Ive got a '78 F-150 with a 400 engine. When its idling it knocks real loud but when I rev it up or the rpm's are high it goes away until the it goes back to idling. I sounds like it is coming from the back of the oil pan near the tranny (c6) I pulled the oil pan yesterday and everything seems tight like crankshaft and bearings but Im not sure what Im really looking for or what it is. I need to put the oil pan back on real soon before it snows any day now. Anybody have ideas about what I should look for? Thanks.

 

Have someone use a ratchet on the crank bolt and move the crank back and forth - trying to reproduce the 'knock'. Check for looseness in the rod bearings and mains. Might even be a good idea to remove and inspect some bearing caps - rods and mains. But be sure to use correct torque specs when you put them back on.

Also check that the nuts on the torque converter to flexplate are tight. That makes a bad knock.

Let us know what you find.

Marty

 

Is the oil p.s.i. low ? Sounds like a main bearing knock.

 

Nick,
I assume that the sound is a true "knocking" sound similar to a rod knock. This is a deep thud, low frequency and has no metallic clicking of any sort?
The rods and crank are tight.
Was there any bearing material in the pan?
Could be a cam bearing thats spun and worn out.
Could be a piston pin has burned and worn out.
You can get by on a worn cam bearing for quite some time.
A piston pin is another situation. You won't make it long with that problem.
If you could elaborate a bit more, I could probably fill in the blanks.
regards,
KingFisher

 

Wow, thanks for all the quick replies. To answer some questions: I did turn the crank bolt but could not reproduce the knock. I did check for loose bearings but not real thoroughly. I'll do that tonite when I get home from work. I didnt think to check the torque converter to flexplate nuts. When I put the engine in last year I put them on with lock-tite. It was a used motor with I assume high miles. It has had very low oil pressure since I've had it. Also when I bought it I replaced the oil pan gasket and could not believe all the stuff in the pan (bearing material, chunks of metal, you name it!). This time I dropped the pan it looked pretty clean. I would call it a knock and not a clicking sound. A few months ago I did drive it to work one day, 100 miles round trip and almost all highway doing 70 mph and it didnt blow up on me. This was after it started knocking too. Since then I just start it to move it around. It needs brakes real bad but I would like to fix this engine problem if I could first. When I drove it to work I didnt have a choice,my car was leaking gas real bad. I thought for sure the motor was going to come apart on me. The knock hasnt gotten any worse since it started, but if someone is standing within 100 ft of it when its idling they look at me like "whoa, you got serious engine problems!'

 

Moved to proper forum.

Please read the Read First: threads on the top of the 73-79 forum.

 

I checked all the mains and rod bearings last night and everything was tight. A few of the torque converter to flexplate nuts were kind of loose. I had to turn them a few times to tighten them up. I wouldnt think that it would knock so bad because of this or would it? I did notice about 8 places on the flexplate where it looked like as if someone started to drill a hole about half way thru. They were about an inch or two from the teeth all the way around and unevenly spaced apart. What is that all about?

 

Nick,
The flex plate is balanced and the holes are drill marks to remove weight for balancing. Don't sweat it.
I have never had a loose flexplate so I couldn't tell you what would happen. The mass of the torque converter and flexplate are suppose to smooth the pulse from individual cylinders firing. The flexplate also helps align the torque converter. I imagine some pretty ugly things could arise from a loose torque converter. I hope you torqued the flexplate to the crank correctly.
Good Luck,
KingFisher

 

Thanks Kingfisher, I guess I never noticed those holes before. I believe I torqued the flexplate to the crank correctly. The motor ran fine and didnt knock when I first put it in last year. Oh well, sounds like my problem might be something internal that I cant fix. Thanks.