Many codes, one problem

blk90notch

New Member
Dec 11, 2004
2
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Maryland
I have been pulling codes 12, 41, and 91 from the car I just bought and it is running very rough. It also backfires through the air cleaner when you give it gas and is not really drivable because it has no power. I have gathered that it is pig rich because it smells like gas. It was running ok (rough, but no backfires and adaquate power) when I drove it home after purchasing it, but I pulled the upper intake to jack the motor up to fix an oil pan leak. When I put it back down it ran like ass, so I got a new gasket for the upper plenum and it really helped. I have around 18 in. vaccuum at idle and up to 22 when decel. My fuel pressure is 34psi on vaccuum and 44 off. Here are a list of parts replaced. The previous owner said he replaced one bad injector and that he was getting an o2 sensor code sometimes, how do i check the others? This was posted on the corral and I tried all of the advice gotten. Thanks alot for any input. It had a massive oil leak so the o2 plugs were covered in oil if that matters. :shrug:

Both 02 sensors
Upper manifold gasket
Cleaned MAF elements (looks good)
check engine grounding strap and tightened
 
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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. 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

A Cylinder balance test will usuall find bad injectors.

Warm the car's engine up to normal operating temperature. Use a jumper wire or paper clip to put the computer into test mode. Start the engine and let it go through the normal diagnostic tests, then quickly press the throttle to the floor. The engine RPM should exceed 2500 RPM's for a brief second. The engine RPM's will increase to about 1450-1600 RPM and hold steady. The engine will shut off power to each injector, one at a time. When it has sequenced through all 8 injectors, it will flash 99 for everything OK, or the number of the failing cylinder such as 22 for cylinder #2. Quickly pressing the throttle again up to 2500 RPM’s will cause the test to re-run with smaller qualifying figures. Do it a third time, and if the same cylinder shows up, the cylinder is weak and isn’t putting out power like it should. See the Chilton’s Shop manual for the complete test procedure

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

For those who are intimidated by all the wires & connections, see http://www.actron.com/product_detail.php?pid=16153 for what a typical hand scanner looks like. Normal retail price is about $30 or so at Walmart.