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Unread 07-15-2010, 08:53 AM   #1
techRob
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WJ fuel pump issue?

I was driving the jeep more than usual last week while on vacation. I usually only put about 100 miles on the jeep per week, but I drove it about 400 miles to raleigh from atlanta to go camping.

while I was in raleigh, we stopped at a store to pick up some stuff. when I got back in and started the jeep up, it was idling rough. I thought it might act better when moving, so I shifted to reverse and started backing up. this didn't help, and I shifted to neutral to see if that would change anything. then I heard the engine sputtering, so I put it in drive to pull back into the sparking space and check it out. as I shifted, I lost power steering, accel, and breaks and the jeep just coasted into the spot. I took the key out and started it again and it was still rough. I popped the hood and the engine was shaking around pretty violently in the compartment. I shut it down again and let it sit before starting it up again. it seemed fine after that.

I drove it a few times after that with no issues, but my girlfriend's father (a mechanic) thought it sounded like a fuel pump on the way out. he recommended changing the fuel filter to see if that would fix it. I went to advanced and got a purolator filter for about $30 and installed it. I drove it a lot more after that, including the trip back to atlanta without any issues.

last monday I was on the off-ramp getting off the highway when the jeep suddenly died again on me. it wasn't rough or making any noise--it just straight up died again, with the same loss of steering, accel, and breaks. it coasted to a stop again and luckily no one was right behind me. I turned it off and then on again and drove it the remaining 2 blocks to my house. since I restarted it, the check engine light has been on. it's giving me code P0720.

P0720 (M)
Low Output SPD Sensor RPM Above 15 MPH
The relationship between the Output Shaft Speed Sensor and vehicle speed is not within acceptable limits.

P0720 (M)
Low Output SPD Sensor RPM Above 15 MPH
Output shaft speed is less than 60 rpm with vehicle speed above 15 mph (4-speed auto. trans. only).

while driving it again tuesday I noticed when I accelerated quickly (I got to about 3500 rpm) the engine stopped giving me power but continued to rev up. I had to wait for the rpm to come back down before I could accelerate. I cycled that a few times to see if it was a random occurrence but it kept happening. I pulled off to the side of the road and restarted the car and the issue went away. I've been shying away from giving the jeep too much gas since then.

anyway, I don't think the fault code I've been getting is helpful in diagnosing the problem, but it seems to be a problem with the fuel system. does this sound like the fuel pump is on its way out or am I missing something else? if it is the pump, what should I be looking to replace it with? oem or will airtex/bosch from autozone or advanced work the same? lastly, the purolator fuel filter I replaced the oem filter with said to service it every 12,000 miles. should I follow that or should I let it be like I did with the oem filter?

thanks, guys

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Unread 07-15-2010, 12:15 PM   #2
pebloy
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Jeep Grand Cherokee WJ - Transmission Speed Sensors
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Unread 07-15-2010, 02:59 PM   #3
techRob
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thanks for the reply.

I saw that when I was looking up the code, but I don't think that alone explains my original problem. it does, however, sound like it could be the source of the second problem I had with the engine revving up but not giving any power. I'll look into it more as that seems to be a cheaper and easier solution than replacing the fuel pump.
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Unread 07-15-2010, 04:27 PM   #4
pebloy
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Quote:
Originally Posted by techRob View Post
thanks for the reply.

I saw that when I was looking up the code, but I don't think that alone explains my original problem. it does, however, sound like it could be the source of the second problem I had with the engine revving up but not giving any power. I'll look into it more as that seems to be a cheaper and easier solution than replacing the fuel pump.
Just my two cents:

Fuel pump is basically a device which would rarely be working intermittently in driving. The most common phenomenon from a faulty fuel pump is hard-start or long start time. If you would doubt the fuel pump, you better conduct a pressure test since it is really expensive. wjjeeps.com has a TSB on this issue.

I would doubt electrical problems (faulty fuses, ignition switch or sensors like CPS, TPS, Input and Output Speed Sensors ) rather than the fuel pump.
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Unread 07-15-2010, 08:09 PM   #5
techRob
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well I went to autozone earlier and picked up a replacement sensor. I got home, took out the old sensor, and attempted to put the new one in when I found that the new one does not fit the jeep.



so in fiddling around with this, all my transmission fluid drained. so now I can't drive the jeep. I have to wait until my girlfriend gets home from work tomorrow before I can go to autozone to return the part that their computers said would fit my fing jeep and try to convince them to give me tranny fluid. then I gotta go to advanced, where I should have gone in the first place, to get the right part. after looking at the oem one, I can see that on autozone's site it's the wrong size but on advanced's site it is identical.

so yeah. FML. :
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Unread 07-15-2010, 08:20 PM   #6
pebloy
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No worries. You could take the time to do more search into this issue.

Is this the input or output speed sensor? Taking them down wouldn't drain all the transmission fluid.
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Unread 07-15-2010, 08:46 PM   #7
WiCkeDuDe
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Give you tranny fluid? Gimme a break man.
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Unread 07-15-2010, 08:55 PM   #8
techRob
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Quote:
Originally Posted by pebloy View Post
No worries. You could take the time to do more search into this issue.

Is this the input or output speed sensor? Taking them down wouldn't drain all the transmission fluid.
it's the output sensor. I have the 4.0 with the 42re, which only has the output sensor. I didn't really keep track of how long it was draining for but it came out steadily for a couple minutes.

actually I just went and checked, admittedly for the first time ever, and it's above the 2 holes for cold but below the 2 holes for hot. I did this with the engine having been off for 3 hours or so. I'll try it again tomorrow when the oil is actually closer to 70 degrees and if it's still at acceptable levels I'll stick the sensor back in and try measuring it hot. then I'd at least be able to drive the jeep.

thanks for all your help so far.
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Unread 07-15-2010, 08:57 PM   #9
techRob
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Originally Posted by WiCkeDuDe View Post
Give you tranny fluid? Gimme a break man.
thanks for your input, *******. it's not my fault their computers are wrong.
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Unread 07-16-2010, 08:04 AM   #10
techRob
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ok, I checked the transmission fluid this morning while the jeep was running and in neutral. there was still plenty of fluid in there so I put the old sensor back in and drove it to advanced. on the way I had what has now became the usual shifting difficulties. I got the new sensor, brought it home, and put it in. I noticed the fluid came out much slower this time, as opposed to the gushing it did the first time I took the sensor out.

I torqued the new one and hooked everything up and then started the jeep up. I shifted through all gears and then put it back in neutral and checked the fluid one more time. it was still above the bottom hole in the HOT range. I may add a little more fluid down the line but that satisfied me enough to take it out for a test drive. I took it a few miles down the road and then took the highway back home. the jeep shifted perfectly the entire time.

I'm keeping my fingers crossed that this will be the end of my problems. obviously one test drive doesn't necessarily mean it's back to normal, but given how regular the shifting problems had become, I'm confident now that everything is fixed.

thank you so much for your help, pebloy.
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Unread 07-16-2010, 08:20 AM   #11
ras2001wj
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why would you expect free tranny fluid anyways? It would have drained out either way, correct sensor or not.
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Unread 07-16-2010, 08:41 AM   #12
techRob
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Quote:
Originally Posted by ras2001wj View Post
why would you expect free tranny fluid anyways? It would have drained out either way, correct sensor or not.
how do you figure? if the sensor was the correct size the new one would have plugged the hole, minimizing the amount of fluid drained.

it's a moot point now anyway since it didn't drain as much as I originally thought. the level is still within the acceptable range. I was much more angry when I thought I had lost the ability to drive the jeep.
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