Iv,e done a search on the above but can,t find a definitive answer, plenty of info on S4,s but not my particular model. The problem Iv,e got is when the car is at say between 1300/2000 RPM,s level road and going with the flow of traffic the transmission is shunting back and forth, not always but fairly frequent ( manual box ) the kind of feeling you get when your in the wrong gear ( I,m not ) .If you slip the clutch a little you can overcome it, or boot it ..,not ideal though in a restricted area.
Iv,e replaced all vacuum lines , new Beru wires, all earths ,and fuse board refurb ie took all fuses out, cleaned their contacts replaced with new one,s, I have,nt removed the relays as of yet.
Spark plugs are about five years old, but have,nt done real mileage. I think the system is K-jet ? could it be the Dist cap/rotor or Ign coil maybe ? The car idles just fine.

Any pointers would be really good.

Ken
80 928 S
5 Speed

UK

 

Mine does this too. I think it's due to the driveline lash getting worse over time.

 

Not sure but sounds like fuel to me, but i am just learning.

 

drop down 2 gears .... NOW what does it do.


It sounds like you lugging the engine or running in to hi a gear

 

..., basically when your in a 40mph zone in fourth she,ll be fine, then momentarilly for no reason I can see, she,ll start and jerk back and forth, drop a couple gears and hard on throttle, it clears, then settles back to a steady speed. Then it,s ok for a while and resumes it,s shunting effect. I,ve experienced this problem in the past were it seems to do this then it disappears for a while, now it,s back again.
Another situation is approaching a stop sign, she,ll start to shunt, and even in the correct gear for the speed your doing.., dropping down to second does,nt help, untill you stop, and floor it pulling away, then the whole scenario resumes again.

Ken
80 928 S
5 Speed

UK

 

Just my $.02. Check the adjustment on the throttle body micro switches.. The lower switch (idle) could be kicking in too soon, or the upper (WOT) could be hanging on in mid range.

 

My CIS S has a sticky throttle somewhere in the system. It makes smooth transitions at low load/no load levels very difficult. I have lubed the pedal, replaced the cable, and lubed the butterfly. They all seem to operate smoothly individually, but as a whole they are a bit sticky.

Don't know if this is what you are describing, though.

To lube the inside of butterfly plate's bushings, squirt some oil in the vacuum line that enters the throttlebody on the midline of the car (into the upper port) and let it soak for a while. It will slowly flow along the plate's axle and capillary into the bushings from inside.

 

.., thank,s for that, but mine does,nt have any micro switches for idle etc ? It,s a K-jet inj system

Ken
80 928 S
5 Speed

UK

 

.., thank,s for that tip Erik, I oiled the linkages last time I experienced the shunt, but did,nt have much effect. I,ll try your tip regarding the butterfly plate though, as it,s never been lubed before, did,nt even know you could access it the way you describe.

Ken
80 928 S
5 Speed

UK

 

I'd look at driveability issues. New plugs and injectors. Not that it's strongly pulsing from the engine. That it's smoothly pulsing and sets up the vibration. What's happening is the narrow drive shaft is twisting and unwinding.

I'd also jack up the rear and turn the wheels. Some slop from the U-joints is expected but listen and feel for any looseness in the driveline couplers or clutch. Better to inspect directly, but that's more work.

 

Well, "access" would be an overstatement. Just do a little at a time, as the oil needs to flow sideways a bit before even contacting the bushings.

 

40 mph in 4th gear isnt the best, as this is lugging the engine
better to be in 3rd or 2nd

NOTE that at low RPM the oil pressure will also be low and with the trans in a high gear the loading on the drive line is a lot higher than it would be with a higher RPM,
also with the lower oil pressure , you could do damage to the bearings in the engine.

Solution drop down a gear run your engine from 2K on up and it should be fine

 

I started to notice a similar phenomenon in my last few drives of the year. I had the clutch slave cylinder off to do the TB job. I didn't have this issue before. I was driving as before. Could air in the clutch system somehow cause this kind of symptom?

 

Fuel PSI or Volume issues?

 

The only thing I did differently fuel-wise was put Stabil in for the first time ever.