2004 Grand Cherokee lagging/bucking under acceleration
I have a 2004 GC. It has the 4.0L inline six with 154kmi. No engine codes thrown, no wrecks, has been reliable until this issue.
About three weeks ago, it started feeling like it missed a little when I got on the throttle on the interstate. It seemed to progress over a few days to a point where it bucks/jerks whenever I do anything other than baby it. It starts fine, idles fine. It does not seem to do it during the first minute or two of driving. Some days seem worse than others, but it is always there. I can't associate it with outdoor temp or humidity/rain. My commute is mostly interstate. I notice it when I head up the first hill, usually when RPMs get in the 1700 to 2000 range. If I press further on the accelerator, it resists violently jerking hard, almost like a fast stall/start.
I have read all the other threads. Some sound spot-on, but they were either years old and gone cold, or resolved with something that did not help here.
What I have tried so far, not necessarily in the smartest order, and mostly from tips on this forum:
1. Run a tank full of gas additive. No change.
2. Replace the air filter. No change.
3. Replace the Throttle Position Sensor (TPS). (Used a part from O'Reillys)
4. Replace the fuel filter/pressure regulator combo unit (located back near the tank). No change.
5. Replace the fuel pump. No change.
6. Tee-in a fuel pressure gauge at the rail inlet, tape it to the windshield and drive. It flickered a lot when the bucking started, but stayed up around 50.
7. Replace the plugs. Old ones had about 20K miles, looked just like the new ones. Changed them anyway. No change.
8. Caved-in and took it to a shop. They did a 45-minute aggressive cleaner through the intake to clean the injectors. They also put it on the computer and said that my TPS was bad. No change in the problem in relation to the injector clean process.
9. Replaced the TPS again. Measured the output of the new one , smooth voltage variation over the pedal range, proper levels. No change in the problem.
10. Tapped/rapped on the Cat converters to see if they rattled, they did not.
11. Disconnected the O2 sensors and drove. Problem seemed about the same.
12. Dismantled the steering column and disconnected the clock-spring (strange, but for someone a short in there was causing pretty similar problems) and drove. No change.
13. Checked the 12V and GND to the O2 sensors (seemed good, but the 12V was only there for a few seconds when the key was turned to the "key-on, not running" position). The O2 sensor heaters ohm'ed out at around 4.5 ohms.
14. Measured the output of the Crank Position Sensor while slowly rotating the crank. It toggled from 0V to 5V as one thread said that it should. I have NOT replaced it (yet).
Hey , welcome to the forum , I have something very simular to yours , and so far I have replaced the coil pack and the plugs , and going to replace the TPS when JEEP get it in for me (next couple of days) , I hope I dont have to do all what you have done , and still have the problem ......
I think the next thing to look at would be cylinder or PCM
Which TPS from O'Reilly's did you use? If it's the MasterPro brand, it's garbage and should be taken back for either a refund or for the BWD branded part (note, do not get the BWD 'P' series, as they are not the same quality as the 'Select' series). Many of the BWD Select engine management parts will have the Chrysler part number molded in to them as they supply parts for Mopar.
Next, I would suggest checking the coil pack, as I see that's the one part you haven't looked at yet related to the ignition system. Most people would associate a defective or damaged coil with a misfire CEL, however that is not always the case. I recently had a coil on my 4.7 that cracked and would arc against the spark plug tube, no CEL, but rough idle and stumbling upon acceleration. Here is what the coil looked like:
The grey/silver stuff on the o-ring is anti-seize, however just below that you can where the coil was arcing through the hairline crack that travels diagonally across the tip of the coil.
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Thanks for the quick feedback guys. The TPS I replaced twice was a BWD part each time, but I don't have the box anymore and don't know if it was a 'P' series or not.
So here's what I just did: I pierced the TPS signal wire with a pin, connected my voltmeter to it and to chassis. Did a quick check for smooth voltage variation as I rotated the throttle. When closed, it measured about 0.57V, then rose smoothly as I rotated. Then I taped the voltmeter to my windshield and went for a drive. The voltage didn't jump or jitter at all when the lagging/bucking happened. Generally, the voltage was around 1.4V at the throttle level I was at when the bucking happened.
So I think this TPS is working at the moment. Do you agree? What I can't say is that the signal I saw is making it back to the computer since I measured near the sensor.
Talk to me more about the coil pack. Definitely no misfire CEL, but I think I've avoided the coil pack because this is the first vehicle I've had with this kind of a bar. I lift it off the plugs to replace the plugs, but have never taken it out of the car altogether. I guess there is some kind of connector at the rear of it? Where do I find the cylinder that you show in your pic? Is this something I can inspect / test? Or do I have to just guess and replace it?
Sprocket1200 is right, this forum is awesome. Not only is there great help, but I find motivation to go back out there and try again instead of giving up. Thanks to all of you!
Throughout this process, I have done the key-toggle to get the diag codes to come up up the odometer... and also had the auto shop put a reader on the ODB port. In all cases, there have been only one code, other than temporary codes I cause by disconnecting sensors as part of a test. That one is the P1899 - park/neutral switch. This has come and gone for a couple of years, causing the CEL to come on briefly. It has always started without issue, and I had not considered it relevant to this recent issue.
JamesWJ2001 asks an interesting question about the auto-transmission. I have not previously had any shifting issues. But the lagging/bucking does resemble bad shifting... although it happens at times when no shifting should be happening, including under fairly easy throttle at interstate hwy speeds. If I go really light on throttle, it seems okay. A little more, and it starts what I'll call "lagging/surging", dropping rather softly in power and RPMs, holding at the reduced level for a couple of seconds, the coming back to normal power. If I press even further on the throttle, the lag/surge gets harsher - jerking/bucking fast.
By the way, I have not yet put a gauge on the manifold vacuum. Not sure if that would help.
I've had a problem very similar to this in a pontiac g6. The car did what you have described and ended up being a trans issue. The trans would not be able to downshift under load/throttle. It would buck, kick and jerk while trying to downshift. At highway speeds it struggled to stay in O/D and would constantly try to downshift, which would cause the jerking, kicking and bucking. Under regular conditions it shifted well. Upon further investigation I discovered the trans was not rated to handle the output of the engine and thus the trans would fail. I traded it off after that... If your trans is worn and/or In need of service, this could possibly be your problem.
Okay. Thanks for the recommendations. Today I purchased the two transmission fluid filters (in-pan and cooling-line) but have not installed yet. I want to run it first and change it warm. Figured this would be good to do anyway as I tend to neglect transmission filter and fluid changes.
I also thought I might shift it into neutral when it is on the highway and giving trouble and see if it will rev cleanly. Of course, the engine would be unloaded in this case, but I'll still do it and report back. On the slim chance that it still hiccups while in neutral, I suppose that would eliminate the transmission as a suspect??
I also pulled the coil rail. It is not separate packs, but the long thing (see picture). From what I can find so far online, there is no ohm-meter test for it. Seems like it is just an ohm or so between pins on the input side and mega-ohms / open circuit on the spark plug sides (to earth and to each other). It is $100 bucks to get a replacement. Any thoughts on how one might test it first?
My 4.7HO did the same things you've described, the buckling/jerking, especially on the highway whenever the engine encountered any strain. I originally thought it was the tranny so I put on my big girl pants and sulked into the shop ready for a huge bill. Turns out it was a clogged catalytic converter. I took it to a friend's muffler shop, he confirmed 100%. They put a new cat on it and a Flowmaster as well, I drove it home with a big ****-eating grin on my face because it ran better than ever. Try taking it to a muffler shop and have them run some pressure tests, let us know how it goes.