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Discussion Starter #1
Help. My 1999 mustang gt will start and idle but will not rev up at all. I did a smokey burnout on my way home from work. When I got to the house it was low on gas so picked up some gas and filled it with about 5 gallons. After that when I started it it would not rev up and threw four check engine codes. P0300, P0161, P0156, and P1504. I have some modifications done cold air intake, underdrive pulleys, short throw shifter, 3.73 gears, and comp cams xe262ah installed. Any information would greatly appreciated. I bought this car new as a birthday present and I am the only one who works on it.
 

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Before we get into a bunch of tests, check fuses F2.2 and F2.8.

Most of the O2 sensors errors deal with the rear O2 sensors. Any exhaust mods? Inspect the O2 pigtails to see if any have contacted hot exhaust?

DTC P0300 can be cause by an unstable CKP signal due to incorrectly torqued crank bolt or under drive pulleys.

However, note what the 2nd item under P0300 is.

Does the no rev problem still occur after PCM/code reset?
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P0300 - Random Misfire The random misfire DTC indicates multiple cylinders are misfiring or the PCM cannot identify which cylinder is misfiring.
  • Camshaft position sensor (CMP)
  • Low fuel: less than 1/8 tank
  • Stuck open EGR valve
 

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No luck with resetting the PCM. Still will not rev at all. I had an off road h pipe and thrush outsiders side pipes on it but were removed in 2004 ant the stock exhaust was reinstalled. As for the underdrive pulleys they were installed in 2001 and have never been off the motor since. I did have 24 # injectors and a superchip tune but were also removed because the car would shut down while driving.
 

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If the DTC codes remain after reset, likely the problem is still there. Have you inspected the O2 pigtails for damage?
 

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Ok. O2 sensor wires are good no damage. Codes have all gone except P1504 Unknown Powertrain DTC (Manufacturer Controlled, Vehicle Speed, Idle Control, and Auxiliary Inputs). Still getting nothing when I step on the throttle. You can hear air rushing up the intake, and with the throttle wide open it drops to 500 rpm and idles rough. I did notice that COnstant Control Relay Module no longer has a cover. Could this have gone bad?
 

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Sure sounds like you have a vaccum leak. At the min, something is not right with the flow through the IAC valve.

The DTC indicates a bad IAC value or a wiring fault to the IAC.

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Troubleshooting IAC idle problems
http://www.allfordmustangs.com/forums/2260649-post.html

P1504 - Idle Air Control (IAC) Circuit Malfunction
This DTC is set when the PCM detects an electrical load failure on the

  • IAC output circuit. IAC circuit open
  • VPWR to IAC solenoid open
  • IAC circuit short to PWR
  • IAC circuit short to GND
  • Damaged IAC valve
  • Damaged PCM
The IAC solenoid resistance is from 6 to 13 ohms.
IAC valve stuck open
Vacuum leaks
Failed EVAP system
Damaged PCM
The IAC solenoid resistance is from 6 to 13 ohms.​
 

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Discussion Starter #8
:bigthumbsup Thank you for allyour help. I tested the IAC with a volt meter and all looked good. So I happened to glance at the MAF sensor and unplugged it. Went and started the car and she revved up.Odd the error codes never even pointed to the MAF sensor at all. Thank you again for all your help it is appreciated greatly. :bigthumbsup
 
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