1996 Jeep GC multiple misfires, hesitation and bucking

[hoofan]

Well

Back to the drawing board. On the way home, I got a few "misfires" at or above 70 mph for sure but no codes have been thrown yet. I expect the CE light to come on tomorrow and also expect a 12, 43, 43 ,43, 55 sequence at the least.

I could see the TAC shifting around the 2000 rpm range just like before, so I was thinking it was imminent. Must be something about the drive home just after work, it seems to do it every time.

I'm back to square one. It's drivable around town with little problem, but I'm staying off of the highway until I can measure the fuel pressure and change the filter.

Does anyone know whether I should change the TPS or something else based on just the misfires and the fluctuating TAC (It's minor shifting, but not steady at all above 65mph)?

PLEASE help if you can...thanks....

Bryan

[dallas121469]

I dont have much experience with the 4.0 engine in the jeeps but I just bought one and have had similar problems with other fuel injected engines. I noticed today when I tried to start the Cherokee that I bought it wanted to start but just wouldnt. I looked down into the intake manifold through the throttle body and noticed it was full of gas. I have experienced this with the Blazer 4.3 engines. The fuel lines run through the intake plenum and get weakened over time. This leads to fuel leakage into the plenum and essentially starves the engine of fuel. Due to the high pressure nature of fuel injection the fuel tends to take the path of least resistance i.e. the leak. This leads to chugging, sluggish throttle response, stalling etc. depending on how bad the leak is. I am unsure of the layout of the 4.0 intake plenum but depending on where the leak is an excess of gas can enter certain cylinders causing fouled plugs thereby making the problem worse.

Like I said. I dont have much experience with the 4.0's but that is the first thing I am going to check on my new Cherokee. Just thought I'd throw another possibility out there. Good luck all. Steve

[hoofan]

I am still in analysis mode since I replaced the catalytic converter. One thing of note, not a single "CE Light" or code of any kind in over a week since I replaced it. That's significant to me with regard to the 43 misfire codes.

However, I can still force the hesitation or "missing" above 60 mph (especially with the cruise enabled as it tries to maintain that speed). But, there have not been any codes thrown my way, as I've said. Certainly wants to point me to fuel, but I can't get past a cheap CPS either when I REALLY look at it objectively. It was the first CONFIRMED bad part and the Jeep has not been the same since.

I didn't have any other symptoms like this before the CPS completely failed on me before.....so, go figure.....what a mess...

The good news is that it is drivable and seems to run like a top a lower speeds around town. We'll see....

Bryan

[wpatters1229]

That is what it was to the tune of $899 for parts and labor. Way over priced for a pump...but as they say...they gotcha. Car runs fine now. Onto another problem....master cyclinder for the brakes seem to be leaking. UGH...if I wasn't looking for a job I'd send it back to the mechanic but even at that the part is $200 at the local parts house.

[hoofan]

I finally got another engine misfire today as I tried to accelerate around a vehicle on the highway. It really didn't "jump" to accelerate when I pressed on the gas and then it "bucked" twice as I went into the passing lane. I guess this is pointing to a fuel filter/pressure issue, but it is interesting to me still that I didn't have any of these problems before I had the CPS replaced.

Bryan

[2BitJive]

I'm having the same problem. Have you replaced your O2 sensor and / or your TPS? I've been doing some research and these seem to be commom cures for this. I'm ordering them today to see if it makes any difference. Also, my fuel pressure is good.

[hoofan]

I just had the O2 sensor cleaned when I put on a new Cat Converter, so I feel I may be OK there. I'm going to change the TPS tomorrow and I will post my results.

Bryan

[Somemedic]

I have a '94 GC LTD w/ the 4.0. It appears to have the same problem that has progressed for some time. Occasionally the jeep shuts off, without warning, and can easily be started while rolling in neutral. Or so that was the case up until recently. Lately the jeep does the bucking and sometimes won't start after dying so I think the problem may be getting worse. These symptoms may not manifest for days but then it may happen several times in one trip. This is after replacing the coil, wires, cap, and rotor. I may look into replacing the crank position sensor next... it sounds like a darling to do. I have noticed though while perusing the other boards on this same topic that simple CLEANING of the CPS pigtail connection worked for one jeep owner.
Also you may look into, and I apologize if you'd already mentionedthis as I had forgotten, the motor and sensor on the throttlebody. Those were the other two problem children mentioned.
For clarification the CPS is the crank(case) position sensor. Not to be confused with the camshaft position sensor. Sound like the same thing to me though. I had an autozone guy ask me today about that.

[wpatters1229]

Have you tested the fuel pump and the pressure? Have you replaced the fuel filter? I had the same problems of it working sometimes and then it just finally gave it up totally.

[Somemedic]

I'm going to replace the CPS tomorrow at my fire station. I picked up the one from NAPA made by Echlin. It was roughly $20 cheaper and as I recall the better sensor when compared to the Autozone/Advance/Pep... Fuel PSI was tested around 49PSI. I did notice one hell of a bad ground. Its the wire that comes from the battery ground and goes to the block. The bolt in the block and the wires running to it were rusty and corroded. It was rough so it was replaced resulting in brighter lights all around. Still the same problem with the jeep shutting off. One other quirk has been noted besides the bad ground was replaced: When removing the gas cap to fill up there is a large amount of air rushing in. It pulls one mean vaccuum I had wondered if this was the initial problem; i.e. Vapor lock. I have left the gas cap off and very loose to no avail. Jeep still dies without warning. Once I replace this sensor though I really think this will cure the problem

[hoofan]

I took a look at my CPS cable and connector yesterday and it was a little close to the bare metal on my engine for my comfort. It made me wonder whether I could be getting some arcing from the GND side of the connector and wonder if that could be an intermittent cause.

So I re-ran the connector around the outside of some other wire-wrap and it's a safe distance now. I probably still should take a shot at cleaning it a little, just for kicks.

All in all, I doubt this is the actual "fix" but it's worth marking off the list.

Due to the bad weather here today (Snow) I haven't been able to take it on the highway. Test drove it around the neighborhood yesterday, up and down steep hill, gunning the gas and there was no immediate indication of "misfire". and it's harder to tell from the TAC if anyting "funny" is going on. Around town driving doesn't usually throw the code though, as I've said. I need to get up to 60mph or so, watch the TAC around 2K for "bouncing" and then wait....

Will test it tomorrow if possible.

Let me know about the Echlin CPS, I've heard they are really good and may go that route myself.

Bryan

[hoofan]

Oh by the way, I measured my fuel pressure on the rail at 50 psi as well and was told that was good to go....

I just hope that it is not dropping at highway speeds, I have no idea how I will test that...

Bryan

[Ron68]

The symptoms that have been stated as far as the "runs ok until I ask for more power" - ie - idles ok and runs ok at lower speeds until the pedal is depressed more and then the engine stumbles or dies. The engine won't start again until it sits for a few minutes, then starts ok and runs ok for a short while and then dies again sounds like what I experienced with my 97.

The fuel pump in the 96 and later models sits in a can in the fuel tank. The can, once the tank is full, keeps the pump submerged in fuel, possible for cooling, possibly so that the pump doesn't lose prime. There are two plastic strainers on the inlet to the pump. One is located on the bottom of the outside of the can, and one is located at the bottom of the can on the inside. The pump is mounted in the can vertically with the pickup at the bottom. The outlet is at the top of the pump and is connected by a short aluminum pipe to the pressure regulator and filter assy at the top of the can. Also mounted on the side of the can is the fuel quantity float arm and sender unit.

What I found on my 97 was that both of the strainers were blocked with fine rust particles and debris. The inside of the bottom of the can had both water and rust retained there, which was blocking the inside strainer. When the pump picked up enough material/water to block the fuel flow, the engine would die. Once the debris had settled down and fell away from the strainers, the engine would start and run for a while until the pump could not keep up with the demand. The outside bottom strainer was just plain blocked all the time. I replaced both stainers and the pump precautionary and cleaned and wiped out the tank and I have not had any more problems. The new pump and strainers cost about $105 at Pep Boys. If you replace (or have replaced) the pump, make sure the tank is cleaned.

When I kept getting those symptoms, I suspected the pump, but to check it, I got a fuel pressure gage and hooked it up to the fuel rail. When I turned on the key, the pressure charged up to 47 psi, and remained about that at idle. I hooked up a long section of hose and ran it down and back underneath and out next to the driver's door. I taped it to the mirror and took a drive. When it started to cut out the pressure dropped to 10 psi and fluctuated back and forth until I let up on the pedal, so I knew I had a supply problem. I dropped the tank and found what I stated earlier.

[dksob81]

Without reading everyones post (just reading the orignial post), I would replace the CPS again.

If the mechanic did not disconnect the battery when he replaced the original one then that would be the problem, replace the CPS with a good known brand like echlin brand (napa) or MOPAR brand (dealership). and remeber to disconnect the battery, it is very important to disconnect the battery before replacing any electrical components.

[hoofan]

That's what I'm leaning toward doing tomorrow morning. I just feel that the CPS must be the problem now.

I will pass along this, because I've read just abot everything that you can read about this topic and more, based on my and other's symptoms.

Of the folks that had to replace the CPS because it was the known failed part (mainly internittent complee shutdown, which was my own failure point) or the CODE was thrown to indicate failure AND having had the symptoms I have had "post-repair', it has always been a poor brand or poorly installed CPS that was the problem.

Again, I had no problems pre-CPS failure.

My Jeep also initially ran fine after the CPS was changed until I hit the highway and it felt like the engine would buck out of the Jeep for a split second, now and again.

My guess is that is the "Hall Effect" working there. It wants to completely shut down for a split second, but the part is not "broken" just not communicatiig to the computer properly. So it only happens "briefly".

No matter what I've changed or "troubleshot" in the meantime just giving the Jeep a good once over, it keeps pointing back to the original repair....CPS all the way...

I'll pass along what I find this week.....

Bryan