Wow...what a knock.

kz6fittycent

@MazdaMitch you know...when I was probing around exhaust header #4 was fairly loud and had a different tone than the others at idle. I wonder....

Axel-Breaker-Earl

Does your truck have a Carrier Bearing on the driveshaft?

kz6fittycent

Axel-Breaker-Earl No it doesn't. The engine will do this out of gear but only at a certain rpm _2k w/o the AC on. It will not do it under load, or decelerating. It also won't do it until you reach a STEADY rpm out of gear. So, revving it up to 2k, it's not likely to do it, but if you slowly rev it, and hold it, yeah, it's going to make the noise.

Axel-Breaker-Earl

I forgot that you linked a video of the noise with the truck parked and engine running.......DOH!

Axel-Breaker-Earl

When the truck is parked, in neutral, and the engine is running like your video shows.......can you see if the harmonic balancer is moving forward or backward while changing the RPM's up and down.......especially to the point where it is making the noise? Basically varying the RPM back and forth while looking at the Harmonic Balancer for any front/back movement.

kz6fittycent

Axel-Breaker-Earl the harmonic balancer isn't moving. Dead stable. Took the timing belt cover off, and placed the stethoscope on the tensioner bolt. Revved the engine up and it's pretty loud, but it could just be resonating. The belt is brand new (₄₀₀-500 miles on it) and so is the tensioner.

The belt seems "tight" enough and the tensioner was torqued per Mazda spec (can't remember but whatever the manual says). I do need to double-check the timing belt tension to ensure it's per spec but this is a metallic sound.

I had my wife rev the engine and hold it at 2k rpms. Moved the exhaust all around by hand and no change in the sound. So I need to just move on from the exhaust being the problem.

I still need to remove the alternator belt, and the AC belt to see if anything changes.

OGMB2K

I'm not offering a new diagnosis, just an observation that the knock is there at all times, it is just louder in the range you are talking about. It sounds like a mini diesel at idle. As it has been said, at the higher RPM's something may be throwing the engine out of balance which is why it gets louder in a particular range.
What I have not read yet is whether or not this engine has been worked on internally? You mentioned someone already botched the clutch, is it to much to assume that a wrong part was used or something was not assembled correctly internally?
You've done just about everything externally including isolating the engine from the A/C and alt with no change, it may be time bring out the engine hoist.

kz6fittycent

OGMB2K actually, it's not there at idle. It's butter smooth. I know you're hearing something, and that'd be the valve train at idle. But this does NOT make ANY sound until _2k rpms. It's abrupt.

Also, it does NOT make this sound under acceleration nor during deceleration when driving.

I don't disagree with you that maybe someone decided do rebuild this engine and botched it, but I am 10000% going to figure out what's happening before I pull the trigger on buying a rebuild kit, or a rebuilt engine.

I am going to pull the plugs out and rotate the engine, getting each piston on the down stroke, then using something small enough in the spark plug holes to see if any of the pistons move when applying pressure. This will determine or eliminate a bad rod, wrist pin, or bearing.

OGMB2K

kz6fittycent Wouldn't that be easier from the bottom? Just remove engine brace and the oil pan, it may be uncomfortable from the ground looking up, but you will have a better view of the bottom end of the engine and may even be able to wiggle some connecting rods from there .. or at least check some torque specs .. oooh if you do that, please get me the measurements of the bolts that hold the engine brace in ..

kz6fittycent

OGMB2K If it's a wrist pin, not really. I might be able to see up there, but 4 spark plugs vs umpteen tiny bolts and laying on my back again, after laying on it for 2.5 days doesn't sit well with me.

kz6fittycent

It's definitely cylinder #2.

I pulled one plug wire at a time and the knock went away when I pulled #2. I'm going to pull the pan, and check the bottom end. If I can replace the rod bearing on that cylinder, I will and move on. Otherwise, time to order a rebuild kit and maybe one rod. What brands do ya'll recommend? I found several. I'm also going to replace the head - ain't playing around.

I found this rebuild kit on eBay.
Here's a head I found, too.

Cusser

kz6fittycent If I can replace the rod bearing on that cylinder, I will

Yes, one can replace the rod bearings from underneath once the pan (and I think the oil pickup tube) are removed.

kz6fittycent

Cusser Has anyone ever attempted replacing a rod and piston from the underside w/o removing the crank 😛

I'm being facetious but....you know sometimes people figure things out and I thought I'd ask.

Cusser

kz6fittycent Has anyone ever attempted replacing a rod and piston from the underside w/o removing the crank

I think the serious answer is that the cylinder head must also come off for that.

MazdaMitch

There is a lot not used in a kit it's better to just piece together what you need

kz6fittycent

Okay, disaster averted and prayers answered! The rod's okay, the crank isn't prestine, but it's still in spec and it'll be okay for a while until I can get time and money to do a full rebuild. The bearing material was in the the pan, and the oil pick up caught some other bits. The filter did the rest.

Turns out, at some point this truck had a head gasket replaced - the restricter for the valvetrain was in the pan! In addition to that, there was some old RTV, bits of what looked like a gasket, and maybe part of a rear main seal. Whoever worked on this thing in the past didn't do it any favor by "fixing" it.

I ordered new rod bearings. I'm gonna replace them all while I'm under this thing. Here's a pic of the bad bearing.

P.S. Once I'm done getting this thing back together again, I'll post some videos here for your entertainment.

Axel-Breaker-Earl

So, I'm assuming that the rod bearing was SPUN. If so, it will have damaged the big end of the rod to a certain degree. Typically a machinist will resize the rods if they had spun a bearing. This is basically done by grinding material off (about .002" or less) of the rod cap & the rod where the cap bolts to it. This makes the hole in the big end of the rod oblong.....instead of round as it should be. Then the machinist will hone the big end of the rod round again and to the specification that is required......so the new rod bearings fit as intended.

Can you just place a new bearing in the rod and run with it? Who knows? But it is worth trying I guess, if you don't want to fully rebuild the engine yet.

Let us know how it goes!

kz6fittycent

It didn't spin! It was still actually locked in place! I was surprised and grateful, to say the least. I'm going to mix bearing sizes if I have to make this work. I am not going to remove anything, since I have plans to do a full "performance" rebuild or a rotary swap in the future. This engine just needs to last until, well as long as it can.

EDIT: If it did spin and I'm okay admitting that I'm wrong, it was still in it's correct location in the rod, and the rod appears to be in good condition. Regardless, I believe I dodged some major damage with this one.

kz6fittycent

On the road again. All the rod bearings got replaced. #2 needed a mixed set, like I mentioned.

Cusser

kz6fittycent On the road again. All the rod bearings got replaced. #2 needed a mixed set, like I mentioned.

Great ! Keep us posted.

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