Tach Hang/lag, Jumpy On Start And Shutdown

Discussion in 'Fox 5.0 Mustang Tech' started by izodhalo, Oct 30, 2013.

  1. Hello everyone,

    I am just recently getting back into the Fox mustangs. I've had a few in the past then stepped up to an 03 Cobra a few years ago. Now I am back with the Foxes because I started driving with one and can't seem to get it out of my system.

    I purchased a very clean and low mileage 1990 LX 5.0, 5spd about a week ago. A couple of small things concern me. The speedometer jumps at low speeds, but I pretty much know how to fix it. My main concern right now is the odd tach movement. When you start the car, the tach will jump a bit, not much, but like it's having a mild seizure before it gets to idle range. It will idle fine, but when you give it gas quickly, the tach does not move smoothly, as it seems to lag slightly at certain points. Also, when the car is turned off, the tach will bounce as the RPM's approach 0. Also, the other day, during hard acceleration in 4th, it seemed to miss a bit. Idle is for the most part smooth, but doesn't seem as smooth as stock.

    I have heard about jumping tach problems and issues with the pip sensor, but the tach doesn't bounce crazy like it's not grounded, and I don't have any heat soak problems. Car ran for 2 hours the other day and turned off and started up fine after. Just wondering if I can fix this issue some simple checks or do I have to get a completely new distributor.

    Only modifications this car has (from previous owner)
    Mac equal length headers (spark plugs wrapped)
    Off road H
    Cold air intake (inside fenderwell)
    MSD Distributor (stock coil)
  2. Have you check the codes? I just replaced the distributor in my car for the same reason..when i check the codes i had a code 14 for the pip sensor. It still could be a bad pip sensor..my car didn't stall but it had miss sometimes.
  3. It's been so long since I had a fox and tried to run codes so I just picked up a small OBD I digital scanner. After I ran the scanner, it came back clean, no codes. It did pull one out of memory that was 31. Not sure what the 31 would be on a mustang.

    One thing I did notice this time about the tach was that when you shut the engine off, the needle drops below 0 and bounces a few times.
  4. No help for the tach except to clean and check the dash connections for the dash cluster.

    The code 31 is completely unrelated to your tach problems.
    CODE: 31 (KOEO) - EVP circuit below minimum voltage. Vref (5 volt reference voltage supplied by the computer) missing or broken wire or bad connection in circuit. Use a DVM to check for 5 volts on the orange/white wire. If it is missing, look for +5 volts at the orange/white wire on the TPS or MAP sensor located on the firewall near the center of the car. Use the black/white wire for the ground for the DVM.
    With the sensor removed from the EGR and still connected, press the plunger and watch the voltage change on the brown/lt green wire. Pull the passenger side kick panel and measure the voltage at the computer. You will need to remove the plastic cover over the wires and probe them from the backside. A safety pin may prove very useful for this task. Use pin 27, EVR input (brown/lt green wire) and pin 46, signal ground (black/white wire) to measure the voltage. The orange/white wire is Vref and should always be 5 volts -/+ .25 volt. Be sure to measure Vref at the EGR sensor to rule out any broken wires or bad connections.
    Measuring the voltage at the computer helps you spot broken wiring and intermittent connections.
    See the graphic for the 10 pin connector circuit layout.