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My 84 300sd has developed a knock in cylinder 5. The car has 175k on it and has been treated well it's whole life with regular oil changes and maintenance.

When I purchased it a thousand miles ago, I did a valve adjustment, new filters, cleaned the tank strainer (which was plugged solid), and went over everything I could find.

It had recently begun nailing so I rebuilt the injectors with monarch nozzles. Since this didn't fix the issue, I started digging a little more. I swapped the injectors in 4 and 5 and thought I had it, but the noise continues in 5. When the line is cracked, the noise is gone. I have since swapped the injector with #1 and the noise still continues in the back. I'm super nervous it's rod knock, but am still hoping for the best. Can air get trapped in the lines and cause knocking? If the returns are pulled at idle, I get very little fuel squirt. I have an extra filter before the MB filter and have changed that, but I don't have another factory filter here.

I did just have a leaky lift pump and replaced it with a used one. A new style new one is in the mail now.

The knock isn't super loud, but it is louder than normal. It goes away as engine speed is increased to around 2000. A little black smoke at pull out, but not any white.

Compression in that cylinder is around 400.

Suggestions would be very welcome.
 

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Discussion Starter #2
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I adjusted the idle up a bit and adjusted the rack bolt in a smidge. Both adjustments helped a bit. Engine actually moves a tiny bit more, but sounds a lot better. The tap/knock is still there, but less noticeable. The injectors seem to be firing more evenly (as in all sound the same and all lines pulse about the same amount) but they are actually a bit louder. They actually sound exactly what I thought the monarchs would sound like.

Could the valves need to be gone over again? With my compression at 400 I assumed they were still fine.


Still would love any suggestions. Thanks.
 
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