Lean Mixture Code

[Thunderboy]

I have an 87 GT with stock MAF (Cali Model),intake manifold and throttle body. It's had a problem with the check engine light blinking on and off before. I figured it was just a short somewhere, but then I noticed that when I drive the car for a while the check engine light will come on and stay on instead of blinking on and off. I checked it out and it the code that I got on the Engine running test is a lean fuel mixture. What would cause a lean mixture? How might I correct it?

I have installed:
K&N Filtercharger kit
Mac 1 3/4" Shorty Headers
2.5" Magnaflow High Flow Cats (O2 sens. have less than 12,000 mi on them)
2.5" Flowmaster Cat-back system (2.5" from headers to Tailpipes)
Ford Racing Clutch, Flywheel, Quadrant, and adjustable cable w/ BBK Firewall Adjuster.
I replaced the fuel pump in May of '04 just because it sounded a little weird to me.
I also replaced my alternator with a PA Performance 130 amp.
My initial engine timing is set to 10 BTDC.

 

[BlownFiveLiter]

No Mustangs were sold with a MAF in '87, even in California. The 88 5.0 was the first to offer it out there.
How's your fuel pressure look? What is the code exactly that you get? I don't recall for the Fox Mustangs, but with my 95, it will give a different code for either bank that reads rich or lean.

 

[jrichker]

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. 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

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