M37 engine knocking

[hasaboat]

Hello Steel Soldiers and I wish you a pleasant Labor Day weekend.

I have a very strange engine knock that I hope someone can help me to troubleshoot. I was out for a drive a few weeks ago and the engine acted like it was getting hot so I headed towards home. Before I go to the house the engine started making a very strange knocking noise but by this time the thermostat indicated 160 F and I had oil pressure. I have been doubtful about my water pump so I ordered a new one and, while waiting for it to show up, I dropped my oil pan hoping not to see steel shards, nuts, bolts, washers, springs...you get it. Well, the good news - I think - is that the pan was surprisingly clean and except for the expected amount of gunk I did not find anything unexpected. I ran a magnet through it and the small amount of iron filings that one would expect was all that I found. I checked the rods and other things I could see and everything looked right and tight.

I got the new water pump and installed it yesterday. I started the engine and after a few minutes I could hear a faint knock in the engine. The knock becomes louder as the RPMs increase. Now here's the part that stumps me. The knocking is not in time with the RPMs. The knock might happen every 2 to 3 seconds and again only at higher RPMs. The knock might not happen for a minute or two, then a knock or two then nothing for a minute or two. It almost seems like some debris is getting occasionally picked up then falling back before making another appearance. The knocking seems to be low in the front of the engine. At idle the knock is faint or non-existent. The engine is running great i.e. no missing, backfires, etc.

Has anyone experienced this before? Obviously I don't want to damage the engine but I would really like to not have to pull it either.

Hoping for any easy remedy!

Thanks, Todd Burkes

 

[G744]

I'd pull each rod cap and see what the bearing & crank looks like. Not too hard to do.

Seldom do mains fail.

 

[GopherHill]

If the rod bearings look OK, shine a bright light at the wrist pins and piston skirts.
It's a possibility. An aside, be sure the water distribution tube behind the water pump is not rusted away. The #6 cylinder suffers if this item is bad.

 

[hasaboat]

Thanks guys. I will get started on this by Monday and keep you posted. As always I appreciate the help.

 

[hasaboat]

 

[hasaboat]

View attachment 878377View attachment 878378

This is from #1. I am not an expert but I don't see anything alarming. The rod bearing halves are scored but each in a single piece. I an going to pull #2 but if it looks the same that will probably be as far as I go with this. What do you fellas think?

 

[GopherHill]

Just for your peace of mind, I suggest pulling all 6 rod bearings. If you aren't dead broke, replace with new bearing shells. You have already got the oil pan off.

 

[NDT]

You will need to inspect all the bearings. Suggest you use plastigauge.

 

[hasaboat]

I don't know why I would ever question this groups' expertise.

I think the attached pic from #2 tells the story. The bearing halves are imbedded in each other and paper thin. Geez these engines are tough. I will for sure pull the other caps.

when saying bearing shells do you mean the bearing halves? Clearly I have to replace these from #2 but changing all 6 would be a good move, correct? Also I haven't checked the bottom of the #2 rod. How wrong would it be to replace all of the halves to make an upcoming show then fix it right in a month or so? I would drive only about 40 or 50 miles for the show.

Thanks so much Steel Soldiers!

 

[NDT]

When the bearing halves are that bad, you can be almost 100% certain the crank needs to be turned.

 

[Johnbyrdgates]

I agree with NDT regarding the crank’s probable condition. However, I used to read the tech notes section of Military Vehicles magazine and in one article, the excellent author (Turchet?) indicated that replacing those bearings was better than nothing and a lot easier than having the crank ground. So maybe replace them (both “shells” on each cylinder) and go to the event, listening for the difference.
It’s always best to do the job properly in the long run, since your camshaft is probably worn where it drives the fuel pump, the wrist pins are probably tired, etc. But I’d give the bearing replacement a shot.

 

[GopherHill]

I agree with NDT regarding the crank’s probable condition. However, I used to read the tech notes section of Military Vehicles magazine and in one article, the excellent author (Turchet?) indicated that replacing those bearings was better than nothing and a lot easier than having the crank ground. So maybe replace them (both “shells” on each cylinder) and go to the event, listening for the difference.
It’s always best to do the job properly in the long run, since your camshaft is probably worn where it drives the fuel pump, the wrist pins are probably tired, etc. But I’d give the bearing replacement a shot.

For sure, replace all 6 rod bearings. Try it and see.