Jeep 4.0 - not much power reaching 4000 RPMs - JeepForum.com
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post #1 of 21 Old 12-20-2018, 08:52 AM Thread Starter
pablorg
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Jeep 4.0 - not much power reaching 4000 RPMs

HI,

I have a 1995 YJ Sahara 4.0. The problem I have is that the Jeep won't go over 4000 RPMs and it starts to loose power after 3800 RPMs. Instead, I have seen videos of 4.0 engines reaching 4600 RPMs easily



Any ideas what could be wrong.

The engine is recently rebuilt. Bored 60 over. HESCO adjustable pressure regulator. New fuel pump.

Thanks!


- Pablo
95 YJ Sahara AX-15
81 CJ-7 Laredo 360V8-Edelbrock MPI
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post #2 of 21 Old 12-20-2018, 09:02 AM
Mortgage-payer
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Plugged fuel filter?

I call it a distributor, not a dizzy.
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post #3 of 21 Old 12-20-2018, 09:14 AM
jtec
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I do not understand why over 4k in a jeep 4.0/l but my 2 esp if an automatic.

Anyway first is fuel - check the pressure - Re pump New is nt always good.
Exhaust restrictions - a backpressure gauge will tell easily.

MY favorite use a scanner - the data will help - all done quickly from front seat .
Always codes offer a clue. and we can see live data,
look at ECT, MAP, TPS, and fuel trims and o2 sensor readings .

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #4 of 21 Old 12-20-2018, 09:26 AM Thread Starter
pablorg
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The fuel filter and fuel pump are new - changed them 2 weeks ago

- Pablo
95 YJ Sahara AX-15
81 CJ-7 Laredo 360V8-Edelbrock MPI
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post #5 of 21 Old 12-20-2018, 11:31 AM
jtec
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X2 new ≠ good

If you have a schraider valve on fuel rail - an EASY test.

Have you look at codes?

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #6 of 21 Old 12-20-2018, 11:49 AM
gutthans
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Just to check: Are you having this issue in all gears, or just 5th gear? Maybe 4th and 5th? Plugged exhaust (rust or catalytic failure) or too tall tires can lead to this result. Also misadjusted timing settings.
Don't know much about which sensors might contribute (I always rip all of that off and run 'old school')...

Do as suggested, and check fuel output. Your 'new' stuff could be faulty or a line could be plugged. Shake/thump your muffler and cat and listen for junk inside that shouldn't be there...
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post #7 of 21 Old 12-20-2018, 12:26 PM
V8orBUST
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I'm curious about the torque curve above 3500 rpm, the 4.0 makes most of its power in the 2000s I thought. With such an over bore, I would assume your compression ratio is higher possibly requiring more fuel at a higher octane. Timing may also play a part in the increased compression.

Just my thoughts, I can and may well be wrong.

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post #8 of 21 Old 12-20-2018, 07:21 PM
Awesome
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Quote:
Originally Posted by V8orBUST View Post
I'm curious about the torque curve above 3500 rpm, the 4.0 makes most of its power in the 2000s I thought. With such an over bore, I would assume your compression ratio is higher possibly requiring more fuel at a higher octane. Timing may also play a part in the increased compression.

Just my thoughts, I can and may well be wrong.
You are correct, the engine makes most of its power before it hits 3k. Mine actually drops power noticeably after 3200 RPM.

To the Original Poster:

What gear are you in? If you're trying to do 120 in 5th gear, you're obviously going to have a hard time revving past 4k.

The 4th 5-seater YJ in the world.
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post #9 of 21 Old 12-21-2018, 10:59 AM Thread Starter
pablorg
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Quote:
Originally Posted by Awesome View Post
You are correct, the engine makes most of its power before it hits 3k. Mine actually drops power noticeably after 3200 RPM.

To the Original Poster:

What gear are you in? If you're trying to do 120 in 5th gear, you're obviously going to have a hard time revving past 4k.
I am in 3rd gear going uphill. I have 31" tires and 4.56 gears. I am also @ 9800 feet of altitude...

- Pablo
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post #10 of 21 Old 12-21-2018, 11:34 AM Thread Starter
pablorg
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Compare mine to this, for example:


or


- Pablo
95 YJ Sahara AX-15
81 CJ-7 Laredo 360V8-Edelbrock MPI
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post #11 of 21 Old 12-21-2018, 12:10 PM
Awesome
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Quote:
Originally Posted by pablorg View Post
I am in 3rd gear going uphill. I have 31" tires and 4.56 gears. I am also @ 9800 feet of altitude...
Okay, then you have a fuel or air issue. Check for vacuum leaks using the carb cleaner method. Check your air intake for restrictions. At that altitude you're going to be sucking a lot of air down.

The 4th 5-seater YJ in the world.
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post #12 of 21 Old 12-21-2018, 06:41 PM
Boojo35
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Quote:
Originally Posted by pablorg View Post
I am in 3rd gear going uphill. I have 31" tires and 4.56 gears. I am also @ 9800 feet of altitude...
Good lord. 9800 feet? There is no air left up there. Your engine loses cylinder pressure in a big way because there is no air to compress. An engine is an air pump with combustion added in. It needs air to make power.

Next question. How steep is the hill in degrees?

You may not have a problem IMO.
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If you cannot fix it with a hammer then it has to be an electrical problem.
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post #13 of 21 Old 12-29-2018, 08:11 PM
rgb66
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Quote:
Originally Posted by Boojo35 View Post

You may not have a problem IMO.
or as a wise person once told me. "You don't have a problem, you have a jeep."
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post #14 of 21 Old 01-03-2019, 10:48 AM Thread Starter
pablorg
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Quote:
Originally Posted by Awesome View Post
You are correct, the engine makes most of its power before it hits 3k. Mine actually drops power noticeably after 3200 RPM.

To the Original Poster:

What gear are you in? If you're trying to do 120 in 5th gear, you're obviously going to have a hard time revving past 4k.
I am in 3rd gear going uphill..
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- Pablo
95 YJ Sahara AX-15
81 CJ-7 Laredo 360V8-Edelbrock MPI
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post #15 of 21 Old 01-04-2019, 04:29 PM
astjp2
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Clogged catalytic converter, my YJ had the same problem and found the exhaust clogged.
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