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Old 09-28-2015, 12:53 AM
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Rpm and acceleration trouble

Hello guys,

I need help to diagnosis the problem.

I was recently in a 3 hours trip and notice my jeep wj v8 has a performance issue. It accelerate fine up to 55mph. After that, when I try to go faster and floor the pedal, the rpm won't go higher than 2200rpm, then gradually drop back to 1500-1600rpm. But speed will pick up slowly, but sluggish. It could reach 70mph, but cannot hold steady. Cold engine could rev up to 3500rpm without problem. But produce a engine code P0455 And P0523. The transmission would produce a thump/bang/clap sound when shift from reverse to drive.

Things has done:
1. Injector, coil ok.
2. Engine oil level ok.
3. Spark plug ok.
4. Will replace oil pressure sensor on my next oil change.

Please guide me to the right way. Thanks.

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Old 09-28-2015, 01:47 AM
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Re: Rpm and acceleration trouble

Sounds like its going in to limp mode.... Hows ur tranny fluid
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Old 09-28-2015, 04:25 AM
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Re: Rpm and acceleration trouble

Welcome to the garage.
When the engine speed reduces to 1500 rpm, the vehicle also reduces speed?
It can not be in limp mode, because the rpm should increase then, besides the fact that a P0700 and probably a more specific code will be set.
How is the idle rpm? Should be at 650, but when higher, could cause a bang when shifting from R to D. Also play on a U joint can cause a bang.
Do you have a scanner with live data? It would be nice to see what the TPS % is doing when pressing the accelerator. Also the MAP (manifold air pressure) should be checked, as it is the #1 input for fuel injection. Does the engine start without problems?
Checking fuel pressure and cylinder compression would also be on my list.
Also check for a vacuum leak, since you have the P0455 code.
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Old 09-28-2015, 07:46 AM
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Re: Rpm and acceleration trouble

Hello there,

Yes, speed decrease slowly. It idles fine at about 610-630rpm and starts right up every time. No misfire also from any cylinder. The funny thing about the P0523 is that it comes on and off. When it is on, it shows a Check Gage on the dashboard and no pressure . But it could go off after I refuel the tank, or started the car, feels like connection problem.

From 0-35, I feel it drives like new. Even it is a wj. But the speed won't pick up easily after 55. Also, tranny fluid level is good. Would over fill the tranny cause this kind of problem? And do I need to worry about the park to drive and reverse to drive sound?
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Old 09-28-2015, 08:35 AM
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Re: Rpm and acceleration trouble

The P0523 code can be caused by a dirty pressure sensor/switch connector, so i would clean that first. But it most probably is the sensor/ switch itself. 99-03 models have a pressure sensor and real pressure is shown on instrument panel. 04 models use a pressure switch, which just gives a signal for a fixed pressure or no pressure.
Overfilling the transmission could cause slipping clutches, due to air which can be churned into the fluid and the consequently reduction in line pressure. Should not be your problem here.
Regarding the clunking noise, check the propeller shafts joints. Any play in there can cause a clunk when D or R gear is selected. Mostly it will make the same noise when going back from D to R as well.
Do you have a scanner? If not, and you still want to trouble shoot yourself (and have a smartphone), you could dongle bluetooth or wifi dongle and download the free torque app. You can than read fault codes, errase them and see many live data.
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Old 09-29-2015, 11:36 PM
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Re: Rpm and acceleration trouble

Thanks for the advice, I will get back to you after testing over the weekend.
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