Help please...

Mike, I don't know whether its applicable to you, but here in Pa in November they start putting a funky additive in the gas to reduce emissions in the winter. Then in march they go back to the rgular mix. I have found that my gas milage goes to shit with this mix.Preformance sucks too.Do they do this out your way?

they call it"oxygenated" gas.

Dave

I haven't heard anything a/b it... :x

-Mike

-- A happy kid behind the wheel of a 98 Mustang GT Cold air intake FRPP 3.73 gears Steeda Tri-Ax Shifter Flowmaster 40 Series mufflers (self-installed woohoo) Hi-speed fan switch

255/60R-15 rear tires Subframe connectors

How many miles on the engine? Is the O2 sensor the original one? These sensors do wear out. A sign that they're going bad is your gas mileage getting slowly worse. They default to a rich mixture when they start going bad - this is so you don't burn your engine with a lean mixture. Rich mixture means you burn more gasoline than normal - so your gas mileage gets gradually worse.

hope this helps,

Oskar

I did have codes for the rear 2 O2 sensors, but they went away. 122k miles on the car. Dont' know if the sensors are original.. they probably are.

-Mike

-- A happy kid behind the wheel of a 98 Mustang GT Cold air intake FRPP 3.73 gears Steeda Tri-Ax Shifter Flowmaster 40 Series mufflers (self-installed woohoo) Hi-speed fan switch

255/60R-15 rear tires Subframe connectors

Interesting... I just (out of curiousity) romped on it while the car was semi-cold.. almost warmed up. No stutter *at all* to 4,500RPM. I didn't wanna go higher.. and even that was making me uncomfortable. I did it three times.. not a single hint of a stutter.. then I drove it for another 15 mins to let it warm up.. stomped on it.. stuttered a bit =(.

-Mike

-- A happy kid behind the wheel of a 98 Mustang GT Cold air intake FRPP 3.73 gears Steeda Tri-Ax Shifter Flowmaster 40 Series mufflers (self-installed woohoo) Hi-speed fan switch

255/60R-15 rear tires Subframe connectors

It could also be that the fuel trims aren't quite right. Did you reset the PCM after the exhaust install? You might try this just to clear all the trims and let the PCM re-adapt.

It sounds like you've had an OBD scanner on there? True?

O2 sensors can cause this as well as the EGR system. . Dan

2003 Cobra convertible With some stuff and things

I didn't reset the PCM via battery-disconnect yet.. I guess I could. What are "fuel trims"? (Excuse the ignorance please). Yeah.. I've had an OBDII scanner on it (I have a coder reader)... and come to think of it.. the driver-side O2 sensor is about 40k miles old. The passenger side I think is original. I replaced the EGR valve roughly 20k miles ago.. maybe less.

-Mike

-- A happy kid behind the wheel of a 98 Mustang GT Cold air intake FRPP 3.73 gears Steeda Tri-Ax Shifter Flowmaster 40 Series mufflers (self-installed woohoo) Hi-speed fan switch

255/60R-15 rear tires Subframe connectors

The PCM maintains a set of parameters that allow it to adapt the changes in the system over time. Some of these are short term fuel trims (STFT) and long term fuel trims (LTFT) which are time averaged parameters that the PCM uses to shift the AFR. For example, if your car tends to run lean, the fuel trims will cause the mixture to be a little richer to accomodate this. The long term fuel trims are also used by the PCM at WOT as the open loop fuel modifier (OLFM).

It can happen, for a number of reasons, that the trims shift too far and cause some mild drivability issues. It's not very common but it can happen.

. Dan

2003 Cobra convertible With some stuff and things

*shrug* worth a shot. Thx for the info. I'll do that tomorrow.

-Mike

-- A happy kid behind the wheel of a 98 Mustang GT Cold air intake FRPP 3.73 gears Steeda Tri-Ax Shifter Flowmaster 40 Series mufflers (self-installed woohoo) Hi-speed fan switch

255/60R-15 rear tires Subframe connectors Aluminum adjustable clutch quadrant

well poop. that explains a lot. I've been getting poor-er milage since right about then and had no idea why. Nice of them to let the public know, huh? Where in PA are you at Dave? I'm stationed in NEPA.

Chester county

Dave