RPM problems

[Duckserman]

I have a 95 2.5 YJ

I know this is a TJ forum, but I haven't gotten a solution from the YJ forum.

During driving, when I push the clutch to slow down, the rpms jump up to 1500 rpms and holds there until I make a complete stop and slowly comes back down the ~900 rpms, but the idle is always rock solid wherever it lands.
I put some new sensors on it before this became a problem. I'm wondering if the new sensors I put on are the problem. They're all either NTK or OEM.

 

[Chris]

I have a 95 2.5 YJ

I know this is a TJ forum, but I haven't gotten a solution from the YJ forum.

During driving, when I push the clutch to slow down, the rpms jump up to 1500 rpms and holds there until I make a complete stop and slowly comes back down the ~900 rpms, but the idle is always rock solid wherever it lands.
I put some new sensors on it before this became a problem. I'm wondering if the new sensors I put on are the problem. They're all either NTK or OEM.

Since you've already replaced some sensors, it's possible that one of them could be malfunctioning or incorrectly calibrated.

Here's a checklist of things to consider:

Throttle Position Sensor (TPS): This sensor can sometimes cause high idle if it’s reading incorrectly. Make sure it's properly adjusted and that its voltage readings are in line with the specifications.

Idle Air Control (IAC) Valve: If this valve is sticky or malfunctioning, it could cause erratic RPM behavior when you engage the clutch.

Vacuum Leaks: Check for any vacuum leaks in the intake system. Even a small leak can lead to high RPMs during deceleration as it may allow unmetered air into the engine.

Clutch Release Bearing: Sometimes, if the clutch is dragging, it may be causing the engine to rev higher when you push the clutch in.

ECU/PCM Software: If the sensors were replaced with different part numbers or have different characteristics, it might be worth checking if the ECU has adapted to the new inputs correctly.

Check Connections and Wiring: Since new sensors were installed, inspect all connections and wiring to ensure everything is secure and functioning as intended.

Since you've got new sensors, it might be worth going back to see if any of them have become a problem or if there's been an installation issue. If all else fails, consider reaching out to someone with a scanner to get real-time data while you drive—it can provide valuable insight into what may be going wrong. Hopefully, you can get this sorted out soon!

 

[5TTTj]

Good advise above, Im thinking Vacuum Leak.

 

[TJ Jim]

When my tj is in open loop my rpms do the exact same thing. Once it goes into closed loop the rpms act like you expect. Not real familiar with or if the yj has open and closed loop, but I suspect it does. I’m thinking you are obd1 and there should be a way to see if it goes into closed loop operation. How do you read codes? Flashing lights on the dash for feedback?

 

[Duckserman]

Extra detail, since I posted this in a rush.

I've noticed this problem started when I replaced the tps and iac. The tps is ntk and the iac is oem, both test out good, and I've replced them multiple times with the same brands until I settled with the ones I have, since every one of them did the same thing. I should've left them alone, but I don't have them anymore. I'm thinking it's the tps but every one of them tests out good.
All connections are cleaned and sealed with dielectric grease. And the vacuum lines are all new.

 

[TJ Jim]

I’m sure someone will disagree, but dielectric grease is perfect for the rubber seals to help keep moisture out. It is not intended to be used on metal contacts. If you added it to the metal contacts you have successfully insulated them from passing a signal. It is not conductive to voltage/current. If it were me, I’d start by using a good spray contact cleaner and get all the grease off the metal contacts.

 

[Duckserman]

I’m sure someone will disagree, but dielectric grease is perfect for the rubber seals to help keep moisture out. It is not intended to be used on metal contacts. If you added it to the metal contacts you have successfully insulated them from passing a signal. It is not conductive to voltage/current. If it were me, I’d start by using a good spray contact cleaner and get all the grease off the metal contacts.

I remember hearing about it being an insulator, so I put it on a cotton swab and just carefully coated the inside plastic and rubber, as well as the outside where the plugs meet. No grease made contact with the connectors.

 

[TJ Jim]

I remember hearing about it being an insulator, so I put it on a cotton swab and just carefully coated the inside plastic and rubber, as well as the outside where the plugs meet. No grease made contact with the connectors.

Excellent

 

[TJ Jim]

Still, can you verify open/closed loop? Of corse if it’s not coming out of open loop you still gotta find out why.
On the mechanical side…
Is the spring on the throttle strong and is the movement of the butterfly smooth and complete?

 

[Duckserman]

Still, can you verify open/closed loop? Of corse if it’s not coming out of open loop you still gotta find out why.
On the mechanical side…
Is the spring on the throttle strong and is the movement of the butterfly smooth and complete?

As far as the TB, it's clean and the movement is smooth with no hang ups, and the plate snaps shut with some force when I let go.

Something I have noticed is my temp gauge stopped working out of no where, it doesn't go above ~120⁰. I'm sure my tstat is stuck open since according to the temp gun I have while running, it's at ~140⁰ on the housing and ~130⁰ on the upper hose with the lower hose reading ~120⁰. I put a failsafe tstat in there, and I've heard they have a rep for failing more often than a standard tstat.

 

[TJ Jim]

If it doesn’t come up to temp I’d say it’ll not go into closed operation. You may have found the problem

 

[Duckserman]

If it doesn’t come up to temp I’d say it’ll not go into closed operation. You may have found the problem

So by it not getting up to temp/closed loop, this can cause higher rpms? I assume since when it's cold, the injectors fire hard to warm up, so I guess the computer is forcing it to fire harder since it's reading colder

 

[jjvw]

Good advise above, Im thinking Vacuum Leak.

Thank AI. Chris is artificial now.

 

[jjvw]

I have a 95 2.5 YJ

I know this is a TJ forum, but I haven't gotten a solution from the YJ forum.

During driving, when I push the clutch to slow down, the rpms jump up to 1500 rpms and holds there until I make a complete stop and slowly comes back down the ~900 rpms, but the idle is always rock solid wherever it lands.
I put some new sensors on it before this became a problem. I'm wondering if the new sensors I put on are the problem. They're all either NTK or OEM.

My first instinct is a vacuum leak.

 

[TJ Jim]

So by it not getting up to temp/closed loop, this can cause higher rpms? I assume since when it's cold, the injectors fire hard to warm up, so I guess the computer is forcing it to fire harder since it's reading colder

I’m just relating the fact when my tj is first started on a cold morning it idles high when going through the gears. Then when it has come up to temperature the idle is low where it should be.
Start it. 1500ish
First gear reves up and go to second and between shifts back to 1500.
Same with each gear change. By the time I get about two miles down the road the engine is close to operating temp and when shifting it’s down to 650 between shifts. That’s why I’m thinking it’s the closed loop scenario.

Vacuum leak is plausible as @jjvw suggests, but why would rpm drop to 900?

 

[TJ Jim]

Fix the low hanging fruit. Get the temps right and see what happens. Just come back and tell us what you find.

 

[jjvw]

....

Vacuum leak is plausible as @jjvw suggests, but why would rpm drop to 900?

If it is a vacuum leak, then perhaps the hot engine closes up the leak into less of one.

 

[Duckserman]

If it is a vacuum leak, then perhaps the hot engine closes up the leak into less of one.

I thought the same thing, but all the lines are new. They were the one of first thing I replaced once the problem started. I'm at a loss other than the thermostat being faulty.

 

[TJ Jim]

If it is a vacuum leak, then perhaps the hot engine closes up the leak into less of one.

Intake

 

[Duckserman]

Once I find time to flush the coolant, I'll swap the tstat, and see what happens. I appreciate all the input. I'll update this when something good or bad happens.