HELP!!!!
there's a blizzard forecasted 4 days from now, and suddenly my plow truck (combination of an 82 f-100, an 86 f-250, and an 89 f-150 put together) started knocking really bad. the engine is out of a 86 f-250 and its the 300 six cylinder. the engine started tapping at first while driving, then i pulled over and it started knocking really bad. i assumed it was low on oil so i added some oil and attemped to start it again. it kept knocking just as loud. keep in mind that the whole time this was happening the oil pressure gauge read normal pressure. it sounded like either a rod or crank bearing had gone bad, and it was running rough and lost alot of power as i was attempting to get it off the roadway. so now i'm thinking of where i'll find another bottom end and have it in the truck within 4 days. i had my brother drive me home and get my other truck... when i came back to the plow truck to get some stuff out of it i started it back up to see if it still knocked. to my surprise it didn't? it ran just fine and didn't knock a bit. i thought for sure that i had spun or lost a bearing before but now i'm not sure. i tried to drive the truck home but notice the faint knock slowly coming back so i cut it back off and left it for the night.
has anyone ever had this problem before? the engine only has 100k on it. i think i'm just gonna take the pan off tomorrow and replace the oil pump and screen. would that just be a waste of money? could a bearing still be bad? i'm trying to get the truck running again before the snow gets here. if anyone has any input please help me i'm running out of time!!

 

Does it blow blue smoke and does the exhaust smell like oil and do you have heat sheilds on your cats

 

give it a tune up plugs, oil, fuel filter and check your timing. also deff. change the coolent and thermostat. sounds like its getting to hot and begins to knock. (dont rely on your old temp gauge they dont tell you the temp in degrees) possably theirs air pockets in the coolent system and that creates hot spots in the head and results in pre-ignition aka knock.

so coolent change (bleed all air out), and timing check are the biggest things that can cause it. fresh oil change helps to.

 

2nd that above. ive had my engine knocking one time from detonation, let it cool down for a few minutes and it ran fine

 

It would really help to isolate where the knock is coming from a little more. Front, topend, bottom end. You could pull the valve cover and check the rockers as an easy start.

 

the truck does burn a litte bit of oil, but its been leaking from the valve guides for a while now. it doesn't burn much truck doesn't have cat's or emmissions system on it anymore either. the coolant is only a year old? the temp sending unit is about a year old too. the truck wasn't even close to hot when it started knocking. sounds to me like its deffinately a bottom end knock coming from the #2 cylinder. it just amazes me, because it knocked so hard and loud that i KNEW that the internals were trashed. then i started it up and hour later, and it purred like a kitten. i'm hoping its just a clogged screen on the oil pump, but it doesn't sound like anyone else has had thi problem before

 

mine has done that a few times, but not nearly as loud it was just faint, and a long tim ago now

 

How in the world is changing the spark plugs going to prevent it from knocking?

You need to check your oil pressure with a mechanical oil pressure gauge. The stock gauge isn't meant to be a diagnostic tool and can often read incorrectly over time. Furthermore, knocking from detonation and knocking from lack of oil pressure sound completely different.

 

what i would do is pull the motor and tear it all down fresh bearings, rings, seals, gaskets, oil pump, and anything else that needs to be replaced.

 

You got 4 days,you had better start with the basics and get this thing diagnosed.Do a compression test,check the oil pressure with a mechanical gauge,check all the moving parts(pulleys,fan ect) on the front of the engine.Maybe get a mechanics stethloscope or something to find that knock.It doesn't sound like a bottom end,it wouldn't go away,and if it was that bad,oil pressure would drop.It could be something entirely different,I bought a '81F150 that sounded like the thing was going to blow any second,when I got it home,the starter was turning,but not the engine,the center tore out on the flex plate to the automatic trans.So, you can be fooled by knocks,especially when they don't make sense.Look that baby over good,may not be as bad as you think.

 

Several years ago, the electronic sensor (?) located inside the distributor went bad on my son's 1984 F-150 4.9L [all original/feedback system/computor controlled distributor]. The engine made all kinds of horrible noises... knocking and or ticking. The solenoids located on the back and side of the valve cover were clicking so badly, I thought the pushrods were bent. You stated that your engine was from a 1986... 1984-1986 shared the same system.

After the part in the distributor was replaced, all was once again well.

 

Old Blue.....was purchased for $400 because of a bad engine knock. I was in and out of that thing 2 or 3 times trying to find the problem. It got all new rod bearings, a new con rod, a couple new pistons, new rings, a new oil pump, all the required gaskets and seals, before we finally pulled the timing cover and found 4 teeth missing from the timing gears. A couple new gears later, and it purred.

 

I have a F 150 with a 390. Was told when i bought it that it only had 20,000 on it. Has been running fine then I stepped on it a bit on the freeway and started hearing a bad knock. I thought it was a rod for sure. Towed it home and have replaced the rod bearings and checked the rods. they are all fine. I have eliminated a loose or bad harmonic balancer and still hear the knock. Does anyone have any other idea's for me to check before I tear into it? Oil pressure is fine.

 

I know this is kind of an old resurrected threat but I just though it was funny you did all that motor work without ever taking the timing cover off. That takes skill LOL