Caterham Roadsport Hot Idle Fix Throttle Body

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At idle the revs would usually wander up and down, slowly and unevenly, perhaps only by ~100rpm, but just enough to become rather annoying sitting in traffic or at a junction.

To remove the air filter (Sigma 125 engine), just one jubilee clip needs loosening. Then it needs quite a good amount of straight pull, on mine it was just enough to move the engine on its mounts slightly (i.e. enough to make you think something else must be holding it on).

This is the state of my butterfly, car has done ~5300 miles.


The top left opening is the crankcase (cylinder blow-by) vent hose inlet, top is some temp? sensor.

Caterham-throttle-body-butterfly-closed.JPG

Caterham-throttle-body-butterfly-fully-open.JPG

Caterham-throttle-body-butterfly-open-closeup.JPG


The only thing of note was when the butterfly was shut, there was enough oil at the buttom to join up with the butterfly itself forming a bit of a seal. There wasn't really much crud built up.

I cleaned it all out (can easily move the butterfly by manipulating the throttle cable mechanism) with just normal degreasant and kitchen role. (I wedged throttle mechanism fully open with a bit of wood between it and the ecu).

My only tip is to wipe away as much as possible without the degreasant so crud doesn't get washed down past the butterfly and into the manifold.

Anyway somewhat to my surprise that seemed to fix the slightly non-smooth idling, and also makes it idle (noticeably) slightly faster now. Hope this helps someone!

Update

Turned out cleaning it didn't alter the slightly unsteady idle afterall - the cause appears to be having the lights and heated windscreen on at the same time, turning screen off seemed to make the idle smooth (guess it was a slightly warmer day when I 'tested' it after cleaning throttle body). However it does still idle slightly faster and slightly smoother so cleaning it was no bad thing and have learned a little more about the car.

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