JackC 617 #1 Posted March 22, 2015 Cause of death? Neglect by failure to clean grime off of cooling surfaces. I should post this in the engine section but anyone with a P Series ONAN needs to see this. I bought a 1990 416H with a bad ONAN engine a few years ago. I paid what the deck was worth for the whole thing including a new red vinyl wheel horse tractor cover. I finally got around to opening the engine to see what went wrong. The engine had about 1000 hours on it and the tractor was used primarily for mowing. The first thing I noticed was the extreme amount of grime around the cooling fins on the back and bottom of the engine. Clearly the engine overheated and the rod disintegrated. The cylinder wall was fine and the piston was still a snug fit in the cylinder. There was carbon but not enough to be the cause of the failure. I did not check the valve clearance on the back cylinder since it was clear the grime prevented the engine from cooling properly. The valves and valve seats looked fine. After seeing this I will be taking my ONANS off the tractors and giving them a thorough cleaning along with the de-carbon and valve clearance checks at least every 500 hours or so. This may be one of the main causes of the rear cylinder failures on our horses. The pictures of the engine are after a removed a significant amount of grime. Not sure what i will do with it but I did purchase a new rod and piston and new valves. Not sure how bad the crank is. The oil tube got destroyed also. If these engines are neglected and not allowed to cool properly they will be toast. Periodically cleaning the cooling fins on the entire engine is a must do. The way these engines are mounted in our tractors, it is way too easy to neglect the bottom and rear of the engine and it is not just grass accumulation. An oil temperature gauge may help provide a warning sign. 2 Share this post Link to post Share on other sites
boovuc 1,090 #2 Posted March 22, 2015 Amen to keeping them clean and decarboned and adjusted, Jack! Share this post Link to post Share on other sites
JERSEYHAWG / Glenn 4,498 #3 Posted March 22, 2015 Jack, thanks. What a nightmare. Point well taken. Glenn Share this post Link to post Share on other sites
Theroundhousernr 590 #4 Posted March 23, 2015 Yup, Also when low on oil the last thing in the chain is the number two rod and cylinder. Heat+low oil = failure. Judging by the aluminum caked on the crank , it would appear the rod seized on the crank and the rest is self explanitory. The neat thing about onan motors is they rarely damage the cylinder when that failure occurs. One of mine , the wrist pin seized also which scored up the cylinder requiring a rebore of the cylinder. Most of the time the aluminum will clean right off and mine have measured with in spec but some of the cranks I have seen where chipped from shrapnel and ruined the crank. Hopefully you cleaned up and measured the crank before ordering parts. Looks like its savable though! Share this post Link to post Share on other sites
WH nut 553 #5 Posted March 23, 2015 Looks like an oil change or 2 wouldn't have hurt one bit Share this post Link to post Share on other sites
JackC 617 #7 Posted March 23, 2015 I have several tractors with P Series engines. After seeing the grim build up on the oil pan, I plan to drill more holes in the engine mounting plates under the engines to provide better air flow up to the engine and to allow inspection for grime build up on the cooling fins under the engine and back cylinder. I am not sure if the oil transfer tube can be replaced without totally dismantling the engine. A total tear down is probably what should be done in order to clean and spec everything. There are how to videos on the internet. With all the grime on the engine, I sure could use a parts cleaner. Share this post Link to post Share on other sites
Theroundhousernr 590 #8 Posted March 23, 2015 Something to consider is drilling holes will only cause more dirt to easily find its way in there. The build up is usually from an oil leak like oil filter base gasket, crank plate, or Welch plug over cam. The oil then soaks the motor and attracts dust. Fix oil leaks, degrease engine, then blow down the motor regularly. 2 Share this post Link to post Share on other sites
JackC 617 #9 Posted March 23, 2015 Something to consider is drilling holes will only cause more dirt to easily find its way in there. The build up is usually from an oil leak like oil filter base gasket, crank plate, or Welch plug over cam. The oil then soaks the motor and attracts dust. Fix oil leaks, degrease engine, then blow down the motor regularly. Good point. Share this post Link to post Share on other sites
mey030 9 #10 Posted March 26, 2015 Roundhouse hit it on the head. I have scene same scenario Oil Filter base gasket is usually the first culprit or dipstick tube lower seal fix them clean it blow it out your good to go 1 Share this post Link to post Share on other sites
