Returning to stock question

Paradox2018

Member
Oct 9, 2019
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Pennsylvania
I have a 2001 Mustang GT that I recently got that came with an SCT tune.

I’m not sure if the tune was a custom tune or just a preloaded tune. He could’ve just changed the fuel octane rating and that’s it in the settings as well, but I’m not sure.

I want to return the car to stock temporarily to diagnose a P0190 CEL I can’t figure out and maybe see if stock makes it go away.

I’m not 100% sure if the car has other mods but all the seller told me was “a k&n CAI, 93 octane SCT tune and borla exhaust”. Can I safely return the car to stock without damaging the car while I drive it?
 
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wmburns

SN Certified Technician
Aug 14, 2009
5,671
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Just wondering. Have you tested to see IF the voltage at the FRPS itself?

For example. With the key on test between BN/WH and GY/RD wire. This should read the 5 volt VREF. If no VREF at the FRPS, it's not possible for the FRPS sensor to work.

We need to know what the PCM "thinks" is the FRPS voltage value. We can then force the FRPS to force the value to change by shorting between the signal return (RD/PK) between the BN/WK which should make the volts jump to 5. Shorting between RD/PK and GY/RD should make the FRPS volts go to zero. These tests will "prove" that the PCM is able to "see" the signal return value.

Measure the key on voltage between the FRPS signal return wire RD/PK and GY/RD. The volt value here should match what the ODB2 fuel pressure PID volts. If not, it means there is either a break in the signal return line or the PCM is bad.

I just don't buy that your problem is being "caused" by a tune. If your theory is true the problem would have been there since the beginning.
 

Paradox2018

Member
Oct 9, 2019
21
2
13
19
Pennsylvania
Just wondering. Have you tested to see IF the voltage at the FRPS itself?

For example. With the key on test between BN/WH and GY/RD wire. This should read the 5 volt VREF. If no VREF at the FRPS, it's not possible for the FRPS sensor to work.

We need to know what the PCM "thinks" is the FRPS voltage value. We can then force the FRPS to force the value to change by shorting between the signal return (RD/PK) between the BN/WK which should make the volts jump to 5. Shorting between RD/PK and GY/RD should make the FRPS volts go to zero. These tests will "prove" that the PCM is able to "see" the signal return value.

Measure the key on voltage between the FRPS signal return wire RD/PK and GY/RD. The volt value here should match what the ODB2 fuel pressure PID volts. If not, it means there is either a break in the signal return line or the PCM is bad.

I just don't buy that your problem is being "caused" by a tune. If your theory is true the problem would have been there since the beginning.
I will try out these tests tomorrow morning when I have some daylight lol, and I know the tune most likely isn't the problem but I still wanted to know if I could safely return to stock without damaging anything. I have only had the car for 2 weeks and got it with the tune already installed. I just want to also know how the car feels when running stock. That will also remove the possibility of the tune causing the CEL somehow.
 

007

Founding Member
Jan 29, 2002
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You can safely return to stock. Those mods are extremely minor (ACI and catback). Thats going to be a pretty cookie cutter basic tune to grab you maybe 10hp. A tune is certainly not required.
 

Paradox2018

Member
Oct 9, 2019
21
2
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19
Pennsylvania
Awesome, I will be returning to stock today and doing some testing then. I’m hoping the car feels better in stock but I’m not expecting too big of a difference. Currently the car doesn’t even feel too bad, maybe just slightly down on power
 

Paradox2018

Member
Oct 9, 2019
21
2
13
19
Pennsylvania
You can safely return to stock. Those mods are extremely minor (ACI and catback). Thats going to be a pretty cookie cutter basic tune to grab you maybe 10hp. A tune is certainly not required.
This is what is strange to me, I drove the car to my college classes today and when I turned my car back on after letting it sit for an hour after my first class the CEL went away. I'm confused why it went away without me doing anything and without me returning it to stock.

After I get out of my last class of the day I plan on cleaning the MAF Sensor and IAC Valve and also doing the test that wmburns posted above to see if that changes anything.

The car doesn't feel different when the CEL goes away, but the exhaust sounds slightly louder to me.