Posted by John in La on May 23, 2011 at 17:15:56 from (66.157.26.157):
1998 F150 w/4.2L V6 62,000 miles Dads truck developed a knock. From talking to you guys and looking on the net I find this engine has a history of leaking radiator water past the intake gasket and bending a rod. Was going to have a new engine put in but decided to rebuild the original engine. Finally get it towed the 60 miles to my house so I can work on it.
When it arrived I started it up to check it out. Did not knock for the first split second but did not give it a second thought at the time. Runs with a miss and has what sounds like a deep knock. If you drive it after warmed up it billows what looks to be steam out the tail pipe but the knock stops under a load.
The tail pipe steam and knocks leads me to believe it has the problem above but why does it only knock under NO load. So could it be a spun bearing instead of a bent rod. Does not fit the water lock theory but we will see.
So I pull it in the shop and start taking it apart. Just for kicks I run a compression test to see if I can find what cylinder is knocking.
I get........... LF 0........ RF 182 181..........190 185..........175
Now this has me really stumped. Why would a cylinder that supposedly has a knock from a bent rod due to water locking have 0 compression. It could have blown out all the rings on that cylinder but that seems unlikely. Then I remember the knock did not start till about 1 or 2 seconds after start up. About the time it would take for a hydraulic lifter to fill with oil on a engine that has not been run for the past month or so.
I am really stumped now and about ready to go threw the trouble of pulling the head only to make sure this is not a valve problem. Recon I should pull the whole motor so I can get the pan off and at least check the lower end.
I guess time will tell as I get deeper into this motor but it sure has me guessing right now.
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Today's Featured Article - Engine Valves - Some Helpful Information - by Staff. Intake - On the intake stroke the piston moves doward, sucking in carburized fuel through the open intake valve. Exhaust valve is closed. Compression - With both the intake and exhaust valves closed, the pistons upward stroke compresses or squeezes the fuel into the combustion chamber. Firing - Ignited by the spark, the compressed fuel explodes and forces the piston downward on its power stroke. Intake and exhaust valves are closed. Exhaust valves and seats are exp
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