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Unknown 7.3l IDI knocking

Old 11-24-2018, 04:19 PM
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Default Unknown 7.3l IDI knocking

I have recently acquired a 1991 f350 with a nonturbo 7.3 IDI (174,000mi) with a zf5 five speed, this is my first diesel truck and it is bone stock. It is hard starting all the time, even when it is hot. On startup i get some smoke, then it clears up after a few seconds. I have done basic maintaince and replaced the glow plugs. But there is a very distinct knocking sound coming from the engine, it happens all the time hot or cold and at all rpm levels(thought it's hard to hear at high rpms). I have used a stethoscope to track the sound to the rear left of the engine. I thought it was a collapsed lifter so i pulled the valve cover to check, and all the rocker arms are solid. Next, I thought it could be an injector so I cracked each of them to see it any of them dulled the knocking sound, and they didn't, the truck was just pulled down by the miss, but the knocking continued. My last thought was the clutch on my zf5 transmission, but while I hear the knocking in the bellhousing it appears to be loudest on the engine(cylinder 4 I believe). I am stumped on what this knocking is, by the location of the knock I dont think it is rod knock, I hope. Sorry for the long post. Any help is appreciated. Thanks in advance.

Last edited by bigmoot19; 11-24-2018 at 04:40 PM.
Old 11-29-2018, 03:54 AM
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It is possible that the dual mass flywheel is causing it. Though it can be tough to verify.

At idle does the sound change any of the clutch is depressed vs when it is released?

It it is also possible that due to wear and tear on the injection pump and injectors, that the injection timing has been impacted. Or the timing was adjusted by a previous owner.
Unfortunately you only have a couple choices here. Adjust the timing blind and tune by ear, or use the proper timing equipment to verify timing and adjust. Unfortunately not many people have the proper equipment to do this on these engines. Where are you located?

Get back to us on the answer to the clutch clutch question and then we’ll work through some more possibilities.

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