Merc 310d engine issue
Hi, i bought a camper van with the OM602.940, 5 cyl engine, 4 speed automatic. Which i thought was a bit slow but running ok. However after a particularly long drive i noticed every now and then i would get bursts of much better performance out of it. For example typically goin up an incline at 60mph most of the time it will grumble along or possibly drop back to 55 even if i put my footdown it doesnt make any difference. If one of these performance bursts happens then during the incline i could put my foot down and i can charge up the incline at 70 no problem. It only seems to happen on long journeys of several hours or more and the longer the journey the more often the performance seems to improve. Coming back from london to scotland recently i found the performance burst would sometimes last 20 minutes or more. So i now think this is what the van should run like all the time and there is something wrong with it most of the time. Having googled this a fair bit and spoken to some mechanics the problem remains unresolved, however in the troubleshooting we have replaced the fuel lift pump and added back the pre filter which was missing. Not being an engine type person and not wanting to keep spending money on a treasure hunt, i was wondering if anyone has seen this issue before or can offer any insight. Im planning a trip of several thousand miles starting on Sat and would love to get it resolved before i go. There is no smoke coming from exhaust and one mechanic thinks the engine sounds great and looks great, completely stumped as to wat the problem could be, he doesnt really want to spend hours troubleshooting and take the money from me with no guarantee that he could find the problem.
If i have missed out any key information or anyone can give me some ideas it would be very much appreciated.