Please help this code pulling noob

[David Schwarz]

This applies to the '88GT. When I get on the car and pull heavy boost, the check engine light will come on for a few seconds after I let up on the gas. I should also mention that this CEL problem only started after the passenger side header gasket was replaced. I pulled the codes today (KOEO), and here is what I got:

67, then for continuous readings, 29, 33, and 66.

I bet some of you know these by heart, but for the others, here is the associated fault code

67: Neutral drive switch circuit failure, circuit open; or A/C input high

29: Insufficient input from speed sensor

33: EGR valve not opening properly

66: mass air flow sensor fault, below minimum voltage

When I bought the car, it came without A/C (removed by previous owner). Could this be the cause for code 67? I'm not too concerned by the speed sensor code, but it is the last two that do concern me.

The MAF is a brand new ProM 80mm. Like I said above, I didn't have a problem before the header gasket was replaced. As for the EGR valve not opening properly, what step(s) should I take to solve this problem.

Thanks in advance for helping me. I didn't expect to see this many codes for a new motor with less than 2K on it.

 

[jrichker]

Code 29 - Vehicle Speed Sensor (VSS) is an electronic sender mounted on the speedo pickup gear on the trans. It works the cruse control for both 5 speed and auto trans cars. On auto trans cars, it tells the computer to speed up the idle slightly as you slow down to a stop to keep the engine from stalling. Check to see if the electrical connector is plugged into it. Cost is under $30 and it is easy to replace.


Code 67 - clutch not depressed (5 speed) or car not in neutral or park (auto) or the A/C was on when you ran the test (it is supposed to be off). Possible neutral safety switch or wiring problem. You can generally ignore this code, since it has no effect on engine performance.

Code 66 MAF below minimum test voltage.
Insufficient or no voltage from MAF. Dirty MAF element, bad MAF, bad MAF wiring, missing power to MAF. Check for missing +12 volts on this circuit. Check the two links for a wiring diagram to help you find the red wire for computer power relay switched +12 volts. Check for 12 volts between the red and black wires on the MAF (usually pins A & B).

The MAF element is secured by 2 screws & has a1 wiring connector. To clean the element, remove it from the MAF housing and spray it down with electronic parts cleaner or non-inflammable brake parts cleaner (same stuff in a bigger can and cheaper too).

There are three parts in a MAF: the heater, the sensor element and the amplifier. The heater heats the MAF sensor element casing the resistance to increase. The amplifier buffers the MAF output signal and has a resistor that is laser trimmed to provide an output range compatible with the computer's load tables.

The MAF output varies with RPM which causes the airflow to increase or decease. The increase of air across the MAF sensor element causes it to cool, allowing more voltage to pass and telling the computer to increase the fuel flow. A decrease in airflow causes the MAF sensor element to get warmer, decreasing the voltage and reducing the fuel flow. Measure the MAF output at pins C & D on the MAF connector (dark blue/orange and tan/light blue) or at pins 50 & 9 on the computer.

At idle = approximately .6 volt
20 MPH = approximately 1.10 volt
40 MPH = approximately 1.70 volt
60 MPH = approximately 2.10 volt

Check the resistance of the MAF signal wiring. Pin D on the MAF and pin 50 on the computer (dark blue/orange wire) should be less than 2 ohms. Pin C on the MAF and pin 9 on the computer (tan/light blue wire) should be less than 2 ohms.

There should be a minimum of 10K ohms between either pin C or D on the MAF and ground.

Remember that the computer does not source any power, but provides the ground necessary to complete the circuit. That means one side of the circuit will always be hot, and the other side will go to ground or below 1 volt as the computer switches on that circuit.

Code 33 - Insufficient EGR flow detected.
Look for vacuum leaks, cracked vacuum lines. Check to see if you have 10” of vacuum at the EGR vacuum connection. Look for electrical signal at the vacuum regulator solenoid valves located on the rear of the passenger side wheel well. Using a test light across the electrical connector, it should flicker as the electrical signal flickers. Remember that the computer does not source any power, but provides the ground necessary to complete the circuit. That means one side of the circuit will always be hot, and the other side will go to ground or below 1 volt as the computer switches on that circuit.

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)

http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

http://www.veryuseful.com/mustang/tech/engine/images/88-91eecPinout.gif

 

[David Schwarz]

One word

WOW!!

Now that's what I was looking for. I think you're assuming that I have more mechanical skills than I actually do, but I'll digest your information and take it as far as I can on my own

Thanks a million.

 

[David Schwarz]

More info

I disconnected the battery (for an unrelated project) and cleared the memory. I then ran the car up to temperature at idle and ran the KOEO test again. I received the all clear code 11 for both on-demand and continuous memory.

I then took the car out for a short drive, and still got the intermitent CEL. I now understand what brings it on: if I power the car up to ~4K rpm in any gear, and then let off the gas but leave the car in gear (engine braking), the light will come on. If I pull the codes after such an event, I still get both 29 and 66 (didn't see code 33 this time).

I still need to run the KOER test, but to do this I need to purchase a timing light first (at least according to the manual that came with the code tester I do). So I'll see if I can take care of that this weekend. In the meantime, does this give anyone any more information (other than what's already been provided) that might help me hunt down this problem? I really do want to fix it myself if I can.