831xj

Hey trying to hunt down my intermittent cyl 1 misfire. I've been narrowing it down for months.. Seems when I get on it 4-5k rpms and at the next stop when slowing down the rpms will bounce to 500 sometimes like 250 and come back to about 750 idle. It will shake my whole jeep at a stop. Runs rich but no misfire when driving. I'm fairly certain it's valves I have a tick that I was 99% sure was the exhaust manifold but it could be valves. I have brand new Brown Dog mounts and an APN header in my living room. Anyways... I was under my jeep after driving around and my cat was hot as hell but i could touch my hands on the pipe before and the muffler after. Does that indicate clogged cat?
99 4.0 aw4 231

831xj

I have a obd2 scan tool with live data also but I am not so sure what to look for.

o2s both new
new dizzy rotor
plugs
wires
tps

MichiganCherokee

I've had the same exact issue with my 01.
Tried everything I can think of...new tps, new plugs, coil rail, ect sensor, crank sensor, compression is 160-165 on all six, all 02s are good, injectors are all firing correctly and are new. Dealer thinks valves but everything I've read states bad valves = low compression which I don't have. I have a Clearwater Head on its way as we speak since I have the 0331 head, hasn't cracked but don't want to hAve to worry about it and it might fix my miss. I will report back once it's swapped. Fuel pressure is at 48 psi as well. One night driving home I couldn't go over 40mph because I thought it was going to shake apart, good thing all the stop lights were flashing I didn't want to stop and stall lol.

Turbo X_J

Why are you spinning that tired old motor to 5k anyway? No wonder it's jacked up.

jebmccall

checked the iac? they like to get gummed up with carbon over the years and what the iac does is control the flow of air at idle... idle air control valve... pull the throttle bottls off, give it a good cleaning, pull the iac off and use electrical contact cleaner on the valve. do not try to extend or retract the valve, also clean its seat.

my jeep was doing something similar, when ever i would be crawling in 4 low for extended times at around 2k and then come back to idle the rpms would drop like yours. cleaned it and no more issues.

djb383

What would change about the valves that would cause good-to-bad idle?

831xj

Well dont they rotate @ 3200 rpm? could one be stuck. Compression was even across the board with 10psi max difference.
Iac and TB have been cleaned recently

highmileage

A clogged cat will rear its ugly head with an infrared thermometer or right after driving it at night look for the red glow under your JEEP. Backseat passengers complaining of hot feet is another sign.
I had an older T-bird cook a cat years ago engine never lugged just lost power and seemed to load up and needed a little more skinny pedal, the RPMs' moving around rings a bell, but no backfiring or anything like that. you can take a look at your plugs too what are they burning like, hot and clean may be the cat, dirty may be the valves.
If you make regular trips to 5k bad valve springs could be a winner too.

831xj

I baby it with shift points in the 1.8k-2.5k range. It has stock gears on 33's so I lug around. I am just saying what I have noticed as far as the rough idle occuring after I get on the gas. First time I noticed is when I had it in 1/2 and drove it til it shifted into 2nd. It also has a knock at idle but goes away when I accelerate.

highmileage

sounds like back-pressure, could well be your cat? Old school trick was to punch a hole in the exhaust in front of the cat and see if it drives better, if it did bad cat if not you have a hole to patch.

831xj

What are some inexpensive options for a carb cat? Mine has over 220k on it

MtnHermit

Be sure to check the injector cable between the engine and firewall. The photo below shows the fuel manifold cutting into the cable and causing an intermittent injector short. The cable needs to be well away from the engine.

djb383

No.....they reciprocate. If a valve sticks, it would "hang" in the open position and would show up in a compression test as a weak/low number cylinder.