low speed drivability (like creeping in traffic).

[PabloX]

Originally Posted by DanielDudley

Your throttle position sensor has two switches in it, one for idle, and one for Wide Open Throttle.

You want that adjusted so that the idle switch is only activated at idle. Once you crack the throttle open, you want that switch off, so the engine can make its own determination of what is happening via the other sensors.

I should have been more clear. I happens at 2000-3000rpm in 1st gear, not just right off idle.

Would what you say still apply in that case?

[petercory]

Mine was horrible when trolling through a parking lot at idle. Once I replaced the cracked vacuum lines, the bucking was history even with a 9i3 Octane SW Chip.

[brianlay]

I have the same car with a SW chip.
Mine behaved like yours until I recently adjusted the idle mixture. Apparently it was too rich

[Tippy]

Originally Posted by PabloX

It has a Steve Wong chip in it, and don't most aftermarket chips advance timing anyway?

Advanced for high RPM/high load. You can retard at low load/lower RPM where it bucks the most that everyone experiences.

Beauty of 3D ignition mapping!

[MrBonus]

Originally Posted by kuba_kuba

Interesting - increased bucking at low speed is the only negative from installing a Steve Wong chip on my '88 (and its not that bad)

I have an SW chip on an otherwise stock '89 and I've never experienced any bucking at low or any speed.

[stlrj]

Originally Posted by PabloX

I should have been more clear. I happens at 2000-3000rpm in 1st gear, not just right off idle.

Would what you say still apply in that case?

Seems like a lean surge issue.

If it were my car, I would get into the the AFM and fatten up the mixture.

[Drisump]

Seems abnormal for the RPM range you describe. My 3.2 (stock euro) is OK around town in heavy traffic....the cable clutch is not as refined as a modern car however. My 993 has a light weight flywheel and clutch though, and in certain circumstances it pulses and lightly bucks at low RPM. You have a stock flywheel do you? Cheers

[RetroSC]

Worn tracks on the AFM wiper surface will cause this exact condition. It's fairly easy to modify how the wiper arm sweeps across the track and move it slightly to a fresh spot.

[Hnichols]

Does it go away when the engine is warm?

Does it buck in second gear as well, or only first?

(Mine occasionally bucks between 2-2.5K, but always goes away when the temp gauge needle comes off its peg.)

['76 911S 3.0]

I would look for a vacuum leak first. "Bucking" is usually an indication of a lean condition or not enough ignition advance. Typically, the leaner the AFR, the more ignition advance you want (to a degree - no pun intended). Since these engines are distributed and if your base ignition timing is set correctly, most certainly the problem is a lean condition caused by vacuum leaks.

I'm not well versed on the AFM in these motronic systems, but as others have said, you may have a dead wiper spot at that particular air flow.

[sugarwood]

I don't understand the situation.
Can you post a video dash cam?

In my '86, I can drive slowly with no bucking.
It's a hassle, just as in any manual car.

Once you are going too slowly, you need to push the clutch in.
That's just how clutches work.
When I let off the clutch, it engages, and rolls, but does not buck.

My car is identical to any other stick shift car in traffic.

[jlex]

Judicious use of the clutch controls my low speed bucking. Not to be confused with bucking at speed or when under acceleration. Honestly, I think it's just the characteristic of a car that is meant to be driven at higher speeds rather than trolling around a parking lot.

[Quicksilver]

The car doesn't start "bucking" at over 1000 RPM unless something is wrong. The joy of electronic engine management systems is they can compensate for so many issues that you often can't tell that something is wrong until it is really wrong.
Plus when you add to this the truth that full throttle operation is so basic that you can have damn near everything wrong and it will still run right with the pedal mashed. This means that you can convince yourself there isn't an issue.

Problems that can cause this are kind of a list of a major portion of the management system. Big things I would check are:
- Vacuum leaks. Every chunk of rubber in the engine compartment is suspect and you either need to use smoke, pressure, visual inspection, or a combination of all of them. Plus the intake gaskets are a common failure. (I was surprised to find that the main breather hose isn't supposed to be hard but was actually extremely soft and pliable! All the rubber is probably toast.)
- The ICV is a failure point and there are plenty of threads on cleaning them or diagnosing them.
- The mixture is often out of adjustment because with the self-correcting nature of the DME you just can't tell. Go through the adjustment procedure.
- The Air Flow Meter has a "wiper track" that is a wear item that can cause issues especially in the air flow ranges where the engine has seen the most use. There are good How-Tos on checking and correcting these issues.
- Because it is all electronic, wiring is extremely important. Check your grounds (especially on the driver's side intake manifold) and check that the engine harness. Also check that the injector harness's connection at the front of the engine compartment is firm.
- If there is a lot of flex in the drive system this can cause some limited bucking just because it has a bouncing oscillation and that can even make your foot oscillate/bounce on the gas pedal. With the car off and in 1st gear look at the engine as you rock the car forward and aft hard to see if the engine is moving.

[T77911S]

Originally Posted by jlex

Judicious use of the clutch controls my low speed bucking. Not to be confused with bucking at speed or when under acceleration. Honestly, I think it's just the characteristic of a car that is meant to be driven at higher speeds rather than trolling around a parking lot.

no its not.
I can take my 930 down to slow speeds, and that is with a 1st gear that I can do 60mph in.
now if you are trying to drive it at 800rpm that's different.

[Tippy]

Originally Posted by T77911S

no its not.
I can take my 930 down to slow speeds, and that is with a 1st gear that I can do 60mph in.
now if you are trying to drive it at 800rpm that's different.

Well yeah, a 930 1st gear is tall as a V8 car. Of course it won't buck.

A G50, 915, or 901 would easily buck due to super low gear ratios. Drive a heavy duty truck, and they buck like crazy in lower gears.

[rick-l]

I'd vote for checking the AFM also.
Does the idle hunt for a minute at cold startup also?

[PabloX]

Originally Posted by rick-l

I'd vote for checking the AFM also.
Does the idle hunt for a minute at cold startup also?

It does. Good call.

Also, thanks Quicksilver. Great list.

[rick-l]

It might be out of whack due to injectors flowing different amounts