99-04 Gt, Calling All Mechanical Bug Wizards

TheJarlofNewEdge

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Feb 8, 2017
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New tadpole in the pond here
But I am having some problems, i've thought long and hard about, but i live in the boonies, the mechanical experience is around here but i can't seem to come to a conclusion

- had stock gt w/K&N intake
- running fine

- installed pypes o/r H
- tried eliminators, didnt work
- installed tune, helped but now when i cold start it, itll rev @ 1000 for a moment, and when i want to drive off, it has no power when i want to give the engine any actual load, i give it 3/4 throttle for about two seconds and it feels like it switches over to spit enough fuel again, and the horses return
- other times, about 6/10 drives, give or take, ill get to a stop sign, doing fine idling cold starting it, and itll want to just die on the way down the road, perse, if i was 100%ing the clutch

- one other thing, my rear o2's are registering a lean code, im assuming my car is running rich
- my SCT is supposed to turn those off, why does it come back?

I can retune and have no CEL for about an hour, and then it comes right back up.

Im thinking o2 spacers.

I love my car brothers, can anyone step up to the plate to save Sally's life?

I want her to be my daily driver,
My track car.
My everything.
<3
 

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Davedacarpainter

I think I've messed my pants
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Nov 28, 2015
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Welcome to stangnet.

I'll move your thread over to the '96-'04 tech subforum. That way you won't have to retype.

Look around the subforums for your type of mustang, you'll find lots of help there.
 

squeak93

10 Year Member
Jun 2, 2005
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What codes specifically is it throwing? The rears need the sensor and the heater turned off in order to run an off road pipe and not trigger a cel. They will have little to no effect on the issues you're stating though.

I would suspect that you have a significant exhaust leak that is tossing a lean code (rear o2 will only tell if the cat is working, the front are what trim the fuel).

IF it only does the weird idle with the tune then you need to adjust the idle air motor in the tune to compensate either way.

Post the codes
 

TheJarlofNewEdge

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Feb 8, 2017
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What codes specifically is it throwing? The rears need the sensor and the heater turned off in order to run an off road pipe and not trigger a cel. They will have little to no effect on the issues you're stating though.

I would suspect that you have a significant exhaust leak that is tossing a lean code (rear o2 will only tell if the cat is working, the front are what trim the fuel).

IF it only does the weird idle with the tune then you need to adjust the idle air motor in the tune to compensate either way.

Post the codes

P1131 & P1151, both lean codes if i remember correctly, it couldn't be an exhaust leak, I just had it checked out at two different shops a month ago, no leaks whatsoever, I even replaced my RH gasket, which helped a LITTLE bit, but not much, turns out it had the smallest leak on it, but the problems presume.
 

squeak93

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Jun 2, 2005
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p1131 is the bank two (driver side) sensor one (upstream) issue. Replace that sensor.

p1151 is related to a lean condition that is likely caused by the sensor not working.

Replace both sensors and report back.
 

TheJarlofNewEdge

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Feb 8, 2017
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If i were to take them off, and test them, what should they be throwing as far as voltage goes?

How would I test them, actually?

Any brand you recommend, or should I go all Motorcraft?
Sites you'd recommend?

Aaaaand, did you mean for me to replace both front sensors?

Or the LH rear also?
 
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squeak93

10 Year Member
Jun 2, 2005
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Bosh, buy from autozone/oriellys pep boys. Any of the normal parts stores will stock them.

Ignore the rears. Just replace the fronts. Do both, if one has an issue the other could fail as well.

I'm sure you could test them for load or impedance, I have no idea how. If you don't know the age of the sensors, it's best to just replace them, especially with the codes they are tossing up. Should be right around $100 for both.
 

wmburns

SN Certified Technician
Aug 14, 2009
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Are you using anti-foulers/spacers on the front O2 sensors? If so, likely this is one of the sources of the problems.

This thread does not state the model year of the Mustang. Why is this important? Because the later MY Mustang have a smarter PCM. There's a subroutine that runs from time to time that forces a misfire just to confirm that is seen by the rear O2 sensor. This subroutine was put into place specifically to combat the use of MIL eliminators or anti-foulers on the rear O2 sensors.

Another source of front O2 sensor problems is when long tube headers are used and the O2 sensors are located to far down. There isn't enough heat to allow the O2 sensors to work correctly.

Bottom line. IF there's even the smallest exhaust leak, EXPECT problems with the O2 sensors.​
 

TheJarlofNewEdge

New Member
Feb 8, 2017
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Are you using anti-foulers/spacers on the front O2 sensors? If so, likely this is one of the sources of the problems.

This thread does not state the model year of the Mustang. Why is this important? Because the later MY Mustang have a smarter PCM. There's a subroutine that runs from time to time that forces a misfire just to confirm that is seen by the rear O2 sensor. This subroutine was put into place specifically to combat the use of MIL eliminators or anti-foulers on the rear O2 sensors.

Another source of front O2 sensor problems is when long tube headers are used and the O2 sensors are located to far down. There isn't enough heat to allow the O2 sensors to work correctly.

Bottom line. IF there's even the smallest exhaust leak, EXPECT problems with the O2 sensors.​


- 04 Engine, '00 body (no idea about the computer in it though)
- im not using spacers on the upstreams haha
- just ordered new upstream o2s
- pypes o/r H

and i just have the rear o2s turned off with a tune

das awl
 
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