[v8volvo]

Sounds like you're working on the IP timing side of things. The noisy running could certainly have been because of the cold start device being stuck.

Did a new fuel filter make any difference?

Checking cam timing requires having the special cam locking plate (similar to VW tool, I will look up the Volvo number next time I have my books out), and you have to remove the valve cover and rear timing belt and reset injection pump timing when you reinstall the rear belt afterwards. A bit of a tricky procedure but if it's not right it will make it run poorly... should have been set correctly at timing belt replacement though, assuming the garage that did the job had the right tools and knowledge of how to do it properly on this exact engine. If they did, then no need to check it, but if there is any doubt, then it should be checked by someone with correct equipment and know-how -- and the crankshaft balancer bolt torque should be checked too, since that also requires special tools and needs a massive amount of torque. If it wasn't torqued properly and it comes loose then you can end up with major engine problems

In my '86 740 TD wagon, with a heavy foot and ZF automatic in mostly city driving, I average around 26 mpg (US gallons). Don't know how that compares with the Imperial conversion. 26 US MPG is not bad for a big, comfortable, safe station wagon that can haul lots and is a pleasure to drive.