Anyone got any ideas about a problem with my friends transit van? 1999 2.4 non-turbo diesel. It runs okay round town but at motorway speeds starts to lose power and run rough till it's down to a top speed of 45 mph. It then remains like this all day but is okay again the day after. Water temp is normal all the time. His garage reckon it's something to do with the airbox but sounds unlikely to me.

sounds like dirty fuel filter or blocked tank vent

Yes I was thinking along those lines but it's okay cruising at around 65 for maybe 15 mins and then the rough running starts. If it was blocked fuel supply u'd think it would never go fast (relatively].

2.5 di, check fuel filter and housing, mechanical lift pump on side of block, block off the egr valve to get more power!

I'd agree with dirty fuel filter, my L200 did similar, a new filter cured it, I get a new one fitted at each service now....

Thanks guys I'll check it out.

Fuel filter pipes make sure the unions on the pipes are tight. adjust the air flap so it opens fully on half to full throttle also fuel lift pumps go weak should be raplaced at least twice in the vehicles life, tappets and has it had the leake off pipe mod as in if its still to the back injector then its not had it can make a world of differance if you can cure the problem by knocking off an injector then thats the problem if you need the mod details shout me best of fun..

Is it misfiring?? they have a mod where you move the leak off return pipe from number 4 to number 1 injector as it collects air and wont get rid of it as number 4 is lower than the rest. Normally a good thrash will clear it.
Valve clearances out are common as muck. Diaphragm and throttle pin in the injector pump wear aswell and give lack of power. Easy change.

Id try the first 2 first.

When it's hot and running rough, slacken off each injector union in turn. You'll need a 14mm and 17mm spanner, the 14mm to hold the injector (so nothing shears) and a 17mm to slacken the union. Put a towel around it (mind the fan), wear goggles, or if on the layby of a motorway just look away while the diesel spurts. No difference? it's air or a faulty injector.

Leak off modification advice is sound. All vans that went in via the normal service route had it done. My 1999 White Arrow shed though, still hadn't at 160K when I acquired it 2 years ago.

I'm no expert, it's just that I've used (and still do) Mk5 smiley's to pull the race car over the country. Not this year though, no job, dwindling savings and frightening fuel costs....