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Discussion Starter · #1 ·
I have a 1997 Cobra with an off road H pipe and am having the following issues. After running some errands in the Cobra for a few hours with the car running normally out of no where as I tried to accelerate when the light turned green the car acted as if it was not firing right( Hesitating, Sputtering). I pushed in the clutch and got normal response from the engine when giving it gas. But again hesitation while in gear and under load. I have had a check engine light on for a while but was told by a buddy who diagnosed the problem at his shop that it was due to the H pipe messing with the oxygen sensors. Now I am getting the same readings which are Oxygen Sensor and Lean fuel. Does replacing the four sensors sound like the answer to my problem ? PLEASE HELP !
 

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looks like low fuel pressure.the check engine light will come on for a thousand different reasons.usually,lean system together with o2 codes is caused by a vac leak or low fuel pressure.
 

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How many miles on the car? Does it still have the front O2 sensors hooked up?

I would tend to agree, it sounds like it might be something wrong with the fuel pressure. If the filter hasn't been changed, it could simply be that.

Depending on the mods to the car, you might be running lean due to them, too. How long have you had the O/R H?
 

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Discussion Starter · #4 ·
I have only had the car a year or so. Ihave noy changed the fuel filter. The car has around 64k miles. And Im not sure how long the O/R H has been in place or do I know if the fronts are still hooked up. I assume they are besides the exhaust there is a cold air intake and the pulley have been swapped. No other mods. Remember It has normal throttle response when in Neutral or with the clutch in.
 

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Pressure

I have only had the car a year or so. Ihave noy changed the fuel filter. The car has around 64k miles. And Im not sure how long the O/R H has been in place or do I know if the fronts are still hooked up. I assume they are besides the exhaust there is a cold air intake and the pulley have been swapped. No other mods. Remember It has normal throttle response when in Neutral or with the clutch in.
The response would be normal - at low load levels - if the filter was starting to block up, or more likely, if the pump was failing. As you demand more from the car, and the system strains to supply sufficient fuel, you can run lean. The CAI may be contributing to the problem, particularly with low back pressure in the exhaust.

You might also look at the IAC, if its sticking, it can create a problem like this.
 

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I have also continued to have these same problems with my 04 GT. Very sluggish and stuttering with poor throttle response. My cel codes kept coming up as o2 sensors slow to respond, so i replaced both upstream sensors as well as replaced all spark plugs and my stang continues to throw a slow to respond o2 sensor on the passenger side. I have no exhaust leaks, no vacuum leaks, idle is good, wiring is good, and o2 sensors r obviously brand new. I tired taking my CAI off and putting the factory one back on and my stang ran like a dream for about a week but then slowly started getting back into its sluggish funk. This has been an ongoing issue for about 4 months now and not even Ford can figure out what is going on. If i needed a new fuel pump isnt that something that Ford would be able to easily diagnose?
 

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Parts

Did you replace the O2 sensors with OE Ford parts? Motorcraft, etc? Be advised that problems with aftermarket sensors have been reported. Since it sounds like you have been dealing with a Ford dealership, moving on...

Did you swap out the CAI before replacing the O2 sensors? Assuming it was afterward, the fact that it helped for a while leads me to believe you may have a problem with the MAF, etc, something upstream of the O2's.

Are you running the stock tune?

As for any potential fuel pressure problems, yes, that is something easily diagnosed by a competent mechanic.

I have also continued to have these same problems with my 04 GT. Very sluggish and stuttering with poor throttle response. My cel codes kept coming up as o2 sensors slow to respond, so i replaced both upstream sensors as well as replaced all spark plugs and my stang continues to throw a slow to respond o2 sensor on the passenger side. I have no exhaust leaks, no vacuum leaks, idle is good, wiring is good, and o2 sensors r obviously brand new. I tired taking my CAI off and putting the factory one back on and my stang ran like a dream for about a week but then slowly started getting back into its sluggish funk. This has been an ongoing issue for about 4 months now and not even Ford can figure out what is going on. If i needed a new fuel pump isnt that something that Ford would be able to easily diagnose?
 

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Yes i am running a stock tune and yes they are Ford Motor-Craft sensors. I am planning on getting my mustang dyno tuned this summer, however i figured i should resolve the problems i am experiencing with my o2 sensors before i get the tune. As far as the MAF goes, i immediately assumed that something may be wrong with the sensor but the ford mechanics ruled that out seeing as how it is not throwing any codes in relation to the MAF. The mechanic assumed that my car was running better after re-installing the factory intake bc the computer brought itself to a complete relearn and eventually over time slowly recognized the same problems created by the o2's, but it also recognized the correct amount of airflow created by the factory intake.
 

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How are you bypassing the rear O2 sensors with the offroad midpipe? MIL eliminators?
 

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How are you bypassing the rear O2 sensors with the offroad midpipe? MIL eliminators?
I am using plug-in MILs, however i do not think that they are functioning correctly bc my rear sensors are still throwing a PO161/141 code which is some kind of sensor heating malfunction. I called AM and they are sending me a new pair so hopefully the new ones will get rid of these codes. But regardless of the fact that those rear sensors are throwing this code they still shouldn't cause any driveability issues such as stuttering, surging, and loss of throttle response. This is why i am so confused as to what is causing my upstream passenger sensor to continuously throw a code for "slow to respond" even though i have no vacuum/exhaust leaks, good wiring, new o2 sensors, and no thrown codes in relation to the MAF.
 
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