98cherokeeguy

Ok guys I was sitin out in my 98 Cherokee today and I was pumping the brake and relised it made my jeep idle up where it's suppose to be idling which is between the third mark and the 1000 mark
My jeep currently idles at the third mark on the tach not in-between like it's suppose to have any ideas greatly appreciated

Gee oh Dee

Brakes have vacuum going to them, naturally pumping them will create a fluxuation in RPMs.

I don't know what the hell marks your talking about. You should idle around 750-1000 RPMs, maybe a bit higher depending on the condition of the truck.

Each line and numberI on the tach represents a RPM, btw.

98cherokeeguy

Ya I kinda figured that on the tach on the 98 it had three marks under the 1000 on the tach and mine idles at the third mark still can figure out why but cylinder 6 is give me heck it's always misfiring any idea there and I've already done a compression and fuel tune up I think my dist isn't set right

Gorillaxj

1st dash is 0 rpm, 2nd is 330 rpm, 3rd is 660 rpm. 4th is 990 (1k rpm) but roughly 700 and is normal but a tad low. if you have a missfire your idle will be lower due to the engine having more dead weight to throw around then normal, which is slowing it down.

and like stated above pumping the brakes uses vacuum and makes the engine idle up.

the real problem is the missfire. In my 2000 the fuel rail clogged up with rust and ruined my cylender 6 injector. Not saying this is your problem but its a possible fuel delivery issue too.

what was done on the tune up? just plugs? or wires, cap, rotor too? its always good to have timing set at the same time as these.

98cherokeeguy

I did the cap, rotor, wire , and plugs didn't take the dist out and reset the timing but gotta feeling that's playin a little bit of a roll as well but how would I go about check the ejectors

Gorillaxj

injectors can be hard to trace. if compression is good. spark is good, timing is correct. then swap the injector with a known good cyl, I did cyl 1 as its easy to get to. then drive it and see if the missfire moved with the injector. if it did its the injector, if it did not then it is still back at cyl 6... this is how i would recomend doing it as its free and they are easy to get to on the 4.0L.... if it moved to cyl 1 then its easy from here change it out and your good to go! but check for rust/crap in teh rail (just look into the hole. should be abviouse, mine had rusty poo looking goo come out lol

to set timing you don't take it out. you use a timing light and set it per the repair manual to the proper advanced timing with the computer disabled. haven't had to do it on a jeep as mine is coil-pack.

98cherokeeguy

Alright will give it a try this weekend and see what happens cause I can sit in my jeep while it idle and watch the tach move up and down

lynskynbnd

There are some TSB's on 4.0 misfires.
-extra heat shielding to protect the fuel rail from too much heat that can cause fuel to vaporize in the rail causing a miss.
-carbon build up on the valves can stop the valves from spinning and holds the valves open causing a misfire.

98cherokeeguy

I know theres no carbon build up
What do you mean by tbs
I'll give the heat shield some thought but it does it while cold too

deekay911

I am interested in your comment about the valves 'spinnning' - are you suggesting they are able, and do, turn during the normal open-close cycle? I would be very surprised if they did since they are under a fair amount of tension from the valave spring.

lynskynbnd

Technical Service Bulletins are put out by manufacturers for common problems.

I have access to them with ALLDATA Online at my work.

98cherokeeguy

That's pretty cool

lynskynbnd

They do spin. not much each time they open and close, but they slowly spin/turn in the valve guide. Look at the top of a used valve stem and you'll see a circular wear pattern. If you see a linniar wear patturn the valve has been binding in the guide and won't move properly.

Gorillaxj

^ this is correct!