I am working on a 2005 Grand Cherokee Limited with the 3.7 (WK I guess). It suffers from a generally rough idle. At start up, is sounds kind of rough (a little bit like it is missing), but after a second or so after starting it smooths out. When it is sitting there idling, you can feel a bit of a stumble or miss in the seat of your pants. It is also a gutless pig that can barely get out of its own way. There is no light being thrown. It has about 125k on the clock and to the owner's knowledge it has never been tuned up.

Obviously, the first order of business is new plugs. I am unfamiliar with the 3.7, but is it generally known to not have much power and suffer from rough idle/start problems? Are these trucks pretty good about throwing codes when issues arise, or are there often problems with them that don't trigger codes? For example, if a coil pack goes bad or has an intermittent issue, does it usually result in a code or is it often not detected by the computer.

I know there is no substitute for good diagnosis, I just wanted to get a lay of the land from those of you with experience with these engines/vehicles - places to look first or things to consider.

Thanks in advance for any assistance you guys can offer!

 

The ECM had to be dumbed down by the factory so it wouldn't throw misfire codes. When OBDII first showed up in 96 misfire codes were very common because the ECMs were too sensitive. Today's fuel is a major problem, it requires more KV to ignite and puts a lot of stress on the ignition system. That is why almost all new engines don't have plug wires anymore, the wires wear out a lot faster when you're trying to pump 80KV through them. The days of just replacing the spark plus are over, replace the wires every time you do the plugs. If you have COP you should replace the boots when doing the plugs. The fuel we're forced to run is killing the fuel system and ignition systems.