Engine rapping noise during deceleration

[lockman]

Sent: 9/23/2007 9:49 PM

Last run out we climbed a fairly steep hiway hill in our 78D23 with a 440-3 and 68,000 miles. I noticed a high speed intermitant miss that I hoped had gone away with the fuel filter replacement but nothing special happened. I kept her at about 2500 rpm and let her pull all the way up. At the top we entered and followed a nice flat valley for about 5 miles till we reached our camping spot. There was a strong rapping sound coming out ot the left side of the engine when we slowed down to exit. Engine temp. oil pressure and engine vacum we all normal, just as on the hill. No leaking or burning fluids or exhaust, running fine, just strong rapping at idle. Noise goes away as rpm increases (hidden by exhaust sound?). Pulled each passenger side spark plug wire one at a time to check rpm drop. Number 7 ? (third from the front left side) stopped the noise instantly when plug wire removed, started again when reinstalled. Pulled spark plug, looked normal, run without plug for a few minutes to blow out any carbon chunks that may have broken off in cylinder. No good, still has sharp rapping sound that goes away when plug not firing. Any suggestions or next dianostics to do? Thanks





From: denisondc
Sent: 9/24/2007 12:32 AM

My suggestion wont be a source of happiness.....
Get a 440-3 from a RV in a Salvage yard and start to rebuild it this winter, if you have a place to work indoors. I think you have a piston damaged by detonation.





From: killme10
Sent: 9/24/2007 2:53 AM

Sounds to me like a wrist pin, rebuild time. It could also be a broken piston skirt, Also rebuild time. check your oil for possible chunks or shavings. Marty






From: Im-still-Lefty
Sent: 10/11/2007 9:42 PM

Sorry to be the bearer of bad news, but I have to agree with the previous diagnosises. From the conditions that you gave for when the problem occured (High strain, moderate rpm, extended length of time) I would agree that it sounds as if the piston on the #7 cyl. is damaged. Most likely, it has cracked. But it is also possible that the sound is a spun rod bearing.
A crack in a piston will cause a knocking sound whenever the gas in that cyl. fires, but will go away when the piston is not under a load (such as when you remove the plug or plug wire). What you are actually hearing is the crack expanding due to the pressure of the cyl. when it fires, causing the sides of the piston to hit the cyl. wall. The effect is devastating to a cyl. bore. and will eventually cause the entire piston to break free, destroying the block.
If it is a rod bearing, you may can salvage the engine by removing the crank and having the rod journal turned to an undersize. And replace the bad rod. or have it reconditioned.
Rod bearing noise usually doesn't go away as the engine speed increases, instead it tends to get louder.
You can usually hear if a intake valve is burned by listening for it in the carb throat as the engine is idling. You'll hear a distinctive puff as the pressure from the cyl. gets blown past the non-sealing valve.
For a burned exhaust valve, take a dollar bill and hold it by one edge just over the exhaust pipe. If you have a burned exhaust valve, the bill will get strongly sucked into the pipe every time that cyl. starts downward on the intake stroke, as the vacuum in the cyl that usually pulls intake air into the cyl. instead pulls exhaust gas in. This also has the effect of reducing the fuel that reaches the cyl. and also leans it out, causing it to burn weaker. A lopey idle can result.

Compression tests will reveal if a piston has broken rings, or a hole in the top of the piston, but may or may not reveal if the piston is cracked, depending on whether or not the crack is severe enough to leak out the pressure.
Wrist pin tolorances are measured in ten thousandths of an inch, and cannot be checked without disassembling the engine and physically measuring the parts with a micrometer. But you can usually see the damage on the sides of the pistons and cyl. walls if the wrist pin is loose.





From: denisondc
Sent: 10/12/2007 5:45 AM

Unless you dismantle an engine while its still making the rapping noise (before the final failure), its not likely you will ever know where/how a piston was cracked or melted. When the piston fails it gets hammered to unrecognizable bits in that moment before the crankshaft stops. Even at mild RPM this can ruin other parts; crankshaft, con rods; stretch the main bearing bores out of -true-, or crack open a hole in the side of the block.
Only once in my life I have had the opportunity to see the pistons from an engine that had been making the 'rapping noise' before final failure. I only saw one piston (of 8) that seemed damaged; a chunk of the rim above the top compression ring was missing, with the piston top pockmarked by the pieces. But a machinist showed me the hairline crack at the bottom of the top ring groove for that piston. The crack was only visible when you pried-up on the top of the groove, and it ran almost halfway around the piston. The other pistons also had serious scuff marks on their sides.
I have had plenty of chances to see what the guts of an engine looked like -after- piston failure.





From: lockman
Sent: 10/19/2007 7:43 PM

Is it possible to replace or repair just the one damaged piston or wrist pin? We dont use the "Old Girl" as much as we would like to any more. Less than 2000 miles a year now, and the cost of a full rebuild is a little prohibative. I think I can handle getting the piston out, but can I do just the one, and would it be worthwhile to be able to keep her running a few more years. Otherwise the engine is very strong and runs well.





From: Im-still-Lefty
Sent: 10/19/2007 9:31 PM

Well,
If you can get a parts supplier to sell just one piston and you are able to hone out any marks in the cyl. wall without needing the block bored oversized, then you possibly could replace just one.
Personally I wouldn't recommend it, due to several possible problems this might cause.
Some of them are:
1: New piston is different weight from old piston causing an out of balance condition.
2: Some needed parts usually only come as full sets, usually rings and pistons are not sold individually.
3: It's usually cheaper to purchase a full engine rebuild gasket set than to try and buy each individual gasket needed.
4: You'll still need oil, oil filter, antifreeze, etc... either way you go.
5: The new piston and rings may seal tighter than the rest of the engine resulting in a power imbalance between cyl.





From: denisondc
Sent: 10/20/2007 5:15 AM

I also would still recommend replacing all pistons/rings, since all of them would have experienced the same stresses that caused the one to fail. If you replace them all and proceed with a general rebuild, you have more peace of mind about using the RV, and a better story to tell a future buyer: plus having a better likelihood of future trauma free seasons of use. Replacing a single piston isnt a 'rebuild', its just a 'fix', unless you inspect/micrometer the other pistons.
As and lefty points out; by the time you are able to remove one piston, you have done a LOT of labor already.
As an alternative to spending the extra money for 8 of everything and maybe valve jobs on both engine heads, I would price out getting a used 440 from a salvage yard from a pickup truck or van. It wont be the 440-3, but it would probably still haul your motorhome around well enough. (Though my RVing is very seldom in the Real mountains!) I priced out pistons, rings and bearings at www.rockauto.com for the 440-1 in a 72 D300 pickup. About $400 for a set of pistons, about $100 for rings, $160 for bearings. (All in U.S.D.)
I wouldnt go looking for a used 440-3 from an RV in a salvage yard unless you planned to rebuild it or had some Good indication it had recently been completely rebuilt itself; because it might have experienced the same kind of stresses that your engine has.