Back knock after intake manifold gasket change

1998 CHEVROLET S-10
190,000 MILES • 6 CYL • 4WD • AUTOMATIC
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IANOBETZ
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Last week I had coolant leak on my truck. I spent this weekend looking for the leak and found it in the intake manifold gasket.

I spent yesterday and today changing the upper gaskets on the truck. I made sure to mark the distributor, followed all directions, then put it back together (almost). I could not remember where the ECT sensor I had removed the terminal from was, so left it hanging. Maybe I got a little antsy, but I wanted to hear the engine fire up and run. I double checked all nuts, connections, etc and fired the engine.

It cranked, fired right up, sounded great, then shut off after about ten seconds. So I fired it up again, it started, but this time it had a really bad knock in the engine. I found the ECT sensor (thinking that might have been the cause), plugged it back in, but no luck. The engine will not throw a service engine code when left running for a few minutes. I do not want to leave it running much longer than three to five minutes because it sounds so bad.

Any suggestions?
Dec 1, 2010 at 5:20 AM
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FACTORYJACK
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Is there any possibility that something had dropped into an intake port or made its way into a cylinder? You might be able to isolate it to a specific cylinder by removing spark one at a time. It is possible that a chunk of carbon may have came loose, resulting in a knock. If you can identify a cylinder, remove that plug and allow it to run, the compression may force whatever it is out of the hole. If you are not sure that you had all the fasteners upon reassembly, that may be a cause as well. If that is the case, it will not be long before damage occurs. Try removing the spark plugs one at a time to see if there is any indication that something was in the cylinder.
Is it a fast knock or a slow knock?
Dec 1, 2010 at 5:42 AM
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IANOBETZ
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Nothing dropped in the engine. As soon as the manifold came off, a rag went down there and paper towels in all of the holes. I took extra care that nothing went down there.

I removed each plug wire one at a time, but the knock did not change at all.

This knock is loud, not a chunk of carbon (which would probably sound dull). I do not want to run the vehicle for very long for fear something might break.
Dec 1, 2010 at 5:56 AM
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STILISON
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Same thing happened to me. I had to take off one of the cylinder heads to remove a washer that fell in somehow. Bad luck I guess. All fixed now and running great.
Aug 28, 2018 at 5:43 PM