My tow vehical a 1991 ford van has about 180,000 miles on it.  And after seeing how much a new van will cost, i'm hoping to keep this one running.....

But,  the engine is super tired.  van can barley get out of its own way esp with the hills up here and the freeway with a slight incline is full throttle.  I replaced much needed plugs.  what is next?  check compression,  clogged cat? mabe cap and rotor?

Where could all the power have gone?

possibly new fuel injectors? i have a 94 E150 and am considering the same thing. When i run a couple bottles of injector cleaner in it, it seems to temporarily restore power. Just a thought though.

mat

In 91, the 351 Windsor engine barely had enough power to pull it's own weight when new. Damn good engine though, and well worth rebuilding and some upgrades to keep things moving. At that many miles, I'd recommend a rebuild, with a good truck cam, and a good set of aftermarket cyl heads. If the budget is too tight for that, a rebuild with a decent set of pistons to bump the compression half a point might get You the power You need.

Good luck with it. Mac.

I would check for an exhaust restriction (cat) first.

http://www.walkerexhaust.com/support/exhaust101/inspections.asp

Then  you can check compression on the engine.... 

^^ Jim is right. Def check the converter for restriction first. If the converter is hollow, then the substrate might have blown into the muffler causing a restriction.

Get underneath and wiggle the catcon (COLD!). If it rattles the cat is likely clogged. Make sure its not the heat shield rattling.
But, I agree, that engine is getting tired. It's getting grey whiskers.

Got under her a bit today.  Still thudded.  but it was hard to get a good whack to it with the heat shield.  Neighbor came by and said his was packed with unburnt carbon and his truck is a year younger than mine(said headers and a cat delete really helped his...) .  Unfortunately need to cut it out to check further.  I was looking for a good project for the welder.