300TDi losing power

I'm hoping you can help me diagnose a problem with the 300TDi engine in my Defender 90 losing power. I have found I've needed to change down a gear when going up hills that it normally has no trouble with. But more significantly, I'm struggling to get it above 55mph in 5th gear. The engine runs as smoothly as normal at lower speeds but the power starts to drop off when I get above 50. (I can feel it sporadically backing off and then pulling forward again - if it were a petrol engine, I would describe it feeling as though the engine were misfiring.)

Interestingly, the problem occurs at the same speed even if I'm going downhill, which suggests to me that it is RPM rather than power related. The weather has been cold here over the last few weeks, but not so cold that I can blame it on fuel waxing (the problem happened last night at about -1degC). I have had no problems starting the engine, and it idles smoothly (or, at least, as smoothly as a 30TDi ever idles). I was wondering whether the RPM governor in the injector pump ever gives problems (I have a lifetime's experience with petrol engines but very little with diesels).

I am going to replace the fuel filter because that's a cheap and simple step to take, but I am not entirely convinced that this will fix it. Have any of you come across this type of symptom before? Any ideas what might be the problem?

Many thanks in advance for your advice!

Roger

Replace the lift pump - cheap and easy to do. Also check the internals of the air intake hoses as they can delaminate and restrict the air flow.

classic lift pump failure no grunt on motorways etc

I replaced my lift pump the other week, has so much more go now in any gear. Use a delphi branded one.

Thanks, guys. I've ordered a replacement Delphi lift pump for delivery tomorrow morning. Hopefully that (and the replacement filter) will solve the problem in time for me to head up to the Highlands on Sunday.

Roger

My Delphi pump arrived this morning, and much swearing the cursing later, it was successfully installed (the front bolt was almost impossible to get to because of the plumbing coming out of the injector pump). Unfortunately it made absolutely no difference to the power problem! However, when I replaced the fuel filter, normal power was restored. At least my Landy has a shiny new lift pump now...