Really bad gas milage

bal · Aug 30, 2011

Hey everyone,

I been driving around my car for the past year as a weekend car. When i first got it i was getting about 300 km to a tank. I drive it on weekends so its spirited driving, but not drag racing light to light.

I threw on a pypes off road X-pipe around October, i didn't buy mils so i had check engine light come on. Gas milage didn't change much. Just recently things have gone downhill. Im getting 180KM to a tank about half of what i was getting. Driving a little more conservative compared to before.

I have changed plugs, oil changes, fuel filter,PCV valve, air filter was cleaned.

I ran a scanner came up with these codes, cleared it once and 181 code comes on after 10-15 kilometers. With no Cats i assume some are okay codes until i get MILS but any idea with these other codes.

P0141 02 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)
(assume cat related)

P0181 Fuel Temperature Sensor A Circuit Range/Performance
Not sure??

P0173 Fuel Trim Malfunction (Bank 2)
(assume cat related)

P0753 Shift Solenoid A Electrical
or
735 - gear 5 incorrect ratio
this one scares me, makes me think tranny is on its way out. Its 5 speed drives great. could larger wheels cause this?

any help would be appreciated.

also i did try the DIY mils, light came on right away so i assume those didn't work. or possible one side?

Mods
Jet Chip
BBK Air intake
BBK 78MM throttle body
Flowmaster 40's
Pypes Off road X-Pipe

wmburns · Sep 2, 2011

I have a question. Is this a manual or automatic? I'm assuming since the post states that the car is a "5 speed", that means it's a manual.

But, since the thread does not state the model year of the Mustang, that leaves us to guess if this is a 96-04 or a 05+.

Sooooooo. If this is a manual, what does it tell you when a DTC related to an automatic transmission comes up??????

I have helped many ppl over the years. In almost every case when the PCM starts doing really strange things, the problem was traced back to dirty/wet/corroded PCM/chip pins. If this were my car, that's were I would start looking.

bal · Sep 2, 2011

hey thanks for the response,

Car is a 2001 GT manual.

should have mentioned i was using a cheap code reader and after some research on that shows sometimes wrong codes show up. The 181 code i got is known to be read wrong should be 171.

I cleared the codes took a quick 30 KM trip and checked again nothing registered as of yet. Last thing i did was change the PCV valve and notice the car pulls lot harder. Im thinking that might have been one issue.

The 735 code i thought might possibly be because the tires are mismatched. I bought the car used i got 245/30/20 up front and 245/45/20 out back?

The PCM being wet could be an idea, the car is parked 5-6 days a week in a garage. Probably hasn't seen rain since summer started. Something i could check. Is the PCM under the hood, or by passenger footwell?

let me know if you got any more ideas?

Also what codes should be seen if cats are pulled without MILS?

wmburns · Sep 2, 2011

P0171 is a Lean on bank 1. Vacuum leaks and exhaust leaks lead the list in possible causes.

My reference does not have a P0735. I'm not a tire expert. But wider tires do not normmaly cause ABS issues unless the tires rub the sensor wires. Taller tires do. The front and rears must be within 8% of each other. The typical symptom is an ABS waring light. I'm assuming you would have mentioned if true. So IDK.

The PCM is in the passenger's kick panel. The Jet chip should be on the PCM itself. I would pull the chip and clean the pins with electrical cleaner. Re-grease with a very small amount if silcone di-electric grease (same kind used on spark plugs).

bal · Sep 2, 2011

I'll check out the PCM later today and clean it up.
ABS lights do come on, the very rare time on the highway I guess the difference in tires does turn off traction And abs light comes on. But after a quick restart of the car they go back to normal.

Bank 1 is driver side or passenger side. Some of the rubber hoses do look little cracked maybe should change all of them. Otherwise maybe after the O/R pipe was installed there was a exhaust leak.

I'll do things one at a time and see the results. Thanks for sending me in the right direction.

bal · Sep 6, 2011

Updated - did my first gas tank since changing the PCV valve. KM's have improved from 180KM to 225KM a tank. That might have been one problem as the old PCV was very loose maybe unmetered air was entering at that point?

Check the PCM and it was dry and clean, i cleaned it anyways again.

Still getting the same codes - 171, 141.

After researching gas tank, shows capacity is 15.7 gallons. Noticed on fill up i only got 10.4 gallons in. Wondering if gas light is supposed to come on that early.?

wmburns · Sep 7, 2011

One possible reason may be water in the gas tank.

What else did you do in an attempt to locate the vacuum leak or the exhaust leak?

Maxpowers · Sep 7, 2011

Just an FYI:

The proper way to test MPG (or in your case KMPG) is to fill your gas tank up. Drive the car and refill at the same station to as close as possible as you did the first time. There are slight differences between the pressure of the gas coming out of the hose, so some gas stations will shut the pump off before others.

Doing the above will give you a much more accurate reading than per tank. With the per tank, you have to rely on your fuel gauge being very accurate.

As far as a possible fix, if MIL Eliminators are in your future, buy/install them now. I have heard of people who haven't used them and lost a few MPG.

Lugnuts · Sep 8, 2011

I agree with max and burns. You are getting pretty good info. Just FYI, I have seen a fair number of instances where a check engine light will cause up to a 10% loss in fuel economy. Even if you know what the code is and do not associate it to the problem, it is a good practice to eliminate it from the equation. I would put the MIL eliminators on it or use this as an excuse to get a programmer and turn off the rear O2s. The programmer will give you a better picture (more accurate according to your info above) and you can see what your computer is seeing.

bal · Sep 8, 2011

Thanks for all the info guys. I actually drive down to the states and fill up at the same station. I'll try the same pump.

Vacuum leaks I'll have time tomorrow. Some of the rubber hoses on the intake and throttle body look bad, I'll. Try replacing them.

Would running seafoam through the intake show vacuum leaks or only the possible exhaust leak ?

Really bad gas milage

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