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Unread 06-29-2013, 11:07 PM   #1
LoveHate
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rpms jumping under load?

was towing a uhaul small trailer today about 120 miles round trip. when I was on the way back my zj started jumping rpms and bucking if I tried to accelerate. had to gain speed as slow as a semi just to get to a spot to pull over, checked all fluids and nothing seamed out of norm, figured it wouldn't hurt to add some seafoam I had in the jeep to the gas incase it was bad gas. well when I opened the cap it had a lot of suction. put the cap back on loose and it was fine for a few miles then started jumping again. also my fuel gauge read half tank before opening cap and then quarter tank after opening cap but quickly went down when I started driving. I stopped at the nearest gas station and put more gas in and it seamed to ride ok for the rest of the trip which was only like 5 miles. also I in the past week have done a oil change and trans filter change, the trans fluid is a little high though. so what could cause this? was it bad fuel, bad gas cap, to much trans fluid, or? also in neutral the engine (4.0) idles and revved fine, despite the rpm jumping the trans also seamed to shift fine and it wasn't slipping. thanks for any and all help.

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Unread 06-30-2013, 01:26 AM   #2
jm8881
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You have a blockage in your system somewhere that isn't allowing the space the fuel was taking up to be replaced by air. A vacuum is being created by the fuel pump and causing your problems.
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Unread 06-30-2013, 10:34 AM   #3
LoveHate
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Quote:
Originally Posted by jm8881 View Post
You have a blockage in your system somewhere that isn't allowing the space the fuel was taking up to be replaced by air. A vacuum is being created by the fuel pump and causing your problems.
So its safe to say the gas cap is bad and not venting. I kinda knew that but wanted to make sure it would be causing my problems and not something else also wrong. Thank you.
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Unread 06-30-2013, 10:38 AM   #4
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Those are symptoms for a bad TPS aslo
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Unread 06-30-2013, 03:21 PM   #5
LoveHate
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yeah I was thinking possible tps but in neutral it revs up great and holds rpm at any place i want. would seafoam clean the tps if ran threw break booster or gas tank, reason I ask is I did the seafoam last week and if it wont reach it then I will take it out and clean it manually.
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Unread 06-30-2013, 03:26 PM   #6
PolkaPower
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Not the TPS. The IAC can be cleaned and it's exposed to the intake the TPS is not.
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Unread 06-30-2013, 03:43 PM   #7
LoveHate
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Originally Posted by PolkaPower View Post
Not the TPS. The IAC can be cleaned and it's exposed to the intake the TPS is not.
ok mixed that up lol semi new to the 4.0s happen to have a write up or quick explanation where the tps is located. thank you.
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Unread 07-15-2013, 12:07 PM   #8
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Ok well jeep was acting fine up til yesterday. I did get a new gas cap but Yesterday it started jumping rpm again. I'm gonna go head and put a New tps in and hope it solves it. Is the bwd one from advance good or do I need mopar?
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Unread 07-15-2013, 12:12 PM   #9
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Wait ... wait ... what do you mean by "jumping RPMs"? Is the tachometer jumping around, or are the Jeep's RPMs suddenly surging, or is the Jeep acting like it is running out of gas?

If the tachometer is jumping around, then that doesn't sound like the TPS. If the engine is struggling to accelerate or has delayed acceleration ... well, then that may be the TPS.
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Unread 07-15-2013, 12:19 PM   #10
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Quote:
Originally Posted by dellis View Post
Wait ... wait ... what do you mean by "jumping RPMs"? Is the tachometer jumping around, or are the Jeep's RPMs suddenly surging, or is the Jeep acting like it is running out of gas?

If the tachometer is jumping around, then that doesn't sound like the TPS.
When I'm driving and the rpms are over 1500 they jump a few hundred rpms and the jeep is very sluggish feels like it will die when I go to accelerate. The fuel systems been cleaned and new filter and gas cap was replaced. And it idles fine and revs fine and hold rpms fine when in neutral or park. EDIT....From everything I've read I'm almost certain its the tps. Just not sure which one to get
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Unread 07-15-2013, 12:36 PM   #11
dellis
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Originally Posted by LoveHate View Post
When I'm driving and the rpms are over 1500 they jump a few hundred rpms and the jeep is very sluggish feels like it will die when I go to accelerate. .... And it idles fine and revs fine and hold rpms fine when in neutral or park. EDIT....From everything I've read I'm almost certain its the tps. Just not sure which one to get
OK ... TPS would not be my first thought (particularly because the behavior is not happening at idle and in neutral ... it seems to me that a bad TPS would evidence problems there as well at some point), but you are closer to all the symptoms than I, and if your symptoms are matching with what you are reading, then go for it.
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Unread 07-15-2013, 12:50 PM   #12
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Quote:
Originally Posted by dellis View Post

OK ... TPS would not be my first thought (particularly because the behavior is not happening at idle and in neutral ... it seems to me that a bad TPS would evidence problems there as well at some point), but you are closer to all the symptoms than I, and if your symptoms are matching with what you are reading, then go for it.
Any other ideas as to what it could be? It basically feels like its feathering the gas when I go to accelerate. And there's no codes so idk what it could be lol just throwin parts st it till its happy at this point.
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Unread 07-15-2013, 01:12 PM   #13
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Originally Posted by LoveHate View Post
Any other ideas as to what it could be?
You seem to indicate that this is happening under load, so weak fuel pressure or restricted cat come to mind first.

I'm not sure how either would contribute to a tachometer jump of 200 RPM ... but I'm still not really clear about what you are experiencing with the RPMs.
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Unread 07-15-2013, 01:19 PM   #14
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I'm leaning towards the TPS too. Sounds exactly like the symptoms I've had.
It "seems" to be fuel related, but there's no way to be certain. without more info.

If it were carbureted, I'd say there's water in the fuel or a fuel pump going out and you might make a case for that anyway....although it's low on my short list. It could even be a timing issue...( not advancing) but no sense in going there, yet either. TPS gets my vote at this point unless someone else has a more logical idea.
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Unread 07-15-2013, 01:27 PM   #15
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Quote:
Originally Posted by dellis View Post

You seem to indicate that this is happening under load, so weak fuel pressure or restricted cat come to mind first.

I'm not sure how either would contribute to a tachometer jump of 200 RPM ... but I'm still not really clear about what you are experiencing with the RPMs.
We'll there is no cat on the jeep and the fuel pressure is fine or so says the shop. Ok so say I'm going 30 mph at 1500rpm and I push the gas in to speed up to say 50. Well instead of going to 2000rpm and accelerating it will go to 1700 then back to 1500 then back up to 1700 then back down and it will keep doing it til I let off the gas. It also feels like its speeding up a little then slowing back down over and over. I guess you would call it surging?
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