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rough idle/running. bad IDM?
I have a 1996 F350 with 300k miles. Relatively new to me with no known service history.
It developed an intermittent misfire at idle that eventually became not-so-intermittent. Developed to full blown runs on 7 cylinders. Did some digging and found a decent list of suggestions. (most point to injectors.)
Started with valve cover gaskets. found some issues with the connectors and the harnesses as well. Figured "hey, this must be it." Replaced the gaskets, repaired the harnesses. No change in running though.
By unplugging the harness connectors on the valve cover gaskets with the engine running i narrowed it down to the two cylinders on the rear of the drivers side bank. Replaced both of those injectors with two other injectors from a junkyard (so not necessarily known to be good). One of the injectors that was removed was in bad shape. Looks like it didn't seal to the combustion chamber properly and had some carbon around the bottom of the injector where it is supposed to be shiny. Thought "ok, THIS must be it."
After that replacement, I now still have a misfire, but I also now have smoke. maybe/hopefully fuel smoke as that injector is now working?
I also now have a noise. At idle, it seems to be the same (7 cylinders ish). At light part throttle its pretty smooth. As throttle increases, I get what I can only describe as knock? (i'm a gas engine guy mostly.) sharp and seems to be only one cylinder. It is clearly load (or throttle position) dependent though.
So, slowly running out of things to do/check. What is next on the list?
I have no CEL to point me to anything specific. I am assuming generic OBD2 codes work on early vehicles like my 96.
What i have not replaced or checked is the IDM. What are the symptoms of a bad IDM? Next step is probably to try to find another one to swap in, and to check the wiring between the IDM plug and the valve cover gasket plugs. Any other suggestions or comments?