Hello, I have a 2000 Grand Cherokee with about 160,730 miles, and I had a question about rough idle. Within the last few months I have replaced the fuel filter, spark plugs, crankshaft position sensor, air filter, cleaned the throttle body but at idle it is still pretty shaky and not smooth and consistent. My last resort is the IAC Control valve but thought I’d ask for a second opinion before I do anything.

 

My opinion is change your injectors and do a carbon cleaning by using water to blow out the carbon or some sort of cleaner like Seafoam.

 

I'm guessing your 'Check Engine' light isn't on? If it comes on, switching the ignition from 'Off' to 'On' (but NOT starting the Jeep, turning back to 'Off' as soon as the dash lights all come on), and leaving it in 'On' the third time you do so, the odometer will show the codes for you, and display 'd0ne' once all codes have been displayed.

I'd check the emissions hoses too, as there could be an evap leak (VERY common on WJ Grand Cherokees, especially the 4.0s... mine has all three evap codes: P0442, P0455, and P0456). Check the gas cap as well to make sure it's sealing off the gas tank correctly. If you want, any good automotive shop can do a smoke evap test to test for leaks.

-Vince

 

Sounds like it needs a good long high speed run, but there aren't too many places in Alaska to do that, is there? Short trips and low speed will carbon up the engine quickly, especially the intake valves, so they don't seal very well at idle but will run good otherwise.

Warm up the engine to normal temp, then block the throttle at about 2500 rpm and slowly pour a couple of glasses of water down the throttle bore, but not fast enough to kill it. The water will soak into the carbon, turn to steam, and break loose most of the carbon. You'd be amazed at what comes out the tailpipe.

 

Yes but will the loose carbon mess up the cat converter? I wanted to do the same to my 4.0 with 163k on it but hesitated for a messed up cat! I like the old days when the emissions were not on the car, yes they did not run as efficient as today but I liked to work on them more simple design !

 

I've done it several times on both my WJ and XJ with no problems and the converters on my WJ are 256K miles old. The carbon won't come off in chunks but as fine particles that should eventually blow clear anyhow.

One thing that adds to the carbon problem is the CCV system that pulls in oil vapor to be burnt in the engine. When my WJ engine got old before I replaced it, I put an oil catch can on the CCV line between the valve and the manifold and was capturing about a cup on oil per week that didn't go thru the manifold and crap up the valves. I had to hot-tank that manifold before using it again to get all the crap out of it near the intake valves.