help diagnosing rough idle (this should be fun)

I've posted this on Jeep Forum, as well as the Jeep section on Expedition Portal. Haven't really had any specific answers as to what my problem is caused by.

1999 4.0 322k miles very very meticulously taken care of.
there is a leak from the header. as well as from between the cat and muffler
new bosch injectors (rough idle before and after install of injectors)

the rough idle throws a check engine light. the codes consist of misfire in cylinder 1, misfire cylinder 2 cylinder 3 etc. as well as random misfire in multiple cylinders (think that ones P0300?)

it happens when i get on the gas a little to hard, high revs seem to cause it, and high revs seem to cure it.
example of this: get on the gas to pass someone on highway, it downshifts revs high, pass and were good. when i come to a stop it idles rough. if i put it in neutral and rev it for about 20-30 seconds it clears right up.

what we have noticed it when it is running rough removing the wires from injector one does nothing, if you rev it up and remove the wire it runs poorly so the misfire is ONLY AT IDLE when up to speed the motor is fine!

lastly, i have noticed with the new injectors it seems to be ever so slightly worse.

here is the theory of a 19 year old kid who grew up around cars (FWIW)
when i get on the gas the O2 sensor reads that the engine is running lean because of the exhaust leak at the header, because of this it causes the motor to run rich (which i know it runs rich, spark plugs smell like fuel pretty bad and the leak at the header is rich of fuel)

when i put the new injectors on they are brand new as compared to the old ones so they run alot better so im guessing its slightly worse with them because they are getting better flow from the beginning and when it makes it run rich its getting a better flow than the older injectors so dumping slightly more fuel causing it to run ever so slightly richer

yal think thats a likely scenario? i just cant figure why it would clear up with more revs... and why its only at idle.
any way i can help prevent this a bit till i can afford a new header? maybe adjust the gap to give a hotter spark?...

let me clarify on this statement
"what we have noticed is when it is running rough removing the wires from injector one does nothing, if you rev it up and remove the wire it runs poorly so the misfire is ONLY AT IDLE when up to speed the motor is fine!"

at idle removing wires from cylinder one's injector has no adverse affect on the idle. it still runs ****ty leading you to think that injector one is not working properly, test of wire shows it is sending a current. when you rev up motor and the rpms smooth out then you remove the wire the rpms become rough so its safe to assume that for some reason at idle cylinder one is misfiring but at rev (sorry stupid cluster has no tach so idk what rpm its at) it runs fine. thus we are led to believe problem is only at idle. though affects on MPG are very present still.

also at idle removal of any other wires results in an even rougher idle so they are good. it is just cylinder one that is messed at idle.. well. in the case of when we checked. if there is random misfire in multiple cylinders im guessing the outcome of that experiment would be different.

i have been told i should do a compression check, which i am waiting on the right fitting for (have the gauge but the part that screws into the head is the wrong thread) i was told if the compression is low in cyl 1. that i may have a worn or missing valve seat. i was also told i should get a leak down test, which unfortunately i have no idea what that consists of. at the moment I'm in college, no job, and after dec. i should be shipping off the the military so im limited with what i can do. i already have a ton wearing down on the jeep (needs: u-joints, tires, brake job, rear main seal, tranny filter/fluid, fluid change in t-case, oil change, the exhaust work, etc) so i can't just start replacing random stuff and hope it fixes the issue. my dad has said it has had the problem for quite some time before the jeep was given to me (we are the original owners) when i got it, it would misfire if you drove faster than 72mph. replaced cap and rotor and now its just at high revs...

thanks for your help guys. I'm kind of in desperation with the jeep at the moment...

i should have just named this help diagnosing a few problems lol

what would cause this, randomly the airbag light will come on and the speedo will quit working (drops to zero mph) while driving. when my sister was using the xj to get to school she had said it happened quite often, i did the cluster check (hold trip reset button while turning ignition on) and everything worked fine, it happened to me few weeks ago though. dropped to zero. i kinda tapped the gas a couple times and it started working again. light went away. im sure hitting the throttle didnt help but thats just what i did any thoughts on this as well as the before mentioned issues?

thanks

Insofar as the gauges, this is a known issue. The connector on the back of the cluster gets dirty and loses connection eith the harness. This momentary lose of data causes the cluster to reset which is why the gauges drop and the air bag light comes on. Remove the cluster and clean the contacts on the back of the cluster. It will help but, it is a problem that will reappear...

I agree that you absolutely need some testing of the internal condition of the engine. Compression test is something that you can do yourself (perform both a dry AND a wet compression test; GOOGLE search for that if you are not familiar), but a cylinder leakdown test is even better at isolating root cause. Start with the compression test.

The compression spec. for the Jeep 4.0 engine is 120-150, with not more than a 30 psi variation between cylinders.

And pay attention to that P0300. That means you have a "random misfire", which often causes the other misfires. A P0300 can mean you have a vacuum leak, bad coil, poor fuel pressure (check it with a gauge; you should have 49 psi at idle, plus or minus 5 psi)

Here is a link to a good article on the P0300 random misfire.

http://www.random-misfire.com/

Make sure that you do have adequate fuel pressure. You also want to fix that exhaust leak as soon as possible. With the leak in from of the oxygen sensor is is giving your computer false readings and incorrect amounts of fuel because of it. Until the basics are good you can't find the real source of the problem. Fix the exhaust leak and any vacuum leaks. If it has a external fuel filter and you can't remember the last time you changed it do it. They are inexpensive, easy to change and extremely dirty ones can cause intermittent running issues.

X2 on fixing the exhaust leak, personally I wouldn't do anything else until that's done. An exhaust leak (before the cat) will cause all kinds of misfire issues.

If you can't afford a new header you might consider pulling the header and checking the flange with a straight edge (metal yardstick works), then with a large flat file (I've used a hand held belt sander) get it flush. Cracks can be welded but you'll need to find someone that really knows how to use a MIG or TIG welder to do it right, and then it may crack again down the road. New gasket and clean the header to down pipe connection.

alright i will have to find the time to take the jeep to my dads and take out the header and have it welded up, if its something he can't weld then ill see about finding another one. and while im there i will do the compression check and fuel pressure. i haven't thought about the coil being bad, any way to test that?

bldeagle10 said:

alright i will have to find the time to take the jeep to my dads and take out the header and have it welded up, if its something he can't weld then ill see about finding another one. and while im there i will do the compression check and fuel pressure. i haven't thought about the coil being bad, any way to test that?

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There may be, but before you start throwing parts at it eliminate the exhaust leak, then go from there. With the kind of mileage you've got there may be a bunch of things that are working (or not working together) that may be causing the misfires. You sound like you're way beyond your years in ability...you'll get it.

I remember from working on old in-line six-cylinder engines (with carbs) that number one and six cylinders tended to run a little lean because of the longer intake runners. Even thought this is a direct injection engine it may have a contributing factor along with a borderline weak O2 sensor(s).

Your throttle body should be cleaned too (throttle body cleaner, not Gumout).

ability wise i can do mostly everything minus machine work. i grew up working on race cars. my only set back is my borderline severe financial situation at the moment.

i keep the fuel and air system cleaned out almost religiously. keep a bottle of carb cleaner in the back of the jeep. i feel like even after i do fix the issue i should go out and buy as many of the sensors as i can think of, o2, oil pressure, cps, tps...etc. i highly doubt my dad ever changed them. he did maintain the motor well with oil changes and stuff, but when i got it i had to replace the dist cap, rotor, plugs, wires, air filter...the front hubs were original so they had 300k miles on them, the front u-joints just went out on me and i found out they are the originals... there's a lot that he didnt touch. so i've got my work cut out for me. i think what will happen is the jeep will get parked and once im in the military (dec. i should be enlisting) ill send him cash every now and then and have him replace stuff little by little. just figuring out the main issues now helps out A LOT. so thank yal for the help

bldeagle10 said:

i keep the fuel and air system cleaned out almost religiously. keep a bottle of carb cleaner in the back of the jeep.

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If that can doesn't say "sensor safe" on it, it is probably harmful to O2 sensors.

"safe for oxygen sensors and catalytic converters"
that is good advice though cause i would not have thought about the cleaner affecting any sensors. i do feel the need to go sensory happy and replace alot of them cause i doubt any have ever been replaced in the 322k life span.

i did do an oil change, added a can of some engine restore stuff. new cap and rotor and the motor does have alot more get up and go and it rolls into the revs aloooooot smoother. but still, to high of rev and it misfires. i will get ahold of my dad when he is back in town and see about having the header welded up asap.

I'm new to the site but I'm having a similar problem. I have a 1996 4 cylinder manual Jeep. I just had an oil change done, new rotor, distributor cap, wires and battery done. My check engine light is still on and I experience the high revving when in neutral and off/on while driving. I also have this weird rattling sound close to the the cross member and I was wondering if anyone could tell me what my problem is and how to fix it?

I had the same issue with my gauges. Cleaning them did nothing, and eventually they stopped working altogether. I spent $20 for a new cluster on eBay, and they are now working perfectly.

The most annoying part is not knowing how much gas is in the Jeep. Speed can be tracked with GPS, but gas cannot.

longenasia, pull the codes and post them first.