Loss of power at 4000rpm.

[gropar]

Now : I do not go there often. But when I do, it's to get some power.

But when I get there, I can feel the engine loose all it's guts.
When that happens, I usualy upshift : so I figure it is not impossible that power might come back a few seconds later.

Anyhow : I am about to go for a pump rebuilt (it leaks) and an injectors check-up. I suppose that might be the cause. Yet if you have any other idea...

[fspGTD]

I'd guess it's your stock governor mechanism working as designed by the factory, to reduce high RPM power.  At about 4000RPM in a 1.6lTD at least, that's when the governor intermediate spring starts to kick in, which noticeably reduces power output.

in the following G-tech plots, the stock govenor ('84 1.6lTD) is shown in blue.
(modified governor shown in black.)  Note that changes in power between the 2 curves below 4000rpm are due only to inconsistent environmental factors (air pressure, temperature, etc) and possibly air intake manifold or fuel temperature differences.  IE: under more scientific test conditions, the power between stock and modified governor plots below 4000RPM should be the same.  But the plot is useful nonetheless to see the different relative shapes of the power curves.

[gropar]

I tought something like this might a cause.

But I am shure that the point at wich the power go down used t come later (let's say : this summer).

[Dr. Diesel]

there's a certain amount of revving resistance by nature of the 1.9L's much bigger stroke too.