Engine moves around a lot on ignition switch off

[marcusa]

When I switch off the engine in my roadster, the engine moves around quite a lot for about 1 sec after ignition switch off. I have just replaced the front centre mount. Any ideas?

[Thrumbleux]

Just a trait of inline 3 cylinder engines (rocking couple). Check your rear mounts if you think the movement excessive or there's noise.

[tolsen]

You mean engine run on?

Suggest you read below article on Wikipedia:

https://en.m.wikipedia.org/wiki/Dieseling

[marcusa]

Hmmm......it certainly didn't used to do that. Is it possible that because I have replaced front mount it now may have caused rear mounts to break (they looked OK on inspection before front mount change)

[Thrumbleux]

The rear mounts may have been damaged during fitting the front mount if the front of the engine was moved significantly up or down straining the rears. The rears would have to be near the end of their life for that I'd have thought. But, the front was shot, so possibly.
If Tolsen's scenario is correct, it will sound as though the engine is still trying to run. Otherwise it's just what triples do - with possibly more lee-way if the rear mounts are goosed. Easily accessed to check...

[tolsen]

Is this "moves around a lot" thing engine shaking or engine running on (rotating) for a second or two?

[marcusa]

I would say its rotating for a second or two, so if I am looking at the engine from the bay when the ignition is being switched off I can visibly see the engine shake to a stop

[Thrumbleux]

It can only 'run on' when hot. Try starting it from cold, waiting only long enough for the idle to stabilise to low, then switch off. If it still jumps around then it isn't dieseling causing it. If however it stops without jumping around.... then from hot pull the fuse that serves the fuel pump while the engine is running. The fuel pressure will decay so rapidly running on will be impossible and the oil will be thin enough not to mask the triple's natural tendency to rock on shut down. If it jumps then - it's what it does. Only the rear mounts can be making it worse.

[marcusa]

OK tried this and it still jumps around, checked all mounts and they are fine...

[10:45PM]

When I switch off the engine in my roadster, the engine moves around quite a lot for about 1 sec after ignition switch off. I have just replaced the front centre mount. Any ideas?

I reckon you're just noticing it because you've just done that mount? ….

There's a simple cure for this , put the engine lid back on and stop looking.

[Thrumbleux]

….

There's a simple cure for this , put the engine lid back on and stop looking.

But, if the desire to look persists..... check how someone else's smart behaves on shut down as comparison.