s2racer

the damnedest thing happened to me yesterday.....my '96 993 had been parked for a couple of hours, and when i started off from a stop sign, the car started bucking like a bronco. this is the first time it's happened since i've owned the car ('98). anyone have an idea what this is all about?

tia

Ray Calvo

This shortly/right after you started it up? In FLA, so car was super hot? Thoughts:

a) Might be vapor lock; switch to some decent gas

b) Might be ISV (idle stabilization valve); clean it - if it's still acting up, might replace it

s2racer

no, it wasn't hot at all (a very pleasant 87), and i use shell 93 (exclusively-my contract requires it ;-))....my initial thoughts are with the clutch, as when slipping it to launch, is when the bucking started. i was wondering if this could be a sign of deterioration in the clutch.

tom_993

Could be the notorious ECU low speed fuel cut off issue.

Here’s the description. It’s basically a fuel economy and emissions issue. When you’re coasting, the ECU cuts fuel. There’s no reason to supply fuel to an engine that’s coasting down hill. The ECU cuts fuel when the clutch is engaged and there is no throttle. It only does this above a certain RPM. Below that RPM cutoff, it starts putting fuel in. When you’re at speed, this is a good thing. At low speeds, this can cause the bucking you mention. Picture this: You’re in first gear, above the fuel cutoff RPM. You lift, cutting throttle. The ECU cuts fuel. This results in massive compression braking, and you lurch forward in your seat. As soon as you touch the throttle again or the RPMs drop below the limit, the ECU adds fuel and the compression braking is replaced with torque. Now the car lurches forward, and you get shoved back in your seat.

Does this seem to fit with the bucking you experienced?

’95 993
Tom

s2racer

very interesting....i've never heard of that. what you descibe is similar to happened, and when i stopped trying to apply throttle, the car smoothed out and i was able to accelerate normally. do you think because i was accelerating somewhat aggressively, the engine management wasn't keeping up?

tom_993

Actually, accelerating aggressively doesn’t cause the problem. It’s minor on and off the throttle that triggers it. One of the worst cases is traffic that’s flowing right at the RPM cutoff speed in first gear. You make minor adjustments to the throttle, and that causes fuel to be cut off or turned on.

Sometimes you get stuck in this bucking mode and the only way out is to either punch the gas or press the clutch in.

Tom
’95 993

s2racer

what a drag!...any way to fix this?

tom_993

There are things to do to minimize it, but nothing I’m aware of is a complete fix.

You can adjust the ISV for a higher idle. This way, when you lift and the ECU cuts fuel, the compression braking is not as strong. Also, make sure you keep your ISV clean.

A lightweight flywheel can actually make it a bit worse, as it has less inertia.

Some aftermarket chips or ECU’s help a little bit.

Replacing worn motor mounts can also help. You might want to check yours.

Be glad you don’t have a ’95. They are much worse in this regard than the OBD II cars.

Tom
’95 993

s2racer

thanks for the info.