YJ won't rev past 3k RPMs (automatic transmission)

[Ryan Hawkes]

Hello

95 YJ 4.0 auto

Okay, first, everything is brand new.

For some strange reason the Jeep will not rev past 3k rpm.
In park, 3k rpm
In neutral, 3k rpm
While driving, will not accelerate past 3k rpm.

As far as I can tell, this does not feel like a rev limiter issue. It feels like the engine starts to break down during the combustion cycle and gets out of time. I really don’t know and would appreciate any insight anyone might have.

 

[Flyer58]

Sounds like it's being starved for air either from an exhaust restriction or blocked intake / dirty filter. Can you verify the throttle plate is opening all the way? Also check for engine codes even though you may not have a check engine light on.

 

[Ryan Hawkes]

Gets good fuel pressure, has a clean cold air intake, brand new exhaust, brand new throttle body, new vacuum lines, truthfully everything is brand new. It has OBD 1 and no codes come up.

 

[Ryan Hawkes]

This is what she looks like

 

[Flyer58]

Looks great! Not sure where to start with the RPM limit.

How do the spark plugs look? You sure the firing order is correct? Did this start after any new parts were installed or did it run good then have this problem?

Trying to cover the basics first. I've surprised and kicked myself many times.

 

[Ryan Hawkes]

It ran fine before, then I rebuilt everything. The machine shop did the reassembly head down on the motor and I know he does outstanding work. Otherwise I put everything in and back together. Swapped plugs, wires, distributor and every sensor. I also replaced the flexplate because that is a thing when dealing with this issue. I also made sure that all the grounds were installed correctly and bought a new ECM. Non of these things have helped. Ill be swapping to a Mopar fuel pump, rather than the Napa one now. Hoping that helps. I hate to replace new parts with new parts, but I've tried everything, asked very experienced mechanics, yet they always tell me that its probably a jeep thing and jeeps are known for these silly issues.

 

[Flyer58]

Huh, head scratcher. If you get good fuel pressure at all RPM's with the NAPA pump then switching it may not make any difference.

Do you have the old ECM or is the motor different and requires a different ECM? May be worth swapping it back to test it it was working.

 

[Ryan Hawkes]

The pressure starts to slug and drop a couple psi at around 2500 rpms. I was told that it was fine, but signs are pointing toward it isn't. I'm doing it this week. I have a new bumper and tire carrier to put on, so dropping the tank makes life easier, figured installing the pump should be an easy experiment.

 

[Flyer58]

You mentioned rev limiter and somewhere I think I read they do have one but it kicks in around 5500 or 6000. I never tested my 2.5L

The input for that is probably the Crankshaft Position Sensor and there might be a difference between the manual trans and automatic ones. I think the auto trans have to be adjusted in depth somehow. Does that sound right? Do you have the old one to swap back to?

 

[Ryan Hawkes]

I haven't heard of an adjustment, I would definitely like to hear more. Early on during the troubleshooting signs pointed to the CPS and or the flexplate or the combination of the two. Easy fix, bought Mopar parts swapped them out.... zero change.

 

[Flyer58]

I'm on my phone and don't have the FSM manual here. Go to the resources section in this forum to find the factory service manual. That should help with any sensor adjustment. I don't have the auto so I'm not sure of the difference in sensors.

Look here...

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