

I haven't replaced the fuel filter. However, I would think a clogged fuel filter would exhibit driveability issues during WOT.
Click to expand...

Ditto on the FFilter but I just changed mine and the car felt a we bit better. When I made my runs at Etown it was with original Spart Plugs, Fuel Filter and a 20k air filter.

I now want to go back and see if it improves on the 15.1?

You can just take the air filter out and drive to see if the instant mpg changes or not?
 
Plug wires ever been replaced? Are they arcing or breaking down? How about your coil packs. I know those hardly go bad but never hurts to swap one out and check.

Did you get a bad tank of gas?
 
+1 on the air filter and wires.

And you can see a plugged cat with a vacuum gage.:)
 
Quit towing a trailer around

But Obvs replace air filter every 20-30,000 same with fuel filter It's a 97 so it coil on plug, if a boots bad you will have a misfire if no missfire no bad boot

What kind of plugs are you running...best results with motorcraft, or autolite but usually only run motorcraft in my fords
 
Stangman said:
Plug wires ever been replaced? Are they arcing or breaking down? How about your coil packs. I know those hardly go bad but never hurts to swap one out and check.

Did you get a bad tank of gas?
Click to expand...

The engine uses the Coil On Plug design so there are no spark plug wires.

The engine completely smoothed out after several minutes so I don't think it was a bad coil. From my experience, coils usually work or don't work. They're not intermittent.
 
87TTOPGT420 said:
Quit towing a trailer around

But Obvs replace air filter every 20-30,000 same with fuel filter It's a 97 so it coil on plug, if a boots bad you will have a misfire if no missfire no bad boot

What kind of plugs are you running...best results with motorcraft, or autolite but usually only run motorcraft in my fords
Click to expand...

I haven't been using it for towing since I've got a truck with a powerful Cummins engine to do that. However, the car did surprising well considering the load I towed.

I'm running Autolite platinum plugs. The boots are original.
 
Ed Hall said:
I haven't been using it for towing since I've got a truck with a powerful Cummins engine to do that. However, the car did surprising well considering the load I towed.

I'm running Autolite platinum plugs. The boots are original.
Click to expand...

Since it's cheap, try using Autolite 764's copper. Might not fix the problem but platinum is overrated. Glad to hear you quit using the Mark for towing.

With my air on and running at 75, mine averages at around 25 to 26 mpg and I have 32 psi in the front and 31 psi in the rear.
 
NoLimit95 said:
Since it's cheap, try using Autolite 764's copper. Might not fix the problem but platinum is overrated. Glad to hear you quit using the Mark for towing.

With my air on and running at 75, mine averages at around 25 to 26 mpg and I have 32 psi in the front and 31 psi in the rear.
Click to expand...

If you're getting that kind of gas mileage at 75 mph, there definitely is something wrong with mine.
 
This is what mine shows on flat hwy. Some get better than this, like 27, 28

Photo0522.jpg
 
ed you have something real wrong because i can get 24 mpg at 75 with 4:30 gears in my 97 lsc. and for my gears thats some good gas mileage. even around town i average 14.8
 
Ed Hall said:
From my experience, coils usually work or don't work. They're not intermittent.
Click to expand...

not always, when our coils fail (which is about as often as your air systems need servicing) they often work great most of the time and intermittently miss fire more and more. they usually end up getting replaced long before they fail completely.
 
I have only experienced the miss 3 times over a period of 1 year. The miss only happens when the engine is first started but it goes away after several minutes. Does this sound like a defective coil?

Also, the highest I ever see on the instant fuel economy meter is 48 mpg if I completely let off the throttle while going down hill. Shouldn't it be higher than this?
 
Have the.O2 sensors ever been changed? Just because you don't have a CEL doesn't mean they are working properly. They do effect the mileage.
 
97MarkJA said:
Have the.O2 sensors ever been changed? Just because you don't have a CEL doesn't mean they are working properly. They do effect the mileage.
Click to expand...

Yes, the O2 sensors were changed but it was an exercise in futility and on top of that, they were not easy to change out. I noticed no difference with the new O2 sensors other than a lighter pocket book.
 
BAD97LSC said:
ed you have something real wrong because i can get 24 mpg at 75 with 4:30 gears in my 97 lsc. and for my gears thats some good gas mileage. even around town i average 14.8
Click to expand...

yeah. its gotta be major. i've seen 26 on an open highway with 4.10s and still do 16 ish in town. i can drop into the teens on a hilly road (like to school and back) and driving hard but 19 highway is nuts.
 
Ed Hall said:
The air filter looks clean after banging out the dust that it had collected. I thought the air filter cleanliness doesn't affect fuel economy on a fuel injected engine but maybe I'm wrong.

I haven't replaced the fuel filter. However, I would think a clogged fuel filter would exhibit driveability issues during WOT.
Click to expand...

Have you ever tried wrapping a scarf around your face and going for a nice long sprint down the road?
 
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