My Z is a 2004 auto. Recently, it's been jumping like a little nudge of gas is given when I let go of the gas pedal and slow down. It'll hover around 1000 RPM and then finally come down to around 650 RPM at idle. Sometimes, it also delays going into the second gear by few seconds. I've checked the transmission fluid and it's pretty clean (clear-ish red). It was swapped at Nissan dealership at 100k mileage by previous owner and now it's at 120k. I have changed the throttle body and valve cover gaskets couple months ago. Can it be the the motor mounts or some vacuum leak?

 

Vacuum leak would cause this exact issue of raising rpm off throttle. I had this ones and it was a intake coupler that cracked.

 

Did it do this prior to you replacing the throttle body?.. Since you had to disconnect the Electronic Throttle Control Actuator you may need to do the 'Throttle Valve Closed Position Learning' and the 'Idle Air Volume Learning' procedures here:


Throttle Valve Closed Position Learning
DESCRIPTION
Throttle Valve Closed Position Learning is an operation to learn the fully closed position of the throttle valve by monitoring the throttle position sensor output signal. It must be performed each time harness connector of electric throttle control actuator or ECM is disconnected.

OPERATION PROCEDURE
1. Make sure that accelerator pedal is fully released.
2. Turn ignition switch ON.
3. Turn ignition switch OFF and wait at least 10 seconds.
Make sure that throttle valve moves during above 10 seconds by confirming the operating sound.


Idle Air Volume Learning
DESCRIPTION
Idle Air Volume Learning is an operation to learn the idle air volume that keeps each engine within the specific range. It must be performed under any of the following conditions:

Each time electric throttle control actuator or ECM is replaced.

Idle speed or ignition timing is out of specification.
PREPARATION
Before performing Idle Air Volume Learning, make sure that all of the following conditions are satisfied.
Learning will be cancelled if any of the following conditions are missed for even a moment.

Battery voltage: More than 12.9V (At idle)

Engine coolant temperature: 70 - 100°C (158 - 212°F)

Park/neutral position (PNP) switch: ON

Electric load switch: OFF
(Air conditioner, headlamp, rear window defogger)
On vehicles equipped with daytime light systems, if the parking brake is applied before the engine is started, the headlamp will not be illuminated.

Steering wheel: Neutral (Straight-ahead position)

Vehicle speed: Stopped

Transmission: Warmed-up
For A/T models with CONSULT-II, drive vehicle until “ATF TEMP SE 1” in “DATA MONITOR” mode of “A/T” system indicates less than 0.9V.


Without CONSULT-II
NOTE:

It is better to count the time accurately with a clock.

It is impossible to switch the diagnostic mode when an accelerator pedal position sensor circuit has a malfunction.
1. Perform EC-42, "Accelerator Pedal Released Position Learning" .
2. Perform EC-42, "Throttle Valve Closed Position Learning" .
3. Start engine and warm it up to normal operating temperature.
4. Check that all items listed under the topic PREPARATION (previously mentioned) are in good order.
5. Turn ignition switch OFF and wait at least 10 seconds.
6. Confirm that accelerator pedal is fully released, turn ignition switch ON and wait 3 seconds.
7. Repeat the following procedure quickly five times within 5 seconds.
a. Fully depress the accelerator pedal.
b. Fully release the accelerator pedal.
8. Wait 7 seconds, fully depress the accelerator pedal and keep it for approx. 20 seconds until the MIL stops blinking and turned ON.
9. Fully release the accelerator pedal within 3 seconds after the MIL turned ON.
10. Start engine and let it idle.
11. Wait 20 seconds.
12. Rev up the engine two or three times and make sure that idle speed and ignition timing are within the specifications.
13. If idle speed and ignition timing are not within the specification, Idle Air Volume Learning will not be carried out successfully

 

Good call! Simple first!

 

If you have an android device an a bluetooth OBDII scanner there is an application called NDSIII (nissan data scan III) that will be able to perform the idle air relearn without having to do the pedal dance.

 

I'm thinking it's a vacuum leak too. I took off intake and other pipes couple times when I replaced the valve cover gaskets and throttle body myself couple months ago. The PCV pipe snapped in half while changing valve cover gaskets so I also replaced that. What would be the best way to check for a leak?

 

Yes, you're right, I did replace the throttle body when I had gotten a code for it in my ECU. I have done all those procedures countless times after replacing the throttle body.

 

Thank you. I'll look into this. Maybe I have done the procedure wrong or something, hopefully this can fix it.

 

Video of what happens. Hopefully you can get an idea

 

Anything else? Sorry it's just that it's still happening and I'm tight on budget to take it to a mechanic

 

A vacuum leak could cause it but seems a little odd given the correct idle speed. Maybe try cleaning the MAF sensor and if your really careful clean the throttle body. You will need to do another idle air relearn if you cleaned them.

If you choose to clean the throttle body make sure the battery is disconnected. Try to discharge the ECU by pushing on the brake pedal and then waiting at least a few minutes. Then disconnect the throttle body. When cleaning don't move the throttle plate. This is overly cautious but it's easy to kill the throttle bodies on these cars.

If that doesn't fix it you can start inspecting the intake after the MAF sensor for cracks. you can also check all the vacuum lines. But the best method for checking for vacuum leaks would be to do a smoke test.

 

Edited

 

Sorry for bumping this. I was thinking of making a new thread but I decided it's proper to just keep this one going. I'm still having this issue sadly. I have cleaned the MAF sensor with MAF cleaner and cleaned the fairly new OEM throttle body with cleaner. As a desperate measure, I even tried using a vape to smoke it into the pipe that goes below the intake but I didn't notice any smoke coming out from any vacuum leak. I have made another video to show what I'm taking about. I'm very careful these days to not give extra gas since it suddenly jumps few RPMS, even when I'm braking.

 

Hey, to look for leaks around the plenum and throttle body, on a budget, I saw a video a while ago on Youtube, were the guy uses plastic film.
He uses a small piece (probably 4x4") going 360 degrees around the plenum, and actually found a leak based on that. An idea where you can rule the plenum and throttle body gaskets out (on a budget).