incurable misfire an 18 month saga

[6TIME]

If you have a lobe going flat, it'll first show up at idle. Then as it wears you'll steadily lose that cylinder and it'll miss progressively worse until total misfire occurs. Pull the cover and measure lift at the pushrod with dial indicator. Also, have you done a compression check on all cylinders to eliminate any other mechanical issues? Several years ago, I had a 232-4 go 76k before losing a lobe on it due to using the wrong oil the whole time....

[bandofthHawk]

Sounds like exact symptoms of what im having. First it was at idle and would appear once a blue moon...now its chronic.I'm taking it in Monday for investigation. I will post any results. Thanks guys. "oh yeah you can run whatever kind of oil".....live and learn.

[SilverXJ]

Nor using the Comp Cam anymore. Lobes went flat on a nitrited cam. I had an Isky cam that I liked, but I ended with a custom Jones cam with the later snout that uses the thrust plate. There is no quality roller cam for our engines.

[bandofthHawk]

sounds like comp needs to pick things up in the reliability department...well I guess more ill have more info once he pulls the valve cover, head and all that. If you were in my shoes with this 4.6L would you replace with a different/ better quality cam? or replace parts with as similar to the original parts list as possible? also what oil should be used for maximum cam longevity? my apologies for the barrage of questions.. I looked on Comps website I see the 68-231-4 (not impressed seeing as yours and mine have failed)the 232 looks the same just they say its for FI. Are the 68-239-4 and 68-235-4 any better?

[Retlaw01XJ]

Found a nice article on OBDII misfires:
http://www.underhoodservice.com/Article ... gines.aspx

Some highlights:
>>>Most engine control systems with OBD II monitor the speed of the crankshaft between cylinder firings to detect misfires. <<<
>>>Misfires can be caused by one of three things: fuel mixture problems (too rich or too lean, or not enough fuel), compression problems, or ignition faults.<<<
>>>A much more common cause of fuel-related misfires on fuel injected engines is too much air and not enough fuel (lean misfire).....
>>>Vacuum leaks have the most effect on the air/fuel mixture at idle, and less of an effect at higher engine speeds. If you suspect a vacuum leak, look at the fuel trim values on your scan tool when the engine is idling, and again at 2,500 rpm. If the engine is running lean only at idle, it has a vacuum leak. If it is running lean at all speeds, the problem is in the fuel supply or control system (weak pump, leaky regulator, bad sensor inputs to the PCM, etc.).<<<

PCM:>>>Lean misfire also may be caused by a bad fuel injector or a bad injector driver circuit in the PCM.<<<

>>> Compression misfires will occur if a cylinder fails to hold compression because of a leaky head gasket, bent valve, burned exhaust valve or broken valve spring. Low compression also can cause misfires if a cam lobe has rounded off.

These types of problems also can be found with a vacuum gauge. A flickering vacuum reading typically indicates a cylinder with a compression problem. You can follow up with a power balance test, a compression test and/or a leak down test to further isolate the fault.<<<

Best to read the whole article. Sounds like solid advise.
I still recommend putting a vacuum gauge on there as a first step.... it's easy and cheap.

[6TIME]

When mine went flat it began missing a bit at idle and it also began ticking pretty good at cold startup for a few secs. After a month or so it was ticking solid all the time along with the miss getting worse. Many times with a lobe going flat, you won't see a noticeable drop in cranking compression because the cylinder is still sealing up fine. Along with the flat lobe, it cranked at 155-160psi on all 6. The valves were just not opening up enough to promote effective intake/exhaust flow thru the head... It's a good test to rule out ring/valve seat issues tho.

[bandofthHawk]

Taking the jeep in first thing tomorrow morning . Will post results after further investigation. Thanks to everyone for pointing me in the right direction

[bandofthHawk]

ok guys heres an update. Did compression test again. found low compression on cylinder 4... 85 psi. pulled valve cover everything was tight & solid. no bent pushrods, no bent rocker arms, springs were fine. they all appear to be stock replacments. From what I understand the issue is a valve problem. very glad I had everything checked because we discovered I have the wrong exhaust manifold gasket. Its holes do not line up with the ports...I have the older head and they used the newer gasket...common sense tells me major exhaust restriction is not good. the Jeep stealership definatley screwed me on that... looks like im going to need a valve job. doing head gasket and getting head cleaned up too obviously. will be replacing header and motor mounts while were in there as well. thanks again for helping guide me in the right direction.

[SilverXJ]

Check the lift on the cam too. The sound could be coming from the exhaust gasket. Check preload as well.. too much can hang a valve open. A leak down test would be helpful, or just pump air into the cylinder @ TDC and listen to where it is escaping from. Then remove the rockers and see if you hear less air. Don't discard the idea of damaged rings or pistons yet.

I would check preload before you pull the head.

[6TIME]

Was the head rebuilt? There's also potential for a burnt valve if whoever did the valve grind went too deep into the factory exhaust seats... At least you can focus on #4 now and pinpoint the issue. There's only a couple reasons for low cranking #'s in a isolated cylinder. Should be a no brainer. What'd the rest of the cylinders crank at?

[bandofthHawk]

yeah the head was definatley rebuilt previously.
compression test results:
cyl 1 - 125
cyl 2-125
cyl 3 -140
cyl 4 -85
cyl 5 125
cyl 6 145

somehow when the test was done he was able to get the gauge to get up to the 120's on #4. he seemed very confident it was a valve problem. I just cant imagine that a motor with only 25k or so miles that's been well maintained would have shot piston rings, cam lobes, and lifters. I mean now im having to do valve job, head rebuild, gasket, header & motor mounts. To me that's a big expensive job I cant do myself...my only concern if he pulls the head and says oh hey yeah your cam is bad etc... Im screwed and basically looking at a complete rebuild...silverxj im pretty sure he did the test you mentioned with the air in the cylinder. also im told that while we do all this it would now be a good idea to port the intake manifold for better flow? what do you guys think ?

[6TIME]

Those compression numbers vary quite a bit.. Pull the head and inspect the valve seats for proper seal. Then check all of the cylinders for excessive/irregular wear or scoring. Once the head is off, your problem should be easy to spot. As for the cam wear.. it all depends on quality of lifters used and proper oil/break in procedure. Porting the intake will not benefit you much at this point.. You're much better off just getting the right exhaust gasket and locating the cause of the erratic compression numbers. This will give you more power gains than anything at this stage in the game. Good luck.

[bandofthHawk]

an update here. we've got some serious issues. silver you called it man. pistons are looking horrible. cylinder walls very scarred. lifters were very worn and cam lobes definitely wiped out. found head gasket was blown in two spots.( ive got pics its not pretty) the 7120 head I have is very lightly ported at best. head would need ALOT of work. my engine builder said whoever built this motor used cheap pistons (Kb silvolite) and never machined anything correctly. so the good news is this jeep isn't a daily driver anymore. its just sitting here in the driveway with the motor all torn up. would like to get her running again but I don't want to spend thousands of dollars on machine work and rebuild. Doesn't have to be perfect. Id honestly be happy with a used 4.0. again its a 2000 xj. I have a good banks header here, good intake mani. & 62mm tb, plenty of different sets of injectors to choose from, any suggestions? not in any hurry here. thanks guys

[6TIME]

Was that cylinder #4 in the pic? With that many problems goin on in that motor, It is very apparent that the assembly and machine work was waay sub par. Did you ever figure out what springs you were running? There's nothing wrong with Silvolite pistons either... If that motor was setup better they'd be perfectly happy in there... Bummer man.

[sly-jeeper]

that's a bummer man it doesn't even look like the cylinders were honed properly or at all for that matter