I have a 2000 excursion 6.8 with 99k on it. Truck purrs like a kitten when idling but as soon as I put the in gear and press the accelerator an annoying continuous click can be heard from the engine. I am also experiencing power loss, I live in New England and when trying to go up even a semi steep grade the truck can barely pull its own weight let alone the 10,000 lbs it claims to be able to pull. My air conditioner will switch from blowing in your face to only out of the defrost. I read that could mean a vacuum leak but my rpms seem fine at idle. Please help. Thank you in advance

 

When was the last time the spark plugs were changed/torqued? How do the exhaust manifold studs look? When your vents switch to defrost, are you sitting at idle, accelerating hard, or just cruising throttle?

 

First, WELCOME to FTE!

Yes, the vents switching to defrost is a vacuum leak somewhere. On long pulls, engine vacuum is low, but there are check valves to keep the vacuum supplied to the HVAC system. This may or may not have something to do with your lack of performance.

I would suspect a fuel pressure problem more than anything. Check the fuel pressure under load and at idle.

 

 

A ticking sound under acceleration can be a sign of a spark plug getting ready to eject itself. You need to, at minimum, re-torque them all to about 24ft/lbs. If you're going through all that trouble you may as well replace them while you're in there. The ticking could also be a sign of a broken exhaust stud or two. That's easy enough to check visually, but a royal pain to repair.

Definitely sounds like you also have a small vacuum leak somewhere, as full throttle conditions provide for the least amount of vacuum generation.

 

First thing I would do is torque all the plugs just in case one has worked loose, then check all the other stuff mentioned.
Pull one plug and if it is a Motorcraft AGSF22WM they are OEM.
The current replacement is a Motorcraft SP-479, Ebay - 10 for $30.
Might as well change the boots, too, Denso 671-0001 - Ebay 10 for $30.
Our Excur had 104k when we got it and blew an OEM plug at 108k.

 

Come to think of it, I have been remiss. Clicking/ticking sounds on acceleration are an exhaust leak (broken studs) or loose plug. Check the plugs ASAP.