got a code of 41..what...

this might help

http://fordfuelinjection.com/index.php?p=31

sounds like the 02 is bad, or a problem in the wiring...

Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor.
The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more 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

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.

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.

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.

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.

Here's a book that will get you started with how the Ford electronic engine control or "computer" works.

Ford Fuel Injection & Electronic Engine Control 1988-1993 by Charles Probst :ISBN 0-8376-0301-3.

It's about $20 from Borders.com see http://www.amazon.com/ . Select boo...very good, and I found it to be very helpful.

Did you measure the voltage as directed in the test path?

If an O2 sensor fails, the computer goes full rich on that bank of cylinders. This kills you gas mileage and some performance, but it protects the engine from damage by pinging or other lean mixture problems.

Note that the cylinder banks are arranged cylinders 1-4 (passenger side) and then cylinders 5-8 (driverside)

Time to get the Book...

The engine will run with NO working O2 sensors. It goes into "Limp mode" when the O2 sensor(s) quit working. Limp mode is a collection of preset values for air/fuel & spark advance that are "safe" for engine operation until the fault is corrected.

Are you getting spark to the sick cylinders? If you disconnect the spark plug wire from cylinders 8,4,3, or 2, while the engine is idling, does the engine change speed?

89GtConvert5spd said:

i do have a shop manual...

Click to expand...

A shop manual is good, but the "Book" is the Probst book, Ford Fuel Injection & Electronic Engine Control 1988-1993 by Charles Probst :ISBN 0-8376-0301-3.
It is a must have book if you are going to be working on 5.0 Mustangs.

Very well put by JR. One aside - I use a timing light on problematic cylinders - it is a quick and dirty way to see if they are firing. Is your knowing those cylinders are out via a cylinder balance test? Just curious....

A noid light can be used for injector harness testing - the connectors at the injectors can go south (I think it is a little more common on Fox IV's than fox IIIs).........

None of that is here nor there.

Good luck.

HISSIN50 said:

A noid light can be used for injector harness testing - the connectors at the injectors can go south (I think it is a little more common on Fox IV's than fox IIIs).........

None of that is here nor there.

Good luck.

Click to expand...

Take Hissin's advice and make this your next step.

Go back to my orginal post describing the 41/91 code. You will notice that a vacuum leak can cause the code 41/91.

Look for a vacuum leak. That means carefully examine all the hoses & replace if necssary. Make sure that the carbon canister line is connected to the carbon canister and that the canister isn't cracked.