A few remedies:
There is no "magic bullet" to cure the stumbling Idle problem. It is charchteristic of the H.O. Motors and is seen often in other 4.0 motors as well. The "stumble" is usually so insignifigant that it does not even register on the tachometer. Most people do not even hear it except for the most critical and maintenance-minded 4.0 Owners. It is more audible in engines with a slightly louder aftermarket exhaust.
If power, acceleration, start up and MPG is fine, and you do NOT have a CEL light on, and you are still having the minor occasional "hiccup" or "stumble" at idle, there is no real cause for alarm. If you are encountering other issues succh as hard starts, overheating, smoke, and/or STEADY mengine miss throughout the entire RPM range....then you have an actual PROBLEM that must be addressed that is totally unrelated to this.
A few things I have donehave lessened the severity and frequency of it to the point of no one else but myself noticing it.
- QUALITY ignition components: Copper terminaled cap and rotor, good high quality plug wires, and CHEAP Champion truck plugs or Autolite 3294's. (Thanks, Bruno.).....Do NOT think you will improve power, economy and idle stumble with multi pronged, platinum spark plugs. You WILL have problems. Stick to copper terminaled ignition componenents, and basic, cheap plugs as outlined above.
- SENSORS. Ensure that all of your sensors are clean and up to par. Most specifically your Idle Air Controller and 02 sensors. Either of these in a worn or dirty state can contribute to a crappy idle condition.
- Vaccuum system. Even a small vaccuum leak can cause an erratic idle. Look for cracked hoses and loose connections, then look at the intake manifold and TB base for possible vaccuum leaks.
- THROTTLE BODY: Dirty throttle bodies can contribute to an erratic, stumbling idle. Disassemble, clean and torque to spec if it is dirty. Pauy attention to getting all of the crap out around the idle circuit. Dental picks, wire brushes, compressed air, and throttle body cleaner are your friends when doing this job.
-CCV System: The orifices on the return side of the CCV (think PCV system) are smaller than the intake side, and can clog prematurely, especially at the elbow on the valve cover. Disassemble, clean, re-install ensuring there are no leaks.
- Electrical Grounds/System. Make sure all electrical grounds underhood are snug and corrosion free. Ditto for battery terminals. Clean, consistent voltage helps systems signal better. Ensure your alternator is putting out proper voltage and your battery is holding the same. Weak electrical systems can play havoc with computer-controlled vehicles.....to include an erratic, stumbling idle.
- Proper TEMP Range. 4.0's NEED to be operating in the 210 degree temp range. If you have anything other than the proper thermostat in there, get rid of it ASAP and get the proper temp T-stat. The computer will throw excess fuel and operate in "warm-up" mode until it reaches warmup temp. If it never makes it to 200 - 210 degrees, idle quality and MPG will suffer as a result. DITTO if it is running too hot (over 225 degrees.) Maintain thayt cooling system within the 200-215 degree range and the computer will operate properly resulting in a proper idle.
- Exhaust System. A plugged catalyctic converter and/or crimped/crushed exhaust components can affect idle quality, resulting in stumbling at idle. Inspect and replace as necessary.
- Fuel system: When was the last time you replaced your FUEL FILTER?
- AIR filter. Same as above. It matters. Really.
.......These are the basics. Even the most seasoned ogf mechanics can overlook a basic or two. Once youve checked all of the above, your idle should be a little better. About the best you are going to get it using conventional tune-up and maintenance teqhniques. If you want to further your attempt, use old school hot rodding techniques:
- 1999 and up intake manifold: Re-designed in 1999 as an equal-length intake to increase HP, reduce emmissions, and smooth intake charge, this intake flows smoothly without restrictions and is a basic junkyard upgrade that improves stumbling idle...a little bit.
- Free flowing exhaust. Quality aftermarket mufflers and free-flow cat converters have helped increase HP and clean up the idle quality a bit. The problenm is....now that it is slightly louder, it will be more audible as a result, though it will do it less frequently.
Damn if this didnt get longer than it was supposed to. Basically, keep your engine in top form, maintained and use quality replacememnt components, and it will lessen the severity of a minor problem that is inherent to most 4.0 motors.