Its a 2000 Classic with the 4.0L, 151,000 miles, pretty sure it has a cracked head but ive owned it for over a year now and its ran fine until this past week.

Started off idling rough but would go away as it heated up, steadly lasted longer until now where it runs rough all the time except when at speed and coasting, also seemed to be worse with 1/4 tank in it as opposed to now where its 3/4.

I pulled the coil pack and checked ohm resistance and it was fine, 1 - .9, compared it to a new one at O'reillys. Pulled the plugs and all were fine except the 2 cyl was coated in fuel, so i replaced all plugs, autolite coppers, gapped .035. Ive cleaned the IAC valve at it ran like a slightly better turd but same problems, ive also cleaned the MAT sensor i believe but not 100% sure, regardless no change.

these are the codes its throwing...
P0303 - cyl 3 missfire
P0300 - multiple missfire
p0301 - cyl 1 missfire
P0302 - cyl 2 missfire
P0138 - downstream 02 sensore (prev owner took out the cat so this is normal)
P0305 - cyl 5 missfire
P0113 - IAT volt high

the last one could be the problem, idk where the IAT is tho im confused with that and the MAT sensor i havent seen a clean picture to straighten that up, but anyways if its getting high volts its prolly not the sensor but the wiring right?

please help, hopefully ive provided enough info

oh its also got what sounds like a valve or lifter tick that wont go away, i was thinking worry about 1 thing at a time, but idk if they could be connected thanks

 

it did also stall a few times during the week at red lights but not doing that now

 

Check the fuel pressure.

 

possible heat soak issue if it's misfiring at idle/stopped at a light or after a hot start....also check your injectors, see if any of them are leaking. The o rings could be worn out. Also try running pure gasoline in it for a tank or two. That would be no ethanol gas. Do a search for stations in your area where they sell it. Clean the throttle body too.

 

did fuel pressure check, 48 psi at idle and didnt drop after 5 min when i turned it off

 

dont think its heat soak bc it does it when its cold and hasnt ran at all, like this morning i thought heat soak only sets in when the jeeps at op temp? Im gonna clean the throttle body now and if i had a leaky injector wouldnt the fuel pressure test tell me, like when i killed the engine if i had a bad oring or injector the psi would drop right away right?

 

TB cleaned, no difference. Thinking of replacing TPS

 

if it's doing it cold then it's definately not heat soak....you said you think it has a cracked head? is you coolant diasappearing?...

also you ohmed the coil pack-but you did it without current running through it? I would think (not an expert) that it might ohm out ok but then when it sees current it can't handle the load. Just thinking out loud.

 

Yeah it's been drinking coolant since I bought it. Burns up and comes out exhaust. That's a good thought I'm not good w/electrical stuff but that makes sense to me. I'll try swapping the coil rail tomorrow if I can

 

I was doing some research came across a post about a toyota 4 cyl truck. truck has 2 coil packs, 2 cylinders a pack. both ohmed good, but when he switched the coil packs the misfire followed the bad pack. So it is plausable that you have a back pack even though it ohmed good. I would think to do a sure fire test you would have to run some current through them. I'm no expert on motor vehicle electrical, but whenever we do tests in the field in my line of work, we run voltage with current through the wires. You might have a "leak" in a wire and it's not going to show with some fire running through it.

 

This is what you should concentrate on first. If it's using coolant, it's either leaking it or burning it.

If you can't find a leak after a comprehensive search, then it is very likely that you have a cracked cylinder head. Not uncommon for the 2000-2001 with the infamous factory 0331 cylinder head.

Start reading up on that head. Knowledge is power when it comes to the 0331 head issue. You need to diagnose if you have this issue before doing anything else.

 

I havent magnafluxed it, but im pretty sure its cracked. i pulled the VC yesterday and saw the crust build up in between the 3 and 4 cyl. but no visible crack. i believe its been cracked since i bought it a year ago but its ran fine until last week. i also have low oil pressure but ive been running 10 - 40 oil in it and it seems to do better. I might be able to get my hands on a compression tester, that could give me a better idea.

ive been looking into replacement heads, alabama and clearwater brands, id like to get one of those 2 b/c they are recast. i understand that i probably have caused some bearing wear but the jeep has ran fine until now so im alright puttin a new head in.

but for now its missing not knocking so what would the cracked head have to do with that? And what other ways can i check for a cracked head

 

The visual of the head in the 3-4 area is likely telling your story. But you should confirm. You can try running a cooling system pressure test on it (often times you can see the coolant seeping through the crack as the system is pressurized) or run a cylinder leakdown test. Compression test rarely tells you anything with a cracked 0331 head; don't waste your time as it can give you a false sense of security.

If you confirm that you have had a cracked head for a while and you mention that your oil pressure is low, I'll be honest with you and recommend that you consider installing a different engine in there. Or a rebuild. AND a Clearwater cylinder head. (Alabama Cylinder Head went out of business).

I've seen too many guys install a new head and then not far down the line they have to replace the engine. But don't take my word for it, search and you'll see for yourself.

 

ive done a good amount of research and i know that its bad news. I just dont have the money to rebuild it for sure, ive called a couple machine shops to see pricing and its very costly. ive seen long blocks online for about $2,000 which i dont think is bad but i cant afford that at the moment. I really need to get this running right for now and see if i can swing a summer car and work on this one til the winter. but even with a new engine i think im suffering from an electrical problem, ive seen a lot of your posts on other threads tjwalker, i believe i can trust your opinion, do you think i can get it running right an worry about replacement later or is the head causing this and im just days away from the end of the engine?

I have a 79 z28 that i can drive when it warms up but not friendly on insurance and gas, and it needs a tune up badly

 

Those multiple cylinder misfires could be a lot of things, from a flaky coil pack, weak valve springs, intake or exhaust leak, ground or harness issues to a cam sensor that's not correctly indexed.
Unless the CEL is flashing, it's more an annoyance than an immediate problem. The cracked head on the other hand, that's an immediate problem as it will keep getting worse and eventually destroy your bearings. I'd make that first priority, keeping an eye on the oil pressure afterwards.