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1985 F150 engine knockPosted by jimks on July 14, 2003 at 10:55:37 from (65.150.168.60): I've got an '85 F-150 with an inline six engine. I've had it for about three years, and it's always had a knock in the engine. I've driven it most every day for these three years, and it doesn't seem to be getting any worse, but it's certainly not going to get better. A couple years ago, I decided seek and destroy this knock, so I removed the oil pan and replaced the rod and main bearings. They had some copper showing, and it helped my oil pressure out, but none were bad enough to cause a knock. While I was under there, I inspected each piston skirt when it was all the way down- they all looked almost new. Tried rocking them side-to-side, no noticeable play. So I put that back together, still had the knock. Maybe six months later, I decided to tackle the leak at my tappet cover gasket, so while I was at it, I replaced all the lifters and used a flashlight and mirror to look the cam over- I checked out all the pushrods and rocker arms just to be safe- I didn't really see anything wrong, but put new lifters in anyway and took care of the leak. At some point my fuel pump went out, so I replaced it, and while I was at it, I took a look at the fuel pump eccentric- looked great, wasn't loose. At this point, I think I've replaced or inspected every single reciprocating part in the engine. The truck's odometer says 32,000 or thereabouts... of course that means at least 132,000, and I tend to believe that 232,000 is just as likely. I'll describe the knock: I don't hear the knock at idle, I hear it very clearly when moving down the road with a moderate load on the engine, but it goes away at if I let off the gas, OR if I floor it. The knock is directly proportional to engine speed. If I push in the clutch or have the transmission in neutral(with the truck moving or not), I can hear the knock when I rev the engine, but only while the engine is under a bit of load while the RPM's are increasing, so I know that the noise is in the engine. The knock becomes much quieter when the engine warms up, but I can still hear it at highway speed. Any ideas? The only suggestion I've heard that sounded possible to me was that maybe it was carbon built up on a piston causing the knock... but I just don't see carbon lasting for 3+ years while smacking a cylinder head dozens of times per second. Also, the fact that the knock changes according to engine load makes me think that it has something to do with the pistons (maybe?), but like I said, the piston skirts looked new... the engine uses little if any oil, and I've changed the plugs a couple times, and saw no evidence of any funny-business. Thanks for any suggestions or ideas- I'm all out.
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