gtroth

I don't drive in a lot of stop and go traffic, but I have been the last couple of days for some reason, and it seems like sometimes when I let up on the throttle in 1st, the car "engine brakes" rather sharply, maybe as if fuel is being cut off too abruptly. It doesn't bother me that much frankly, but it jsut doesn't seem quite right. Any ideas?

Operator

dont all cars do this? especially at lower rpms?
then again hows your rubber centered clutch?

gtroth

Well, yes, all cars do this I guess, but the abruptness I'm experiencing is a tad abnormal. Again, as though the fuel isn't tapered off but just cut.

Spring centered clutch has ~40k miles on it.

GlenL

Mine does this as well. My S&G driving gives plenty of time to experiment with and be aggrivated by it. Doesn't seem to be related to the TPS actuation, which was my first guess.

Feels like the there's some slop in the driveline. I've got the rubber clutch (I believe) but it behaves itself ortherwise. I'm starting to suspect excess slop in the tranny, differential or CV joints. Maybe all three?

special tool

How abrupt?
Aree you sure that the e-brake is not sticking?
Maybe you should see Rage2's thread from a few days ago about what immediately preceeded his engine failure frrom broken rod........................

gtroth

Doesn't really feel like drive train to me. Axles are brand new too, so no slop there. Couldn't find Rage2's broken rod thread - probably better off not reading that, anyway.

GlenL - I think it does happen before the TPS actuates. My current best guess is that I found a non-linear spot on my AFM wiper, unless there's another way for the DME/electronics to decide much less fuel is quickly needed. But who knows. It doesn't bother me that much - I'm mostly interested in case it may be related in some way to my other pesky problem (severe hesitation / cut-outs when the car is wamr and it's very cold out...but that's another thread).

Many thanks, guys.

GlenL

I've recently put in a rebuilt AFM so I've discounted that. (Danger?)

I don't notice any engine cut-out. It just feels like there a split second of float before it grabs hard with engine braking.

Used to really embrassing before replacing from shocks and getting stiffer springs. Bet other drivers thought "What the hell's wrong with that Porsh?"

APKhaos

Check the throttle position switch. It should be set to click just as the throttle plate is fully closed. The switch signals fuel shut down to idle state.

Also check that the replacement AFM is compatible with your model year.

apierce918

i dont think its anything to be worried about, anything over 3k rpms letting off the throttle it is going to be an agressive jerk

GlenL

This is low RPM. Like 3 MPH in first (1200RPM?)

gtroth

Yeah, GlenL, sounds like you know exactly what I'm talking about. Only annoying really when creeping along in stop and go traffic. As far as I know, the cut-outs are completely unrelated.

TPS is new and perfectly adjusted (click and signal at DME connector are just right).

Peckster

Sounds to me like a bad on-off gas transition, which is usually related to slop in the drive train or suspension.

Mine started getting really bad in slow traffic when just crawling along, really jerky.

Replacing some rubber pads up high in the rear suspension fixed it, the old ones were worn and flattened out, allowing too much slop.

jpk

I'm pretty sure this is normal for the NA cars. I think this is par for the BOSCH DME system. bottom line is that at low speeds/small airflow/small throttle opening; the system is not very responsive.
If you're running in traffic, say at 10 mph in first (maybee 2500 rpm) under light load, the throttle opening isn't much above idle. The AFM is also barely open at this point. When you lift off the throttle, there isn't much of a signal change to the DME, but one of the things it does is cut fuel as soon as it sees a throttle closed signal (from the throttle position switch). Fuel is supposed to come back on at 1000 rpm or so, but in reality, the idle will drop below 500-600 rpm before the system responds. You should see this if you rev it in nuetral the let off - the speed will drop below normal idle, them come back up and stabilize.
F.R. Wilk has some good write ups about this on his site (look for the "shudder" topics). He suggests a pretty cheap/simple/revesible fix by disabling the throttle position switch so you never send the DME a throttle closed signal, hence it doesn't cut the fuel off on overrun. The only drawback is emissions.

A rubber clutch center failure will be a pretty solid clunk that happens whenever power is applied or removed; so you'd hear it not only when you let off the gas, but when you got back on it as well.

gtroth

Hey - you know what fixed this? O2 sensor. Comes off throttle at all times much more gracefully now. I didn't expect this - guess she really likes running closed-loop. Cheers.

sharky47

Mine does this - if you are more careful with the application of throttle, you can avoid it.