5.7 knocks under load

[dead boat]

I have a 1996 5.7, mark 5 ignition, and it knocks at 3000 rpm while under load only. I changed the spark plugs and timing and it still knocks. What could it be?

 

[mkast]

Re: 5.7 knocks under load

Bad bearing.

 

[dead boat]

Re: 5.7 knocks under load

My mistake, i originally said 300 rpms, i meant 3000 rpms under load, at idle and out of water in gear it runs smooth.

 

[rs2k]

Re: 5.7 knocks under load

overheating? You should check the compression.

 

[dead boat]

Re: 5.7 knocks under load

No, not over heating. I just check compression, cylinders 3 and 5 are at 50 pounds. the rest are 150 +or-5 pounds. What should i do next? and wuold this cause knocking at 3000 rpms only under load?

 

[mkast]

Re: 5.7 knocks under load

When you tear the engine apart to fix the cylinders that read 50 psi, you'll find the knock.

 

[rs2k]

Re: 5.7 knocks under load

My guess is that you either have a blown head gasket or a warped or cracked head.

My 305 knocked under load and it had a huge cracked between 3, 5 and between 4, 6.

 

[Bifflefan]

Re: 5.7 knocks under load

Bad main bearing.

 

[rs2k]

Re: 5.7 knocks under load

What's your oil pressure read?

 

[dead boat]

Re: 5.7 knocks under load

Oil reads 20 at idle and 40 at revved. I put a spoonful of motor oil in the spark plug hole in one cylinder and the pressure went from 50 lbs to 75 lbs. Could a bad motor run fine not under load?

 

[rs2k]

Re: 5.7 knocks under load

Does your knock sound deep and short like this (Like a jack hammer):
http://www.youtube.com/watch?v=1ZBbKMaOlK0&feature=related

-or-

Does it sound sharper like this exhaust leak?
http://www.youtube.com/watch?v=uTmaQ1XBASk

-or-

Does it sound more high pitched and/or longer duration (Spark knock)?
http://www.youtube.com/watch?v=XiBzxxvy3FQ

 

[ringmaster72]

Re: 5.7 knocks under load

Nobody can tell you over the computer. If it was a main bearing, I believe the oil pressure would be low. If it is spark knock, it would quit if you retarded the timing. To retard the timing you turn the dist. clockwise. Eitehr way, you need to tear it down to find out why the comp is low, so just get it done.

 

[dead boat]

Re: 5.7 knocks under load

It sounds like a very loud pinging, the timing was 8 degrees advanced, I fixed that when I changed the plugs. The you tube vids didn't match, thank you, though. Could it possibly be the "knock module"?

 

[rs2k]

Re: 5.7 knocks under load

My guess is that you are getting spark knock caused by the cracked head or blown head gasket. I would pull the head and have it inspected.

 

[gadget73]

Re: 5.7 knocks under load

Bad bearings won't explain low compression. I'm betting the compression problem is directly related to the noise. Pull it apart, and find out why the compression is low. SInce its an adjacent cylinder, its likely a cracked head or popped head gasket.

 

[dead boat]

Re: 5.7 knocks under load

Head gasket blown between 3 and 5, thanks all who have helped.