Low Fuel Mileage

[SALEEN961]

Hello,

After driving my mustang for awhile and keeping track of my mileage over several tanks I must admit I'm a bit disappointed. My car is getting 14mpg city and 24.5 highway (71mph, 2150RPM) on average. My car has a brand new Accel Coilpack, MSD plug wires, NGK IX Iridium spark plugs, and a freshly cleaned K&N air filter (mileage didn't change significantly after filter was cleaned), I also ran a can of Seafoam through the engine but that too had little if any effect on my mileage.

I'm thinking I should change out the front 02 sensors to better regulate my engine's air/fuel ratio since the car seems like it runs a little rich to me. I don't plan on replacing the rear 02's since they only report on catalyst efficiency and as such won't affect closed loop fuel trim.

But before I go spending money I was wondering if there was any common or likely cause that I may be missing? For reference my car is a 1999 3.8L 5-speed.

 

[wmburns]

Clean the MAF. Use only a product designed for the job. If using an oiled air filter, the oil will contaminate the MAF more than typical.

Check for vacuum leaks. This is important. Do not cut corners here. Even the smallest leak will cause problems.

If over 100k miles, recommend replacing the front O2 sensor(s). This is just good maintenance.

What are the results of a compression test? Are your tires inflated to the level recommended by the owner's manual?

Consider using the "driver" mod. This can improve mileage by keeping RPM's below 2000 RPM. Coast up to lights.

OBTW; the highway mileage looks Ok to me. How much better did you have in mind? Looks like the city MPG could be better.

FWIIW, I frequently get 18+ on my 2000 GT in the city.

 

[SALEEN961]

I'd like to get somewhere between 28-32mpg highway and the factory rated 20mpg city. I know my highway mileage isn't terrible but it's more typical for an automatic then it is for a 5-speed.

I'll have to try cleaning the MAF as that's really the only thing I hadn't tried yet, other then replacing the O2's.

 

[wmburns]

I'll have to try cleaning the MAF as that's really the only thing I hadn't tried yet, other then replacing the O2's.

So what are the results of a compression test?

 

[SALEEN961]

So what are the results of a compression test?

Engine up to temp, cranking at WOT with a jumppack on the car.

Cylinder 1: 182 PSI
Cylinder 2: 181 PSI
Cylinder 3: 181 PSI
Cylinder 4: 179 PSI
Cylinder 5: 180 PSI
Cylinder 6: 181 PSI

 

[wmburns]

Compression looks very good. That means that the base engine is in good shape.

Clean the MAF.

Consider replacing the front O2 sensors. This is especially true if over 100k miles. As the O2 sensors age, their response time slows. This tends to affect city mileage more than highway (the load changes less during highway driving).

 

[SALEEN961]

I've been driving around for a while now with new front O2 sensors and a clean MAF but my mileage still isn't improving and at this point I'm really out of ideas. I put 560 highway miles on my car last night and my average mileage was 25mpg @ 71mph, sustained speed with no traffic, cruise control on, A/C off.

 

[wmburns]

The Mustang's rear brakes slide pins will gall to the bracket. This will keep the brakes from centering over the rotors. More importantly for you, the brakes will drag on one side.

Check and clean the rear brake slid pins. Regrease with high temp silcone grease.

Check your tire pressure.

What is your city mileage now?

 

[SALEEN961]

I'll definitely check the rear brakes, I know my fronts are good as I recently replaced them and greased the slides thoroughly. My tire pressure is still at spec and with all the highway driving I've done, I haven't done any sustained city driving recently so I can't say whether or not my city mileage improved just yet.

 

[The Flex]

This may seem obvious but have you checked the tire pressure? Is the car dog tracking?

I have seen many tire shops set tire pressure at 30-35 psi irregardless of the tires. This will hurt milage on a car with 6 or 8 ply tires that need 45-55 psi of pressure.

A really bad alignment could cost you major mileage also.

 

[SALEEN961]

I was finally able to run a few tanks of gas through my car in the city and my city mileage suddenly jumped up to 18.5MPG when driving conservatively and 18MPG when I'm romping on it. I'm not sure what my highway mileage jumped to but it was 24.5MPG before my city mileage jumped up.

I haven’t changed anything recently so I'm a bit puzzled by this, but I really can't complain about my mileage suddenly jumping up for no apparent reason.

 

[robb15033]

Fuel additives and ethanol can be a factor negatively. Have you or do you regularly Seafoam the upper intake plenum and the pcv induction line ect.?? Seafoam, fuel filter, spark. Try it if you haven't. It should be done at some time if not on a schedule.

 

[Skymarshal]

I've gotten around 24 MPG with mostly highway driving and driving like a grandma.

Several months ago, I noticed that one of the stations near me had a "special unleaded" 88 octane grade between the regular and the mid grade. I tried it and now I'm getting 26.5 or so. All the gas here has ethanol, so I'm at a loss to explain this...