Misfire under load

I've a Defender with a 200TDi Discovery engine. It runs smoothly most of the time, but it suddenly developed a heavy misfire under load. It feels like fuel starvation - heavy vibrations - but it can be driven through by dropping down a gear. It's worse when towing on a motorway.

I've changed the fuel filter, bled it, and then cracked open each injector, and none are different from any of the others.

I've been concerned that it could be the head gasket blowing between cylinders, but it definitely feels more like fuel, and it's covered more than 500 miles since starting to play up, without getting any worse.

My next plan is a new lift pump, but I might as well order other bits while I'm at it. Any suggestions?

Sounds like lift pump. If it was an air leak it would be more constant.
When removing pump remember where the arm went on the cam.

Hicap phill said:

Sounds like lift pump. If it was an air leak it would be more constant.
When removing pump remember where the arm went on the cam.

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Thanks. I've ordered a new one.

Well, the new lift pump made no improvement.

Could it be an injector fault? Maybe the main pump? They're pricey items, so I'm reluctant to diagnose by ruling them out.

Stick some clear pipe in the feed to the injector pump, I wonder if you are drawing air in?
Could be injector fault or inj pump.
The head gasket pretty much always blows in the same place, when its running put your hand around the back of number 4 and if shes blowing you will feel it, mine took the paint off the bulkhead!
The valve lash caps like to break for fun, easy to change and cheap.
Checked cambelt for timing and dust out of the bottom drain hole?
A cheap laser temperature probe on the exhaust manifold will show if one cylinder is running different temps to the other three, ie not doing its fair share of the work.

have you done any work on it prior to power loss
Mine had an issue just this week when tank was low and the gauze on pick up pipe was well gummed up gave it a good spray with carb cleaner and topped up tank