Engine thumping. It may be a paper towel in a valve.

[hlev00]

I started my Yanmar 4JH4E on the hard getting ready to launch. It fired right up and had good water flow sucking out of a bucket. Then I noticed a thumping sound from the back end. The frequency seemed half of the RPMs. I ran for a minute or two while I tried to isolate then shut it down normally. When I got home and thinking about I got a sick feeling in the pit of my stomach.

So last fall I may have placed part of a blue paper towel in the air intake. Usually I place plastic bag around it, held in place by rubber bands. However I also removed my exhaust elbow so I placed a paper towel in the output of the exhaust manifold. Maybe I did something similar with the air intake. I really can't imagine that I would have stuffed it in there so I would notice, but it's all I can think of at this point. (Yes, if I can't keep track of this stuff I'm going to stop trying to be my own mechanic.)

It was still thumping when I shut the engine down. No horrible noises but I know that doesn't guarantee anything. I was going to try to remove the silencer and take a look with a friend's endoscope to see if I can see any evidence in the intake manifold. Whether I see anything or not, I expect my next step is to have the head pulled to see what might have been damaged.

I've found a couple of posts suggesting that a paper towel isn't likely to live long enough to do any damage but the fact that the thumping sound didn't go away makes me think something is semi stuck.


The link is the audio from the back end of the engine. The thumping is hard to detect with good speakers or ear buds / earphones, but live it definitely sounds different than I remember having listened to this engine for over 10 years.

https://drive.google.com/file/d/1yTm...usp=drive_link

Any other thoughts on next steps or other causes and diagnoses to pursue?

Thanks all,
Harry

[skipperpete]

Before trying to address the paper towel issue, I have a question. Is there any chance that the engine was accidentally in gear?

[Bellinghamster]

I agree, it doesn't sound normal. Bummer! Take the intake manifold off and you should be able to see if there is paper towel stuck in the intake valves.

[Foswick]

Hard to tell with the audio sample, but it does sound like a valve issue. To my ear, it sounds like a valve is not seating correctly. I couldn't hypothesize beyond that without being there.

[hlev00]

Re: is it in gear? My thought too, but no.

[skipperpete]

Probably wise to roll it over manually 2 full turns to see if it has a hard spot, I definitely would avoid starting it again until a bit more static investigation has been done. I gotta say, the audio doesn’t sound too terrible to me…. But I’ve seen a JH series do a similar thing after a bunch of soot detached and ended up in the combustion space, we already had 2 instances of this in the past on the forum, from memory both were lockups though, not “thumping”

[Scrimshaw4]

Have you checked your motor mounts? One might be starting to go or be loose and that would allow the engine to thump on it as it lifts and drops. Just a thought.

[hlev00]

Thanks for all the input.

I used a scope to check the manifold side of the valves. I couldn’t see it all but no sign of debris where I could. I put everything back together and ran it a while. It still seems a little different but nothing terrible. Sea trial after we launch then I will call it good until I can get an experienced mechanic to check it out.

[Lepke]

A sticking fuel injector can make an engine sound like a rod knock.

[RaymondR]

I will be very disappointed to find that a Yanmar engine could not eat a single paper towel and spit any remains out (unless you have very tough paper towels where you live)

Take the tappet cover off and check the valve clearances.