QuestionI have a 1994 dodge van with a 3.3 engine. It runs fine for about an hour, then once it gets up to operating temp then it backfires through the intake, spits and sputters and has no acceleration. Recently replaced the fuel pump, new spark plug wires, and coil pack. The codes we have are 12, 21 and 55. This mostly seems to occur when it sits and gets hot, but the temp gauge doesn't rise. It drives fine for awhile and then starts dying. If I shut it off and let it sit for about an hour it goes away.
AnswerHi Robert,
The 21 code says that there is no variation in the signal from one or both* of the oxygen sensors that are located on the inner surface of each of the two exhaust manifolds, on one of the exhaust pipes of each manifold (the pipe closest to the rear of the engine). I would begin by inspecting the electrical disconnect for each sensor, the wires from the sensor to the disconnect, and wire from the disconnect to the harness. There are four wires for each sensor. If you have an ohmmeter disconnect the plug and measure the resistance between the two white wires for each sensor, it should be 5 to 7 ohms if the heater coil is patent. If either or both read much hight or infinite, then that means the sensor is bad and needs to be replaced. You can also test the function of the sensors with a voltmeter: Use a straigth pin to pierce the black/light blue wire of each sensor, and a straight pin to pierce either the tan/white (right sensor) or the Black/dark green wire (left sensor). Attach a digital voltmeter across the pins on each sensor, one at a time, and start the engine. Note the voltage should be 0.1 to 0.2 V when the engine is cold, and within a couple of minutes should start changing between 0.1 and 0.9 volts. If it doesn't get to that switching back and forth stage at all or in a very long period of time then the sensor is bad as well. So that may help you identify if both sensors are bad or only one.
Be very careful when replacing the sensors not to damage the tip where the wires are attached, using a box end wrench. The new units will come with a compound on the threads that prevents they sensor from getting locked in place in the manifold in case it needs to be removed at a later date.
In the absence of other codes, I believe your problem is with one or both O2 sensors.
Roland
*There may be only one sensor (as is the case on the '93) on the '94 engine as well( but by '96 there were 4 sensors!) but look for one on each manifold as I described. If there is only one on the engine, it will be located on the top of the rear exhaust manifold. In that case the heater wires will be dark green/black and black/tan, and the signal wire will be black/dark green and the signal ground will be black/light blue. If there is only one sensor on your engine, and because you have the 21 code and the wires on either side of the plug and plug too looks o.k., then I think it is safe to just buy a new sensor and replace the old one without testing it. That should solve your problem.
