1996 Jeep GC multiple misfires, hesitation and bucking

[hoofan]

Unreal!!!! That explains a lot and I hope it ends up being the solution for me as well.

It would also explain why sometimes I get a FLASHING CE Light (especially on steep hills or on inclines with CRUISE enabled), but it always comes up 43 (mult cyl misfire) and never coded an exhaust or emission code.

I definately drive less than 3K RPM most of the time, but tried something interesting recently. I started driving with the Jeep NOT in OD and driving at higher RPM. I still eventually got misfires, but not until after I took the OD off for a while.

I've tried a few things with the wiring, but now this is starting to make sense a little bit....I'll pass along what I find...

Bryan

[protozee]

The jeep shake or shudder, misfire may occur when the vehicle is operated between 50 - 70 MPH


I have a 2000 Grand Cherokee Laredo with 84,000 miles. have notice that my Jeep "sputters" when I accelerate heavily (i.e. not when I gradually accelerate but really lay on the pedal to get into traffic).


I tried 2 jeep dealers and they could not fix this, one said it was the coil, replaced it no help, and tune up, injectors cleaned too no help. The other on said it was the torque converter and pump it was under warranty, no help. So I think they just want to sell you something or they don’t know any more then we do, or maybe they throw the service bulletins away. So I did some research on the internet and this is what I found the trouble. So far
I have paid a lot of money at the dealers.

Late closure of an exhaust valve may be the result of no valve rotation and associated build up of carbon on the exhaust valve stem.

So I bought a 16 oz. bottle of Marvel Mystery Oil, put half in the gas tank and the other half in the oil. No more jeep shake or shudder or misfires.

Mabe I drove a couple of round trips to work 120 mi or so, now My power is back saved $500. I guess we should add a little just befor every oil change, just to clean up carbon build up.

By the way the $500 at the dealer for decarburizing



4.0L MULTIPLE CYLINDER MISFIRE
Date: 08/01/03
Model Year(s): 1999-2004

Description: NOTE: THIS BULLETIN APPLIES TO VEHICLES EQUIPPED WITH A 4.0L ENGINE. This bulletin involves inspection of all engine exhaust valves and a decarbonizing procedure if necessary.

Details: The customer may experience an incident of engine misfire during certain vehicle operating conditions. The misfire may occur when the vehicle is operated between 50 - 70 MPH and under light loading conditions, e.g. slight uphill road grades. This condition may occur at all ambient conditions, but is more noticeable when ambient conditions are less than 0 C (32 F).

If the vehicle is equipped with On-Board Diagnostic (OBD), a MIL illumination may also have occurred due to Diagnostic Trouble Code (DTC) P0300 - Multiple Cylinder Misfire. Various single cylinder misfire DTC?s may also be present. If the frequency of misfire is high the Powertrain Control Module (PCM) may place the engine in ?Limp-In? mode.

The misfire condition may be caused by one or more engine exhaust valves that are slow to close. Late closure of an exhaust valve may be the result of no valve rotation and associated build up of carbon on the exhaust valve stem.

This condition may occur when the engine is not allowed to run at engine RPM?s that are greater than 3,200 RPM. At 3,200 RPM or higher the engine exhaust valves will rotate if not impeded by high carbon deposits. Low engine RPM?s and high carbon deposits are associated with short trip driving where the vehicle engine is not allowed to fully warm to normal engine operating temperatures. Cold ambient temperatures will increase engine warm-up time and add to the opportunity of carbon deposit build-up on the stem of the engine exhaust valve.

[Somemedic]

Alright...
So I replaced the crankcase position sensor yesterday at the fire station. While it was blowing and snowing up to 2" yesterday here in Chicagoland, yours truely put taxpaying dollars to work. It's a bee-yootifull thing to work in relative comfort . Once I unbolted the NEG off the battery I began. With the help of 2-10" extensions together and a swivel the bolt came out without complaint from the underside. Looked like the best way to do it. Putting the new part in from the top wasn't too hard. I did have to uncouple 1 vaccuum hose to get better position. Starting the bolt back through the CPS was a bit of a challenge with my ham hock hands. The new part seemed very similar to the OEM part with the sensor wires even being protected by the same type of corugated plastic as the old. Put the NEG terminal back onto the battery and it fired up.As I drove around in the parking lot there was no noticable dips in RPM's. I applied the brake while leaving the gc in drive as to simulate waiting at red light. The RPM's didn't budge, staying right around 500. Even when I moved the steering wheel from lock to lock with the power steering pump working the RPM's didn't fluctuate. I feel this may have been the cure BUT without driving it for several days I won't have any confirmation. Time will tell.
As a side note this may not have anything to do with my problem but... the radio hasn't worked it seems since about the time that this problem first manifested. Your radios may be different since mine is a '94 with cassette/clock built in. Sometimes it comes on for no reason and works for a little while. Up until recently it wouldn't even come on or display the time but now it occassionally comes on. It might be a coincidence or bad ground someplace. Did anyone else have any electrical gremlins affecting the radio?

[bringselpup]

Having disco'd the battery you cleared the PCM so it's going to rebuild it's table for fuel air ratio. I think it keeps the first 50 times the vehicle gets up to operating temp and averages that info for the best mix. Give it some varied operating scenarios and a little time before drawing a conclusion but I bet you fixed it.

[2BitJive]

I've been having the same problems and to date I have done the following (in order):

- Replaced plugs, wires, cap and rotor
- Replaced Throttle Position Sensor
- Replaced Ignition Coil
- Replaced Crankshaft Position Sensor

I'm still having the same problem. My Jeep is a '93 Limited w/ the 6cyl. Would the tip from protozee help me out? The years listed on the service bulletin he posted state 1999-2004. Also, I'm not getting any code at all. I'm stumped at this point.

The weird part is that sometimes it will not start for me at all. It will crank but I'm not getting any spark or fuel (this is why I replaced the CPS). But then out of nowhere it will start and I'll have both spark and fuel. Fuel pressure was tested in all ranges and all seems to right with that. Could it possibly be the PCM? I already replaced it once in Sept or Oct because of it preventing the fuel pump from turning on. Is it possible the new one (refurbished) could be screwing up too? I'm not getting any codes which is why I've not considered this, last time I got a code everytime I tried to start it. Also, as a side note. My windshield wiper delay stopped working a day before the other symptoms popped up. Could they be related?

[Somemedic]

My trip odometer reads a little over 100miles since I changed the CPS and 5 days have passed. The jeep hasn't inadvertently shut down once. I have to assume that the sensor was the problem. To note the radio has also mysteriously started working as well. That one I cant explain but the old lady sure digs it.

I suspect that if you havent changed the wiper relay yet AND you do change the CPS I wouldnt be surprised to hear that the wipers have begun functioning. I would love to know if it happens and more importantly why.

[crazy_mtb_2002]

I seem to have the same problem with my jeep as a few of you, it idles fine, but once i give it gas, it stubbles like crazy, kind of like when a chain saw gets stuck in a log and you rev it, and doesn't stop until i release the gas, and SLOWLY accelerate, then it only stumbles a bit. i have tried injector cleaner, sea foam, etc, and gasline de-icer, but nothing, seems to me like i have an issue. Just wondering what some of your guys's results are, and what you did. Mine doesn't die or anything.

[Somemedic]

After a few weeks the jeep hasn't died on her yet. CPS was the culprit. Radio however does seem to work as long as its dry outside. If the jeep gets wet it won't work util things dry off.

MTB, try with cheap stuff first, filters mostly. Replace them all. Then begin by doing a tune up. If that fails run another search on google for "jeep 4.0 stumbles" or won't run. Play a few word games and then read some threads. This little thread saved me some buku cash.

[barryec]

Jeep 4.0, 1996 – problem similar to other postings, with a twist: had crank angle sensor replaced to cure very occasional stalling, which it did cure; but new problem developed immediately thereafter. (don't know the mfg or if mechanic disconnected negative terminal.)

Original problem – would stall at traffic light once every couple of weeks, could not restart for about half an hour. New CPS completely eliminated that problem.

New problem – after sitting overnight, engine misfires until it's been driven for a spell. Then it runs like a top under all conditions. The misfiring (bucking) is sometimes bad and sometimes very bad, but it's always there. When warming up at idle it's usually minor, but codes are thrown for all cylinders. While driving, the frequency varies but is usually several times per minute. Some hiccups are slightly longer than others, in a random way. Problem exaggerated under acceleration.

Clearing the problem may be done in various ways. Simply letting it idle to operating temperature wont' get it. Hopping on freeway will clear it up after 8 or 10 miles. Driving for a bit and letting it sit for 10 minutes before driving again works. Just turning off and on won't work.

Jeep needs to sit for 5 or 6 hours to bring problem back, otherwise it's ok all day and night.

Things replaced:
-crank angle sensor
-plugs, wires, cap & rotor
-distributor (rebuilt) – shaft had lots of play
-coil
-catalytic converter
-computer
-tried advancing and retarding distributor one tooth
-monitored fuel pressure during malfunction

thinking about throttle position sensor and oxygen sensor, but the postings seem to favor a second CPS.

Has anyone else had experience with major misfires going away after a good warm-up?

PS, Why is the solenoid between the air filter and the fender pulsing during idle @ around 200 cycles per minute? (both in bucking mode and normal mode)

Thanks much for any help which is forthcoming!

[Somemedic]

Would like to hear how you fix this. Keep us posted.

[hoofan]

That's almost exactly the same situation I have to the letter.

I have done everything that you have done as well except:

-computer
-tried advancing and retarding distributor one tooth

I have really come to the conclusion that the new CPS must have been installed wrong or it is a flaky part.

Was your CPS (barryec) MOPAR and done at the dealership or did you have it done elsewhere with an aftermarket? Mine was done in a reliable shop, but I have no idea to date what brand they used or whether they followed the right procedures to install it.

I have also tried the TPS (WELLS brand though) and returned to the original with no change or improvement. The O2 sensor is not throwing any codes to me at all, so I don;t see any need in trying either of those unless somebody thinks I should. They cost about the same as a new CPS. The only codes that are ever read out are misfires on each cylinder at very random patterns. Just like yours, it runs great at points, terrible at others.

The common denominator for me has been that my Jeep ran like a top until I had to replace the old CPS (just like you) because it was bad and shutting down the Jeep while driving. So, I know the original was bad.

I am probably going to have a new MOPAR part put on while I'm on vacation. I don't kow what else to try. If I get to it, I will report back in July the results when I get back.

Thanks for the post, it really confirms some of my thoughts on this problem (now going on 7 months with no solution).

Bryan

[barryec]

Bryan,

Possibly some good news: I solved my problem with the misfires. (Replacing with a MOPAR CPS might have worked also, I'm not sure.) My replacement which gave the well- known problems was an AC DELCO. My solution was to file a bit of a neck in the (single) mounting bolt, which allowed me to install the sensor about .010" deeper into the bell housing. (Secured the bolt in the chuck of a drill motor.)

I have the Haynes manual, which was not much help. I believe your stick shift has a two-bolt mounting, whereas my automatic uses a single bolt. Haynes claims the OEM bolts determine the depth of the 2-bolt system, while the single bolt does not. They're wrong. They say the 1-bolt sensor body has an elongated hole, and the face of the sensor is furnished with a paper spacer to be butted against the flywheel before tightening the bolt. Actually, there's no paper spacer and the hole in the sensor body has a precision metal insert which allows about .011" clearance around the bolt (.0055" on a side) - not much room for adjustment. Obviously the need for adjustment is not anticipated, hence the bad record for after-market sensors.

I went through a lot of grief coming to this conclusion. My misfiring problem was always temperature sensitive, which is something I didn't see in any other postings. Once the car was thoroughly warmed up the problem went away, and returned only after a cool-down of several hours. The idea that the flywheel would expand with heat and change the sensor gap seemed so obvious that I assumed they had engineered their way around it. The paper spacer thing showed how critical the spacing is.

So I assumed the replacement sensor was defective. On your advice I picked up a NAPA Echlin. I put it in when the car was cold (my worst condition) and was really ecstatic with the results - the answer to many prayers! (I also found out why the mechanic had charged me $220 for the first job!)

It started right up and idled beautifully. I went for a spin and accelerated and drove 75, all without a single hitch. About 15 min. into the trip I started getting occasional misfires. By the 20 min. mark I was wondering how in hell I would get home. It got so bad I had to let it cool down, and then limp home in low range. This was ten times worse than the AC DELCO had ever been.

The routine with the NAPA turned out to be 100% repeatable, just like the DELCO was, except opposite, so to speak. When the car was cold it started great and ran perfectly under all conditions - until it was thoroughly warmed up. The DELCO, on the other hand, misfired UNTIL it was thoroughly warmed up, then ran perfectly.

I began to experiment with the small amount of adjustment which the mounting bolt allowed. With the DELCO the misfiring on cold startups was noticeably worse at the shallow limit than at the deep limit. At the risk of rubbing the sensor on the flywheel, I had to try going deeper. It worked! I'm assuming the NAPA unit would work better if it were LESS deep, but I haven't tried it yet. It seems like we have to compromise between the optimum cold setting and the optimum hot setting. Bad engineering?

By mistake the counterperson at NAPA brought out the CPS for a different Jeep model. It had a gear on it, like a distributor, and a little plastic window. Obviously such a device wouldn't be effected by heat expansion. So what made me think the change in performance was caused by expansion of the flywheel rather than warming up of the sensor itself?
When I first switched back to the DELCO, the car was hot but the sensor hadn't been used for three days. The car started immediately and ran perfectly under all conditions. Could the bell housing have pre-heated the sensor while I was fishing the cable up through to the engine compartment? Maybe. The clincher was the change in cold startup performance brought about by moving the DELCO sensor that small amount (.011") allowed by the clearance in its mounting hole. Also, it's counterintuitive that between two supposedly identical units, one should consistently malfunction only when hot, the other only when cold.

Why did the NAPA and the DELCO behave so differently: too deep and too shallow? (Hopefully the MOPAR would be "just right.") I measured the perpendicular distance from the face to the mounting hole and both were identical within maybe .002". So if we used the paper spacer method of setting the face to the flywheel, the problem would still be there. (And would you do it when the flywheel was hot or cold?) Even though the sensor housings are the same, the electrical components inside may be positioned differently, or have different "potencies," requiring compensation in depth setting.

[Ron68]

Originally Posted by barryec

Bryan,

Possibly some good news: I solved my problem with the misfires. (Replacing with a MOPAR CPS might have worked also, I'm not sure.) My replacement which gave the well- known problems was an AC DELCO. My solution was to file a bit of a neck in the (single) mounting bolt, which allowed me to install the sensor about .010" deeper into the bell housing. (Secured the bolt in the chuck of a drill motor.)

I have the Haynes manual, which was not much help. I believe your stick shift has a two-bolt mounting, whereas my automatic uses a single bolt. Haynes claims the OEM bolts determine the depth of the 2-bolt system, while the single bolt does not. They're wrong. They say the 1-bolt sensor body has an elongated hole, and the face of the sensor is furnished with a paper spacer to be butted against the flywheel before tightening the bolt. Actually, there's no paper spacer and the hole in the sensor body has a precision metal insert which allows about .011" clearance around the bolt (.0055" on a side) - not much room for adjustment. Obviously the need for adjustment is not anticipated, hence the bad record for after-market sensors.

I went through a lot of grief coming to this conclusion. My misfiring problem was always temperature sensitive, which is something I didn't see in any other postings. Once the car was thoroughly warmed up the problem went away, and returned only after a cool-down of several hours. The idea that the flywheel would expand with heat and change the sensor gap seemed so obvious that I assumed they had engineered their way around it. The paper spacer thing showed how critical the spacing is.

So I assumed the replacement sensor was defective. On your advice I picked up a NAPA Echlin. I put it in when the car was cold (my worst condition) and was really ecstatic with the results - the answer to many prayers! (I also found out why the mechanic had charged me $220 for the first job!)

It started right up and idled beautifully. I went for a spin and accelerated and drove 75, all without a single hitch. About 15 min. into the trip I started getting occasional misfires. By the 20 min. mark I was wondering how in hell I would get home. It got so bad I had to let it cool down, and then limp home in low range. This was ten times worse than the AC DELCO had ever been.

The routine with the NAPA turned out to be 100% repeatable, just like the DELCO was, except opposite, so to speak. When the car was cold it started great and ran perfectly under all conditions - until it was thoroughly warmed up. The DELCO, on the other hand, misfired UNTIL it was thoroughly warmed up, then ran perfectly.

I began to experiment with the small amount of adjustment which the mounting bolt allowed. With the DELCO the misfiring on cold startups was noticeably worse at the shallow limit than at the deep limit. At the risk of rubbing the sensor on the flywheel, I had to try going deeper. It worked! I'm assuming the NAPA unit would work better if it were LESS deep, but I haven't tried it yet. It seems like we have to compromise between the optimum cold setting and the optimum hot setting. Bad engineering?

By mistake the counterperson at NAPA brought out the CPS for a different Jeep model. It had a gear on it, like a distributor, and a little plastic window. Obviously such a device wouldn't be effected by heat expansion. So what made me think the change in performance was caused by expansion of the flywheel rather than warming up of the sensor itself?
When I first switched back to the DELCO, the car was hot but the sensor hadn't been used for three days. The car started immediately and ran perfectly under all conditions. Could the bell housing have pre-heated the sensor while I was fishing the cable up through to the engine compartment? Maybe. The clincher was the change in cold startup performance brought about by moving the DELCO sensor that small amount (.011") allowed by the clearance in its mounting hole. Also, it's counterintuitive that between two supposedly identical units, one should consistently malfunction only when hot, the other only when cold.

Why did the NAPA and the DELCO behave so differently: too deep and too shallow? (Hopefully the MOPAR would be "just right.") I measured the perpendicular distance from the face to the mounting hole and both were identical within maybe .002". So if we used the paper spacer method of setting the face to the flywheel, the problem would still be there. (And would you do it when the flywheel was hot or cold?) Even though the sensor housings are the same, the electrical components inside may be positioned differently, or have different "potencies," requiring compensation in depth setting.

Nice job on the troubleshooting and a clear and concise description of your deductions and reasoning. Glad you solved your problem and thanks for sharing the information with us.
Would you like to take on my daughter's Ford Contour......? ( I believe the next thing is to park it on a lonely dark side street with the keys in it with a can of gas and matches next to it......) Definitely the biggest POS that Ford ever produced.

[hoofan]

That's a great summary of what you did and it explains a lot actually. I have always been skeptical about the original CPS that was installed as a replacement and what brand it was or whether it was even done properly to begin with. Your info echos that possibility as well.

Given the fact that I have no time to do it myself and it poses a significant challenge because of the location, I am having the dealer (and a Jeep mechanic I know and trust) install a new MOPAR CPS for me next Tuesday morning. I am VERY confident that this will correct my problem, however, if not, at least I will know that the part is MOPAR and it should be good. Right now, I have no confidence in the part that is in there.

Like I have said in many posts before, my Jeep was running like a top before the last CPS shut me down and went bad. Just seems like too much of a coincidence to be anything else and the only code that is ever thrown is 43 on all cylinders.

I'll let you know what happens...good or bad results....

Bryan

[redpreludeowner]

So did you ever figure out what was causing the bucking? Mine is doing the exact same thing and we have replaced everything we can think of. Would appreciate info.