88 Gt Erratic Cutting Out

88scar50 · Feb 23, 2016

I have an 88 GT, I haven't had it very long but since Ive had it seems to have an erratic cutting out/bucking problem. it doesn't seem to matter if it at low or higher speeds. It starts and idles perfect, and doesnt seem to do it when idling. It also doesnt seem to effect the power. When i accelerate hard it seems fine. Mostly happens just on normal driving, like a split second bucking every few minutes. Extremely annoying. I have replaced cap/rotor/plugs/wires/fuel filter/tps. I ran the codes and i get a 11 twice then a 22 twice. Any ideas?

jrichker · Feb 23, 2016

Code 11 is the passed internal diagnostics code when the computer is OK.

MAP/BARO sensor operation and code 22

Revised 14-Nov-2014 to add wire colors for frequency & voltage testing and engine sensor wiring diagrams.

On a Speed Density car, the MAP/BARO sensor is connected to the intake manifold and acts to sense the manifold pressure. Lower vacuum inside the intake manifold when combined with more throttle opening measured by the TPS means more airflow through the engine. As airflow increases, fuel flow through the injectors needs to increase to keep the air/fuel ratio where it needs to be. When manifold vacuum increases, the engine is either decelerating or idling, and it needs to reduce the fuel flow through the injectors.

On a Mass Air car, the MAP/BARO sensor vents to open air and actually senses the barometric pressure due to changes in weather and altitude. Its purpose is to set a baseline for the computer to know the barometric pressure. As barometric pressure decreases, it leans out the fuel flow to compensate for less oxygen in the air. When the barometric pressure rises, it increases to add fuel since there is more oxygen in the air. The fuel requirements decrease as altitude increases, since the atmospheric pressure decreases.

Disconnecting the MAP or BARO sensor will set code 22.

Misconnecting the BARO sensor to vacuum on a Mass Air car will cause the computer to lean out the fuel mixture.

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’s with a price tag under $40 that will measure frequency, but there are some out there.

Map sensor wiring:
black/white - ground
orange/white or +5 volts power
white/red signal out.

Measure the +5 volt supply using the orange/white and black/white wires
Measure the signal using the black/white and white/red wires.

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

Baro or MAP test using a real 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. And oscilloscope is very useful if you have access to one or know of someone who does. With an oscilloscope, you can see the waveform and amplitude.

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.

a9x-series-computer-connector-wire-side-view-gif.71316.gif

eec-iv-computer-connector-for-5-0-mustang-gif.88243.gif

Diagrams courtesy of Tmoss & Stang&2birds

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs

Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs

88scar50 · Feb 23, 2016

Whoah, thanks for all the info. So after reading through it your saying its likely a bad map/Baro sensor?

jrichker · Feb 24, 2016

That's what the code 22 points to. Clear the codes by using the instructions that came with your code reader. Then drive the car for 2-3 days and dump the codes again. If you still get the code 22, it needs troubleshooting or replacement. Be sure to troubleshoot it first, because the MAP/Baro sensor isn't cheap.

Mustang5L5 · Feb 24, 2016

^ what he said

88scar50 · Feb 25, 2016

Sounds good, I'll give it a try this weekend and report back, thanks for the help.

88scar50 · Mar 26, 2016

Sorry it's been awhile and haven't had time to test it. Anyway I ended up replacing the map/bap sensor and it did not resolve the issue. Any other suggestions? I've been searching everywhere and can't seem to find a solution.

jrichker · Mar 26, 2016

Are you still getting a code 22 when you dump the codes?

88scar50 · Mar 26, 2016

I had the battery disconnected for an hour and still got the codes. Is there another way to clear them on odb 1?

jrichker · Mar 27, 2016

88scar50 said:
I had the battery disconnected for an hour and still got the codes. Is there another way to clear them on odb 1?

No, that means that it is really broke. Re-read the Code 22 test path and follow the steps to check the wiring. If you don't find any problems with the wiring, then replace the sensor.

88scar50 · Mar 27, 2016

I cleared the codes, then drove drove it again and the problem happened again and the code came back. I replaced the sensor and there was no change.

jrichker · Mar 27, 2016

Did you check the wiring just as I advised to do before replacing the sensor?

FourEyed · Mar 27, 2016

I replaced the ignition switch and the coil, after having the same issues you described on my 86GT. That was a few years back, never did it again.

88scar50 · Mar 27, 2016

I only have a crappy little voltmeter so it doesn't have anything to measure the hz, however I was able to check the voltage and it was reading 5v.

jrichker · Mar 28, 2016

88scar50 said:
I only have a crappy little voltmeter so it doesn't have anything to measure the hz, however I was able to check the voltage and it was reading 5v.

Did you do the ground resistance checks?
Without a good signal ground, the MAP/Baro sensor won't work properly.

88scar50 · Mar 28, 2016

I haven't checked that because I got nervous about reading something about possibly frying your pcm. Do I just unplug the sensor and stick the black probe in the black wires connector and then put the red probe on the positive battery terminal?

jrichker · Mar 28, 2016

You don't test the sensor for resistance, you test the connections. You cannot damage the computer or the wiring by making resistance tests.

Go back and slowly and carefully re-read the instructions. Everything you need to know to check the wiring is in there. You may want to print the instructions out and have them with you when you do the tests.

88scar50 · Mar 28, 2016

Gotcha, checked all those listed and all were within the spec. They were all from about .5 to .7

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