How and where does the Supurb 1 extract the ventilation air from inside the car?   Is there an extract vent somewhere in the boot, I expect that it is hidden behind the boot trim which is why I would like any clue as to which bit of trip to remove.

 

Reason I ask is that I have checked all the door and boot seals and the exhaust pipe, but when the car is standing and idling I am sure exhaust gas is getting back into the car. YES I know it is dangerous which is why i am going to get it checked.

 

One theory is that the ventilation extract might have a flappy back draft shutter which has stuck open or perished.

They're normally behind the back bumper on the sides. You need to remove either the interior trim or the bumper to get at them.

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There was a theory that having removed the plenum bungs (to cure the water / debris accumulation in the plenum chamber) could cause more exhaust gas in car.

Thanks trundlenut and jimbof.. I havent touched the plenum bungs. (no water ingress yet)  But a "jammed open" extract vent low down on the nearside  just above the exhaust tail pipe would explain a lot.

 

The old bus is 7 years old and has done 110k, so maybe I can forgive it.