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Discussion Starter #1
The trusty 3TN66UJ engine in my restored JD330 started knocking loudly and spewing black smoke then white smoke within 5 seconds of starting to go DOWNHILL on inclines that are 10 degrees or more. This started after running out of diesel and having to bleed the fuel lines. I have re-bled the fuel system to no avail. Runs perfectly normally on level ground or going uphill. Any ideas are welcome!
 

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No solution but some things to ponder. Still sounds like it has air - white smoke can be timing, entrained air, or coolant in the combustion chamber (and possibly other things too). But the black smoke is over-fueling and the knocking at running temperature sounds like advanced timing but could be just the over-fueling.
Is it at all rpms or just rated? If you pull up while going downhill, stop the engine, then try to start it, will it start ok? What smoke does it put out in that scenario? When parked downhill without the engine running, can you hear the fuel pump running? If you can, try bleeding it on the downhill and see if any air is present.
 

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Discussion Starter #3
Thanks Nell. I tried various rpms and it does it just the same irrespective of rpm. I had checked for coolannt in oil and oil in coolant but it all looked good. I will have to test stopping on a hill and restart.
I had this thought: could a plugged return fuel return, i.e. either a line (or nipple) at the injector cause it to go rich on incline? In the bleeding process I did bend the small tube (nipple) at the front injector and now I am thinking that even though I straightened it out it may have remained pinched enough to reduce flow to return causing rich condition on that cylinder. Thoughts?
 

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Definitely worth checking, because that's a change that has occurred since it was running correctly. I'm a little skeptical but not enough to say it's not it. Can you remove that nipple and just run it open briefly to see if it clears up? It'll spill a little bit of diesel but not too much to clean up.
 

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Also check your oil level to see if it has risen, and see if it smells of diesel. If you have access to a known good set of injectors, swap them in because injectors that don't produce the correct spray pattern at the correct opening pressure can "dribble" unatomized diesel into the combustion chamber, and this can both produce white smoke and cause loud combustion sounds / knocking. This wouldn't explain why it only happens when travelling downhill unless doing so affects the return such that it affects the injection event. You might also be able to isolate that by disconnecting the return briefly.
 

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Discussion Starter #7
UPDATE: Indeed the knocking on downhills was caused by an issue in the
fuel bleed return
. Each injector has a fuel return coupling; these three are interconnected with a rubber hose and ultimately back to the fuel tank. During my attempt to bleed the system at the injector, the number 2 coupling rotated enough to bend the brass nipple against the intake manifold. This pinched line prevented the relieving the return flow coming from No.1 cylinder, leading to overfueling (rich) conditions and hence excessive knocking in that cylinder. I was fortunate to source a complete set of three on eBay (thanks georgiadeere!). Although these had a larger outside diameter body, the ID was a perfect match and solved my issue. Thanks for your input!
 

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Good to hear - interesting that it only occurred travelling downhill. If I was to guess, perhaps the return line to the tank is at a level that if you travel downhill, the end of the return is submerged leading to just enough back pressure to cause your symptoms. Usually though the return exits into the top of the tank (or other places depending on design) - can often see it dribbling in when looking inside the tank with the engine running.
Happy mowing!
 
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