This may sound strange but try this:

Elevate the REAR of the van with front bleeder open, engine running at
2000rpm, expansion cap open and ready to fill.  Top off expansion tank and
return cap, close bleeder, turn off engine and let sit until cool.  Run
this routine over the course of an afternoon (about four times) and maybe a
day or two later (about two times) while keeping an eye on the expansion
tank level.  This has helped me considerably.
Hey, you wanted any suggestion, Boston Bob gave me the above.

>>>don't forget that any previous owner may have used radiator stop leak in
your van which may be clogging any part of the cooling system, if in doubt,
check flow on radiator<<<

JB
'89 Wolfsburg GL (Road Whale)
'82 MB 240D (Slo Mo)