dzelanko

1997 F150 4.6L:

replaced the motor in my 97 f150 with a 2003 mercury grand marquis engine, and used my intake and sensors to make it match up right. The engine ran fine, but had a rattling sound coming from the engine whilst under load... it lost power gradually over a few days, and now has LITTLE TO NO compression in cyl 6&7. The leak down test we performed allowed us to find the air coming from the tail pipe... Could this be valve trouble? we cranked the engine over to make sure the valves would've been closed up.

NOTE:

the "mechanic" that worked on the truck did not do a leak down, and replaced the head gaskets and had the heads tested... the heads tested good and needed minor resurfacing on the gasket surface.

Frank_Ford

If you have no compression in 6 &7 you have a big problem. I am suspecting much more than just valves. I am also suspecting that when the motor was taken apart it was not put back together properly and something broke when you started running it.

It is a little late but, I would recommend that you try to find a good motor that you do not have to take apart. You can often find one with a guarantee at a junk yard that if it is broke (like this one) you can take it out and return it and they will at least give you a replacement motor. You can also get a rebuilt motor (for more money) that has a warranty. I don't know your "mechanic" but I think that he should eat this job and install a motor for you that works. If he gives you a hard time, you might have to get it resolved down at the court house. There is nothing wrong with telling him this if he starts jacking you around.

w0lvez

Sounds like the Headgasket is leaking or warped cylinder head.. Instead of listening to the tail pipe try listening directly from the opposite cylinder #6 or #7.