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TAMD40B re-occurring banging sound

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PNW001

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I have a peculiar and rare re-occurring problem which I have consultedmultiple mechanics and other experts on, and never resolved…

On very rare occasions, just 4 times in the last 5 years, our VolvoTAMD40B starts making a very loud banging sound. The rate/frequency of the banging soundaccelerates/decelerates with RPM, and occurs both in gear or neutral. At idle it’s imperceptible or almost imperceptible. It has occurred once after about an hourunderway, and 3 times when underway 2-4 hours. While our boat can run up to 14 knots at 3,600 RPM, we stick to hull speedat 1,700 RPM. We do some short runs, butfrequently do longer trips of 4-8 hours year-round. The engine/boat are 1985 with only 1,500 hours in the first nearly 30 years, and 1,500 of active use in the last 5 years. The engine is in excellent shape and has been very well cared for.

In all four cases, the banging has gone away on its own. Three times after just 1-3 minutes (while tryingturning engine off, low revs, etc.). Onthe fourth occasion in rough water it took almost an hour and finally resolved afterrunning the engine at normal RPM (1,700) and ignoring the banging for a minute or two. Acomplete mystery.

As the problem goes away by itself, it clearly isn’t a permanentproblem such as a damaged piston. Theoil and coolant are always perfect when changed and checked (both consistency and levels). Engine temp and pressure always run at normallevels.

The only outstanding possibilities that have been suggestedto me are that it could be either: a) fuel filter should have been replacedmore frequently, or b) a valve getting sticky, but then freeing up again.

It’s been a while since the problem last occurred, but I’dlike to get to the bottom of what it could be in case it happens again, or toprevent it from happening again.

I would greatly appreciate any ideas or insights.

Thank you.
 
Keep a long handle screwdriver handy for when it happens again (poor mans stethoscope) Go around and listen to different areas on the engine with it running for practice you might be able to isolate the problem area. Be careful of moving parts place the tip of the screwdriver on the valve covers you can listen to the individual rockers, alternator bearings front and rear just put your ear on the handle and search around with the tip. Have you found water in the fuel filter? I probably wouldnt hurt to get your fuel system scrubbed if you havent done so yet. You might have bugs in the fuel tank kinna like a bacteria that lives in the water at the bottom of the tank.
 
Are you on an outdrive or shaft drive? Things that come to mind are things like drive donuts and gimbal bearings etc which on an outdrive will still turn in neutral gear. Another one that I had was a cracked outdrive transom assembly that made a banging noise only when the steering was on a certain angle. In the engine itself you would guess maybe an intercooler with an intermittent leak of some sort putting water into the cylinders and causing early detonation or similar but that sounds unlikely. Could the injector pump timing be moving around a little? Hard problem to work out and a sticky valve could be as good a probability as anything else. The screwdriver stethoscope idea is a good one that I use a lot myself. Another thought is that could it be a carbon build up causing early detonation and once it starts it blows the carbon out and runs good until it builds up again? Haven't come across that myself but if the engine is run off power a lot I guess that it could happen. Could a sticky injector cause this also by squirting too much unatomised fuel? When were the injectors last serviced and a compression test done?
 
kimcrwbr1 said:
Keep a long handle screwdriver handy for when it happens again (poor mans stethoscope) Go around and listen to different areas on the engine with it running for practice you might be able to isolate the problem area. Be careful of moving parts place the tip of the screwdriver on the valve covers you can listen to the individual rockers, alternator bearings front and rear just put your ear on the handle and search around with the tip. Have you found water in the fuel filter? I probably wouldnt hurt to get your fuel system scrubbed if you havent done so yet. You might have bugs in the fuel tank kinna like a bacteria that lives in the water at the bottom of the tank.
Click to expand...

Thanks, will try that. It was so rough the last time it occurred that I would have been impaled by the screwdriver in the process! But if it occurs again and the seas are calm, I'll certainly do this.
 
aliboy said:
Are you on an outdrive or shaft drive? Things that come to mind are things like drive donuts and gimbal bearings etc which on an outdrive will still turn in neutral gear. Another one that I had was a cracked outdrive transom assembly that made a banging noise only when the steering was on a certain angle. In the engine itself you would guess maybe an intercooler with an intermittent leak of some sort putting water into the cylinders and causing early detonation or similar but that sounds unlikely. Could the injector pump timing be moving around a little? Hard problem to work out and a sticky valve could be as good a probability as anything else. The screwdriver stethoscope idea is a good one that I use a lot myself. Another thought is that could it be a carbon build up causing early detonation and once it starts it blows the carbon out and runs good until it builds up again? Haven't come across that myself but if the engine is run off power a lot I guess that it could happen. Could a sticky injector cause this also by squirting too much unatomised fuel? When were the injectors last serviced and a compression test done?
Click to expand...

It's shaft drive. It's probably about time to get the injectors done, a good reminder, and I can get the compression tested at the same time. I will also make a habit of running the engine hotter on occasion to help avoid carbon build-up. Thanks!
 
I am a keen gamefisher and do a lot of trolling in my twin diesel launch. I also associate with a lot of other gamefisher type people including charter operators. One of the 'rules' that most of us follow is that after a days trolling at low power we give the engine(s) a run at full cruise rpms for the last 10 to 20 minutes of the day. With my electronic diesels I am not so sure that it is critical, but with older mechanically injected diesels I would consider this a very good idea. It helps manage the carbon build up as well as making sure the oil is right up to temp and any moisture in it is 'burned off'. I am at 4500hrs on my twin diesels now and they still run like new engines and this is the process I always try to follow.
 
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