Questions and Answers

Jeep Grand Cherokee Black Soot Q. Hi Vincent: I have a 1997 Jeep Grand Cherokee with a 4.0 liter engine. It also has automatic transmission, fuel injection, air conditioning, ABS, powering steering with approximately 64,230 miles. I'm having a problem with "black soot" coming from the my exhaust pipe. This soot was also on my spark plugs and oxygen sensor.

I took my car in for a diagnostic test and was told that my "bank no. 1, sensor no. 1" needed to be replaced along with fuel injectors nos. 3 and 6. I have since replaced both the plugs and sensor but, my "check engine" light still comes on. I also checked my fuel injectors with an ohmmeter and the read out indicates that each injector is 12.3 or 12.4 ohms. Is this correct?

I didn't change these because I'm not sure if the readings are correct or not. Please let me know what the correct readings should be on my fuel injectors. I also want to know if the auto mechanic was correct in his assessment of vehicle. As you may know, it is hard to find a good mechanic anywhere and, we are eager to fix this car!

Can you tell me where the black soot is coming from and what causing it? I do know that the black soot can possibly be a result of the oxygen and fuel not mixing properly. Please guide me and help me fix my vehicle! Does it have anything to do with the exhaust system? Could it be a timing problem?

I know that the timing is factory set and does not require me to do anything with it, but I could be wrong. Could it be that another sensor has gone bad? Could it be my no. 2 sensor? Could it be PCV, EGR, Evaporative Emission Control System? I currently own a Chilton's manual for my vehicle but it does not provide me with the information that I need in order to check, test or fix my vehicle. Please help us.

Thanks!!!!!!!!
Lydia and Charlotte
River Forest, Illinois

A. Okay, first off the readings you got at the injectors is normal Specification is: Resistance reading should be approximately 12 ohms +/- 1.2 ohms at 20°C (68°F). You can plug in a Noid Light or listen to them using a piece of hose as a stethoscope to make sure each one is firing.. I wish you gave me the codes that you came up with, that would have been a big help. But I suspect that you have gotten DTC P0172 and/or P0175.

Black soot usually indicates a rich fuel mixture or incomplete burning of the air/fuel mixture. When I see this the first thing I recommend is a complete tune-up with new distributor cap, distributor rotor and spark plug wires. And even though the timing is not adjustable, it should still be checked. PCMs have been known to go bad. I would also do a fuel pressure test to make sure it is within specifications. High fuel pressure will also cause a rich mixture.

If you got these codes there is nothing wrong with the HO2S. They did their job reporting the problem to the PCM. I don't really think we're looking at a fuel injection problem. I think some basic maintainence will fix this right up.

Spark Plugs Tell A Story