Help! 1988 I-Mark engine knock

Son bought an 88 Isuzu I-Mark (1.5l, non-turbo, carbureted). Prev. owner provided lots of repair bills. Her son had taken the engine out by not monitoring oil level, even with a known rear seal leak. Given
the age of the car, they got a replacement engine from an auto wrecker. Rear seal leaked on that, as well, so they had that replaced.
Motor is clean; didn't make any noise during a warm look-over, or during the test drive the next day. But since then, it has developed a real knock. From above, it doesn't even sound like its definitively engine knock, but we had it on the hoist at my father-in-law's, and from underneath, it sounds like the world is ending....
Engine has to warm up to get noisy. Knock is quite quiet at idle, but gets worse with RPM. Seems to be worse at steady load (as opposed to higher load from acceleration, or no-load when coasting down). Had the pan off; rod bearings have a little scoring (187 000 km engine), but NO visible color change or such to indicate wear. (sorry, didn't have Plastigauge that day to check) Mechanic's stethoscope run around on the engine doesn't point me to anything. Noise seems to be worse from outside. Doesn't seem to be timing belt slap, or anything like that. Engine does not appear to have a harmonic balancer, either.
The only thing I can think of is possibly a wrist pin. Anybody have any other thoughts?
Thanks in advance. DanR
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end away from the house or anything you value and don't want to have to clean. Take off the air cleaner so you can get to the carb inlet throat. Get some Motor Mystery Oil (red can, available at any good parts house) and put it in a squirt oil can (not a dripper). Start the car, and with your hand on the throttle, open the thtottle, and as you're revving it up, squirt the MMO into the throat of the carb. You will make LOTS and LOTS of smoke, and probably the engine will shake, rattle and roll... (well, not roll, but it sounds good) - but if it is carbon on top of a piston, which is what I think it is, you'll be doing your best to dislodge it and blow it free. Other people have used water to do the same thing - I prefer MMO. I'd run about a cup to 2 cups of MMO through it, and take it for a drive. Assuming you don't have a broken piston, a wrist pin that's come loose, or a spun rod bearing, you've probably got carbon buildup on a piston. Of course, you can't hold me liable for blowing up your engine if you actually do have one of those other things, but I doubt that's it, and that's what I'd do if I were you. I hope this helps. r/ Steve
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Thanks, Steve. Don't worry, you're absolved from blame on anything I'm brave enough to try...
So, if it is carbn build-up, can you explain why it gets worse when the engine is hot? Are you thinking it is a piece that comes loose, and bounces around in the combustion chamber?
Also, for my own info... is this an Isuzu-specific thing? Or is your advice more general?
Thanks again DanR
GreenGas wrote:

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To check for a bad wrist pin short out one cylinder at a time. Ron
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Thanks, Steve. Don't worry, you're absolved from blame on anything I'm brave enough to try...
So, if it is carbn build-up, can you explain why it gets worse when the engine is hot? Are you thinking it is a piece that comes loose, and bounces around in the combustion chamber?
Also, for my own info... is this an Isuzu-specific thing? Or is your advice more general?
Thanks again DanR
GreenGas wrote:

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carbon built up on top of a piston, and then a piece breaks free from behind an intake valve and gets embedded in the piston top, and hits the top of the combustion chamber. I'm not sure how the rod/piston may grow, perhaps more than the cylinder/head dimension when cold???
This is NOT an Isuzu-specific thing. I've cured Fords, Chevy's, Mopars, and any other make that came into the shop with a knock, some so bad you'd swear they had a bad rod.
One of the other posters was right, though, if you short out the plugs or simply pull each wire out of the distributor one by one, if it's a wrist pin, you'll quiet it down. Preventing that cylinder from firing will avoid putting pressure on that piston, keeping it's wrist pin from slapping or being pushed around. Hope this helps... r/ Steve
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Thanks, Steve. This was a new one for me - I've never heard of carbon building up to that point. Guess too much of a habit of "blowing it out" from time to time, ha ha...
Based on some advice from a variety of places, I'm sure this isn't piston slap. This one is quiet when cold, and gets worse when warm, rather than the other way around.
Father-in-law pulled plug wires today, and said the knock was tied to #2. So for the $500 beater, we'll get a head gasket, and pull #2...
Thanks again for the assistance! dan
GreenGas wrote:

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Dan, thanks - let us know what you find. Be real careful when you pull the head off, and see what is left in the chamber. When you pull the pan and rod end cap, do the same thing. You never know what you'll find. Also, many times, if a pin is broken, you'll see scoring on the cylinder wall when the piston is at BDC. r/ Steve

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Steve - here's the latest. Father-in-law got impatient, and pulled the head off. Cylinder, piston and wrist pin seem ok. Spark plug check indicated the problem was #2. So he also got some Plastigauge and checked all the rod brgs. Chilton calls for 1 to 2.2 thou (I think that's what he said). #1, #3, and #4 were all towards the 2 thou, but cyl #2 was over 3 thou. That doesn't sound like much, but it seems to be the only thing amiss...
SO: Based on a chat with a former machine shop operator at work, we're going to measure the crank, and (assuming its okay) try to source .001" undersized bearings. F-in-law also said the oil rings were all really dirty, so we'll do mains and rings while we're at it. No detectable ring ridge, so we'll leave the block. (As a side note, the motor in this car was a replacement from an auto wrecker. There was some sludge/mud in the system when I did the coolant flush, and the rod brgs I looked at did have some scoring, so we're thinking that it was either left open, or run far too long without an oil change, because there seems to have been dirt in the engine.)
A bit more than we were hoping to put into the son's first car; feeling a bit guilty as a dad/amatuer mechanic that we didn't find these noises on our test driving (which I thought was quite extensive!). But, that said, the rest of the car is really solid; new carb, new distributor, everything else seems fine. And he didn't pay much. So I think we'll be okay when we get through this. Thank goodness for the F-in-law's ultra-cheap labour rate. Otherwise we would have had to cut our losses!
Thanks again for the assistance. dan
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