AX-15 rpm issue when cold/hot

1999 Cherokee, 4.0L, AX-15.
Supposedly "new" clutch (15K miles) according to previous owner.

When the engine is at operating temperatures and upshifting, my engine RPMs drop to idle and must "bounce" back up to the appropriate speed when i depress the clutch. I've been adding some throttle to compensate for the drop but it takes some concentration and I need to be slow with my clutch release or it is a rough engagement.

When not at operating temperatures, I do not have this issue. It is so smooth, easy, a joy to drive because the rpms slowly drop to the desired rpm when upshifting.


Any advice?

If you have acess to a scan tool, check for current and or pending codes. It sounds like something in the electronics is dropping out when you hit the pedal.

cruise control?

that's about the only thing that would tell the PCM that you've depressed the clutch.

otherwise 'd suspect IAC.

Clean the throttle body

I must be missing something here... When you depress the clutch at any speed the engine is going to drop down to idle. When its cold, the engine is still holding higher RPMs until it it go to closed loop in the ECM.

redneck said:

I must be missing something here... When you depress the clutch at any speed the engine is going to drop down to idle. When its cold, the engine is still holding higher RPMs until it it go to closed loop in the ECM.

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YES!!

If you take your foot off the gas pedal and push the clutch pedal down the engine is supposed to drop to the idle speed. If it remain at a higher RPM than idle then you need to look at what is keeping it running fast. On a cold engine the IAC could keep the RPM high.

The only time I really add throttle when shifting is to rev match for down shifting, ex. 4th to 3rd. You need to get the engine speed matched to the drive line speed or else its going to buck and its rough on the drive line components. If you're up shifting and its dropping all the way back to idle then you're shifting way too slow. The same thing is going to happen. The engine and drive line rpms are not going to match and its going to buck on you. The only time I really let it idle down with the clutch in while moving is if I'm down shifting to a stop.

If you depress the clutch and the RPMs dont come down, i would suspect that youre clutch is not fully disengaging from the flywheel. Some sort of misalignment or a bad throw out bearing could be the culprits. Like I said earlier, when the engine is cold the computer is going to trim the motor with a slightly higher RPM for a min or so for warm up. Other than that, its not normal to have a raised idle and a vacuum leak or idle air control may be suspect.

four_shot said:

If you have acess to a scan tool, check for current and or pending codes. It sounds like something in the electronics is dropping out when you hit the pedal.

Click to expand...

I will do this. Thank you

87manche said:

cruise control?

that's about the only thing that would tell the PCM that you've depressed the clutch.

otherwise 'd suspect IAC.

Click to expand...

I do have cruise control, but this issue occurs when cruise is OFF (have not tested when ON).

First, I cleaned my IAC and it did not help
Second, I bought a JY IAC, cleaned it, and it did not help

I may try a new one (near the end)

Digger87xj said:

Clean the throttle body

Click to expand...

I cleaned the throttle body and it did not help.

redneck said:

The only time I really add throttle when shifting is to rev match for down shifting, ex. 4th to 3rd. You need to get the engine speed matched to the drive line speed or else its going to buck and its rough on the drive line components. If you're up shifting and its dropping all the way back to idle then you're shifting way too slow. The same thing is going to happen. The engine and drive line rpms are not going to match and its going to buck on you. The only time I really let it idle down with the clutch in while moving is if I'm down shifting to a stop.

If you depress the clutch and the RPMs dont come down, i would suspect that youre clutch is not fully disengaging from the flywheel. Some sort of misalignment or a bad throw out bearing could be the culprits. Like I said earlier, when the engine is cold the computer is going to trim the motor with a slightly higher RPM for a min or so for warm up. Other than that, its not normal to have a raised idle and a vacuum leak or idle air control may be suspect.

Click to expand...

Do you find that you need to shift much faster once the engine is warm vs when it's cold?

My issue is nonexistent when the engine is cold but glaring when warm.
When warm, I can shift it fast to "catch" the RPMs w but it is a very rough engagement, so I baby it into position.
When cold, this is not necessary

what fluids in your AX-15?

MT90 made a drastic diffrence in the shifting of mine. I can easily move the lever twice as fast now with no grinding.

Honestly it sounds like your motor is doing what it does. I took the first post to mean that you had a drop in RPMS just sitting at idle when you pushed the clutch in.
If it doesn't dip below the normal idle speed whenyou shift, it's not your IAC. THe PCM does not hold any sort of throttle, when you take your foot off the throttle it's going to idle, and as quickly as it can.

redneck said:

I must be missing something here... When you depress the clutch at any speed the engine is going to drop down to idle. When its cold, the engine is still holding higher RPMs until it it go to closed loop in the ECM.

Click to expand...

i gotta say, i'm with this guy.... i dont think anything is "wrong" i think it has more to do with driving style.
i'm not grasping the reason for upshifting without throttle application. shifting up and down should be a smooth combination of clutch slippage and throttle application that you feel in the seat of your pants...
a colder engine will respond and react slower than a warm engine, but there isnt anything wrong.
i gotta ask, and with no intention of insulting you.
how long have you been driving?
i ask simply to try to understand your problem.
if you've been doing this for years maybe you have a legitimate issue we can help you with, but if you just got your license and you are self-taught it could be something entirely.

87manche said:

what fluids in your AX-15?

MT90 made a drastic diffrence in the shifting of mine. I can easily move the lever twice as fast now with no grinding.

Honestly it sounds like your motor is doing what it does. I took the first post to mean that you had a drop in RPMS just sitting at idle when you pushed the clutch in.
If it doesn't dip below the normal idle speed whenyou shift, it's not your IAC. THe PCM does not hold any sort of throttle, when you take your foot off the throttle it's going to idle, and as quickly as it can.

Click to expand...

I have MT90 in the transmission, shifting is smooth, and no grinding occurs.

It doesn't dip below idle but upon pushing the clutch in (to upshift), the engine will quickly fall down to 750rpm and then bounce up to 1300-1500rpm and then fall slowly down to 750rpm.

Also, bucking/surging sometimes occurs when in 1st, in a level parking lot, when RPMs are 750-1100. This can happen when I'm either applying a tiny amount of throttle or when using no throttle.

The bucking/surging will occur before the RPMs drop below 750.

sidriptide said:

i gotta say, i'm with this guy.... i dont think anything is "wrong" i think it has more to do with driving style.
i'm not grasping the reason for upshifting without throttle application. shifting up and down should be a smooth combination of clutch slippage and throttle application that you feel in the seat of your pants...
a colder engine will respond and react slower than a warm engine, but there isnt anything wrong.
i gotta ask, and with no intention of insulting you.
how long have you been driving?
i ask simply to try to understand your problem.
if you've been doing this for years maybe you have a legitimate issue we can help you with, but if you just got your license and you are self-taught it could be something entirely.

Click to expand...

Yes, I believe that I could definitely be the issue. I have had the jeep (first manual) for 1 yr. I do not have prior experience, another cherokee to drive, or any knowledgeable others to take her for a spin.

My concern is that I do not remember this behavior 1 year ago.

Does the following occur on your jeep?

-When cold, equal amounts of clutch slippage and throttle application are required when upshifting.

-When warm, upshifting 2nd -> 3rd requires a drastically different amount of clutch slippage and throttle application than either 1st -> 2nd, 3rd -> 4th, 4th -> 5th.

It doesn't dip below idle but upon pushing the clutch in (to upshift), the engine will quickly fall down to 750rpm and then bounce up to 1300-1500rpm and then fall slowly down to 750rpm.

Also, bucking/surging sometimes occurs when in 1st, in a level parking lot, when RPMs are 750-1100. This can happen when I'm either applying a tiny amount of throttle or when using no throttle.

Click to expand...

dropping to idle is normal. Surging and bucking is not. Bouncing to 1500 after it hit idle is not.
I'd suspect TPS. put an analog meter on it and make sure it sweeps smoothly when you move the TPS arm.

Yeah, when you push the clutch in with the throttle at zero, it should drop to 750 (idle) and stay there, it shouldn't drop below idle, hunt, or surge.

Check your vacuum lines as well. Does it happen in a more pronounced way if you have your foot on the brake? I had a bad brake booster pushrod seal cause some severe idle issues on my MJ years ago when it was an auto still.

87manche said:

dropping to idle is normal. Surging and bucking is not. Bouncing to 1500 after it hit idle is not.
I'd suspect TPS. put an analog meter on it and make sure it sweeps smoothly when you move the TPS arm.

Click to expand...

I replaced the old TPS with a new Mopar TPS. No change in behavior.
Is there a place that rents analog meters or will I need to purchase one? (have a digital as of now).

kastein said:

Yeah, when you push the clutch in with the throttle at zero, it should drop to 750 (idle) and stay there, it shouldn't drop below idle, hunt, or surge.

Check your vacuum lines as well. Does it happen in a more pronounced way if you have your foot on the brake? I had a bad brake booster pushrod seal cause some severe idle issues on my MJ years ago when it was an auto still.

Click to expand...

I am unfamiliar with what are, where are, and how to check vacuum lines. Do you have a recommended source that I can take a look at to learn?

I will test with the brakes.

vacuum lines are all of the hard plastic ones connected to the intake manifold. Anything connected to the intake manifold can cause a vacuum leak.

Check your grounds as well. When the motor moves a bad ground will present. I had a stumble everytime I let the clutch out, was the head ground.

The bucking a low speeds may not be an engine issue at all. When youre going slow, like in a parking lot and not using the clutch and just loping around in like 2nd gear, youll get the drive train bucking against the engine. This is because the transmission is directly connected to the engine and youre tires rotation is fighting the engines rotation until they equalize (deceleration). You dont get this in an automatic car because the torque converter is a fluid coupler and isolates the drive trains rotational speed from the engines.

Bad engine mounts may be the problem.

Lay a block of wood on a jack and try to slowly lift the engine. From underneath the truck watch the engine mounts to see if the bolt separates from the rubber.

just spitballing:dunno: