What's the engine size? I guessed and looked at a pump for a 5.0L. Nothing in the engine is going to cause the pump to lock up. The first thing to do is autopsy an old one to see if it can be turned by hand. If it can, it's not locked up. A stuck pressure relief valve will strain the pump and make it so hard to turn that the drive shaft could twist.
If the pumps really are stuck, remove the cover and find out why it won't turn. There is likely going to be debris stuck between the gears. If there's metal chunks in there, something is coming apart in the engine and the pickup screen isn't catching it.
Thursday, April 12th, 2012 AT 10:21 PM