1998 F150 4.6 loss of compression in 4 cylinders every other one in the firing order. Driving down interstate at 70-75MPH and had vibration. Stop and would not idle. if you keep the RPMS at 3000 or high it will run. Any ideals on what to start with?

 

How do you know it's losing compression? Please run a compression test on all 8 cylinders and post the results.

 

I have a 2003 5.4 v8. 254445 miles.
Started running rough, similar to COP problem. Took it to my mechanic. He said I had low compression in #5 cylinder. No codes shown. Maybe a stuck open valve. Is there any way to unstick without replacing engine?

 

I would put a compression gauge on #5 and verify the compression.

 

2,3,5,and 8 o compression
1,4,6 and 7 90-100 compression

Put oil in cylinder and the cylinders with compression raised some while the cylinders with no compression did not change

 

If those readings are correct, you have serious engine issues. May be time for a reman engine.

 

Yep, that is what everybody else has said and no one has every heard of that before. I am just trying to figure out if it is worth tearing into it or just go ahead an buy a reman.

 

The problems are most likely caused by the valves. If you pull the heads you may find the problems. If you are lucky, you might get by with getting the heads rebuilt/timing chains, etc.

 

Thanks for the input Roadie, that is probably where I will start. Hopefully i can get a little lucky. I will follow up with an update after pulling heads.