1996 Jeep GC multiple misfires, hesitation and bucking

[hoofan]

From reading many other posts, I see that I am not alone here, no matter the year of Jeep. Let me see if I can provide a history of my problem and get some help. I had my mechanic (non-Jeep guy) originally work this problem, but I fear he may have made it worse.

First of all, I haven't had any significant engine or mechanical problems or symptoms to deal with at all with this Jeep. It's been great. I replaced the fuel pump 3 years ago, but that's really it. I recently had my manifold welded for a crack when it threw out a CE light and the Cat Converter rattles, but nothing that kept me from driving the wheels off this thing.

However, out of the blue in December, it started shutting off on me while driving. I got my mechanic to read the codes and sure enough (Because I don't normally don't read codes, but I do now), it called out the CPS. He replaced it.

Additionally, when I had him replace the CPS I asked him to go ahead and put some new plugs and wires in for me as well as sort of "tune up" that I thought I probably needed. No symptoms, I just thought it couldn't hurt.

Well, my Jeep hasn't been the same since and the odd thing is that I think the CPS replacement fixed my original problem (Jeep shut down while driving) but I'm now wondering if he caused another problem or if the CPS part is just cheap and is intermittent.

After that CPS repair and new plugs/wires installed, the Jeep ran fine for about a day around town between 0-45 mph and then a lesser or milder and much more random hesitation occurred, causing a "Check Engine" light, but not shutting down the Jeep. I drove it back to the shop and after 3 days he determined it was the ignition coil. He replaced it.

Got it back from him again, and after a day of driving, the same problem.

So, my brother referred me to a Jeep mechanic that has been very good to him over the past few years at diagnosing problems and we did not have to go to the dealer to check it out.

He read the codes and it spit out "multiple cylinder misfires". He immediately asked me what spark plugs my mechanic put in and I told him I didn't know. We pulled them all and found that they were Autolite. He said that in ihis experience, this was possibly the worst plug you could run in a 96 Jeep 4.0 6 cyl and generally Champion seems to work the best for some reason. We put in Champion plugs. We also put on better quality wires and dist cap (MOPAR Kit SPC40604AA) that my original mechanic did not use. The "Check Engine" went off just before we changed it and has not come back on. Driving around town was fine. We drove it around (but not at high speeds) with the reader attatched and no misfires on any cylinder.

I drove it myself over the weekend with no problem and this morning, "Check Engine" light and mild bucking/hesitation again, but not enough to have it quit running, same as before. In fact, it is running well enough to drive, for the interim.

However, when I got out on the highway again, I noticed that around 70mph when I set the cruise control, it jerked and hiccupped just once. I backed off the gas to around 60mph and gradually back up to 65mph or so. I drove 10 more miles or so on the highway and then back on secondary roads (35-45mph) all the way home without any problem at all and no CE light. The total trip is about 20 miles or 30 minuted driving time. Some flat driving (highway) and some pretty steep hills as well. Probably misfiring, but hard to tell and not throwing codes.

The difference I noticed this time around was that right before the hesitation at around 70mph on the highway, the TAC fluctuated from 1800 to 2200 or so and then.."bump"...there it was. Then, all was good the rest of the way home, all at 65mph or lower.

At lower speeds like 35-50mph, still not too many problems with the TAC fluctuating or any hesitation of any kind.

I'm REALLY stumped now....

What has been changed/installed so far since initial problem:

CPS (Once)
Ignition Coil (Once)
Plugs, Wires. Dist Cap (Twice, now has a MOPAR Kit SPC40604AA installed and Champion plugs, gapped at .035)
Lucas upper head cleaner and fuel system cleaner used
Combustion Chamber Cleaner (Sprayed into throttle body)

I'm REALLY wondering whether or not I should go to NAPA and get an Echlin CPS (I have seen it mentioned here as a VERY good CPS) and putting it on before going further.

ANY ideas would be great.....I just get the feeling that this MUST be something rather simple.......and thank you in advance.

Bryan

[bringselpup]

You'll find as many opinions as there are spark plugs. I run Autolite 985 and won't run Champion from a personal experience.

When that check engine light goes on get that code and post it here.

[hoofan]

I have only been able to get the codes from my odometer this past week. 12 (Battery), 43 (5 times) (Mult Cylinder Misfire) and 55 (End of code sequence.

I am going to take a peek at the CPS voltages tonight and I'll post them as well.

Bryan

[Wino]

My wife's 97' experienced similar problems on a road trip this summer. The shop we stopped at in Jackson TN. was stumped by the problem also. The mechanic went online to a tech site and through discussion with other mechanics learned that this is a common problem with jeeps with over 100k. As it turned out, it was the upper oxygen sensor. Might be the same on your 96?

[hoofan]

The O2 Sensor is not throwing a code and from everything I've read from those that really seem to know thier stuff, swear that the O2 sensor will throw a code. But you never know, everyone's story seems to be different with this kind of failure.

Tonight, I decided to do a few things.

First, I noticed that the wire from my TPS was looking a bit rough and the insulation was cracked in several spots, so I re-taped it just for kicks. I had not suspected this before, but it was laying on bare metal and there could have been some arcing. It can't hurt.

Anyway, after I did that and gave everything a once over, I decided that it should pull the battery cables and try to reset the CE light (I thought I had read that would work) and clean the posts and terminals while I was in there. Then I was going to probe around a little. However, what I found when I began cleaning them made me go hmmmmmmmm.

My negative terminal was cracked. I measured the resistance to ground and it was a bit difficult getting a short, but it mostly bounced all over the place (and mine Fluke is digital). I scraped and scraped and finally got it down to 0.37 ohms, but it took some work. So, I went hmmmmmm again.

What if my negative terminal, when connected to the battery and running at high speeds, was vibrating just enough from the crack and corrosion to increase the resistance to GND from the negative side of the battery? I wondered if this could possibly be throwing the whole electrical system, PCS and everything out of whack.

So, being that it was too late to go buy another one until tomorrow, I figured that it wouldn't hurt to clean this one REALLY good, along with the post and get it on there as tight as I could and give it a test run. Good to go...fired right up and no CE light as I had hoped, in case I threw another code tonight.

I drove about 10 miles, up and down steep hills, hard acceleration and cruising and it ran just fine. No codes, no hiccups, no "buck".

However, I HAVE been down this road before and I could not get it up to 70mph in my area of town without risking a major ticket, so I'll have to wait until tomorrow to give it a real test on the highway.

I'm not too optimistic yet, but has anyone ever seen this kind of problem with misfires because of a bad GND, battery cable or post??? Just curious, as I head out tomorrow morning.

Thanks for the help.

Bryan

[hoofan]

Drove my Jeep to work this AM and no problems so far, but the real test will be this afternoon on the highway. Acceleration seemed to be OK and not sluggish as I had seen with the misfiring. But again, I went no faster than 60mph this morning at about 1800rpm in OD.

I am still going to replace the negative battery cable (first and then the Pos later) tonight and go from there.

If anyone else has seen this type of problem step from "bad battery syndrome" or just bad electrical connections, please let me know.

I'll post an update tonight after the ride home.

Bryan

[hoofan]

Well....it's a little better, but I doubt it was because of the battery. It "bucked" on me at 70 slightly on the way home, but no CE light. It drove pretty well the rest of the way at 60mph or below. I'm leaning toward the fuel filter now, maybe I'm starving it at higher RPM's...

Bryan

[Woody3d]

I have recently been running into a very similar situation. My 94 Jeep GC would hesitate but almost always during acceleration. Originally my problem started about two weeks ago just after I filled up my fuel tank. I first noticed it when I started my car up and it hesitated for a second and then it was fine. I took a road trip that week and it started to hesitate more but it always did it a little worse when I just had filled up my tank. Finally while I was driving on the highway it completely died. I took it to the closest mechanic which ended up being a Chrysler dealership and they said I had no fuel pressure and replaced my fuel pump and just in case my throttle positioning sensor (actually I broke that, but it had to be cleaned several weeks earlier and was causing the car to die). When I left the dealership it drove fine for a couple of days even at high speeds. About three days later it began hesitating again. I am not getting any codes and I have read several posts about the possibility of the O2 sensor. I have heard somebody refer to it as possibly being "lazy", so I will check that. The hesitation and stalling making it feel like I am still not getting fuel or that possible the fuel air mixture gets thrown way off somehow. I have just replaced my MAP sensor, EGR valve, and pcv valve (was very dirty). My spark plugs and wires are less than a year old as well (don't recall the brand). I replaced my crankcase breather/filter as well. Nobody tells you but at least on the V8s that should be cleaned like every 30,000 miles or so. I bought mine used and I doubt it has ever been cleaned it now has 145k miles on it. My only hope other than the O2 sensor is possibly the fuel filter. I don't think that is the problem since it doesn't hesitate on a more regular basis and mainly at higher speeds now. But it still mainly seems to happen on acceleration no problems at all when it is just idling Does anyone know of any other sensors, switches, or filters which might be causing this type of problem? I don't think I have any vacuum leaks anywhere. I might try the battery solution but that sounds a little odd to me. I know however, that one of my engine mounts is bad so the engine may be shaking enough at the higher speeds to cause some type of problem. Besides I could use a new battery anyway. There is also the possibility that the computer is bad but I am not sure where it is located and I think that will be my last resort.

[hoofan]

After days of analysis and reading the board, I have on my list the following to check/replace:

Bad battery (I can see fluctuations in my dash meter when driving between 12-14 volts or so, but measures 12.5VDC with a Fluke Meter with the engine off).
Bad battery cable(s)
O2 Sensor(s)
Fuel Filter and/or screens
TPS
PCS
Wire Harness to the PCS
Cat Converter (Will probably get this done at a muffler shop next week regardless if it is the source)

At this point, I am going to drive it until Saturday to make sure my symptoms are consistent to my last work (refer to above for the Neg battery cable and wiring from TPS).

Right now, it drives great in the morning when I warm it up after a cold night and drive about 10-15 miles to work around town. 60mph or lower and up and down hills, start and stop, all kids of stuff. No rough idle. You'd never know it if you didn't go more than 60mph.

When I hit the highway this afternoon though and try to go above 65mph, I fully expect a TAC fluctuation and a "buck" and "maybe" a CE light, but it has been 3 days since I cleared the CE indicator and it continues to "buck" at high speed without throwing a code.

We'll see.....if anyone has any input, I'd appreciate it. I will post any successes or failures for everyone else as I plow my way through them....

Bryan

[hoofan]

UPDATE!!!

Well, I broke down and replaced my catalytic converter Friday night at a muffler shop (216.00 and it was going VERY soon anyway) and it appears that I "may" have fixed part of the problem, but not everything. I need to drive it a bit more first, but here it what I have so far today.

I cleared the codes this morning by removing the Neg battery terminal and then took it out for a test drive. Just fine around town, as usual and when I took it out on the bypass near my home I got it WELL above 70mph for 5-10 miles up and down inclines (using the cruise control as well) without one "buck" or "hesitation". I did notice some small fluctuations in the TAC like it had done before, BUT, no "bucking" or "hesitation".

Good news!!!

Before the new catalytic converter, I could count on a "failure" like clockwork before above 65 mph, with rapid variations in the TAC around 1800-2200 and it would be less than a mile at that speed before it happened.

However, I do still feel that it is running a bit "sluggish" though I'm not positive it is "misfiring" anymore. I had one small "hiccup" pulling from a parking lot later today and a "surge" in my neighborhood. Also, had some difficulty getting response from my gas pedal on the bypass when moving up a large incline. So, I'm tending to think that I had a fuel related problem all along in either the filter or low pressure as well.

So, I am going to drive it for several days to make sure that no more codes are thrown my way and then replace the fuel filter and check the pressure.

By the way, I've never checked fule pressure before, is the info in the Haynes manual good enough for a beginner? Can I get a pressure gauge at any auto parts store?

Thanks for the help. I'll post any new info when I have it for those having the same problems. This has been a real adventure....I'm feeling a little better today about this....finally....

Bryan

[wpatters1229]

Sounds almost like my problem. Here is mine. Full tank of gas. Started to lose power like the gas pedal turned to mush and then the car died. Started up again and went a half mile and then died again and could not keep running. Was bucking and acting like mis-fires. Had it towed home. In the morning it started great and I drove it about 2 miles and it started doing the same thing. I replaced the fuel filter since I don't think it was ever replaced. Filter seemed OK...but who knows. Sprayed some starter fluid in the throat of the manifold to help it start and it ran long enough to use up the starter fluid and then died. Had it towed home again. Rolled it into the driveway and hit the key and it started just fine again and then after about 2 minutes of idling it started surging and then died. I then replaced the plugs, wiring, cap and rotor just because and it didn't change a thing. I suspect the fuel pump but I do not know how to check it and if it is bad how to change it out. This is a 6 cyc engine. All the hose etc. look fine. Don't see anything pulled loose or tweaked. My friend said to maybe drain all the gas out and put in a fresh tank since it could have water. I put two bottles of water remover in the tank and that also didn't change anything. Any ideas or explainations of how to test fuel pump pressure etc.?

[wpatters1229]

Before I forget what are the TPS PCS? I have worked on many cars but mostly pre computerized. Another question. When a fuel injection engine runs out of fuel is there some trick to getting gas started into the injectors again or should the electric fuel pump be able to prime then system. Just thought it might be my problem but I would think after spraying starter fuild down the throat of the manifold and the engine running for about 10 seconds it would start pulling in gas. But maybe not.
Again is replacing the fuel pump with a full tank of gas something a home mechanic can do? I understand the fuel pump is inside the tank and there might also be a "sock" type screen filter that sometimes can go bad. Any suggestions would be great. I can take it to a mechanic but funds are limited.

[hoofan]

Yours sounds more complicated than mine because my Jeep is perfectly drivable and really has been since I replaced the CPS (Crankshaft Position Sensor). That was definately bad and you may want to, just for kicks, check this if you haven't changed it before you do anything with the fuel pump.

A bad CPS will cause the Jeep to shutdown randomly and somtimes run for a few miles or more until it quits altogether. But given the fact that you have a "mushy" pedal, that makes it a bit more foggy and don't forget that you "could" have more than one ting going on here.

I replaced my fuel pump about 3 years ago, but had my mechanic do it and it wasn't cheap. My symptom there was that I would start it and it wouldn't hold idle at all at first. Then, later, as it got worse, it would fail altogether and not allow me to even start the Jeep. So I can't really give you any direction in that department.

What I will say is that you should be able to do your fuel filter yourself and I am going to do mine as well, but my CPS is in a bad spot, so I didn't do that myself.

As far as the fuel pressure, I think it is relatively easy based on the Haynes manual and folks I've talked to that have done it before on other Jeeps. You need a gauge (make sure the fitting matches your Jeep) in order to measuere the fuel pressure (and there is a test port shown in the manual). I'm going to attempt it this week after I drive mine with the new cataytic converter to get an idea on any new or corrected symptoms.

Hope that helps....I'll pass along what I find out.....

Bryan

[wpatters1229]

mine is a 93 so it is a bit different. Mine won't even stay running in the driveway. It starts and then immediatly dies. I replace all new plugs, cap rotor and wires...no change. If I disconnect the fuel line from the rail that feeds the injectors and try to start I would expect some fuel to shoot out but nothing. I then removed the vacumn line to the value that regulates the fuel going into the rail and that was suppose to increase pressure...again nothing. I really believe the fuel pump is dead or the computer running it. Either case it seems something that is out of range of the fix it at home syndrome. That is why we have credit cards...

Thanks for the help

[hoofan]

Yes...I would say that is the fuel pump. When mine went out, it went out completely...dead....

Bryan