hooligan603
Recruit
- Joined
- Mar 21, 2016
- Messages
- 2
I love these forums. I've been a marine mechanic/boat builder now for over forty years and I will occasionally search these forums when I come across an issue that I may be looking to short-cut on and find a quick answer. In the last two weeks I had to boats in the shop with Mercruiser 3.0/140 engine in them that both had knocks. The first was an easy repair, a simple valve adjustment. The second had me scratching my head trying to convince myself I didn't have a major issue on my hands. (I did.) I also reassembled a 3.0 for a customers boat and I'm in the middle of tearing down four 3.0/140 engines we have laying around the shop in order to put one good one together.
While on the forums I found some information that concerns me and I felt I should respond by posting something myself. Now, these engines are known to develop a knocking sound and it can usually be attributed to needing a valve adjustment but let me take this a step further; they are also oil sensitive, you MUST maintain the proper level in the crankcase and you MUST change your oil when it needs it. My concern was piqued when some members said that when their brand new boats were knocking since day one, the dealer told them "That's normal with the Mercruiser 3.0 engine and you should just ignore it." NEVER ignore an engine knock, damage will occur, or has occurred already.
Here are the steps I take when I have this engine in the shop for a knock. (This is assuming the engine is otherwise in good working order, i.e. timing is properly set and not needing a "tune-up".
1) Try to determine where the knock is coming from to help pin-point the problem.
2) Remove the valve cover and check for bleed down on the lifters by cycling through the valves (so there is no pressure on a particular valve I'm checking) and pushing down on the lifter side of the rocker arms. (This is to make sure the lifter is still good.)
3) Adjust all of the valves. (Adjustment is done by relieving all pressure as above, loosening the rocker nut so you can spin the push rod, snugging down the nut until you can no longer freely spin the push rod, then tighten the nut one more full turn.)
4) Remove and inspect the fuel pump arm and the cam shaft where the f.p. arm rides. (A bad fuel pump arm usually has a knocking/scraping sound.)
5) Do a compression test. Compression number should be good but I am usually more concerned with compression differences cylinder to cylinder. I find (generally) if it's within 10% from your highest to lowest compression you should be good. (Check on a warm engine if possible.) This is to check for damaged rings or ring lands.
6) Pull the engine, drop the oil pan and inspect the pistons. Look for damage to the skirts, cracks, missing pieces, etc.. Look for discoloration around the piston pins on the piston itself. This is a good sign that there was a lack of oil and the pin damaged the piston. This was the problem I had on the customers engine. After tearing down the four other engines we have in the shop, two of them also had this damage (the other two had cracked blocks, and one of those actually had both a cracked block and a bad pin.
7) Pull the pistons and rock the connecting rods side to side, there should be NO up/down pin movement.
8) While the pistons are out check the bearings and the piston ring lands (grooves where the piston rings set into) and check for any other piston damage.
9) If you still haven't found the problem by this point then check the cam gear clearance and backlash
By now you should have found your problem. (By the way; STOP when you find a problem, don't continue to complete the entire list unless you need to. Parts for this engine can be very expensive.) (Yes, I know there are other parts that I can use from other GM engines that can bring the cost down considerably but that's an entirely different subject.)
I hope this helps somebody out and clears up some misconceptions.
While on the forums I found some information that concerns me and I felt I should respond by posting something myself. Now, these engines are known to develop a knocking sound and it can usually be attributed to needing a valve adjustment but let me take this a step further; they are also oil sensitive, you MUST maintain the proper level in the crankcase and you MUST change your oil when it needs it. My concern was piqued when some members said that when their brand new boats were knocking since day one, the dealer told them "That's normal with the Mercruiser 3.0 engine and you should just ignore it." NEVER ignore an engine knock, damage will occur, or has occurred already.
Here are the steps I take when I have this engine in the shop for a knock. (This is assuming the engine is otherwise in good working order, i.e. timing is properly set and not needing a "tune-up".
1) Try to determine where the knock is coming from to help pin-point the problem.
2) Remove the valve cover and check for bleed down on the lifters by cycling through the valves (so there is no pressure on a particular valve I'm checking) and pushing down on the lifter side of the rocker arms. (This is to make sure the lifter is still good.)
3) Adjust all of the valves. (Adjustment is done by relieving all pressure as above, loosening the rocker nut so you can spin the push rod, snugging down the nut until you can no longer freely spin the push rod, then tighten the nut one more full turn.)
4) Remove and inspect the fuel pump arm and the cam shaft where the f.p. arm rides. (A bad fuel pump arm usually has a knocking/scraping sound.)
5) Do a compression test. Compression number should be good but I am usually more concerned with compression differences cylinder to cylinder. I find (generally) if it's within 10% from your highest to lowest compression you should be good. (Check on a warm engine if possible.) This is to check for damaged rings or ring lands.
6) Pull the engine, drop the oil pan and inspect the pistons. Look for damage to the skirts, cracks, missing pieces, etc.. Look for discoloration around the piston pins on the piston itself. This is a good sign that there was a lack of oil and the pin damaged the piston. This was the problem I had on the customers engine. After tearing down the four other engines we have in the shop, two of them also had this damage (the other two had cracked blocks, and one of those actually had both a cracked block and a bad pin.
7) Pull the pistons and rock the connecting rods side to side, there should be NO up/down pin movement.
8) While the pistons are out check the bearings and the piston ring lands (grooves where the piston rings set into) and check for any other piston damage.
9) If you still haven't found the problem by this point then check the cam gear clearance and backlash
By now you should have found your problem. (By the way; STOP when you find a problem, don't continue to complete the entire list unless you need to. Parts for this engine can be very expensive.) (Yes, I know there are other parts that I can use from other GM engines that can bring the cost down considerably but that's an entirely different subject.)
I hope this helps somebody out and clears up some misconceptions.
