1999 Mustang GT

Chinspector

New Member
May 4, 2021
2
0
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20
Lamberton
So i bought a 1999 mustang gt not running for cheap as a project car. The first thing i did was diagnostic all around the car, found out the fuel pump wasnt priming, so i replaced it with a performance one(had to put in a new battery as well). Then it wasnt reading oil pressure so i slapped a new oil pressure sending unit in it, and it read just fine. I also put in new sparks as the old ones looked carbon fouled. Then i put in a new fuel rail pressure sensor, as i got a code specifically for that because the car is running rich. Today i put a new hose from the pcv to the intake because there wasnt one when i got it. Its still running rich and is having some problems with the anti theft system, but that only comes on from time to time preventing the vehicle from starting. Im not sure what to do next. I had it running one day where it was revving smooth, but now it kind of bogs from being rich again. Rpms drop lower when i try to idle it, killing the car.
 
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wmburns

SN Certified Technician
Aug 14, 2009
5,890
518
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Houston Texas
If asking for an educated guess:
  • check for vacuum leak
  • Is the intake vacuum reference line connected to the fuel rail pressure sensor (FRPS) connected and leak free? What is the fuel pressure as reported by the PCM?
  • check to see IF the MAF sensor is sending "reasonable" values. Does the MAF value change with the throttle?
  • Check to see IF the IAT and ECT sensors are sending "reasonable" values.
  • Check to see IF the O2 sensors are switching. Do they read rich or lean?
  • What are the STFT and LTFT at idle? At higher RPM's?
Do you have an ODB2 scanner capable of monitoring operational PID's? If you did it might make short work of your problem.

ForScan ODB2 scanner w ELM327 USB
 

Chinspector

New Member
May 4, 2021
2
0
1
20
Lamberton
If asking for an educated guess:
  • check for vacuum leak
  • Is the intake vacuum reference line connected to the fuel rail pressure sensor (FRPS) connected and leak free? What is the fuel pressure as reported by the PCM?
  • check to see IF the MAF sensor is sending "reasonable" values. Does the MAF value change with the throttle?
  • Check to see IF the IAT and ECT sensors are sending "reasonable" values.
  • Check to see IF the O2 sensors are switching. Do they read rich or lean?
  • What are the STFT and LTFT at idle? At higher RPM's?
Do you have an ODB2 scanner capable of monitoring operational PID's? If you did it might make short work of your problem.

ForScan ODB2 scanner w ELM327 USB
So this morning, the car started once but didnt want to start again. It gave me a fuel rail pressure sensor code again. Both of the LTFTs read 0%, and both of the STFTs were 25%. I think it was from a freeze frame at 2400rpm. Oxygen sensor monitor and oxygen sensor heater monitor tests werent complete. Catalyst, egr, and evap were all incomplete. There was no gas leak and i couldnt do a vacuum leak test because it wouldnt idle by itself.
 

wmburns

SN Certified Technician
Aug 14, 2009
5,890
518
204
Houston Texas
It gave me a fuel rail pressure sensor code again.
A non functioning FRPS is a "crank with no start sensor". The fuel trims values are not useable when there's a FRPS sensor. Focus first on the FRPS sensor DTC. Focus on the wiring to/from the sensor first.

Also note that the O2 sensors are not used while the motor is starting while in "open loop" mode. The O2 sensors only come into play once the PCM switches to "closed loop" mode.