AndyK

This is a strange one, I think. Normally, when you are going 50mph in 3rd gear, and you lift your foot off the throttle, and let the engine slow down the car a bit before speeding up again--there should be no "slipping" sensation, right? I find that when I let off the gas, the engine compression doesn't slow the car evenly--it kind of jerks back and forth (very slightly). I thought it was the clutch slipping - before I had the clutch replaced. It still happens now.

Anyone have any idea what could be causing this?

Does anyone know what I'm talking about??

Izzy

My car has the same problem. I've heard it could be either a computer problem, or a problem with the fuel cutting off completely as you ease off the gas. There was a site I saw with the fix for the fuel cutting off, it involved disabling the cut-off mechanism on the throttle? But I might be wrong...

AndyK

Hmmm...fuel cut off. Sounds like it could be that. Or the computer that controlls the fuel on/off? It never shuts off completely. More like an on/off/on situation...

Yabo

I thought the fuel cutoff problem was only on early cars, and was solved by chips like the FRWilke one?

Izzy

When you let off the gas, the fuel cuts off and the car drops to around 700rpm, then the gas kicks back in as the car idles around 1000rpm. This is the jerking feeling you're getting. Again, I could be wrong.

AndyK

I watched the Tach last time...no reduction in revs during the "slipping".

What would cause the fuel to cut off, or, slow down, when I take my foot off the gas to decellerate while still in gear??

Yabo

Read this up.. this is about the fuel cutoff....

http://frwilk.com/944dme/shudder.htm

AndyK

Mine's an '89...and my idle is fine.

I find the car actually speeds up, when the engine is supposed to be slowing the car down! So it starts slowing down, then feels like the car shifts to neutral a second and stops slowing down. Then the engine grabs again, and the car slows again.

Izzy

The chances of it being the DME are slim, I just read that problems with the 'shuddering' only occured in 83-85 944s. The shuddering is probably happening due to overrun fuel cut-off. I really don't know how it could be fixed, but the problem could lie in the throttle if not the DME.

Izzy

The point of the fuel cutting off is to basically save fuel, the car figures you're slowing to a stop and you won't need the rpms from the engine as you're coasting.

Sorry didn't read your last post until now. It could still be a computer problem, since you said you're getting some throttle when you're slowing down.

Danno

The issue with the fuel-cuttoff and shudder was that the RPM-point where the injectors are turned back on is too low. Wilk's programming was to raise this RPM-point . The '85.5+ chip fixed this problem with the factory programming.

"What would cause the fuel to cut off, or, slow down, when I take my foot off the gas to decellerate while still in gear??"

It's the TPS that does this. There's a switch that's turned on when the throttle is fully released. What happens sometimes is this switch fails or it's adjusted incorrectly and the closed-throttle trigger is never activated and the fuel is not cut-off. This is not necessarily a bad thing, but what happens is the computer never cuts off the fuel. It thinks you're still running partial-throttle and it will adjust air-fuel mixtures based upon the AFM-signal and O2-signal.

These causes two events that creates the stumbling and bucking that you're getting. First is pressure-pulses traveling up and down the intake tract with the throttle closed. If you hook up a voltmeter to the AFM, you'll see that under closed-throttle coasting, there will actually be rhytmic pulsations that's picked up by the flapper door, you'll see voltage spikes at regular intervals. If the DME doesn't cut off the fuel on coasting, it will respond to these voltage-spikes and inject more fuel because it thinks air-flow is increasing; it thinks you're stabbing the gas or something. However, the real air-flow is constant and is dictated by the closed throttle-plate, and the idle-stabilizer valve. Adding extra fuel from those erroneous voltage-spikes will cause a stumble.

The other issue is the O2-sensor. It will cause the DME to do a sine-wave type of air-fuel ratio correction centered around 14.7:1 at idle and partial-throttle. However, under coasting and minimal air-flow, this dithering action will result in variable power-output as well, leading to the variable speeds and shudder from the engine. Again, the solution is to cut off the fuel on coasting.

So... check your TPS and step in the clutch when you're coasting.

Z-man

Before reading Danno's post, I was thinking TPS as well...
-Z.

AndyK

Is the TPS the triangle unit on the inside left wheel well? Or is that something else?

Thanks for the knowledge Danno!! Sounds like my problem exactly!

PS-- Is it BAD for the engine / clutch to coast while in gear?? I would hate to step on the clutch every time I slow down from 55-50, or 45-40, then speed up again.

Campeck

why not the clutch?

my car jerks real bad when i let off or get on the gas in an gear.

your clutch could be going out.

Porsche-O-Phile

What Danno said. The early car problem is highly overstated. My '85/1 doesn't do it at all.