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My 2005 Ford 500 just had a Fuel Pump Assy. replaced. I filled up with fuel the next day and have driven it for about 100 miles. The car always has averaged about 18.5 to 19.0mpg in city traffic and freeways. Since that repair the mileage has dropped considerbly to 13.6mpg. Strange thing but the car seems to run just fine. No signs of misfire, lugging, fuel smell or anything that would make me feel like something is wrong. The car has 108,450 miles on it. Where do I look?
 

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Phoenix, AZ 85008
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Replace the O2 sensors
Those sensors are old and no longer transmit a signal fast enough. So the PCM is making fuel trim adjustments on old data. That is wasting some fuel as the PCM is just throwing fuel into the cylinders.
The O2 light will only trip IF the O2 sensors do not work at all. If the sensors work in any manner or form the CEL will never come on.

How old are the spark plugs, fuel and air filters?

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If the O2 sensors were not working, yes. AND the CEL would be on.

When the O2 sensors are working but not working as fast as they did when new. the PCM IS getting data on how the engine is running. However the data is no longer in real time. Because the PCM is making adjustments in real time on a cylinder by cylinder basis even at 4000 RPM.
By 100,000 miles the O2 sensors no longer have the speed they once did. This shows up as the PCM throwing more fuel than necessary.

You may have other issues too.
Low tire pressure
Dragging brake
Oil viscosity issues
Restricted air filter
Marginal spark plugs

Or a combination of the above. With no real drivebility issues, the concern is usually back to maintenance items.

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My 2005 Ford 500 just had a Fuel Pump Assy. replaced. I filled up with fuel the next day and have driven it for about 100 miles. The car always has averaged about 18.5 to 19.0mpg in city traffic and freeways. Since that repair the mileage has dropped considerbly to 13.6mpg. Strange thing but the car seems to run just fine. No signs of misfire, lugging, fuel smell or anything that would make me feel like something is wrong. The car has 108,450 miles on it. Where do I look?
Hello Joe645,

I am surprised your car hasn't triggered an engine light getting that low of fuel mileage. Like Action said you can try new O2 sensors, however if the MPG's started declining after a fuel pump replacement, that may be a link to the problem.

To properly diagnose the car, you could either start with a scan tool that reads the short term fuel trim per bank and see where that's at. If it's hitting hard -% values then I would suspect too much fuel pressure as it's running way to rich and the ECU is reducing the injector pulse width by high percentages. Then you could see if there is a fuel pressure port on the fuel rails and attach a pressure gauge and see what the actual pressure is and compare that to what the factory service manual says is nominal.

If the diagnostics confirm the fuel pressure is too high, it could be the fuel pressure sender, the fuel pump controller or the new fuel pump which took out the fuel pump controller.

There is much complexity to a newer car and sure you can load up the parts cannon and shoot it at the car, which is expensive and still might not fix it, or do the proper diagnostics and if you can't pay a good shop that can. It's going to cost either way, one way is just a guarantee that a solution will be found.

As a side note I am shocked that for a 3.0 litre mid sized car you are only getting 18.5 - 19 MPG at best. Geeze, my 54 year old 1968 Ford XL is bigger, heavier and has a 6.4 litre V8 and I get 16 around town and 18 on the highway.

Good Luck
 

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Thank you but if the O2 sensors were the problem wouldn't the car run bad or misfire?
Stick with Motorcraft sensors up top & maybe save with Denso or Bosch sensor(s) down past the catalyst.

I wonder if your stations have switched from summer blend gas to winter blend gas while your car was in the shop too?? How long was it "in for repairs"??

-Jon in TX
 

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Stick with Motorcraft sensors up top & maybe save with Denso or Bosch sensor(s) down past the catalyst.

I wonder if your stations have switched from summer blend gas to winter blend gas while your car was in the shop too?? How long was it "in for repairs"??

-Jon in TX
Howdy TBIRD430,

Just a quick FYI, once a factory car gets about 7-10 years old, there's no guarantee that a Motorcraft part is the same Motorcraft part that was installed on the factory line when it was made. It's just how car companies work at sourcing their parts both originally and afterwards.

Car companies do not make all their parts, they source the parts out, this is true initially and for about 7 -10 years and then thereafter. When a car company makes part X for an initial build, it's made buy another tier 1 company, however it's also extensively tested to meet all the requirements (that was my job). Even for a simple mechanical or electrical/electronic part testing can run in the 100's of thousands of dollars. However amortize that over a few million parts and the final part price still isn't much. That initial run of parts covers production and 7 or so years worth of factory replacement parts.

However once those stocks are gone and if the factory elects to have more made, they can go to anyone and have the part made again without the testing because it will be a much lower volume. GM does the same thing. I tested parts for the big three, imports and the heavy truck (class 8) market. It's just how it works and brand loyalty is a joke.

The best bit of advice is do your research on which tier 1 supplier originally made the part on the car and see if they still make the same part. If they do there is a much higher chance of getting a quality part, even though there will no extensive testing. I will say it takes loads (hours to days) of research to find out who made what for whom.

For instance GM cars of the 90's; Wells made most of the engine control parts initially for the factory and dealer stocks. Buy an AC Delco part now and it's complete crap. Fortunately Wells still makes most of the parts and sells to places like Rock Auto. That part will be far cheaper and better than buying some turd overpriced AC Delco part at a dealer. The same will be true of Ford Motor Company.

Just thought I'd mention this as most people do not know.

You're not supposed to look behind the curtain :)

Cheers
 

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Just my two cents worth, but the answer could be as simple as an erased memory. If the battery was disconnected during the repair, it would have erased the memory of the computer. Ford's system has the ability to learn. As the vehicle ages, the computer compensates for the aging of the sensors, to keep the system running at peak performance. If this happens, you will see things happen that did not occur before the repair. It might just need to have the system relearned. That is a procedure that a driveability technician would know about. He might also spot a sensor that does not operate at it's peak performance and should be replaced, bringing your fuel economy back up.
 
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