Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Low mpg; dealer recommended $59 gas cap

I went to my mechanic to ask suggestions why I only get 18 mpg on my 2003 Ford Focus car with 133,000 miles on it. They said the spark plugs were good & no tune-up was needed. They replaced the fuel pump & there was no change in mpg. They then told me to buy a “special, original FORD gas cap” for $59. Is this good advise or can you guys think of anything better?

It’s good advice for him, but not for you. Just because the spark plugs are “good” does not mean the rest of the car is OK (except for the $59 gas cap?). Find another mechanic and get a good tune up.

Edited: Ask him if you can return the cap and get a refund if it does not work.

1 Like

The remedy does not match the complaint, is the check engine light on with an evaporative emissions fault?

In most cases the fuel economy of a vehicle is up to the driver and not the mechanic. Do you drive in an environment where exceptional fuel economy is expected?


I’ve never heard of a bad gas cap causing low fuel economy. If yours was malfunctioning, the check engine light would be illuminated and you’d get a code related to the fuel system.

If you want to rule it out as a cause, you can go to the nearest auto parts store and buy a compatible gas cap for much less than $59.

1 Like

I, too question the gas cap recommendation. I will question the need to replace the fuel pump, too. Why was it replaced, Did it fail or was it replaced for the mileage complaint? If it was replaced for mileage, you just got ripped off.

This “mechanic” has no idea what he is doing and is spending your money doing it.


from what other people are responding I agree what does the gas cap recommendation will do? and also maybe it looks like one of the fuel injectors are either stuck open or stuck close if someone can correct me if i’m wrong please cant that be a cause for poor gas mileage? I’m just going on a guess here ._.

Can you give a little background, please? What was the typical MPG since you’ve owned it? Was it a sudden drop. or gradual? Have you had any recent work done; new tires for example. Or is a different family member driving it part time?


I often wonder about these posts that don’t give a MPG range. Such as my normal driving in our pickup I get 18 to 20 but when I make a highway trip of about 70 miles it will be 21 to 23, and that is checking it the correct way.

A stuck injector will generally throw a check engine light because the engine will run rich and the ECU won’t be able to lean it out enough.

Short of actually diagnosing the car… code and data scan, compression and/or leakdown check… any other speculation is a WAG at best.

1 Like

i never knew that thank you Mustangman

1 Like

I bought this car as a replacement (for my Toyota, which was totaled) 10 months ago. I have no history of what the previous gas mileage was. But I was consistently getting 18 mpg regardless of fuel used (Arco, Shell & Ralph’s) with about 20% highway miles. The rest of the miles were local free flowing miles with a minimum of red light stops.
A month ago, I was a good Samaritan & allowed a stranger to jump Start his car with my car running. I didn’t know until 2 days later that he reversed polarity & burned out my alternator. When I went to get it repaired (New alternator), I was told that fuel pressure dropped dramatically after the car started each time & that was caused by a faulty fuel pump, which they changed. Since then, the gas mileage dropped to 17mpg, with the exact same driving pattern. The only other thing different is that the fuel tank gauge stays at full for the first 100 miles, then drops very fast day by day over the next 90 miles & almost takes a full tank of gas at a little less than ¼ on the gas gauge. I usually can go about 190 to 200 miles on 11.6 gallons of high tier gas. The mechanic claims the gas cap I have is leaking gas vapor, & since my car is a 2003, the newer caps don’t fit right. I hope this fills in all the info gaps mentioned by everyone.
By the way…thanks for all your inputs on this matter

No engine light was ever on

Without a check engine light on it is unlikely that there is a vapor leak that needs to be repaired.

Additionally, it is unclear to me whether all of this “parts throwing” has been done by an indy mechanic, or by the folks at a dealership.
Can the OP clarify who is recommending these repairs that may–or may not–be necessary?

Let me see. You have a squeaky fuel tank sender. Could be something else, but that is a possible explanation of non-linear read out. This gauge issue does not at all affect the actual gas mileage of car. It could convince you to run out of gas if you believe it. Keep it above 1/2 level to be sure.

A number of things can cause low gas mileage. Dragging brake. Bad wheel bearings. How you drive. Something wrong with transmission, but not sure what.

Go to the Mechanix files page on this URL, and find a good mechanic in your area and get a correct diagnosis. You can spend a lot of money with mechanics who can’t diagnose their way out of a wet paper bag and just want to install the most expensive parts. There are good mechanics out there. Find one and when you do, treat him like a precious asset.

…plus, a cooling system thermostat that is stuck in the “open” position…
…plus carrying around a lot of “junk in the trunk”…
…plus something as simple as underinflated tires.

The action of your gas gauge is totally irrelevant to your gas mileage. The only way to measure the mileage from a fill up to the next fill up. This should be repeated over 4 or 5 tanks because of variations of automatic shutoff valves, driving conditions, weather etc. The difference between 17 and 18 mpg on one tank is not statistically significant. Either way there is something wrong with your car or the way you drive.
I am not saying you do these things, just asking. Are you in a hurry when you drive? Angry? Do you accelerate until you have to step on the brake. Do you wear out the brakes very often on a car? Remember, every time you step on the brake you are wasting momentum you built up by burning gas and generating heat that you can’t recover.

The fuel level unit may have been bent or damaged when the fuel pump was replaced. You might ask if they warranty their work.

The fuel economy for this car is listed at 18 to 24 MPG city depending on engine/transmission in the vehicle so your fuel economy may be normal.

1 Like

Next weekend take it on a reasonably flat freeway drive, on a day that isn’t overly windy, non-stop at 60 mph for an hour, so roughly a 60 mile drive. Fill up just prior to the on-ramp, and again when you leave the freeway at the end of the hour. Measure the mpg. If measures more in line with what you think it should be, you probably don’t have a problem. The mpg you are getting is what you should be getting given your normal driving terrain and driving style.

If the mpg still isn’t very good though, first things I suspect for poor mpg is the thermostat is partially stuck open (a pretty common things on thermostats over 5 years old), or the computer’s engine coolant temp sensor is inaccurate. That can cause double pulsing of the injectors.

1 Like

The gas cap is leaking fumes and that’s why your mileage is too low? Man, that’s creative. Bullcrap, but creative. The cap only seals the fill tube. If it were leaking, you’d have an EVAP fault code, and the hydrocarbon-saturated air in it (normal) would be drawn into the engine via the purge system, as it is when the system is operating normally. The fumes are captured by the charcoal bed (see below) and when you start the engine a “purge valve” opens and allows the engine to breath them in.

It’s a 12 year old car with 133,000 miles. The first thing that needs to be done is to evaluate the condition of the engine. A compression check and a vacuum gage are great places to start. They’ll tell you whether the engine is badly worn, and can even suggest bad valve timing and/or a vacuum leak. An engine analyzer is also a great idea, as it can tell a mechanic whether the ignition system is sound.

Since the fuel pump apparently needed replacement, there IS a possibility that the charcoal canister is saturated. The gas tank, as the gas is pumped out, breaths in through the charcoal bed in the canister. The activated charcoal prevents hydrocarbon molecules from escaping through the vent hole next to the fill hole. IF the charcoal is saturated, a vacuum will develop in the gas tank’s airspace as the fuel is pumped out, forcing the pump to work harder to overcome it and adversely affecting the engine’s ability to run with the power it needs to be efficient. It also can cause premature pump failure.

You really need a good diagnostician to assess the engine and these other fuel system possibilities. The problem is NOT the need for a $59 OEM gas cap.