Rebuilt 340 knocking sound

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Confusedcuda

Confusedcuda

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Hello. My torture continues. Got just about done with my 68 barracuda build. I bought a rebuilt 340 about a year and a half ago on here I primed it. Used high zinc break in oil never revved it up very high. Drove it in total just about 75 or 80 miles. Just drove it to get exhaust done , an alignment, top estimate etc. on the way home from the alignment shop I noticed it started to sound a little different. Also my oil pressure at stop signs was getting way low. When I sped up it would go up a little bit. Once I got home it was starting to sound like a knocking in the lower end. I drained the oil and it had a silver grey look about it , way dirtier then oil that only had 75 miles or so. I put a new filter and 5 qts of new break in oil and it still doesn’t sound good. It has oil pressure when I first start it but as it warms it gets down to almost nothing. I did a compression check on all 8 cylinders and they are all 150 psi or more I’m stumped. Anyway going by a machine shop to see when they can get it in, at least the rotating assembly. I’ll post a couple videos. Tell me what you think

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150 psi on a new 340 ?? that is piss poor smog 318 numbers something serious wrong . who rebullt this 340 ? How low was the oil pressure . A video of the engine running would help . From your description sounds bad . Sorry to hear this should not happen on a new rebuild . Plain sucks
 
Sounds like it's time to pull it back out and apart.
 
I thought 150 to 160 psi was pretty good? I was doing it by myself so I just cranked the key for just a little bit
 
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Confusedcuda said:
I thought 150 to 160 psi was pretty good? I was doing it by myself so I just cranked the key for just a little bit
Click to expand...
Questions.

  1. Engine hot or cold
  2. Was throttle open or closed during cranking
  3. Did you oil the cylinders before each test
 
Throttle needs to be wired open, all plugs pulled out, and cranked off of starter relay until pressure stops climbing on the gauge. But overall, feeling bad for you on this one... What was oil pressure before it started going away? There are a lot of things that can cause what you're talking about, from dirty assembly (by far the most common) to clearances being okay but things not being as round as they should be to a cam bearing walking out or to an internal oil gallery plug coming out.
 
Engine was cool, i just did one plug at a time, with all the other plugs in, cylinders dry. I still need to move it before I tear back down. It fires right up, just starts to knock. wish I could get the video on here for you guys. I bought from a member, was supposed to be ready to run just needed to be primed. I have been a member here for over 15 years, if anyone is interested send me your phone # I will text you the video
 
Is this an auto trans car? if so i would for sure check the flexplate to convertor bolts
to ensure they are tight and not rubbing.
 
Confusedcuda said:
I thought 150 to 160 psi was pretty good? I was doing it by myself so I just cranked the key for just a little bit
Click to expand...

The camshaft has some control over measured compression as well. some cams bleed off some of the compression depending on overlap.
 
Compression doesn't have anything to do with knocking, probably a assembly error by whoever built it. Where are you located?
 
Compression is fine for how he did the test with 7 plugs on a cold engine with carb closed.

Sorry, you’ve got a bearing problem, take it out.
 
Confusedcuda said:
does it in park
Click to expand...

Not that i don’t agree the silver in the oil is a tell tell sign. the flexplate and convertor spin as long as the engine is turning park or
not.
 
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Yeah, the silver oil without pressure is a dead giveaway on bearing trouble. It could have even been originally caused a stopped up pickup screen. Everything has to be taken all the way down for thorough cleaning.
 
Confusedcuda said:
I thought 150 to 160 psi was pretty good? I was doing it by myself so I just cranked the key for just a little bit
Click to expand...
It's not bad. I normally draw the line on the low side about 130 or so, but even lower cylinder pressure can be made to run well. Sounds like you have bigger issues, though.
 
Confusedcuda said:
KCMO
Click to expand...
If you wanted to ship it, I'd be willing to tear it down, document what we find on FABO, remachine as needed, reassemble and test run it. At a nice discount to help a mopar brother, just send me a PM if your interested or have any questions!
 
Very generous! :thumbsup:The only better offer the OP will get is a refund from seller.
replicaracer43 said:
If you wanted to ship it, I'd be willing to tear it down, document what we find on FABO, remachine as needed, reassemble and test run it. At a nice discount to help a mopar brother, just send me a PM if your interested or have any questions!
Click to expand...
 
First order of business is don't start it again.... Drain the oil, cut the Filter open. You might see a little bit of junk from the machining but it better be minute... If you see fleck and shiny things, just stop and pull the motor. Anything else is just wasting time and money.....

JW
 
Try not to buy used rebuilt motors from unknown sources. Especially not local. And if so, look for documentation of what was done with pictures and measurements. Having video of it already running on an engine stand is a big plus too. And running it gets a lot (not 100%) of the cam going flat issues out of the way.

Now it's a he said - she said deal. Maybe you didn't break it in right, maybe seller got a bad part, etc, etc. The current damage could be masking old reused parts or bad clearances... But can't tell now.
 
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First off, low oil pressure and cylinder cranking pressure are TOTALLY DIFFERENT AND UNRELATED THINGS.

Here's what I'm observing, of importance.

1. You were driving it. All was well. You started hearing noise, and low oil pressure.
2. Checked oil and it's silvery.

Bearings gone. Cam, crank, rods, any of them. Silvery oil equals bearing material, most likely. Pull the engine, find the rod or main bearing that's faulty and repair. Check clearances etc and property lube.

Compression has nothing to do with any of this. 150 is maybe a bit low but not awful.
 
Agree with Mean416. It has wiped brg. Nothing else really fits the evidence.
 
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