RPMs Stumble/surge (not idle)

Was the engine at operating temp when you ran codes?

Is/are your MAF/EEC calibrated for/accomodating the 24 pound injectors?

Amongst other things, I would check for a vacuum leak.

Good luck.

If the car was above 170*, you might want to check your ECT calibration.

Code 41 or 91 Three digit code 172 or 176 - O2 sensor indicates system lean. Look for a vacuum leak or failing O2 sensor.

Revised 24-Sep-2009 to correct computer pin description for 94-95 5.0 Mustangs

Code 41 is a RH side sensor,
Code 91 is the LH side sensor.

Code 172 is the RH side sensor
Code 176 is the LH side sensor

The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.

Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Testing the O2 sensors 87-93 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


Testing the O2 sensors 94-95 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 43 (LH O2 with a red/black wire) and 44 (RH O2 with a gray/lt blue wire). Use the metal next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


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. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness.

Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.

If you get only code 41or 91 and have changed the sensor, look for vacuum leaks. This is especially true if you are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives. Replace the tubing and check the PVC and the hoses connected to it.

A secondary problem with only a code 41 or 91 is for cars with an intact smog pump and cats. If the tube on the back of the heads clogs up, all the air from the smog pump gets dumped into one side. This excess air upsets the O2 sensor calibration and can set a false code 41 or 91. The cure is to remove the crossover tube and thoroughly clean the insides so that there is no carbon blocking the free flow of air to both heads.

sweep your mass air and see if theres a dead spot in it.

onefast89 said:

sweep your mass air and see if theres a dead spot in it.

Click to expand...

That isn't going to happen. If you know how to sweep a Mass Air Sensor, I want to know how you do it.

You can mesure the sweep of a TPS with a analog voltmeter to look for dead spots and dropouts.

AIbandit said:

My LH O2 was .28 and my right was 128 Milli Volts

so that means my o2 on the right is bad?

Click to expand...

Yes. The O2 sensor voltage should switch between .2-.9 volt at idle.

AIbandit said:

Any tips for getting a seized 02 sensor off?

Click to expand...

Heat the bung the O2 sensor screws into with a propane torch. Soak a couple of shop rags in a bucket of ice cubes, salt and water. The salt will melt the ice and chill the resulting water down to freezing. Once the the bung is hot and the rags are very cold and wet, grab the O2 sensor shell above the bung with the iced rags. Have your big wrench ready, because the O2 sensor will shrink for a split second before it starts to heat back up. During the that split second, pull on the wrench as hard as you can. The O2 sensor should come loose. Above all, be careful not to burn yourself or set anything on fire...

AIbandit said:

Alright alot has changed since I posted this but I still have Idle issues.

I changed the 02 sensors and I lost the surging problem and she's putting ridiculous power to the ground.

I went to longtube headers but I had to temporarily remove this little guy.


would that cause Idle issues/vac leak?

Click to expand...

No. That's the air supply tube from the smog pump. If you have cataylic converters, it's a necessity. If not, isn't needed.


See the "Surging Idle Checklist for help with all your idle/stall problems.

The quick and easy way to dump the codes is in there too, and all you need to do it is a paper clip! The first two posts contain all the updates to the fixes. At last count there were 24 possible causes and fixes] for surging idle/stall problems. I continue to update it as more people post fixes or ask questions.