Unexplained surging

[davidrbroder62]

I have asked before and have not gotten any help with this. So I'll try again.
I have a 1997 Z3 1.9 manual trans.
When going downhill the engine will surge whether in gear or coasting.
I have no idea what's causing this.
It's dangerous as the car will increase revs speeding the car up when in gear. If in neutral the revs go up out of control unless I step very hard and very fast on the accelerator.
Any Ideas?
Here's an update. I noticed that the surging does NOT occur when the ASC is turned OFF.

 

[Zephyr]

Idle valve?
You need a code reader, otherwise it will be trial and error untill you sort this out.

 

[ChrisD]

Agree on the use of an OBD scanner.
On overrun, foot off the acc pedal, the engine should be in fuel cut off therefore there should be no chance of the engine increasing power. This feature is controlled by the throttle position sensor. Use a scanner to check for any faults on the TPS.

Also, mechanically check throttle cable and throttle actuator. You’ll be looking for the throttle ‘hanging up’ or not returning to the idle position. A scanner with live data will allow a passenger to monitor the throttle position during these events.

 

[Pingu]

Surging is usually caused by unregulated air getting into the mixture somehow.

 

[the Nefyn cat]

If a code reader doesn't find anything a smoke test may be next on the list. There's plenty of places air can get into these things.

 

[Duncodin]

I have asked before and have not gotten any help with this. So I'll try again.
I have a 1997 Z3 1.9 manual trans.
When going downhill the engine will surge whether in gear or coasting.
I have no idea what's causing this.
It's dangerous as the car will increase revs speeding the car up when in gear. If in neutral the revs go up out of control unless I step very hard and very fast on the accelerator.
Any Ideas?

What kind of hill? Steep? or slight slope?

If the car is parked, ie not moving, on a hill facing downhill with the engine running does the engine rev higher? Parked facing uphill the revs are normal?

 

[davidrbroder62]

If a code reader doesn't find anything a smoke test may be next on the list. There's plenty of places air can get into these things.

How does one do a "smoke test"?

 

[colb]

View: https://www.youtube.com/watch?v=EkaXD_IhPPE

I made my own smoke machine using a new 5itre paint can, resistance wiring some wire nuts and bolts and some plastic tubing. Loads of how to make these on Youtube.

 

[the Nefyn cat]

There's a lot to be said for google.

 

[davidrbroder62]

What kind of hill? Steep? or slight slope?

If the car is parked, ie not moving, on a hill facing downhill with the engine running does the engine rev higher? Parked facing uphill the revs are normal?

Both steep and slight slope. If the car is in gear or in neutral. It only seems to occur if the car is moving. It doesn't happen if the car is moving uphill.

 

[Duncodin]

Both steep and slight slope. If the car is in gear or in neutral. It only seems to occur if the car is moving. It doesn't happen if the car is moving uphill.

Happen in neutral - So car rolling down slight slope in neutral it will rev high. But does not happen if the car is stationary with engine running?

 

[Zephyr]

Can we have a photo of the engine compartment of your Z3?

 

[davidrbroder62]

Happen in neutral - So car rolling down slight slope in neutral it will rev high. But does not happen if the car is stationary with engine running?

Correct