Sorry about taking so long to update, been very hectic few weeks. Not the coolant, all the liners from #3 were spotless and 3 of the pistons showed signs of heat scoring. It was all EGTs. By all means get that green crap out of there and put the right stuff in. I don't care what the TM's say, the green isn't good enough even if fresh because of cavitation. You could put an additives package in it and that will be fine but it needs to be the stuff for wet liners. It has to be.
My liners from engine #2 showed serious pitting including etching starting to creep under the top o-ring. Red herring. In my testing the only thing I subjected sample rings to that made them sticky was high heat. So while there was some deep pitting on motor two it was still EGTs that killed it and the liners would have chugged along even compromised probably forever with the right coolant. When I have time I am going to use old pistons and liners in a jig and see if I can work out at what temps exactly piston swelling starts to stick them in the bore. Downhill fast after that, which is ironic because even the smallest downhill run would have saved them. Have not pulled liners from the first motor we lost.
So on the NHC and NTC Cummins 250 they have all the tough as iron internals of all the 855 motors but lack the oil squirters needed to cool the piston skirts to prevent bore sticking. For 95% of you this will never be an issue which is why I have been tearing the heart out of motors no matter how much I baby them every year and almost no one else has. On the 250 with normal fuel buttons (none of mine had the fuel turned up) it takes a very specific set of circumstances to wreck the motor, long slow uninterrupted grades. A 5 mile 1% climb with no break will do it. It takes almost nothing to cool the liners back down, back off the throttle for a few seconds to look at a junk car, figure out what a deer is going to do, anything and liner temps rocket back down. Just traffic or backing off for a corner is enough but if you have a long clear open stretch of empty highway and you are just puttering along at 50-55mph mile after mile on a gradual grade with no break you are going to poach it. Not a lot of places where those kinds of things can happen but guess what Montana is full of…
I think on the flats into a steady headwind it could happen also. Any situation where you are constantly under load, even small load, with no change in throttle position. (remember EGT can go from 1200 to 500 within 3 seconds of backing off the fuel) I put a pyrometer in and drove by it when limping the truck to the shop it was hard to keep EGTs below 1000 out here. It had a used piston, bearings, random set of rings and the best liner of the lot pulled from motor #2, even a used headgasket and it ran like a champ with no smoke and very little blowby. The 250 is incredibly strong except for just that one thing. It just turns out that I did that one thing to it day after day out here.
Should a naturally aspirated de-tuned motor be able to burn itself up driving empty up a very shallow hill or into a stiff breeze? Well no but the 250 will. It takes a very specific set of circumstances but it will, and every place I look out here those exist. That is why I have been killing them and no one else has. I suppose I put a lot more miles on them than average running commercially, but milage isn’t a factor here really. The first one I drove, the second I babied, the third I put on a glass pedestal but it was puttering up the 7 mile 1% hill at 45mph on the way to the tours that killed them. I bet the old 45mph speed limit might also have had something to do with motor life in addition to safety.
Those that follow our company, remember two years ago (first year running the 925a1, 2015) that mystery fuel issue that would cut in and cripple power? Regardless of how many times we tore the fuel system apart, the filters, hoses and pick-ups changed it came back? I am very, very sure at this point it had nothing to do at all with fuel, it was climbing that ever so gradual but mile after mile hill sticking pistons in the bore. I can’t even remember the number of times I said in frustration “I start to lose power, it gets worse fast then the motor dies. Feels just like I am starving for fuel but I check the system and nothing is wrong. The truck fired back up and runs normally.” In the end we were down to wondering if a clear plastic bag was somehow in the bottom of the tank. I shortened the pick-up by 3 inches and babied it (more so than I already do) the rest of the year with no issues. Babied it…
If you have ever had a mystery fuel issue where no culprit could be found (air leaks, failing hoses, plugged filter) you could be in the same boat. Contact me I think I can walk you through how to figure out if it is a fuel gremlin or if you have stuck a piston in the bore. If it makes you feel better that motor ran for another 1000 miles before I killed it for real. I think it might have run much, much longer on like parade duty.
Side note, we looked into a squirter retrofit (making them indestructible) but it wasn’t a simple project. You would be better off with a 290 or 300 block and putting the 250 internals in it. That is a whole new issue however.
Company update:
Motor #2 is back together on the stands, motor #3 we did an inframe on and it was running again last Sunday with a huge miss and belching smoke on throttle input. Turns out one of the brand new injectors was misfiring, swapped with an old and the miss went away. We added the duel fuel to the back head while it was off and it was weeping fuel so rather than taking heads 2 & 3 back off we cut the hole in the firewall that others talk about. Worked a charm and because mine is a winch truck the winch controls will hide the patch. I will try to put together everything I learned about killing them, rebuilding them (in and out of frame), timing them (topstop and non-topstop injectors or even the special unicorn NTC that I have which is topstop with a non-topstop timing marks) or if you are in the middle of something right now email me at
acme66@yahoo.com or call the company at 406-546-9787 (phone broke wait a few days) and I will see what I can do.
Oh and get that green crap out regardless.
Ken
PS: on the phone "Cummins" auto-corrects to "Cummings". If that is all you have to contribute you might think of nodding in smug superiority to yourself and just moving on.
