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I have a mostly stock LT1 in my 95 wagon with 225,000 miles on it. It's the original engine and been running wonderfully without burning any oil. Suddenly on a road trip of 10 hours it started having a constant and consistent ticking knock that worsens significantly as load increases. It even occurs at idle but it's faint. It also seems like a cylinder is misfiring or not firing. My trusty antique Autoxray scan tool just died too.


With my limited time and lack of scan tool at the moment I've narrowed down the problem a bit but any ideas and direction would be appreciated.


Here is what I can tell so far...


The sound seems to originate from the drivers side of the engine.


Disconnecting plug wires one at a time narrows the culprit misfire down to the middle two cylinders on the drivers side. It does not stop the knocking though.



Spark plugs in all cylinders looked great and identical even after having driven it slowly this way for several hours.



Spark at each plug is strong and consistent, even when using a tool that views a spark inline with the installed operating plug.


Compression check on the cylinders on that side were not consistent but I had to do it cold. They were good enough that I don't think that it's a bent valve. I had 129, 95, 100, and 142



Timing chain and opti were done about 5000 miles ago.


Although I realize spark could still be the problem, I was going to check fuel injectors when I get the chance.


I'm hoping that it's not something major with the heads but I suppose that it may be. Would a bad lifter do this?



Has anyone gone through something similar?
 

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Could this be a manifold leak instead? Knocking at idle is not all that typical. Could the spark plugs that are not "firing" be sparking to ground through the coil wires or opti and that's the sound you are hearing?



BTW ... your compression numbers are pretty far apart. Engine wear ?
 

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Constant engine knock at idle that gets worse with load, and better when pulling spark plug wires is usually a rod bearing. Get under the car with the engine running and poke around with the stethoscope. Rod knock sounds can travel and the scope will help you isolate it. Pull the oil filter and cut it open to look for metal. I'd wager a bet about what you'll find.

Your compression test is not "fine". The standard acceptable variance is 10% between cylinders and you're close to double that. Not surprising given your mileage, but just putting that out there. Your compression in the middle two cylinders is low. Not zero, but lower than the outer two. There's a chance you have a weak HG between them and it would not leak into the water jacket or the oil. Do a leakdown test on one of the two cylinders in question and pull the plug out of the other and stick a mechanic's scope in the plug hole to listen for air.
 

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Get the driver's valve cover off to examine springs/rocker/pushrods.
I have never heard a valvetrian issue "knock" - they tick, sometimes loudly. But for peace-of-mind, start at the top. It's only 4 bolts.
 

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Your detail op reapin' good early returns! Sure can't add nothing technical, so only offer 1st kneejerk was/is age, wear, dirt, in that order. It sounds as though you've had the car since new, so likely not neglect issue. But for example I found a 3-year old L-250 had holed a lifter and gave a real scary odd tick/almost knock tap. Next, more than once I've had something I worked on affect something else. That's where 'dirt' came to mind. You'd know yourself if some recent maint/repair could have added a piece of grit to a pr or lifter. I'm braced and sitting down, so did 'someone else' do anything to it recently?


At 225k popping the covers would be a perfect chance to eyeball even a perfectly good motor just to check things. But biggest hint I've found is getting another set (or two) of eyes and ears on it before almost anything. Amazing how the sound description and location moves around to different people. You'd hate yourself if it's something goofy like a rubbing exhaust or suspension part.
 

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don't rule out an exhaust leak, cracked manifold, air injection system leak.
one of mine was almost split in half, to an untrained ear someone would think knock knock.

a length of garden hose or heater hose to the ear will reveal exhaust leaks.
 

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Whatever else 'ol Scotty knows he's lost 2/3 his creds not even knowing how to light a cigar. Crimminy. My SSRI and even the swissed box on the other one both hiss like crazy, but ain't no leaks with either.



Oh and the laziest way to find exhaust leaks = Sea. Foam. Hahaha
 
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