I have a '99 Cherokee Sport, 4.0L, with 111, 000 miles, Airaid intake and flowmaster Series 40 muffler. Other than that, everything else is stock. I have replaced the cps, the distributor cap, spray cleaned the iac without remvoing it from the throttle body and yet am still having a rough idle (tach will bounce around the 700 rpms range at idle) when the vehicle has reached operating temperature 195-210.

Was wondering if anyone had any ideas at what may be causing the rough idle now. I have taken it to one mechanic and he recommended a valve job. Was hoping there may be another cause.

Oh, also have run Seafoam, Chevron Techron, Lucas's High Octane boost, and a couple of Gum Out products through the tank and have had a mechanic do an injector cleaner through the fuel rail.

All suggestions and ideas would be appreciated.

 

Removing the iac is the only way to properly cleaning it. I would start there. Check for vaccum leaks around the intake manifold and check vaccum lines and connectors. Do the valves sound especially loud?

 

No. When the mechanic suggested a valve job, he stated that it was the exhaust valves of cyclinder 1 + 2 that had build up. That the engine had not been rev'ed high enough during the accumulation of the miles and the cylinders would fail to get hot enough.

I dont hear any out of the ordinary once it warms up. The exhaust sounds like it cuts out/back fires/ pudder (however you want to term it) when it warms up.

 

I notice that when I add Lucas Oil octane booster with 93 oct. gas, I have less of a rough idle problem. I took the iac out this weekend and clean both it and the hole it goes into. The cleaning helped a bit, but still is a bit rough when idling after it has reached temps.

I would figure if it is an exhaust valve going bad in 1 or 2 cylinder, wouldn't I notice it on acceleration as well a when it idles?

 

Mine doesn't idle all that smooth. Certainly not like my 2002 F350...but it's a '94 with 200k. I would not be concerned unless it runs rough at speed.

 

mine does the same thing when its hot and sitting at idle. if u let urs heat up then shut it off then go to start it up within a couple of mins does it idle really badly/sound like the exhaust is choking out/just run like crap? kuz if so do a search for "heat soak" kuz apparently its common between '99 - '01.

hth

 

I wrapped my entire fuel rail, and all 6 injectors with heat tape (AZone $16). It did make a difference. Seafoaming, dumping the whole can in every fill up for the last 3 fillups, and it is calimg my idle down a lot. I read the 99+ has a carbon buildup issue and the valves stop rotating.

 

My 01 has a similar issue. It idles at the half way point between 0 and 1000 so I am guessing that its idleing at 500 rpms. It bounces just below the 500 and I can watch the needle wiggle. Feels a little rough like its just barely stumbling. No missfires, no backfires just very quiet yet slightly rough. I have 144k on the speedo so I believe a rough idle is to be expected. I do plan on wraping the fuel rail and injectors but I believe mine could be resolved with new injectors all together. I can't really afford them right now so I will just deal with it until it gets really bad. I am thinking that my spark plugs are part of the problem also. I used the cheapest plugs when I changed them( the 1.19 champions). It had Autolites in it before. Otherwise, I'm just glad it starts every time every day.

 

I may try to wrap those things in a heat tape. Thanks for the suggestion.

The mechanic that stated the Jeep would need a valve job stated that he tried to rotate the valves on cylinder 1 and 2 and that they wouldn't rotate, even after treating them with some thing (which was seafoam or an equivalent).

 

The Champions are what they put in them at the factory, I put a set of those in mine, it idles fine as far as the plugs go. Mine is carbon of time for a new head.