My '86 6.9 started idling rough ( like one cyl was missing ) and making a knocking sound that got louder and increased frequency with the RPM. I replaced the injectors, checked the glowplugs ( all intact ). It started up easily enough but the knock is still there. I listened closely. The knocking noise is more like a regular burst of escaping pressure loud through the intake. It sounds like an intake valve is stuck partly open. This is a daily driver with about 145k miles.
I pulled the valve covers and ran it. All seems normal visually. My next step will be finding out which cylinder is bad. I can either check and compare resistance with the disconnected glowplugs ( after the engine heats up ) OR use an infared thermometer sensing gun and find where the coolest spot is on the exh manifold. Am I on the right track? Could this be the symptom of bent rocker arm, bent pushrod, damaged valve?? Or is this a common symptom of a blown head gasket? Oh yeah- there is some new oil leaking out the back of the engine... About a teaspoon in a few days. I haven't pinpointed the source of the leak yet.

 

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Normally the 7.3 is susceptable to valve guide wear, but with a loud woofing in the intake could mean a bad intake valve/guide.

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the idi engines do sound strange with the intake opened up anyway. Short out the injectors and determine which cylinder is different. Have you seen the oil leak since you installed new injectors? Could be seeing fuel mixing with dirt on the way down from a leaking return line o ring OR could be seeing fuel from the weep hole in the pump running thru the valley and down. Is it using water too?? Just some thoughts. Sounds are tuff for me to figure out with all that is goin on in them diesels.

 

I would do a compression check to see if one is really low compared to the rest of the cylinders

 

Sounds like your in the early stages of head gasket failure. Our 84 6.9 with less than 40k (its a bucket truck) sat for a year when went to start it it was noisy it the intake, and made white smoke. Over several moves it got worse and worse. Finally it would bearly move. invistagation found no compression on one side of block. You can do a simple compression check by removeing the glow plugs and placing your finger over the hole (don't press hard) if there is no compression it will not pop your finger off. If there is it will forcefully pop it up. The torque on the head bolts is very low for a diesel engine.
75 ft# on an 84. Get a book and read before taking anything off. Intake manifold and head bolts must be taken off in reverse install seq. 1/4 turn at a time. Else you get warped manifold and heads. Cleaned up the valves and replaced the blown head gasket and the other one. Runs good and no smoke or intake noise. You can use a streight edte and a flash light to check the head for warp. No light no warp. The Haynes book has the info in it you need but you have to work at using it.

dj

 

This isn't a powerstroke, the injectors are controlled mechanically. In order to isolate, you have to take the fuel lines loose to keep it from pressuring the injector. Don't have to take completely loose, just enough to release the pressure. Messy, yes, but that's the way it has to be.