Drastic MPG drop after repairs

[InvalidUserID]

So this is on my girlfriend's 2001 Mustang (V6). Late last year, it developed bad hesitation under acceleration and a shaky idle. Initially was told it was the fuel pump & catalytic converter restriction which were replaced but the symptoms remained. The spark plugs & wires were then replaced and the symptoms disappeared. It drove perfectly fine the way it did except the MPG dropped a good 20% or so. It never got good MPGs anyways but it's now at about 15 MPGs down from about 20 MPGs. Here is my checklist that I've gone through that have been replaced recently:

- Spark plugs
- Plug Wires
- All four catalytic converters
- Air filter
- Fuel filter
- Fuel pump
- Brand new tires, inflated properly (didn't effect MPGs with old tires to new)
- ATF fluid was done a little over a year ago along with the filter.

With 150k on it, the only thing I didn't find in the paperwork was O2 sensors. There are no CELs and no smell of fuel. Other than replacing the O2 sensors, where else should I look?

 

[147_Grain]

Be patient as it will take a week or so for the computer to relearn things with all the new replacement parts.

 

[spasm3]

Any time the cats are changed, i would replace the upstream O2 sensors.

 

[Leo99]

Originally Posted By: spasm3
Any time the cats are changed, i would replace the upstream O2 sensors.


I'd replace the O2 sensors before I'd even consider replacing the cats.

Cats are expensive.

 

[InvalidUserID]

147_Grain, that's what I thought as well so we gave it a few weeks to see if the economy improved but it never did.

I've got the o2 sensors already, just gotta find time to install them.

 

[Doog]

Did you try pulling the negative cable on the battery for an hour to see if it will reset the computer?

 

[InvalidUserID]

Originally Posted By: Doog
Did you try pulling the negative cable on the battery for an hour to see if it will reset the computer?


Yes. Tried that, didn't work.

 

[Nate1979]

How many tanks of gas have you gone thru to know the mpg has dropped?

 

[InvalidUserID]

It's been about 10-12 tanks.

 

[bvance554]

One time I replaced the spark plugs on a Saturn with a reputable brand - they may have even been OE. It was all I changed. MPG dropped by more than 25%, but it ran just fine. I replaced the plugs with a different set and everything was back to normal. I'm not saying change your plugs, but new parts do not always mean good parts.

 

[Clevy]

So it's a 3.8 litre engine then.
It has to be lazy oxygen sensors. Now that the entire exhaust stream is new and unrestricted the O2 sensors aren't relaying data fast enough if at all. And the ecu has its learned parameters which no longer apply so it will take many start cycles for the ecu to get into its optimal operating range.
The sensors after the cat only monitor the cat. Nothing else. They have no input in relation to the air/fuel delivery. The sensors before the cat are the important ones since it's their data that tells the ecu whether to richen or lean out the air/fuel mix.

 

[SnowDrifter]

Your plugs gapped properly?

 

[Joel_MD]

Is this drop in fuel economy actually hand calculated by dividing the odometer reading by the fuel you are adding, or is this just based on the MPG displayed by the car's computer?

 

[bdcardinal]

What type and gap on the spark plugs? Also are any of the IMRC bushings broken? The little bushings on the ends of the IMRC rods can break and not let them butterflies open all the way.

 

[InvalidUserID]

MPG was calculated by taking the miles driven since last fill-up and dividing by gallons put in. She mentioned herself that she's at half tank well before she used to (in terms of miles). Car doesn't have an onboard MPG display.

Plugs were Motorcraft OEM specified in the manual.

No idea on the IMRC rods...

 

[Lubener]

If the O2 sensors are original, I'd replace them, then go from there.150k on them, they are long overdo.

 

[Oro_O]

Certainly can't rule out sensors or plugs, but also:

1) have you checked the PCV valve? Did not see it in your list. Dirty EGR does it on some vehicles, too.

2) Don't neglect coincidence. Check brakes for a sticking caliper or other driveline drag.

I spent 6 months after a timing belt job trying to figure out why mileage was off. Never raised each axle and checked brakes. The pins were sticking and binding on two calipers.

 

[Gokhan]

Probably MPG hasn't changed but just the way it's measured and calculated or the vehicle is driven is different.

It sounds like she needs a Prius.

 

[Gasbuggy]

O2 sensors. Get Advanced or another store to price match Rock Auto so you can get them fast but for much less $$

 

[zzyzzx]

First of, I have over 150K on a Bosch O2 sensor in my Escort, and as far as I can tell, it still works fine. The OE NGK one died at around 70K. And I also think the cat replacement might have been unnecessary, I have to take the tailpipe test of my Escort and even with the original Cat at 228K I still easily pass emissions. I want to know what brand spark plug wires and model if applicable. I get better results with Motorcraft, even though they are made by Belden, the Motorcrafts are still better.