Engine violent bucking and loss of power?

Tiny
TOMTOM21013
  • MEMBER
  • 1999 JEEP CHEROKEE
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 85,000 MILES
When going up steep hill @ 45mph and 3000rpm. The jeep will buck and seem to lose power. Not sure if it is trans. Or engine.
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have the same problem?
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No
Sunday, August 17th, 2008 AT 7:30 PM

26 Replies

Tiny
SERVICE WRITER
  • EXPERT
Check the fuel pressure when the problem is happening by running a gauge out to the windshield so it can be monitored. I am suspecting a weak fuel pump.

These guides will help us

https://www.2carpros.com/articles/engine-has-low-power-output

and

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Please run down these guides and report back.
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Friday, January 18th, 2019 AT 6:52 PM
Tiny
TOMTOM21013
  • MEMBER
Fuel pressure is 47-48psi and steady when problem happens
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Friday, January 18th, 2019 AT 6:52 PM
Tiny
SERVICE WRITER
  • EXPERT
Okay fuel pump is fine based on The specs (44-54psi) for this one: 1999 Grand Cherokee Larado 4.0.

IS this what you have?

Does this happen when the engiine is cold or not?
Check for codes in the pcm, if possible check the O2 Sensor wave pattern.

Have any carbon removal procedures been done?

Check coil pack resistance.
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Friday, January 18th, 2019 AT 6:52 PM
Tiny
TOMTOM21013
  • MEMBER
I have been driving the Jeep a little this week. It is a 99 Cherokee Sport with 4.0L /auto trans. I find it will miss or slip at any rpm but must be under a load. At first it seemed to only act up in overdrive but have found it to show the same problem in drive(3rd). Pulled two plugs (2, 5) both looked clean and in good shape. Plugs, wires and cap were replaced 3.5 years ago (60,000), when the crankshaft sensor went up. Cap showed some pittng. Does not seem to happen with the engine cold but will after 15min. Have not done anything else to the motor (cyl. Or inj). Jeep has 90,000 miles now. What should the coil res. Be cold and hot?
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Friday, January 18th, 2019 AT 6:52 PM
Tiny
SERVICE WRITER
  • EXPERT
I guess it's a good thing it isn't the grand Cherokee.


https://www.2carpros.com/forum/automotive_pictures/30961_jee_2.jpg



I don't have a hot or a cold spec other than this.

I found this on mitchell, and wanted to throw ot out there.:
Cust comlaint:
Rough idle in high ambient after hot soak. Clears in 30-60 SEC
Cause:
Exhaust heat causing fuel in #3 and #4 injector to vaporize.
Repair:
Install heat sleeve, PN56028371AA, on injectors

Long shot, but.....
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Friday, January 18th, 2019 AT 6:52 PM
Tiny
TISHA GREEN
  • MEMBER
  • 1999 JEEP CHEROKEE
  • 201,200 MILES
It will not do it everyday maybe every 3day but it will run bout 15minutes and cut off. And it is not the battery. When I put the truck in park again and restart it will start right back up.
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Saturday, August 1st, 2020 AT 12:51 PM (Merged)
Tiny
JACOBANDNICKOLAS
  • EXPERT
If the check engine light has come on, have the computer scanned to identify any trouble codes. Also, check to make sure the fuel pressure is within spec. See directions on our home page.
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Saturday, August 1st, 2020 AT 12:51 PM (Merged)
Tiny
DOBYMAN
  • MEMBER
  • 1999 JEEP CHEROKEE
  • 4.0L
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 66,000 MILES
It seems like its starving and has no power when its cold, but after it gets warm, it runs good. I changed the fuel pump, filter and MAP sensor. Nothing changed.
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Saturday, August 1st, 2020 AT 12:51 PM (Merged)
Tiny
CARADIODOC
  • EXPERT
Good guesses, but the first step is to read and record the diagnostic faults. Chrysler makes that much easier to do yourself than any other manufacturer. Cycle the ignition switch from "off" to "run" three times within five seconds without cranking the engine, leave it in "run", then watch the code numbers show up in the odometer display. List those codes here, then we'll figure out how to proceed.
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Saturday, August 1st, 2020 AT 12:51 PM (Merged)
Tiny
DOBYMAN
  • MEMBER
Will try that later today and report back. Thanks!
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Saturday, August 1st, 2020 AT 12:51 PM (Merged)
Tiny
DOBYMAN
  • MEMBER
I tried the code thing like 5 times. It didn't do anything. No codes, no speedo or tach movement, nothing.
Today I changed the coolant temp sensor and I think that solved it. I will know more tomorrow when the wife drives it cold.
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Saturday, August 1st, 2020 AT 12:51 PM (Merged)
Tiny
DOBYMAN
  • MEMBER
Not sure if it matters, but my clock spring is broke. I have an airbag light on all the time. I have the part, but haven't changed it yet. Can that have anything to do with the codes?
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Saturday, August 1st, 2020 AT 12:51 PM (Merged)
Tiny
CARADIODOC
  • EXPERT
The speedometer and tach aren't involved in reading codes. The numbers should show up in the odometer display. If they aren't, the people at most auto parts stores will read engine codes for you for free.

If the coolant temperature sensor is open, which is very rare, there would be a fault code set related to "Coolant temperature voltage too high". It would be much more common to find a broken wire or corroded connector terminal. Once a code is set, the Engine Computer would not use its readings in the fuel metering calculations. It would "inject" an approximate value to run on based on other things like intake air temperature and the length of time the engine has been running. It's unlikely you would notice a change in performance but the Check Engine light would be on.

The main thing to remember about replacing the clock spring is it can only rotate about a half revolution more either way than the steering system can, and since new ones come perfectly centered, you must have the wheels straight ahead when you install the new part. If the steering system is off to either side when the clock spring is installed, you'll be able to turn the clock spring beyond its limits one way. The internal ribbon cable will either become tight and snap off or it will unwind too far and the end will fold over on itself and break after doing that repeatedly.
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Saturday, August 1st, 2020 AT 12:51 PM (Merged)
Tiny
DOBYMAN
  • MEMBER
OK ill be careful on the clock spring. I will take it to auto zone tomorrow and see if they can get any codes.
The reason I mentioned the speedo and tach is someone told me when you do the ON/OFF thing 3 times, the tach and speedo will go all the way clockwise and odometer will show codes.
Ill report back if I get some codes tomorrow.
Thanks
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Saturday, August 1st, 2020 AT 12:51 PM (Merged)
Tiny
CARADIODOC
  • EXPERT
Dandy. I apologize if you're waiting a long time for my replies. I had a house fire over a year ago so I have to drive into town to use the wireless internet at various places. Since I'm busy tearing the house down, I have only been making the trip every other day. I do that because I can't bare the thought of you not having the benefit of my wondrous wisdom!
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Saturday, August 1st, 2020 AT 12:51 PM (Merged)
Tiny
DOBYMAN
  • MEMBER
I appreciate the replies. I am determined to fix this. Only wish I didn't spend $400 on unnecessary stuff. Not a problem with the timing. Its drivable in between attempts, so its not like its tying me down.
Thanks again!
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Saturday, August 1st, 2020 AT 12:51 PM (Merged)
Tiny
2CARPROS MIKE
  • EXPERT
Dobyman

7/715
I had some time today so I changed the cap, rotor, wires and plugs, and believe it or not, its running pretty damn good. Tomorrow it might be back to horrible, but for right now, I am a happy camper.
I ran it over to AutoZone and they hooked up a scanner to it, and there were no codes in the history at all. That's weird too.

7/8/15
I spoke too soon. Got up today, started it, drove a block and it was fine. Turned it off for 30 minutes, started it up and its spitting and jerking like before. I cant figure this out.
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Saturday, August 1st, 2020 AT 12:51 PM (Merged)
Tiny
CARADIODOC
  • EXPERT
Rats. My suspicion is this is indeed fuel-related since the supply system is not monitored by the Engine Computer. You said it runs better when it's warmed up. That reminds me of a recall modification on the K-cars years ago where fuel vapors could condense in the hose going to the MAP sensor and block it. Your sensor should be plugged right into the intake manifold so it won't develop that problem, but that is the one with the biggest influence on how much fuel goes into the engine.

It's odd no fault codes set so far. Any chance the battery was disconnected? That would erase any codes.

The next thing is to connect a scanner so you can watch the sensor readings while you're driving and the problem is occurring. I can watch the oxygen sensor readings to see if they're switching between "rich" and "lean" about twice per second, but for the other sensors, I know what "normal" is, but I'm not experienced in watching them while driving. Engine performance specialists will recognize right away when a reading is wrong, and they will introduce artificial lean and rich conditions to see how the sensors and computer responds. You also want to look at the "long-term" and "short-term fuel trim" numbers. Ideal is all zeros meaning the computer is not adding or subtracting fuel from the factory pre-programmed values, which themselves are real close but only approximate for most conditions. You can expect to see plus or minus 2 or 3 percent, but you don't want to see a high number like 8 to 15 percent. Those mean the computer is really modifying the amount of fuel it needs by a lot, but it doesn't know why. That's what we have to figure out. A high positive number means the mixture is way too lean and the computer is commanding more fuel in an attempt to fix that. Low fuel pressure would do that.

For those cam and crank sensors, the better scanners have graphing capabilities that let you watch their signals. As soon as you see what normal looks like, you'll recognize right away when one is breaking up or pulses are missing intermittently. You have an ignition module too, but I'm familiar with that. Anything electronic is prone to failure just because of the environment it lives in. Heat, dust, and vibration are real hard on that stuff and their connectors.

Most of the other experts here will also suggest connecting a fuel pressure gauge so you can watch it while the problem is occurring. Pressure on Chrysler products doesn't have to be perfectly at specs like on some other brands. On my Grand Caravan, for example, when the pickup screen in the gas tank was plugged, the engine still ran fine with no symptoms down to 20 psi. At 15 psi it started to sputter. Normal is 45 to 50 psi, so that's quite a drop.

The other things to consider are ignition-related, but almost all the time those act up when the engine is hot. The crankshaft position sensor and the camshaft position sensor in the distributor often fail by becoming heat-sensitive, then they work again after they cool down for an hour. There was a common problem in the early '90s with worn bushings in the distributors on Dakotas. Those caused erratic ignition timing which caused running problems.
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Saturday, August 1st, 2020 AT 12:51 PM (Merged)
Tiny
JOAN OBRIEN
  • MEMBER
  • 1999 JEEP CHEROKEE
  • 4.7L
  • 4WD
  • AUTOMATIC
  • 131,000 MILES
Driving normally then the rpm start going down and then it will stop and shut off.
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Saturday, August 1st, 2020 AT 12:52 PM (Merged)
Tiny
KASEKENNY
  • EXPERT
Hi Joan,

You said in the vehicle description that you have a Jeep Cherokee with a 4.7L. I am going to assume you meant you have a Grand Cherokee because the Cherokee did not have a 4.7L. This is normally not a big deal but in this case it could be the difference in what the issue is, so I just want to make sure that is correct.

The cam and crank sensor in the 4.7L are what are called Hall Effect sensors. These sensors are susceptible to heat and will actually start to fail when they get hot.

If you haven't changed your spark plugs since you past 100,000 miles, I would go ahead and replace them and the cam and crank sensor. Then retest the issue.

https://www.2carpros.com/articles/symptoms-of-a-bad-crankshaft-sensor

Let me know what you find and we can go from there. Thanks
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Saturday, August 1st, 2020 AT 12:52 PM (Merged)

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