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RPM reving up/down while slowing down

I have a 2006 Scion Tc, its a manual and 2.4L engine. It has about 75,000 miles on it and everything in it is stock. I have recently changed my ac compressor due to the old one went out. It was properly drained and recharged after I replaced it with a brand new one. It has been working great since, but I noticed in the past couple days that if im driving and I put the car in neutral while still moving (to save gas) my RPM’s will shoot down (like normal) but then it will start bouncing up/down until I come to a complete stop. Once im a complete stop the RPM will stabilize again like if there was no problem. Take in mind that in im getting of the highway coming to a stop light I will put the car in neutral and begin to slowly brake, the clutch is not engaged nor im I stepping on the gas. Why would my RPM’s go up and down. also the AC was not on and the RPM will not do this while driving. Any ideas would be great. Thank you.

check your coolant level.

Coolant level is normal.

I once saw a Honda with a stuck thermostat do that.
Also get the coolant temp sensor tested. It could be out of calibration but not cause a check engine light.

Im not sure how my thesmostat would affect my RPMs but I will get it checked.

The coolant temp (actual or as read by PCM) is important to how the PCM figures out the fuel mix. That’s a reason to check out the coolant temp sensor/thermostat.

Shifting to neutral when coming to a stop isn’t ever going to save you any measurable amount of gasoline. All it does is leave you with no way to accelerate the car if you suddenly needed to do so.

If you just sit at smooth idle and mash the brake pedal firmly, what happens?

nothing, it stays at 750+ RPM, when the AC is on its about 850+.

Could it be a lose/leaking Vaccum hose?

Im sorry I take that back, I just tried it again and everytime I hit the brakes the RPMs go up a little bit, once i let go they go up higher. I dont think it had every done that before.

The power brake booster uses engine vacuum.
It’s normal for the booster to affect the idle slightly and momentarily when you work the brakes.
Did the idle change as much as when it acts up?

it only changed a little bit, so no. I tried something else and maybe this will be a better description. If im going on the highway 35-60 mph and I go into neutral (without going to another gear) and cruise, my RPM gauge will go down to 800+ (when i first put it in neutral) and then it will start shooting up to (1500+) and go back down to (800+) consistently until I do something. If I brake and stop completly and it will stabilize or I can jump into a gear and it works normal.

I’m sticking with low coolant, air in the coolant, or coolant temp sensors then. Hondas do exactly that when they’re either low on coolant or the sensors aren’t working properly.

The reason is because when the sensor stops reporting or the temp sensor is exposed to air instead of coolant due to lack of coolant or a big air bubble, the ECU gets the report that the engine is not at operating temperature, so it goes into the cold engine mode, which uses more gas and makes the engine rev higher. Then the sensor starts working, or coolant covers the sensor again, and the ECU is told that the engine is now at normal operating temperature, and so it goes back to normal, which revs the engine down. Get enough air in the coolant or a flaky enough sensor and you sit there bouncing back and forth between warmup and normal.

How could i check if there is an air bubble, would i have to drain my coolant fluid? and would it show a code for that? Im having diagnosed today to check for codes.

Most cars…Park on a hill with the nose pointing up…then open the radiator cap and let it sit for an hour…Any air in the system usually makes it way to the top of the radiator.

All you guys are awesome, im going to check on the codes and also try to let the air bubbles out. Thank you for all your help so far. Hopefully it will take care of it.

Hey, did you get that fixed? What was the problem? My car is also a 2006 scion tc and it’s doing the same thing.