'91 GT running rich. No codes. 10-12mpg, help

those are all really good places to start! Do you have access to a vacuum pump? If so check the egr valve for proper function. An easy way to check is remove vacuum line to the egr valve while the car is running and supply vaccuum to it, car should immediatley stumble. If it doesnt you have a blockage somewhere in the egr system or a bad egr valve which could cause ur problems!!

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.

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.


Testing the MAF
The MAF body and sensor are designed to match each other. Therefore you can't swap sensors between different part number MAF bodies and maintain proper calibration. The assembly is designed to match the computer’s internal program, and swapping a different MAF can upset the computer's calibration.

There are three parts in a MAF: the heater, the sensor element and the amplifier. The heater heats the MAF sensor element causing the resistance to increase. The amplifier buffers the MAF output signal and has a resistor that is laser trimmed to provide an output range compatible with the computer's load tables.

The MAF element is secured by 2 screws & has 1 wiring connector. To clean the element, remove it from the MAF housing and spray it down with electronic parts cleaner or non-inflammable brake parts cleaner (same stuff in a bigger can and cheaper too).

Changes in RPM causes the airflow to increase or decease, changing the voltage output.. The increase of air across the MAF sensor element causes it to cool, allowing more voltage to pass and telling the computer to increase the fuel flow. A decrease in airflow causes the MAF sensor element to get warmer, decreasing the voltage and reducing the fuel flow. Measure the MAF output at pins C & D on the MAF connector (dark blue/orange and tan/light blue) or at pins 50 & 9 on the computer. Be sure to measure the sensor output by measuring across the pins and not between the pins and ground.

At idle = approximately .6 volt
20 MPH = approximately 1.10 volt
40 MPH = approximately 1.70 volt
60 MPH = approximately 2.10 volt

Check the resistance of the MAF signal wiring. Pin D on the MAF and pin 50 on the computer (dark blue/orange wire) should be less than 2 ohms. Pin C on the MAF and pin 9 on the computer (tan/light blue wire) should be less than 2 ohms.

There should be a minimum of 10K ohms between either pin C or D on the MAF wiring connector and ground. Make your measurement with the MAF disconnected from the wiring harness.

Diagram courtsey of TMoss & Stang&2birds




See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Wiring & Engine Info

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel pump, alternator, ignition & A/C wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Computer,. actuator & sensor wiring
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Fuse panel layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

Vacuum routing
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

The O2 sensor heaters have their own ground (HEGO ground) coming from the computer. This is different and separate from the O2 sensor ground. It is an orange wire with a ring terminal on it. It is located in the fuel injector wiring harness and comes out under the throttle body. It gets connected to a manifold or bolt on back of the cylinder head.

txstang347 said:

I'm not following why you mentioned this, JR. Should I be checking this? If so, what am I looking for?

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You are looking for a loose orange wire coming out of the passenger side engine fuel injection harness. It is supposed to be grounded to the engine block or firewall to insure proper O2 sensor operation.

O2 sensors for 5.0 engines are 3 wire. Sometimes the parts guys will sell you 4 wire O2 sensors, which won't work without a re-wire.

txstang347 said:

Ok, it's been a while and the exhaust fumes have settled down quite a bit. Maintenance I've done since first post in order to get my MPG up from 10 mpg:

All Motorcraft parts:
1. New rotor cap/button
2. New plugs
3. New PCV valve
4. New fuel filter (which was a PITA with the IRS in there now BTW!)
5. New O2 sensors (Bosch)
6. Cleaned MAF sensor
7. Cleaned and oiled K&N filter
8. Replaced vacuum hoses
9. New gas cap

So, after all that I've gone from 10mpg to a best of (approx):
203 miles/13 gallons = 15.6 gallons on street driving
230 miles/13 gallons = 17.6 gallons highway

That's a pretty significant change, but still don't think I'm getting the most out of it and think the MPG are still kinda low for a mostly all stock 5.0. Anyone have any other ideas? Would new plug wires, coil really make that much of a difference?

I was thinking about checking for a leaking fuel injector? Would there be an obvious symptoms of a leaking injector? I have none right now other than low MPG. Starts right up, no bog, runs fine. Picked up some 19lb'ers with 50k miles on them and adj FPR. Just nervous about breaking another old bolt while pulling the intake like I did in my timing chain cover last year.

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Any improvement since this last post ?
My old 92 Vert 5.0 could hit 27mpg freeway, but the notch I just got is stuggling to make 10 mpg so I'm looking for ideas / solutions. Thanks