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Broken (melted) piston ledge x 2 times


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Hello, I'm new to the forum.....I have an 1986 maxidyne 250 four valve engine and have (twice now) melted the rear most piston at the ring ledge Same hole both times? the second time I changed the injector although all tested out ok, this was the beginning of last construction season and thus far the problem has not risen its ugly head,,,,, I have a pyrometer and have always been very careful not to exceed the 1050* heat max. and would like some input into this...also like to know if anyone else has experienced this problem....oh, it was an econodyne engine and I did open the fuel up on it a while ago (before the first meltdown) which I don't believe is related...I've been wrong before.

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Timing will do this and also setting the fuel up will do this as well as the fuel pump calibration should be pulled and sent to fuel shop.One pump may be fueling more than the other five.The piston oil cooler will in most cases score the shirt of the piston if its bent.

No scoring, just melted down over the edge of the piston to the point of breaking the ring and then the ring ledge......the pump was completely gone through when the engine was rebuilt two pistons ago.....and the timing was checked when the engine was rebuilt and possibly when the first piston was replaced....the second time the piston was replaced and the injector in the same hole was replaced but nothing else was done. The fuel is maxed out but as I said in my OP I am very careful of heat, My concern is the fact that its the same hole each time so the over fueling in that one hole is deafinatly a possibility I'd not considered......However I take your an anyone elses info and advise to heart and appreceate any and all input.

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I agree with Glenn. Perhaps when the pump was rebuilt the #6 plunger was set incorrectly. Did the shop flow test the pump after rebuid? If so, look at the individual flow results.

Ken

HOF City, PRR Country, and Charter member of the "Mack Pack"

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I agree with Glenn. Perhaps when the pump was rebuilt the #6 plunger was set incorrectly. Did the shop flow test the pump after rebuid? If so, look at the individual flow results.

I don't know that they did any flow testing, I'll have to check and see, thanks for the help....both you and Glenn, I hope to get to the bottom of this before anything happens to this engine. This should help me do that.

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Im no hot rodding type of person but trying to think of how it could happen leads me to think of it like this

If only one cylinder is running hot the pyrometer wont pick that maybe because the overall heat spread across six pots will still measure ok so although you may be watching the pyrometer like a hawk the one cylinder getting hot might not show up due to the rest of the cylinders running cool if that makes any sense

Im just thinking out aloud as stated above the most likely cause is a fuel problem and that can only be two things to much fuel or the timing at least that is my understanding

good luck

Paul

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Im no hot rodding type of person but trying to think of how it could happen leads me to think of it like this

If only one cylinder is running hot the pyrometer wont pick that maybe because the overall heat spread across six pots will still measure ok so although you may be watching the pyrometer like a hawk the one cylinder getting hot might not show up due to the rest of the cylinders running cool if that makes any sense

Im just thinking out aloud as stated above the most likely cause is a fuel problem and that can only be two things to much fuel or the timing at least that is my understanding

good luck

Paul

I agree with everything you say, The pump WAS calibrated when it was rebuilt......the only thing I changed the last time around was the piston, rings and sleave AND a new injector, I did the injector change because I suspected the spray pattern was bad, That being said I have run the truck one season since I made this change and so far so good. The only way to know for sure is to pull the head and have a look. I'm not going to do that...oh, and the timing was checked too so too much fuel COULD be a problem but that really don't explain why it's the number 6 hole both times and the EGs staying within spec. It is beginning to be a head scratcher at the moment.

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Yeah I can see I didnt explain my self very well

if all six exhausts are going into the one manifold then exhaust gases all get mixed together and turn into one gas the result of the six different gases mixed and the pyro temp would be the average of the six

so it is quit possible that one cylinder could be 200 degrees C and the other and another 300 and two at 400 and one at 1000 and I dont know the last one at say 550 degrees C

so if we add them all together we get 2850 divide that by six = 475 degrees which on the pyro gauge would look all ok but as you can see the temps are all over the place and one of them to stage it's gunna melt a piston

Dunno if I have helped to explain or made me look more of a fool :D:D

Paul

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Hello, I'm new to the forum.....I have an 1986 maxidyne 250 four valve engine and have (twice now) melted the rear most piston at the ring ledge Same hole both times? the second time I changed the injector although all tested out ok, this was the beginning of last construction season and thus far the problem has not risen its ugly head,,,,, I have a pyrometer and have always been very careful not to exceed the 1050* heat max. and would like some input into this...also like to know if anyone else has experienced this problem....oh, it was an econodyne engine and I did open the fuel up on it a while ago (before the first meltdown) which I don't believe is related...I've been wrong before.

has anyone looked at that( piston cooler) spray nozzle

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Yeah I can see I didnt explain my self very well

if all six exhausts are going into the one manifold then exhaust gases all get mixed together and turn into one gas the result of the six different gases mixed and the pyro temp would be the average of the six

so it is quit possible that one cylinder could be 200 degrees C and the other and another 300 and two at 400 and one at 1000 and I dont know the last one at say 550 degrees C

so if we add them all together we get 2850 divide that by six = 475 degrees which on the pyro gauge would look all ok but as you can see the temps are all over the place and one of them to stage it's gunna melt a piston

Dunno if I have helped to explain or made me look more of a fool :D:D

Paul

You done great explaining what you meant to, but that doesn't explain how that could happen, if all injectors are being feed from the same pump...and the pump has been calibrated and known to be accurate, the air is being driven into the engine with the same turbo and the cooling system is working as designed and the water temp never exceeds 200 (very rarely that high) then what could cause the same piston (hole) to burn two pistons to the point of breaking the ring ledges and the other five out live that one by now 2 x's (the original piston after the rebuild....another after that and another season on all five of the others on top of that. That is why I'm scratching my head, I appreceate your input too, thanks

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Off handed, but has the valve train for that cylinder been looked at to see if it's fully opening and closing?

"OPERTUNITY IS MISSED BY MOST PEOPLE BECAUSE IT IS DRESSED IN OVERALLS AND LOOKS LIKE WORK"  Thomas Edison

 “Life’s journey is not to arrive at the grave safely, in a well preserved body, but rather to skid in sideways, totally worn out, shouting ‘Holy shit, what a ride!’

P.T.CHESHIRE

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remember you have a jerk pump each cylinder has its own plunger that rides on a cam shaft inside the pump and a rail that turns the plunger to increase or decrese the helix thru which fuel is metered, so if one plunger is out of adjustment it will cause this issue. and yes pump tecs can goof! i fought with a excavator that had a knock that we coulnt get rid of after a rebuild, it turned ot that the pump shop goofed and was over fueling one cylinder

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get a heat temp gun and ck each manifold(ex) hole after you warmed it up on a small drive and see what temps you are getting at each port

Great Idea, I hadn't thought of that......that should tell me if there is a temp difference for sure.

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remember you have a jerk pump each cylinder has its own plunger that rides on a cam shaft inside the pump and a rail that turns the plunger to increase or decrese the helix thru which fuel is metered, so if one plunger is out of adjustment it will cause this issue. and yes pump tecs can goof! i fought with a excavator that had a knock that we coulnt get rid of after a rebuild, it turned ot that the pump shop goofed and was over fueling one cylinder

That's a possibility albeit a slim one in this case, the pump builder is a very reputable and long serving shop that is used by most everyone in the area....but everyone makes mistakes, I 'll certainly keep that in mind as a place to look if I find a temp difference in the exhaust.

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Off handed, but has the valve train for that cylinder been looked at to see if it's fully opening and closing?

That should be evident when adjusting valves shouldn't it? If so than yes that has been addressed, the valves have been adjusted multiple times during these rebuild/ piston replacements so I believe them to be working like they're suppose to.

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The only thing that I've done differently during this whole incident is the last time I replaced the piston I replace the nozzle in that hole.....and up to now the engine runs good, however I still worry about it because of its history, at this point I think the heat gun comment is my next move at least that should show me if that cylinder is still running hotter than the rest or not. If the temps are near the same across all 6 I may be home free now. I'll update when I have that info. Thanks to all for all the help.

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remember you have a jerk pump each cylinder has its own plunger that rides on a cam shaft inside the pump and a rail that turns the plunger to increase or decrese the helix thru which fuel is metered, so if one plunger is out of adjustment it will cause this issue. and yes pump tecs can goof! i fought with a excavator that had a knock that we coulnt get rid of after a rebuild, it turned ot that the pump shop goofed and was over fueling one cylinder

this is the type of thing I was trying to explain I didnt do a very goo job :huh::huh:

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this is the type of thing I was trying to explain I didnt do a very goo job :huh::huh:

I appreciate the effort :) I understand whats being conveyed here and I'm reasonable sure that isn't the case. That being said I did order a heat sensor today and will be checking the cylinders individually to be sure they're all doing about the same amount of work and number 6 hole isn't getting an excessive amount of fuel/heat. I'll post the results here when I have them.

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That should be evident when adjusting valves shouldn't it? If so than yes that has been addressed, the valves have been adjusted multiple times during these rebuild/ piston replacements so I believe them to be working like they're suppose to.

My though was not adjustment, but the Valve Lift or if the intake valves on that cylinder are opening fully. If not you could get an over rich condition. Paul

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"OPERTUNITY IS MISSED BY MOST PEOPLE BECAUSE IT IS DRESSED IN OVERALLS AND LOOKS LIKE WORK"  Thomas Edison

 “Life’s journey is not to arrive at the grave safely, in a well preserved body, but rather to skid in sideways, totally worn out, shouting ‘Holy shit, what a ride!’

P.T.CHESHIRE

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mack had different pistons if I remember right. The "dish" of the piston had a different depth and center. could you have been given the wrong pistons? just a thought

Not likely, The kits came directly from Mack ..... and if so it has six wrong pistons and that wouldn't explain why just number 6 hole is the only one that failed multiple times, I also just learned that the failure that instigated the first rebuild was also a burned piston/broken ring ledge in number 6 hole....that brings the total to three. I'm now beginning to think that the nozzle was indeed the problem. I have a heat gun on the way and will be checking the individual cylinders for heat variances. I'll let you all know the results when I have them.

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