Running on 3 cylinders....??

[D&DCAMPA]

Sorry for sounding abit daft - but I noticed the van wasnt sounding as good as normal! (ha!) and was losing revs and cutting out when I slowed down at r'bouts etc, and someone commented that it was only running on 3 cylinders.......
so OH has checked the spark plugs, rota arm, dizzy cap, and leads....all ok...
Please can someone suggest what to check next as we're driving it to Okkyfest next week and would rather it was running properly!
Also someone has said it could be the computer thingy that tells the engine which cylinders to run on??? is this an ECU??? How do we check that???
Its a 1.9 DG Petrol if that helps.
Any advice would be welcome as we've just moved house, have no internet access (doing this on phone) and cant find the manual at the mo!!!
Thanks in advance.

[Mickyfin]

When was the last service? Have you checked the condition of your plugs?

[Aidan]

ok with it running if it's running on 3 you can tell which cylinder by removing the HT leads one at a time from the disi cap, be aware mega volts so insulated pliers/rubber gloves sensible precaution, the belt otherwise can sting
if that lead/plug is ok engine revs will drop, and then rise again when you reconnect, if no spark in cylinder removing the lead won't make any difference
often leads breakdown isulation and spark to ground outside cylinder via the metal plug cap shroud, even when the lead checks out for continuity and correct resistance end to end
Hall sender (in dissy)is the elctronic part that triggers the spark in connection with the TCI unit (left side of engine bay on firewall in front of coil), and the TCI is liable to failure, usually they get a bit intermittent, worse when hot, then they fail, common part on most VW carb engines and some FI engines of the era

[D&DCAMPA]

Thanks for that Aiden, will give us something to check tomorrow!

[mblake4u]

Failing Aidan's advice on removing the leads one at a time to see if you're getting a spark in all your cylinders, a compression test is a quick way to see if there's something wrong with one of your cylinders.

Does your engine make any puffing/wheezing or backfiring noises, or is it very smokey?