So my old bus has always started lumpy.
It'll fire up with some cylinders missing, which is especially noticeable if you try to rev it up immediately after starting. If you just leave it alone, chuck it in reverse and back out the driveway, by the time you select drive and pull away its usually got all 8 firing just fine. Alternatively if you bring the revs up to say 2000rpm and just hold it there, you can hear it sort of sort itself out and the cylinders come online.
So cue the other day, i go out and start it at lunch time, after it having sat for best part of two weeks. It started, but sounded REALLY unhappy, like half the cylinders were missing. I tried to rev it up a bit, and the throttle was unresponsive, RPM's were super low and oil pressure light was on. Mashed the throttle a bit more and eventually the revs picked up a wee bit and oil pressure warning eventually went out but it was stumbling and generally running like absolute crap. It was about this point i realised it was pouring white smoke out the exhaust like a steam train.
Wondered if it had done a head gasket or something, but i had kids to collect from nursery up the road, so decided to drive it and see what would happen. Revved it some more to try and "clear" it up, and managed to fill most of the driveway with smoke, but it picked up a few cylinders and was now actually responding to throttle, so i set off.
By the end of the street it was running more or less normally, exhaust still appeared steamey, but its cold and well, not entirely unexpected given conditions etc.
Drove to nursery fine (perhaps a mile max), went in and got kid, and came back out, checked the coolant level which was low but ok (it leaks so its often low). This time it fired up easily and drove off without a problem. Drove to the sports center with no issues. Parked for an hour and a bit while in the sports centre.
When i came out of the sports centre, turned the key and it started, and again, wouldnt respond to the throttle, and this time was belching black smoke out the back... However after 5 or 10 seconds it sorted itself out and i drove home. Coolant level was still the same when i got home, so no massive water consumption.
So at this point i'm thinking head gasket or slipped liner or something... And its fixed itself...
Driven it multiple times since then with no issues whatsoever...