Quote:
Originally posted by Robert J. Yates
Not that I disagree with that John but what does one do when neither a code is thrown or stored yet the Jeep runs like crap? How do you propose to find the problem then? This is a 2000 XJ - service is up to date.
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Hi Robert,
There are many things that can cause the engine to run badly and not set a code. Does this XJ have the distributorless ignition?
If so, this is a waste spark type system. This means that one coil fire two spark plugs at the same time. The spark goes down through the top of one plug and returns to the coil through another plug from the bottom up. The second plug actually fires backwards up the spark plug. So when the two cylinders fire at the same time, one is on the compression stroke and the other is on the exhaust stroke. That is why the second plug doesn't affect anything when it fires, thus the spark is "Wasted"
What this means to you, is if you have a coil or a spark plug going bad it can cause a misfire on 2 cylinders resulting in a very badly running engine.
Unless this misfire happens more than 20% of the time within 1000 revolutions the PCM will not set a code. This is an over simplified explaination of the misfire monitor, but that might give you an idea of what could happen.
Another possibility could be a fuel problem like a leaking fuel injector. The PCM will not set a code, but this will show up in the Long Term Fuel Trim as a negative value. This means the computer is taking away fuel to compensate for the extra fuel it is getting from the leaking injector. (Adaptive learning)
You will need a scan tool to see this data.
Bottom line is that the computer cannot see mechanical problems. It can only react to them. This is why Auto Zone got sued for selling an O2 sensor to everyone that had an O2 sensor code in the computer. Most of the time, an O2 sensor code is the result of some other problem but it is the only thing that the computer can see.