Hey guys, I recently had the clutch replaced in my manual transmission car and I don’t know if I am overthinking this or what but I never noticed this until now.
Whenever I am about to stop in traffic, if I put the gear to neutral I can see the engine’s rpm hovering around 1080’s rpms, and it only drops to the “idle range” of 700 - 800 rpm if I stop the car completely. I did a test going downhill in neutral and I can see the engine rpms going up a little bit if I let the brakes go. I plugged my obd scanner and used OBD doctor on my phone to keep track of the rpm graph. Here is a pic… you can see the rpms going 1000’s steady and then dropping abruptly when I completely stop the vehicle.
So, is this normal? Thanks.
When you stop the car completely and the idle is in the 700-800 range, what happens if you take your foot off the brake pedal? Do the rpms rise back to the 1080 range?
Can you provide the year, make, model, and mileage on your car?
The year, make, model, and engine (if options exist) of your car is essential here.
It’s probably normal, the computer keeping the rpms elevated a bit between shifts to make gear changes smoother. That feature combined with synchros makes for a smooth experience.
Of course, if it’s a '29 Hupmobile I’ll have to rethink my answer. 
Yup, looks normal to me too. Some cars do this so you don’t clash the gears or overly slip the clutch when shifting. And that only needs to happen when you are rolling, not stopped. The car is doing FOR you what we old timers used to do with our right foot when downshifting.
Depending on your unknown vehicle and its electronics.
My 08 Expedition 6 speed auto does exactly that.
( I tried it as a test. My old trucks with no electronics do not. )
The computer knows your road speed and anticipates your putting it back in gear again.
Just think . .IF it allowed the RPMs to drop to idle and you dropped it in gear WITHOUT matching your previous rpm ?
Thanks for all the replies. The vehicle is a 2000 Mazda Protege ES Manual Transmission.
there’s been a lot of banter here about how little to no fuel is saved by the hypermiler’s who think that coasting in neutral is a big gain.
YOU have just hit the nail on the head about why not.
( another reason I tested mine , to see if WE actually knew a basis for our comments . . and sure enough )
Unless they have an older vehicle that does drop the rpm, they’re gaining nothing
Well I don’t coast in neutral… I only go to neutral when I am about to come to a stop, about 3 feet from the vehicle ahead of me because I feel it is easier for me to brake. And I started to notice the RPMs thing because I could hear the engine a bit even when I am in neutral, this is when I started to worry if it was normal or not. Thanks
It is easier for you to brake. You’re doing everything right and your Mazda is operating properly.
You can sleep soundly tonight.
There is a mechanical link between the brakes and the engine. The brake booster’s vacuum input comes from the engine intake manifold. And the brake booster takes a lot of vacuum to work, so applying the brakes could cause the engine rpm to drop in theory at least.
Have you tried idling in a parking lot, then stepping or not stepping on the brake pedal while you watch the engine rpms? Might provide a clue.
If the brake booster springs a leak in its diaphragm, that could allow unmetered air into the engine, which would increase the engine rpms. That’s not an uncommon problem. You could ask a shop to test for that.
There is a quick test you can do on the booster.
With the engine off, pump the brake pedal until it becomes hard.
Then, while pressing on the brake pedal, start the engine.
The brake pedal should become softer and drop a bit.
If it doesn’t the booster is not functioning properly. The most likely culprit is a leak in the diaphragm or the valve on the brake rod that controls wherein goes the vacuum from the engine.
The booster is basically a diaphragm with a valve inside a canister. When your brake pedal is unloaded, the vacuum is directed equally to both sides of the diaphragm. When you push the brake pedal, the attached rod slides forward and an attached valve directs all of the vacuum to the front of the diaphragm and vents the back of the diaphragm, The pressure difference on the diaphragm then assists you in applying the brakes.
I still maintain that your car is operating normally, but checking the brake booster is fast, easy, and (my favorite word) free, so why not?
I use a similar technique as OP when stopping my 2010 Kia Forte SX 6 speed M/T. If it is a stop sign I have transmission in 1st gear and clutch disengaged. If it is a known “forever” red light I have transmission in neutral. A safety tip. If on level ground I still keep brake pedal depressed to illuminate the brake lights. I have never noticed any idle change. The Kia has “throttle by wire” which I am not fond of. I experimented today. Normal idle speed is 700rpms indicated. When I stopped with transmission in neutral and foot off of clutch there was no change in idle speed. When I stopped with transmission in gear with clutch pedal fully depressed idle was 800rpms until fully stopped when idle returned to the normal 700 rpms. No wonder I haven’t noticed it in 6 years.
Does anyone know if the OP’s 2000 Mazda Protege had the smarts to raise the idle unless in a complete stop? I suspect it was too old to have it.
@Triathlor:
When you stop the car completely and the idle is in the 700-800 range, what happens if you take your foot off the brake pedal? Do the rpms rise back to the 1080 range?
Perfectly normal. Do this as an exaggerated test… Idle the vehicle in neutral in a parking lot… While stationary just sitting in the parking lot… Start pumping your brake pedal… watch the rpms dance…
@GeorgeSanJose basically summed it up for you…and he is spot on.
Blackbird
Elevated idle speed during deceleration is an emissions strategy to maintain clean exhaust output and prevent overheating of the catalytic converter. The PCM uses speed sensor input to determine when the vehicle has stopped , then return the idle speed to normal. This has nothing to do with the power brake booster.
Honda Blackbird: I tried that today with my Kia. Still no RPM change in neutral or was that a test for a bad brake booster?
I agree that rpms are elevated as an emissions strategy, but two things appear to be unknown at this point:
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Did Mazda employ that in their 2000 vehicles? Or did they begin doing it more recent years?
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The car is 17 years old. It’s only prudent to do simple tests to determine if the brake booster is not leaking.
It usually works @sgtrock21 you need to pump the brake pedal rapidly to affect an idle speed change. But you saying it didn’t work makes me think about todays engines superior ability to adjust their idle…very much unlike the olden days equipment.
Also now that you said that I need to try it in my GTi… I know it has a vacuum reservoir in addition to the brake booster…it also has a constantly adjusting and variable IAC valve AND…an electronic fly by wire throttle. So…maybe with all these nice computer controlled devices we wont see an idle fluctuation induced by vacuum loss from pumping the brake pedal. Certainly possible.
Older vehicles will surely show a fluctuation… Hmmm… Guess what I’m going to do right now?
Blackbird