Couple of 6.9 issues

Ghuds

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Picked up my first 6.9 IDI a few weeks ago. 83 F250 XLT 4x4. I just replaced the glow plugs because they weren’t working. Had to start it on ether when cold. Starts great now...
The current issue is white smoke when cold, and a miss at idle and higher rpm. Sometimes it runs fine though. The guy I bought it from said he replaced the injectors, but I wonder if one is bad. Or possibly the IP has issues. But like I said, it doesn’t run rough all of the time. Also, when it’s missing, the tach goes crazy.
I don’t think I have any air intrusion problems, there are no leaks and it fires right up after sitting several days. Any advice on where to start with these issues? I would think that if it had cylinder or valve damage it would run crappy all of the time, but I’m no expert on these engines.
 

franklin2

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How long does it run bad? If it has a miss and white smoke for awhile, you can go around and crack each injector loose while it's running till it starts leaking. This will kill that cylinder temporarily. If the engine gets worse you know that cylinder was good. If there is no change, you know that is one of the cylinders not firing. Sort of like pulling the sparkplug wires on a engine to find the dead cylinder on a gas engine.
 

Ghuds

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Off and on. It rarely runs like it should for a long period of time.
Should I loosen the injector or the line that feeds it?
 

chillman88

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The line on top of it. You just want to interrupt the fuel flow so it won't fire. Like he said, basically like pulling a plug wire on a gasser.
 

Ghuds

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That’s a possibility. Someone bypassed the separator at some point. I don’t know if it was leaking or wasn’t working. I’ll have to look into that one. Thanks!
 

franklin2

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Bypassing the water separator on the older 6.9's is standard procedure. Apparently they did leak air.
 

The_Josh_Bear

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Another tip is to fill the fuel filter with ATF or your favorite diesel lubricity additive and run the engine for 30 seconds or until the engine quiets down a bit. Shut down and let it sit overnight. This will clean the IP and injectors.
 
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