Help! Need advise on idle surging/stalling out

[Hellfire50]

Hey guys,

I put a '98 GT40P motor into my '89 Mustang. The motor has 82K miles on it. Here is what has been replaced or modified on the motor:

TFS1 Cam
1.6 RR
Alex Parts Valve Springs
65mm Professional Products Throttle Body
1 5/8" Long Tube Headers
MSD Coil and Distributor
Double Roller Timing Chain
New Oil Pump
3G Alternator
Sensors replaced: O2's, Temp, ACT and TPS
Also, the Heads were pulled off and had a Mondello Valve Job & Springs were set up by a reputable machine shop.

2 weeks ago I put it in and fired it up. Started up first try. Tried to set the timing as best as I could at 10 degrees with the spout out and motor surging. I cleaned my IAC and then tried doing the idle reset procedure. When doing that,I checked my TPS and it was reading high voltage. I cleaned the 10 pin salt and pepper connectors. I found a bad spot SIGRTN wire and repaired it, but was still getting a high voltage reading. Upon further investigation when pulling the ECM out, I found that the car has a superchip installed. It turned out the SIGRTN was fried inside the ECM and I did the jumper repair that Joel5.0 posted. The repair worked and I now have correct voltage readings. I tried doing the idle reset procedure again and even with the IAC unhooked it still surged and the RPM's would not go below 1000. I then bought an ECM code reader and got the following codes:

22 - BARO Sensor out of range
31 - EVP or PFE Circuit below minimum voltage
81 - Air diverter solenoid circuit fault

I went to the local you-pull-it salvage yard and grabbed a MAP/BARO sensor out of a '92 Mustang 4 Cyl. Came home, put it in and I am left with the same issues and codes. I put a DVM on both sensors and they are getting 4.98 readings.

The car has no smog, EGR or A/C hooked up.

I can and have been driving the car around. Overall, it runs hard (full throttle). But when I come to a stop or at an idle, it stalls or surges very badly.

HELP!

 

[jrichker]

Fix the code 22 problem first. When it has cleared, the idle surge will have decreased or cleared. Be sure to disconnect the battery & turn the headlights on after replacing any sensors. This will clear the computer of any old codes or settings.

MAP/BARO sensor operation and code 22

Revised 19-Jul-2011 to add functional descriptions for MAP and BARO operation.

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 acts to sense 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.

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.

 

[Hellfire50]

Is the baro sensor from the '92 mustang 4 cyl ok or should I get a new one specifically for my '89? Thanks!

 

[jrichker]

Is the baro sensor from the '92 mustang 4 cyl ok or should I get a new one specifically for my '89? Thanks!

Visit one of the online parts stores like Autzone, Advanced Auto Parts or NAPA ONLINE and cross check the part numbers.

 

[Hellfire50]

jrichker,

I went to Autozone and got a BARO sensor specific to my car and so far so good. Thanks so much, you are a tremendous asset to this site!

Jeremiah