As seen in Speedboat Magazine’s September 2018 Issue.
Got a weird one for you. I am stumped.
I was working in a buddy’s boat last weekend. He bought a bare hull and stuck a 496 and Bravo One drive in it that had been sitting around for a while. The interesting thing is it will run fine for several minutes, then slow down, then seize. When I hit the key, it seems pretty well seized. Let it cool for a few minutes and it’s good to go again. No funny noises or anything.
At first, I thought it was water reversion and hydro locking, but it happens even at 2,000+ rpm. Plus, I pulled the plugs and there is no water in the cylinders.
Is there any chance the starter is maybe not disengaging from the flywheel and pulling motor down after it heats up without making any unusual noises? Could it be the oil pump, or something in the upper half of the drive? Valve stems sticking on guides? Pistons skirt or wrist pins? Spark doing something funny and jumping a cylinder ahead?
It’s got great oil pressure, and nothing funny in the oil on the dipstick. The problem pulled the motor down from around a constant 2,000 rpm as recently as yesterday. Yet two weeks ago, the motor ran for around an hour on the hose and never had it happen.
Any help or ideas?
San Francisco, CA
Bill, I doubt that the problem is being caused by the starter motor not disengaging. If that were the case you would be able to hear noise from the starter while the engine was running. As you know, the upper in a Bravo drive is rotating whenever the engine is rotating. I am assuming that the drive is properly filled with oil. If the drive was causing the motor to stop because of some heat related issue, it would be very hot to the touch. The only other thing that is rotating is the gimbal bearing. If for some reason the engine was so far out of alignment, it could cause the bearing to get hot. But it would have to be so far out that you would have difficulty putting the drive on.
So, let’s focus on the engine. The problem is apparently heat related. If the 496 had been rebuilt by an automotive engine rebuilder, it is possible that the piston to wall clearances are too tight. Performance marine engines need to have more piston to wall clearance than automotive engines and the ring gap also should be increased. As the pistons warm up, they could be expanding to a point that they are too tight in the bores. I doubt that it is valve guide related because if the valves were sticking in the guides and not closing completely, you could detect a miss and hear it in the exhaust. A 496, as well as most stock engines have the wrist pins pressed into the small end of the rod as apposed to being full floating pins. Accordingly, the only movement of the wrist pin is in the piston. The pins are lubricated by oil directed from a passage from the oil ring land. As the pistons warm up, the wrist pin clearance in the piston will increase slightly. The wrist pins would also not have enough leverage to stop the motor.
The most likely cause is related to the main bearings and possible lack of lubrication. To prove this, after the engine stops, try rotating the engine by hand with a socket and breaker bar on the snout bolt. Even though you stated the engine has great oil pressure, there could be a lack of lubrication to the crankshaft main bearings for some reason. If the engine was rebuilt, a galley plug or something could have been left out. Also, if the engine was rebuilt, the main bearing clearance could be too tight. Typically, big block Chevys need around .003” clearance on the mains.
Check to see if the oil cooler and filter hoses are routed properly. Oil will not flow though your filter backwards because it has what amounts to a check valve on the inlet side of the filter. It is actually like a rubber flap. The oil flows into the outside of the filter and comes out the center. In some cases, depending on where the pressure sender is, you can see good oil pressure while oil is not getting to the bearing. This is only an outside chance because if the engine was being starved for oil, you should also be able to detect valve train noise as the lifters would not stay pumped up creating the lash.
Overheating at Idle
I have a 33’ Hallett with a pair of 580 cubic inch, 725-hp EFI naturally aspirated engines. They run great but one of them starts to get warm at idle. Once I put the boat on plane, it cools right down to the same temperature as the other one. I checked the water pump impeller and it look fine. I also noticed that when I pull the boat out of the water, there is a little water in the bilge on the same side. What do you think could be the cause?
Bill, one of the first things that I would have told you to check is the impeller and pump housing. Sometimes, even though the impeller looks good, there is scouring in the ends of the housing that cause the impeller not to seal on the ends which makes it loose prime when it needs to draw water. But, let’s assume that the pump is good. A few times we have had this problem which is hard to find. The clue that you are also getting water in the bilge indicates that you probably have a hose leaking somewhere. If it is an inlet hose to the pump, it could be drawing air in at idle when the hose has negative pressure in it and then leaking once the boat is under way and the hose has positive pressure in it. If the boat is equipped with sea strainers that have pop off vales or check valves that have a through hull dump above the water line, a piece of debris could be holding the valve open which allows air to inter at idle speed. It is easier for the pump to draw air than water. If all that checks out, it is time to check the inlet hoses that come from the drive. Very rarely I have found that the inner liner of the hose has delaminated from the wall and creates something like a flap that closes off flow during suction. Once the boat is underway, the water pressure pushed the flap out of the way and lets water pass. If you check that inlet water hose coming from the drive to the pump or sea strainer, you may also find your leak.
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