RPM drop while shifting

makaio · 12-27-2005, 06:21 PM

I thought I may have seen this posted here before, but the search function turned up no results.

Car is 86 911 3.2 97,xxx miles.

My problem is when I press in the clutch pedal to upshift or down shift while driving, my rpm's drop rapidly. Not to where the engine stalls, but back down to idle. The car idles rock steady from stone cold to blazing hot, this is the only hiccup the car has. I just put 400 miles on her last week and it performed as good as the day I brought her home, except for this annoying idle issue. I haven't had the time lately to look through the Bently for trouble shooting help, but I plan to hopefully in the next week.

Any thoughts or helpfull insight?

jtratza · 12-27-2005, 06:32 PM

I don't see the problem as long as it doesn't die. Many years ago I had a C modified autoX car with a very light flywheel, lightened crank etc. It would rev quickly and drop off to idle like you are describing. That's what it was supposed to do I thought. Maybe you are comparing it to the BMW.

masraum · 12-27-2005, 07:14 PM

Yeah, I think it should rev up and down quickly. That's one of the things that I loved about my 3.2.

My miata needs some lightening so it'll rev up and down as quickly.

450knotOffice · 450knotOffice's Garage

Are you sure you don't have a lightweight flywheel? That's what they do.

makaio · 08:11 PM

I'm positive it doesn't have a lightened flywheel, because a new OE one was installed less than 2,000 miles ago when I had the clutch replaced. I had a lightened flywheel in my old 78 track car and it wasn't as noticeable as this.

Maybe the car is just mad since I haven't driven it much this last year.

wholberg · wholberg's Garage

Does it get better after the trannie is warm? It could be caused by excessively high viscosity transmission oil, i.e., if you had 85W-140 oil in the trannie rather than 75W-90, and the temp was cold.

Having said that, I suspect that is very unlikely. My 3.2 will drop to about 2000, rpm from a shift at 6000 rpm, in about 1 second.

911scNorway · 12-28-2005, 02:19 AM

My first post so here we go:-)
If you have one, check the deacceleration valve vacum line or the deaceleration valve itself, if it is disconnected then, when you lift off the gas the butterfly throttle valve closes and your not getting enough air the cylinders , result a rapid change in rpm downwards, the deacceleration valve should open at about 18" Hg vakum ( if my memory is not wrong ) it´s function is to open and bypass air over the throttle butterfly valve to stop rapid changes in rpm when one changes gear and keep the afr correct .

Mark. A Englishman in Norway

svandamme · svandamme's Garage

i don't get it , the rpm should drop since you're going to a lower gear
that is , you don't have your throttle pedal mashed down when you clutch in , so the engine rpm should go down regardless.

flywheel will keep some momentum up , but it won't stop your engine from slowing down if the throttle is out, it's only supposed to give the engine enough momentum to idle, and make it a bit easier to drive...

you mention it goes down when you up or down shift
how fast is that? typical upshift clutch in/out is just a quick tap on the pedal, we're not talking seconds here , downshift maybe a tad slower, but still not seconds...

does your engine rpm go down that fast?? <1/2 sec? <1sec?
if it does, then something is effectively braking your engine
because it can't decellerate that fast on it's own , not even if the intake is fully shut , not without a lightened flywheel

how long does that engine take to rev down from say 6000 RPM if your not moving , and the tranny is in neutral?

how long does it take , still standing still, but this time with the clutch fully in ?

maybe that clutch does not work properly , and maybe it doesn't fully engage, enough to be drivable.. not enough for the engine to spin freely...

na2ub · 12-28-2005, 06:31 AM

I second the deceleration valve. It is designed to slow down the decrease in RPMs during shifts. I have heard that some people even disable it because it removes RPM control from the driver.

Doug

kjb · kjb's Garage

I have an 86 as well. The RPMs drop quickly, as you describe, but idle is rock solid.

I have attributed it to the injectors shutting off when you get your foot off the gas. They come back on below a certain RPM to keep the engine from stalling.

/ Johan

masraum · 12-28-2005, 08:00 AM

Quote:

Originally posted by kjb
I have an 86 as well. The RPMs drop quickly, as you describe, but idle is rock solid.

I have attributed it to the injectors shutting off when you get your foot off the gas. They come back on below a certain RPM to keep the engine from stalling.

/ Johan

I think the RPM is 1800.