80 mile each way run to the coast, lovely fun. The Berry Patch is the best food on H'way 30 Oregon btw if you're in the neighbourhood of Westport.
Her Ladyship gets a bit het up when I get this car moving.
It does make some odd vibrations when over 90 that become a nuisance above 100. Wondering about greasing prop shaft and looking at tranny mount.
Also noticed recently that when I check the radiator level, it's low in the header tank but the overflow tank has plenty of water. Crossed my mind that the tank might not be sucking water back into the radiator as it cools. Is this a sign of something bad gonna happen ??

 

Quick thing to do is to is to just replace the rad cap and the hose leading to the overflow tanks. Sometimes the hoses get soft and will vacuum shut.

A coolant pressure test would also be called for.

Since its very low vacuum a small leak at the upper part of the motor can cause the issue.

On a bad note I have seen head gasket leak only when cooled down. Because the head gaskets are so high on the LT1 it would only affect the upper coolant level. That issue will should show up with cold pressure test or any test that looks for hydrocarbons in the radiator.

Prop shafts are on boats., From the factory your drive shaft is not serviceable, even if it was by the time you can "feel" it one or two joints will need replaced. If the u joints feel tight they are probable not the issue unless something knocked the shaft out of balance. How old are the tires? what was the last time they where balanced? that is where i would start. Tires that sit for long periods, like the winter, tend to get flat spots on them unless you store them properly. At tires get older they are more likely to retain that flat spot. Even a small flat sport would be noticeable at 90MPH

 

Thanks Gorn. Prop shafts is the english still in me !!
Will do the cap and hose thing first then start worrying if they don't clear my problem.
Tyres are newish as in not many mikes but have stood for a while by previous owner.
Balancing is on the cards as I might have pressure washed a weight away.
Other than these ****les, pretty much loving tooling around in it.

 

I ran that highway almost daily in one of these trucks for a few years,hauling wood chips from the Hampton lumber mill in Warrenton and to the paper mills at Wauna and Longview,Wa.

 

also check the overflow tank line/pipe under the battery. the metal pipe the hose goes to at the tank has been known to rust and fail, not the best spot to put a metal pipe.

 

If you want to get fancy on the vibration you can download a vibration app on your phone and read the cycles per minute of the vibrations. Then get out you calculator and figure out what is spinning at those RPMs.

 

I have the battery tray off and tank out. The rubber pipe is well soft and replacement is in hand. There was some sludge in bottom of tanks.
Main point of interest has been the broken tray retaining bolt !! Prolly never been removed after its build. Tray has some rusting that I'm treating with a coat of POR15.
Will drill and tap for 5/16" bolt so should be good for another 25 years.
Thanks for the clues chaps. great help from you all and much good info on this site.