So I've noticed this for awhile but never really asked about it because it happened so seldom, but now I'm interested if anyone knows what this is, and if it's normal.

When I'm in first gear going down a hill with my foot OFF the gas (effectively letting the engine keep me going slower), my RPMs will start rising until they get to about 2600 and then the car will jolt and drop down to about 1900-2000 before they start rising back up. Obviously the incline of the hill is going to matter, because some hills aren't steep enough to even get you up to 2600. My description sounds kinda stupid when i reread it, but there isn't really a better way to describe it. Does anybody know what I'm talking about?

 

Does it feel like jerking in a manual transmission?

 

Could be the transmission's hill descent control to keep you from traveling too fast down a steep hill without having to ride your brakes.

 

My 2008 does the exact same thing going down a couple of the hills near my house. I drive with a bit of a lead foot so I figured that it wanted to keep it in the lower gear incase i needed some quick acceleration. The only way to fix this problem I have found is to shift into manual mode and then move it back to automatic. It then realizes that it is in too low of a gear and then shifts up. If it is only going to 2600 RPM you should be fine, these engines can take it.

 

Sorry, I should have specified, I drive a 6MT.

It does feel somewhat like jerking in a manual transmission, but it is just one lurch that brings the car from 2600 down, not the judder like I'm about to stall.

Hill descent control? Do we have that on our cars? If so, that is almost certainly what it is, I've just never seen that. The mechanics of what is happening is definitely to stop you from riding your brakes, I just find it interesting that i only happens in 1st gear (although i could see the argument both ways).

 

I don't think the manual transmissions have "hill decent control"

If you are going down a hill with your foot off the gas, the engine RPM should NOT change without you doing something. Does your clutch slip when under full throttle in the upper gears?

 

When it 'jolts' what happens to the vehicle speed? Is it the same as it was before at 1900 rpm?

 

No slippage on the clutch at all. I actually just got a new everything under there (Slave, CSC, Clutch, Flywheel)

So I'm just going to walk through this again and maybe be more descriptive... hopefully not more confusing...

My grandfather's driveway is somewhat steep (not super steep) and a little lengthy. So rather than ride my breaks in neutral or 2nd gear while going down the driveway, i try to stay in 1st gear. As i go down the hill with my foot off the gas the car slowly picks up speed based on the downgrade. Once the car gets to about 2600 RPMs at whatever speed, there is a quick jolt (not super violent or anything) and the car will slow down to about 1900 RPMs and the speed will decrease accordingly. Once at that point, the car will begin slowly gaining speed again and the process repeats.

 

The only thing I can think of is it's some sort of fuel or ignition cut kicking in to limit rpm because the throttle is closed or has stayed closed in excess of a certain amount of time.

Can't test this on my own as we have no hills here, at least none where the car would get up to 2600 rpm in 1st.

 

I'm not sure if the manuals have it, or even if the autos do, but FWIW I have a 7AT and when I went down a steep hill once, the RPMs kicked up to around 3000 I'm assuming to keep me from rolling down too fast.

 

I don't think it is a fuel cut issue, since the fuel is normally cut when we are moving in gear and lift off the gas.

Sounds like the brakes are being engaged briefly to slow the car. Over the next few days I will try to duplicate the issue. Does it make a simulate sound when the stability control kicks in?

 

^^That was my next guess but can't see what would trigger stability/traction control in this scenario.

 

I'm almost positive it is not the brakes engaging as it is clearly the engine taking the load, much like when you down shift. Actually... that's a great way to describe the feeling, the lurch/jerk kinda feels like you downshifted and didnt match the revs very well.

I also dont think it is traction control kicking in because the slip light isnt coming on, and also, I dont really see a reason why that would happen. I wouldnt know the feeling because most of the time my traction control comes into play, I'm either gassing it pretty hard, or I'm in the snow... neither of which apply here.

 

I have a 2009 G37S and a 2014 Q60 IPL, both auto trans, always drive them in manual. I definitely feel what OP is describing, and it's normal. Try turning on the cruise control on a downhill freeway; the speed stays the same, even on a very steep downhill; while I'm in the 7th gear in manual, I feel the engine jerking back subtly, which is how it manages to hold even 65 MPH on a steep downhill. The monitor still shows 7th manual. My lexus rx 350 did not have this feature and just sped up on the same downhill freeway. That's an awesome feature, whatever it's called!

 

^^I think you missed the point.. The OP has a true manual, not an auto manual . Driving an auto in manual mode is not the same thing as driving a manual manual. Well, unless it's a DCT without a torque converter.

The jerking felt is the transition in the driveline from aceleration to deceleration as the computer tries to maintain set speed.

Might be clutch slippage, but that can't be possible with a new clutch.

I wonder if it's somehow temperature related. To the op, have you tried this when the car is at full operating temperature vs from a cold start?