91mustang gt chk engine lite when on throttle

Dump the codes and see what the computer says is wrong…

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.

See http://www.troublecodes.net/Ford/
OR
See http://www.dalidesign.com/hbook/eectest.html for more descriptive help
OR
See http://www.mustangworks.com/articles/electronics/eec-iv_codes.html

Since we're on the topic, I get an error 41 on my car indicating an always lean condition, yet the car is dumping gas. My question is does the always lean readout mean the car is trying to compensate for an always lean reading and dumping gas as a result or is my car just psychotic? I do get check engine lights, primarily when the car is at or near operating temp.

My car also flashes "check engine" when accelerating, then goes off when cruising. I haven't gotten around to getting the code read yet. It started last summer when we swapped everything from my old wrecked car, including the engine, over to the new car.... so there's no telling what it could be.

Black Sun 5.0 said:

Since we're on the topic, I get an error 41 on my car indicating an always lean condition, yet the car is dumping gas. My question is does the always lean readout mean the car is trying to compensate for an always lean reading and dumping gas as a result or is my car just psychotic? I do get check engine lights, primarily when the car is at or near operating temp.

Click to expand...

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.

Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. 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 ground is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

"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."
Charles O. Probst, Ford fuel Injection & Electronic Engine control

OK, so you pretty much gave me the info that I expected, which is good. Let me expand on this now. The O2's were put in about a year ago, and my gas mileage did not improve. Then the engine light started coming on. I had the shop check the O2's, I was told that there was a problem with them not switching, but a bad ground was located and fixed and the O2's are switching now. Well, the engine light still pops up, and my mileage still sux. Could an improperly grounded O2 be damaged by this? By the way, jrichker, I messed around with the wiring that you told me to check out the other day. I didn't find anything specific, but the voltage has been pretty normal now. I must have jolted something the right way. Thanks again.

The best diagnostic advice I can give is to do the voltmeter test as described in my previous post. That would tell you if the O2 sensors are working properly. The O2 sensors have a common power ground and signal ground. It is the black wire on the O2 sensors. Disconnect both o2 sensors and use and ohmmeter to measure the resistance between the black wire and chassis ground. You should see less than 1 ohm. The O2 sensor ground comes out of the injector wiring harness just below the EGR valve if I remember correctly. While you are at it, check the engine to chassis ground from the back of the engine to just below the windshield wiper. If you have a 3G alternator, you need a 4 gauge ground from the engine block to the chassis where the rest of the battery grounds are.