1991 Ranger 4.0 V6: "Thumper" has lived in my glovebox for many years. The sound is intermittant. When it is present, if I grab onto the top radiator hose, I can feel it pulsating. Doesn't happen all the time; when it does, the engine is at idle speed and fully warmed-up. No visible coolant leaks. I did read a couple of posts regarding the problem. If anyone solved the problem, can you provide me with the specific fix? Thanks!

 

Located the actual service bulletin for the problem at:
Coolant System By-Pass Kit - Ford Truck Enthusiasts Forums
read somewhere the kit costs $200. One of the possible solutions on the bulletin is for 1991 rangers, replace the 13psi radiator cap with a 16psi radiator cap. I think I'll try that first.

 

Did replace my 13psi cap with 16psi cap (motorcraft RS-90). The new cap does reduce the noise somewhat but I still have the problem. Don't want to spend $250 for the coolant by-pass kit so "Thumper" will continue his 20 year existence in my glove box.