Background: Replaced head bout a month ago (it was cracked).
also replaced at the time: plugs, plug wires, distributor cap, rotor button.
upon completion, ran great for about a day, then started noticed a slight miss. It is progressively worse. bucks like mad now and acts starved for fuel. does it when cold or hot...don't matter....worse when lugging the jeep...will vibrate the whole jeep....if I tromp on it....it stops until I smooth out again.....Has never stalled out...not once. does it every time i drive it. never runs smooth. can feel it at idle as in i miss, but don't think it is a spark problem.

I have checked all connections, plug gaps, wires. pulled one of the "new plug wires" off and it came apart.
replaced all wires again with a better set....no change.....
replaced coil, no change, although it was cracked......
replaced fuel filter.....no change
have replaced all plugs again.....no change.
NO CEL unless it was one i created myself....
no vacuum leak....
checked fuel pressure and it is constant at 32 psi with vacuum line plugged
constant 41 with vacuum line unplugged.
Other posts say unplug certain sensors to test....
unplugged tps....no change other than not shifting tight
unplugged iac...no change
unplugged upstream O2 sensor....no change
wiggle computer connection....nothing happens
There is no CAT. was removed by previous owner
don't think it is the CPS, it always starts fine and the TAC never drops out.
Not losing coolant after replacing head, no leaks....

I'm leaning toward injectors although they are only about a year old (rebuilt)
MAP sensor??????

What else am I missing???? Driving me nuts. Love this jeep and it has always ran smooth and been great.

THANKS!!!

 

Oh...And I have ran seafoam thru the tank twice.

Also.....had an alternator code a while back....could that cause this...wouldn't think so.

 

i am having this exact same issue in my 92 and have done all these things.. im running a msd 6al with a blaster coil... replaced everything you have.... and cleaned throttle body. also pulled crank sensor..cleaned it it went away and now is back,, although i have a leaking rear main i dont think that is the issue.. my alternator does sound lik its going to blow screaming bearings in it...i had a check code for 34 wich states cruise control.. but i removed mine and capped the line have no va leaks or anything. if you figure this out please post cause mine has been doing this and then stops starting..soo..if i find it ill post for you plese anyone with info help out.

 

One time my 88 did this and all it was was the vacuum line to the MAP sensor fell out of it's hole on the TB. You could check for that...

 

Renix and other year MAP sensor hookup are different, but yes that needs to be checked.

It sounds like a fuel delivery problem. Unplugging a bunch of sensors and not observing a change tells you that they probably aren't working. For example the TPS: you need to TEST it with a multimeter. If you unplug it the engine doesn't know what position the throttle is in and fuel delivery is messed with. Similar story with the O2: lack of signal should cause it to either run super lean or super rich in closed loop. Is the throttle body clean? What spark plugs did you put in there?

 

Good points Salad!^^ I would double check vacuum lines with a diagram make sure none of them are missing or mixed up. I know with the MAP sensor, if you dont have it hooked up properly the motor will run like complete crap.

With that being said, you also mentioned that you did some work on the head. Since you have to remove alot to get that out go through your repair manual and double and triple check your electical connections and once again check vacuum lines. Like Salad said, if your unplugging sensors and nothing is happening then something isnt right.

Keep us updated!

 

Thanks for the replies.

Unplugging the tps while idling does change the speed a little and plugging it back in may/may not kill the engine if it is idling. So I guess it is doing something, but driving it is no different. I agree I need to put a meter on it, will try to get that done.

I guess I wasn't clear on that, but the aic/tps/map/cps do cause changes while unplugging at idle, I was talking about my driveability issues when i said "no change", as in "still bucks/bogs down.... Sorry for the confusion.

Unplugging O2 caused no change whatsoever. was under the impression (from other posts) that you could run unplugged to look for change. And if it ran ok, then the O2 was bad.

I have checked the electrical connections/vacuum several times and have sprayed starter fluid along vac lines with no change. I believe they are fine.

Have cleaned throttle body.

The plugs are autolite platinum gapped at .035

Unplugging injectors while running: All present some change but some more than others, which leads me to believe maybe some are clogged and starving it for fuel.

I wonder if it is something I did or just coincidence. Like I say, ran good for a several miles after head replacement.

I painted the valve cover, may be I shouldn't have painted it orange, lol.

 

Autolite platinums? And it started running bad? I'd rule out those plugs - 4.0s are reknowned for not liking anything other than the OEM Champion or NGK coppers. Under your hood you should find a part number like RC12LYC, RC9LYC, RC12ECC etc. Get whatever it says there and see how that works out

 

I have tried two sets of autolite platinums and pretty sure that is what I ran in it before.....maybe just doesn't like them anymore???

 

Have you verified spark on all cylinders? Easy way to do that is hook a timing gun up to all the wires and pull the trigger. Make sure the light is a steady pulse.

Do you still have your old injectors? If so try to swap those out.

 

Pull your crank pos sensor and clean it.re install see if it lessens the issue.i did this yesterday noticed allot less bucking when befor it was missing like crazy.also noticed the hotter and longer it ran the worse the symptoms.im running e-3s with extremely consistent spark on all cylinders(msd box helps) although my alternator still sounds like its going to spin a bearing I don't think tht is consistent with your or my issue.the bucking your talking about.it feels like u lost power for a brief instant.and idle is missing?maybe I'll just put the jeep on a video and post on here to see if we're talkin about the same rhing

 

You briefly skipped right over the probable cause.

""""no vacuum leaks"""""

How did you test for that?
visual, carb cleaner, propane etc often miss the leak.

The best method - engine off - remove a vacuum line on the manifold - blow cigarette smoke in the manifold.
Fix wherever it comes out.
The smoke will come out the throttle body - tape the top closed or plug with a rag.

The most likely place since you just did the head - intake manifold/head joint. The smoke will find it in moments.

 

CAO - ditch those E3;s - try the right plugs. E3's will cause detonation in a 4.0.

The multi electrodes make a shadow in the flame front, splitting the flame front into several fronts. Then they collide - detonation, and destruction of rings and pistons.

Early signs are misfires, then major problems.

NEVER milti-electrodes no matter what advertising lies say.

 

Roger.had them laying around brand new for the jeep. But regardless if them the bucking was there befor

 

The E3's may not be the bucking/stumbling problem now - especially when it did it before the plug change, but they will cause trouble later on. Seems like most of the people who try them say they work good for awhile - 100-200 miles, then the troubles begin.
Not easy to diagnose when you have 2 problems at the same time masking each other.

OP - if the smoke test doesn't reveal anything, I'd go back to the plug wires. The symptom you described sounds like eiher a vacuum leak getting worse or an ignition problem - like wires.

If you know someone with a scope it'll show an ignition problem within seconds.
Try this - one at a time pull off a plug wire at the plug while it's running. Let it jump to ground while you look at the spark.
It should be a crispy thin bright blue spark on each cylinder.

ANY COLOR OTHER THAN BLUE IS WRONG.
Red, white, yellow etc show a weak spark.

If not blue only on one cylinder, replace that wire. If on all, test at the coil tower,, if OK there, replace coil wire, if still not blue, replace coil.

Some folks say to never pull off a plug wire it will cause damage o the coil.
Not true, the voltage will go up to 30kv like it should without damage.
Just don't leave it disconnected for a long period of time, it could overheat the cat.


Let us know.