Questions and Answers

Jeep Cherokee Speedometer, No Start Q. I have a 1995 Jeep Cherokee 4.0 liter, automatic transmission, 4WD, 115000 miles. Last week the Check Engine Light came on. I pulled the codes, key ON-OFF-ON-OFF-ON, and got a code 51 indication that it is running too lean, possibly the O2 sensor. Since the sensor had never been replaced, to my knowledge, I replaced it. Started the engine, no CEL.

Went to get some gas, driving about ½ mile. After filling the tank, I drove about one mile when the car stalled. While trying to start it, I noticed that, strangely, when the key was in the "ACC" position, the speedometer read 10 MPH and the odometer was moving! The fuel pump relay was clicking.

After several hours of futzing around, the engine started, all the other odd symptoms, speedometer, odometer, chattering relay, disappeared, but it loped while idling. After driving about two miles, it stalled again. Same symptoms: 10mph, odometer moving, fuel pump relay clicking. It cranks very good. After 24 hours it still won't start. No Spark.

Could the aftermarket O2 sensor have caused a problem? I matched up the four wires according to the color codes. Why would the fuel pump relay "chatter"? I tried swapping relays, only those with the same part number on them, and that didn't help. The fuel filter may never have been changed. I bought this car with about 80,000 miles on it. Could the computer be damaged if the O2 sensor is the wrong type?

Oh, while looking for problems, I did find a few other things: The negative battery cable at the engine block was not connected, so all return current was going through the smaller wire attached to the fender. This may have been like this for a long time.

Also, the vapor line to the canister was disconnected, split at the end, which explained the fuel odor I have had for a several days. Worst of all, I found that the block has a crack on the passenger side where the motor mount bolt fastens to the block, and the other bolt to the block on that side is missing.

Whoever worked on this vehicle before I bought it should be shot. I have also learned that I should have had an expert check this out before I bought it.

Thanks...

A. The code 51 is: "Lean O2 signal. Oxygen sensor signal indicates lean fuel/air ratio condition during engine operation".

This does not indicate a bad O2 sensor. It's saying the O2 sensor is detecting a lean condition beyond it's ability to compensate. Basically the O2 sensor did it's job in trying to compensate and reporting the failure to do so.

I must admit, I have never seen a speedometer and odometer do what you have described. I guess it is possible that a wrong O2 sensor could cause problems with the PCM. If the B+ voltage for the O2 heater was routed into the PCM through the signal wire, it could very well damage the PCM.

And keep in mind that it is an aftermarket O2 sensor and is designed to fit several other makes and models so using the wiring colors to connect it is not a good way to hook it up.

I would try unplugging the O2 sensor and see what happens. It may start for you. Another thing to try is this: Make up some fairly heavy ground wires and run them from the engine block to the firewall and then to the negative battery cable. "Chattering" relays are almost always due to a bad ground.

If you have no ignition or fuel pump, I would suspect a bad ASD relay, although that would not account for the speedometer weirdness.

And yes, you should have had it checked out by a mechanic of your choice.