Jeep runs way too rich

I’ve got a '98 Jeep Grand Cherokee, 4.0 6 cylinder. It’s not showing any codes, but runs so rich that you can easily see the black smoke coming out of the tailpipe. Without any codes to point me in the right direction, where do I start?

The problem might be caused from a partially stuck open thermostat. If the engine coolant doesn’t get hot enough for the computer to go into the closed loop mode the engine will run rich.

Another thing that can cause this is a defective coolant temperature sensor for the computer. If this sensor is telling the computer that the coolant temperature isn’t up to operating temperature when it is, the engine will run rich.

A defective coolant temperature sensor won’t always set a trouble code because it’s one of the primary inputs to the computer. So if the computer gets a signal from the sensor even though it’s an erroneous signal, the computer thinks it’s valid and doen’t turn on the Check Engine light.


It could also be a bad fuel pressure regulator. A fuel pressure test can point this out. If the fuel pressure is too high, the injectors will squirt too much fuel per pulse. The regulator is supposed to keep the pressure under control. Most ECMs don’t monitor the fuel delivery system, only the function of the fuel injectors.

How long have you owned this vehicle? Is the black smoke constant? WHEN, exactly can you see black smoke??

Well, it’s the same old story: it’s my daughter’s car, she brought it to me and said it uses way too much gas and blows black smoke. The black smoke can be seen at idle, reving in neutral, and going down the road. In other words, always. The temp sensor and fuel pressure suggestions make a lot of sense. I’ll start there.

If the air filter is dirty, it could be starving the engine for air, resulting in a rich running engine.
Clean the MAF (Mass Air Flow) sensor with MAF spray cleaner, and clean the idle air control valve air passages, and the throttle body and throttle plate with Throttle Body Cleaner.