Club 80-90 Forums

I recently replaced the rear oil seal on 1.6td and when I started the van up it went again. I took it to a garage and they said that the air pressure is all cockeyed. The van originally had a petrol engine but a previous owner installed a JX engine but left the petrol air filter etc. The garage said they have managed to make the air flow out the bottom, to release the pressure. The problem I have now is that after driving for 45 - 60 mins smoke starts coming out of the rear vents. I am planning a trip from Manchester to Derby on the 9th of June and am concerned that this will make my engine overheat. Also does anyone know a garage that can fix this problem?
Cheers in advance