Pop through intake / miss at cruising speed.

How old is the motor? Is there a chance that the timing chain is worn?

Have you replaced the Cap, Rotor, and wires? Even if they're pretty new? I ask this because I once installed a brand new set of FMS wires and pulled my hair out a month later trying to pin down a similar problem.

Spray down the intake with ether? Find anything?

Not off the top of my head, no. If someone doesn't chime in with that info, I'll go out and pull a meter reading for you though.

If you have another set of wires, cap, and rotor though... swap them in to see if anything changes. All it takes is a small break in a wire to cause silly stuff to happen. That set of wires that I had, had maybe 50 miles on them before I swapped them out as the culprit.

Did you get any codes?

Code 22 or 126 MAP (vacuum) or BARO signal out of range. The MAP or BARO sensor is pretty much the same sensor for both Mass Air & Speed Density cars. The main difference is where it is connected. Mass Air cars vent it to the atmosphere, while Speed Density cars connect it to the intake manifold vacuum. Its purpose is to help set a baseline for the air/fuel mixture by sensing changes in barometric pressure. The MAP or BAP sensor puts out a 5 volt square wave that changes frequency with variations in atmospheric pressure. The base is 154 HZ at 29.92" of mercury - dry sunny day at sea level, about 68-72 degrees. You need an oscilloscope or frequency meter to measure it. There a very few DVM with a price tag under $40 that will measure frequency, but there are some out there.

The MAP/BARO sensor is mounted on the firewall behind the upper manifold on 86-93 Mustangs.

Baro or MAP test using frequency meter - run the test key on engine off. The noise from the ignition system will likely upset the frequency meter. I used a 10 x oscilloscope probe connected from the frequency meter to the MAP/BAP to reduce the jitter in the meter's readout.

If it is defective, your air/fuel ratio will be off and the car’s performance & emissions will suffer

Some basic checks you can make to be sure that the sensor is getting power & ground:
Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter.
Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white wire on the EGR and the same wire on the TPS. It should be less than 1 ohm. Next check the resistance between the black/white wire and the negative battery cable. It should be less than 1.5 ohm.

The following power on check requires you to turn the ignition switch to the Run position.
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 EGR sensors. Use the black/white wire for the ground for the DVM.

Do you have an aftermarket Cold Air Induction kit?

BK_CAULEY said:

I pulled an evap code in the KOER test and got no codes on KOEO. just finished my compression test and all cylinders are good. and yes i do have an aftermarket cold air intake.

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Try rotating the MAF assembly. Clocking the MAF can move it from a turbulent air stream in to a smoother one which helps the MAF do a better job of measuring the airflow.

If the air filter is in the engine compartment, that is a bad thing. The hot, turbulent air from the fan causes all sorts of problems with airflow through the MAF.

Do you run a closed or open breather on the valve cover?

I have the same problem to a tee...may car is doing this also...I have no idea what else to do...Everything is new on the car....Maybe a defective MAP sensor..im thinking maybe a bad fuel injector...im lost I even have the stock Air filter assy my car is an 87 GT with speed density....

BK_CAULEY said:

the vent tube on the filler neck is routed back to the intake. and the pcv valve is new.

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i think he means on your valve cover is your oil fill neck a closed cap or does it have a breather style cap

Count me in

Count me in with this issue as well. I've been fighting it since day one with this car.



Dunno.