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Thread: truck knocks on startup :/
01-28-2012, 08:29 AM #21
- Join Date
- Nov 2008
- loudoun county va
02-02-2012, 04:34 PM #22
im gonna post a video i took the other day so you guys can hear what im talking about ..
02-04-2012, 08:26 PM #23
- Join Date
- Nov 2011
- Madison Lake, MN
That knock is usually caused by low oil pressure at start up. Culprits can be worn bearings, worn oil pump, plugged oil pump sump screen, or leaking oil galley plug. I think I would start by dropping the pan and inspecting the sump screen, if thats clean pull the pump and check the rotor. It the pump looks good try wiggling rods to feel for bearing wear(although mains usually have more effect on oil pressure). Since its initial start up, I would expect to find the sump screen plugged. Good Luck
02-05-2012, 08:47 AM #24
- Join Date
- Jun 2006
- River Ridge Louisiana-4 miles W of New Orleans-didn't flood-water stopped 800 yards away.
Didn't know this was so common- but many trucks here are pretty old-lotta miles.
In the old days-50's -60's engines were commonly rebuilt and rebuilds happened at 100,000 miles or less.
My 1998 5.7 Suburban doesn't know or make any "funny noises"-218,000 miles- last 23000 miles 5 years are mine-so can't say exactly what the maintenance was for PO but suspect it was well maintained.
I thought the earlier 5.3 commonly had a noise like piston slap that GM finally determined was from carbon building up above the top ring
It was supposedly cured by raising the top ring on later 5.3?? In any case it didn't hurt reliability- but did hurt resale-no one wanted a knocking engine-eve
PS I thought GM gave some folks extended warrantys etc- it was a big deal 7- or so years ago
02-07-2012, 11:08 AM #25
short video so you guys know what im talking about
02-09-2012, 12:06 PM #26
- Join Date
- Jun 2010
- Western MA
- Blog Entries
Is it the dreaded Piston Skirt Slap ?
My 5.7 wakes up clattering like a diesel too - has done it for years. When it began I asked around on the then-current forums and the consensus was "cold piston slap". Apparently the aluminum pistons get a lot smaller than the cast iron cylinder when cold. Then when the engine starts, the piston skirts get slammed against the wall as the piston starts pushing up. The noise goes away because the piston heats up and expands to normal size and small clearances.
There were service bulletins on this that just said it was caused by "interaction between the piston and the cylinder sidewall" during warmup. I guess that's a nice way to say that they don't have very tight tolerances on piston fit. For some reason there is usually one piston that does it worse than the others - maybe its bore is toward the large end of the tolerance. The good news is I've never heard of an engine having a serious failure related to this condition. The street wisdom says it's a matter of getting used to hearing the racket on cold startup. I have found that Wolf's Head 20W50 engine oil reduces the noise. I think a slightly thicker film stays on the cylinder wall below the rings, giving the piston skirt a softer place to land.
I have a 2001 C5 with LS1 that does it too. After running for maybe 10-15 seconds it's quiet. Gotta love GM
As always, prompt cheerful refund if theory is bogus.
Last edited by arby; 02-09-2012 at 12:16 PM.
02-11-2012, 07:47 PM #27
makes sense .. its just kinda scary to hear lol.. anytime ive ever heard this noise its been bad news .. but its been like this for over a year now and the motor still runs great otherwise !
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