acme66
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This is a long post but then it is the full recap of three years of destruction and frustration. I put the list of things I wish I had done or known here at the top just in case you don’t feel like reading all of it. It is an interesting story and there is a lot to learn from my expensive lessons. Everything you do is at your own risk but this is what I do now and what I wish I had done then.
All of these suggestions matter more in a 939 NHC truck than the older ones primarily because of the automatic transmission, I will explain later.
1. Install a pyrometer. Do it now. Never let the thing get above 1200F and try to keep it below 1100F. You will find this difficult and frustrating in a 939 truck. Heat kills and the damage is cumulative. If I had a pyrometer on my first truck it would have NEVER died on me. Get a pyro, period.
2. Keep additive in your fuel. Nothing special, one quart of cheap Wal-Mart brand motor oil per tank fill would be fine. Swap in a quart of ATF once in a while to clean things but always something. The dry fuel has less lubrication. Normally it will not be an issue but it might mean all the difference between a stuck piston and getting away with an EGT overheating situation.
3. Keep the air cleaner clean, don’t wait for the indicator. In the 250 less air = more heat. A dirty filter can make an easy 100F EGT difference on my trucks. Now this is a hard one because the fastest way to put a crap ton of dirt in the system is checking the cleaner all the time. Here is how I do it: Pull the air box cover all the way off every time. Pull and inspect the filter. If it is too dirty then replace after carefully cleaning the air box. If you are not replacing use a vacuum to carefully clean out the entire box and the top (opening) of the filter. Don’t blow the filter out, that is asking for some dust to make it around to the inside of the filter and into your motor. Make sure you have a tight seal, I have always put padding behind the filter, not much, just a little because I do not want to find it leaked running these gravel roads. Side note: I found that removing the intake cap dropped my EGTs by 25-50F at wide open throttle so that is also absolutely a restriction. I have been trying to contact Donaldson about selling me some of those Australian road train air ram intake scoops but so far no response from them.
4. When it comes to driving, she likes it rough. Again this is more of a 939 5 speed automatic issue. I am not saying to flog it like a rented chainsaw but if you are pulling out on the highway start in 2nd and manually shift it to 3rd when the RPMs hit 2000. I still have a hard time with this because the thought of all that unbalanced crap spinning around in there so fast makes me cringe but the manual guys have been driving them like that for decades. Don’t lug it, it builds heat faster that way. In a nutshell if I can only do 45mph then I do it in 4th at 2000RPMs rather than 5th at 1500. This remains hard for me because growing up driving old equipment excessive motor speed was always bad. Refer to #1 and unless you are cruising keep it turning between 1700 and 2100RPM. (I still refuse to go above 2000)
5. Let it warm up before driving and cool down before shutting off. This is sort of standard truck fare but I think it is particularly important on the NHC 250. The shock of cold liners rocketing up to 1000F when you pull out on the road is hard on them but I bet not half as bad as the slow cooking of those o-rings if you shut it down at those temps.
6. Don’t turn the fuel up on a 939 without putting a turbo charger on it… or ever. I used to be a firm believer that the 250 was not a good candidate for a turbo conversion but I have come around to the idea. There are some great instructions on here from guys that have done it but do me a favor, carefully read all of their material before asking questions. I have not nor would I crank the fuel but with all the people looking for more out of these motors I feel you should know that you are asking to become familiar with the inside of it.There are lots of other things I have done to my trucks that I recommend but had I followed 1-5 then I would not have butchered 3 NHC motors and would be many thousands of dollars and hours of frustration ahead of where I am now. Number 1 alone would have been enough. Put. A. Pyrometer. In
.And now… the rest of the story.It has been a while coming but I said I would put everything I have learned down in a single post about burning out 3 separate NHC 250 Cummins motors. The 250 is an older design motor based off uprated NH 220 and 230 motors. They are known for simplicity, reliability and rock solid performance… so how did I wipe out three?
After years of researching, testing and breaking things I think we can finally come to the table with an answer that might be beneficial to others.
First let’s talk about how I have been using them. I run a tour company in the summer, Alder Gulch Summit Tours, where we take people to the top of a mountain and talk about the history of gold mining in the area. I run the exact same route day after day and it has been this repetition that has let me test theories over the years and also destroy motors. I have been running up from Alder MT (5100ft) to Virginia City MT (5500ft) every morning, a distance of 7 miles with a 400 foot rise in elevation. From Virginia City we then run the tours, up to 6 daily to the top of the mountain climbing from 5500 feet to 8000 and back down in a 14 mile round trip. That first 7 mile climb is what has been killing my motors. I am sure now that the EGTs were 1300-1400F every day. I was just ‘driving it’ empty and doing my best to motor at speed.
All of the motors have died in EXACTLY the same way and two literally with in 100 yards of the same location. Running uphill on long grades they will start to stumble, feels exactly like a fuel issue. The first two just stumbled and knocked loudly but never died, the third died briefly. Then… they run normal again. (ok the third one had to be restarted and had a knock after) Each and every one then gave a brief but pungent coolant smell. If your engine breather dumps down alongside the motor you might see it puffing and chuffing like a steam engine, but otherwise they ran and had normal power.
What has happened is the Exhaust Gas Temperature (EGT) has climbed so high that the piston has started to stick briefly in the liner bore. It might not take the liner out on the first go-round. I now know that motor number two was a trooper. If you have followed our adventures on youtube you might remember a mystery fuel starvation issue I was having while climbing up the hill. Motor would just drop out. Tore into that system a half dozen times checking and changing things but finding nothing. Down to thinking there might have been a plastic bag in the bottom of the fuel tank.
We now know that I was sticking pistons. If you have the mystery fuel issue but not the puffing from the breather then you have stuck a piston but got away with it. With princess treatment it might run for many thousands of miles more but you have done serious scoring damage inside. Don’t have it rebuilt yet, drive it but know that you have damage in there. When the liner o-rings finally let go in that motor the following spring there was so much galled aluminum the oil and bottom compression ring were melted in. I ran that motor for thousands of miles like that. Sort of amazing when you think about it.
Eventually all that heat, made so much worse when the pistons start sticking, wipes out the liner o-rings and you fill the crankcase with coolant once the motor shuts off. That is why you must check the oil in these things constantly, bad things happen when you add 10 gallons of water to your oil pan and try to run it. All of mine have kept the water out of the oil while running. One of them I drained the coolant until I was ready to leave then filled it with water quick and drive the thing as is 90 miles and dumped the water again as soon as I shut it off.
The NHC 250’s have become sort of infamous for cavitation caused liner failure. We saw some pitting damage in two motors but none on the third. Chasing cavitation issues was a red herring for us, it distracted me from what I was doing that was actually killing them. Please understand that cavitation erosion of the liner wall IS a serious issue for the NHC wet liner motors. It is from the vibrations of detonation forming little bubbles that implode, the same process that eats props on speed boats.
Get that green crap out of there. Make sure you are running a good SCA additive package and test it with the strips. Your liners don’t have to be perfect now as long as they don’t eat through. Understand that thin spots are going to have heat issues and that is the real killer I have found.To have done it over I wish I cracked that first motor rather than switching trucks.
Not sure if we would have put the heat connection together that early given how sure we were about liner failure (I think one contributes to the other). By the time the second motor died we had already purchased the third truck. It was going to be faster to do a motor swap then rebuild and the tour season is only 10 weeks long so time mattered. But we KNEW the insides of that third motor were perfect. Few miles, new liners surly this one would run forever.
When it died the following spring I was gutted. No one else was having these issues. I had had old experienced Cummins guys basically say I was lying when telling them what I was doing and how long they were lasting. Only once we started sending parts to people did the true picture form. Heat damage.
They were sure I was overheating them but I was positive that none of the motors had ever gotten above 190F. Then one thing kept coming up, “I bet you have a blocked piston squirter”. That was when we started to learn the difference between the rock solid civilian versions of the 855 motors and the special ordered military version… without squirters. Under side oil cooling of the pistons is a huge part of heavy diesel survival. The NHC 250 lacks them.
We think we could see what oil journal they would be drilled into but with no way to flush the block I don’t see how the average guy could drill them. I could see taking a 290 or 300 block and putting 250 stuff into it but you might as well have the whole 290 or 300. Best I can tell the lack of those cooling squirters is sort of the Achilles heel of the 250.
It seems silly that driving an empty 923 or 925 up a gradual hill could kill them but if you do it often enough you will. I recommend following my steps to get a long service life from the motor. It is also worth noting that for every 1000 foot rise in elevation the 250 will lose 3% of its power. It is also getting less and less air which = more heat. We are sure the reason I have been the Sweeny Todd of 939 motors is running them day after like I have been and at this elevation.
I hope none of this will ever be an issue for you and that you can learn from what we have done to keep your own trucks rolling. Happy to answer questions.
Ken
All of these suggestions matter more in a 939 NHC truck than the older ones primarily because of the automatic transmission, I will explain later.
1. Install a pyrometer. Do it now. Never let the thing get above 1200F and try to keep it below 1100F. You will find this difficult and frustrating in a 939 truck. Heat kills and the damage is cumulative. If I had a pyrometer on my first truck it would have NEVER died on me. Get a pyro, period.
2. Keep additive in your fuel. Nothing special, one quart of cheap Wal-Mart brand motor oil per tank fill would be fine. Swap in a quart of ATF once in a while to clean things but always something. The dry fuel has less lubrication. Normally it will not be an issue but it might mean all the difference between a stuck piston and getting away with an EGT overheating situation.
3. Keep the air cleaner clean, don’t wait for the indicator. In the 250 less air = more heat. A dirty filter can make an easy 100F EGT difference on my trucks. Now this is a hard one because the fastest way to put a crap ton of dirt in the system is checking the cleaner all the time. Here is how I do it: Pull the air box cover all the way off every time. Pull and inspect the filter. If it is too dirty then replace after carefully cleaning the air box. If you are not replacing use a vacuum to carefully clean out the entire box and the top (opening) of the filter. Don’t blow the filter out, that is asking for some dust to make it around to the inside of the filter and into your motor. Make sure you have a tight seal, I have always put padding behind the filter, not much, just a little because I do not want to find it leaked running these gravel roads. Side note: I found that removing the intake cap dropped my EGTs by 25-50F at wide open throttle so that is also absolutely a restriction. I have been trying to contact Donaldson about selling me some of those Australian road train air ram intake scoops but so far no response from them.
4. When it comes to driving, she likes it rough. Again this is more of a 939 5 speed automatic issue. I am not saying to flog it like a rented chainsaw but if you are pulling out on the highway start in 2nd and manually shift it to 3rd when the RPMs hit 2000. I still have a hard time with this because the thought of all that unbalanced crap spinning around in there so fast makes me cringe but the manual guys have been driving them like that for decades. Don’t lug it, it builds heat faster that way. In a nutshell if I can only do 45mph then I do it in 4th at 2000RPMs rather than 5th at 1500. This remains hard for me because growing up driving old equipment excessive motor speed was always bad. Refer to #1 and unless you are cruising keep it turning between 1700 and 2100RPM. (I still refuse to go above 2000)
5. Let it warm up before driving and cool down before shutting off. This is sort of standard truck fare but I think it is particularly important on the NHC 250. The shock of cold liners rocketing up to 1000F when you pull out on the road is hard on them but I bet not half as bad as the slow cooking of those o-rings if you shut it down at those temps.
6. Don’t turn the fuel up on a 939 without putting a turbo charger on it… or ever. I used to be a firm believer that the 250 was not a good candidate for a turbo conversion but I have come around to the idea. There are some great instructions on here from guys that have done it but do me a favor, carefully read all of their material before asking questions. I have not nor would I crank the fuel but with all the people looking for more out of these motors I feel you should know that you are asking to become familiar with the inside of it.There are lots of other things I have done to my trucks that I recommend but had I followed 1-5 then I would not have butchered 3 NHC motors and would be many thousands of dollars and hours of frustration ahead of where I am now. Number 1 alone would have been enough. Put. A. Pyrometer. In
.And now… the rest of the story.It has been a while coming but I said I would put everything I have learned down in a single post about burning out 3 separate NHC 250 Cummins motors. The 250 is an older design motor based off uprated NH 220 and 230 motors. They are known for simplicity, reliability and rock solid performance… so how did I wipe out three?
After years of researching, testing and breaking things I think we can finally come to the table with an answer that might be beneficial to others.
First let’s talk about how I have been using them. I run a tour company in the summer, Alder Gulch Summit Tours, where we take people to the top of a mountain and talk about the history of gold mining in the area. I run the exact same route day after day and it has been this repetition that has let me test theories over the years and also destroy motors. I have been running up from Alder MT (5100ft) to Virginia City MT (5500ft) every morning, a distance of 7 miles with a 400 foot rise in elevation. From Virginia City we then run the tours, up to 6 daily to the top of the mountain climbing from 5500 feet to 8000 and back down in a 14 mile round trip. That first 7 mile climb is what has been killing my motors. I am sure now that the EGTs were 1300-1400F every day. I was just ‘driving it’ empty and doing my best to motor at speed.
All of the motors have died in EXACTLY the same way and two literally with in 100 yards of the same location. Running uphill on long grades they will start to stumble, feels exactly like a fuel issue. The first two just stumbled and knocked loudly but never died, the third died briefly. Then… they run normal again. (ok the third one had to be restarted and had a knock after) Each and every one then gave a brief but pungent coolant smell. If your engine breather dumps down alongside the motor you might see it puffing and chuffing like a steam engine, but otherwise they ran and had normal power.
What has happened is the Exhaust Gas Temperature (EGT) has climbed so high that the piston has started to stick briefly in the liner bore. It might not take the liner out on the first go-round. I now know that motor number two was a trooper. If you have followed our adventures on youtube you might remember a mystery fuel starvation issue I was having while climbing up the hill. Motor would just drop out. Tore into that system a half dozen times checking and changing things but finding nothing. Down to thinking there might have been a plastic bag in the bottom of the fuel tank.
We now know that I was sticking pistons. If you have the mystery fuel issue but not the puffing from the breather then you have stuck a piston but got away with it. With princess treatment it might run for many thousands of miles more but you have done serious scoring damage inside. Don’t have it rebuilt yet, drive it but know that you have damage in there. When the liner o-rings finally let go in that motor the following spring there was so much galled aluminum the oil and bottom compression ring were melted in. I ran that motor for thousands of miles like that. Sort of amazing when you think about it.
Eventually all that heat, made so much worse when the pistons start sticking, wipes out the liner o-rings and you fill the crankcase with coolant once the motor shuts off. That is why you must check the oil in these things constantly, bad things happen when you add 10 gallons of water to your oil pan and try to run it. All of mine have kept the water out of the oil while running. One of them I drained the coolant until I was ready to leave then filled it with water quick and drive the thing as is 90 miles and dumped the water again as soon as I shut it off.
The NHC 250’s have become sort of infamous for cavitation caused liner failure. We saw some pitting damage in two motors but none on the third. Chasing cavitation issues was a red herring for us, it distracted me from what I was doing that was actually killing them. Please understand that cavitation erosion of the liner wall IS a serious issue for the NHC wet liner motors. It is from the vibrations of detonation forming little bubbles that implode, the same process that eats props on speed boats.
Get that green crap out of there. Make sure you are running a good SCA additive package and test it with the strips. Your liners don’t have to be perfect now as long as they don’t eat through. Understand that thin spots are going to have heat issues and that is the real killer I have found.To have done it over I wish I cracked that first motor rather than switching trucks.
Not sure if we would have put the heat connection together that early given how sure we were about liner failure (I think one contributes to the other). By the time the second motor died we had already purchased the third truck. It was going to be faster to do a motor swap then rebuild and the tour season is only 10 weeks long so time mattered. But we KNEW the insides of that third motor were perfect. Few miles, new liners surly this one would run forever.
When it died the following spring I was gutted. No one else was having these issues. I had had old experienced Cummins guys basically say I was lying when telling them what I was doing and how long they were lasting. Only once we started sending parts to people did the true picture form. Heat damage.
They were sure I was overheating them but I was positive that none of the motors had ever gotten above 190F. Then one thing kept coming up, “I bet you have a blocked piston squirter”. That was when we started to learn the difference between the rock solid civilian versions of the 855 motors and the special ordered military version… without squirters. Under side oil cooling of the pistons is a huge part of heavy diesel survival. The NHC 250 lacks them.
We think we could see what oil journal they would be drilled into but with no way to flush the block I don’t see how the average guy could drill them. I could see taking a 290 or 300 block and putting 250 stuff into it but you might as well have the whole 290 or 300. Best I can tell the lack of those cooling squirters is sort of the Achilles heel of the 250.
It seems silly that driving an empty 923 or 925 up a gradual hill could kill them but if you do it often enough you will. I recommend following my steps to get a long service life from the motor. It is also worth noting that for every 1000 foot rise in elevation the 250 will lose 3% of its power. It is also getting less and less air which = more heat. We are sure the reason I have been the Sweeny Todd of 939 motors is running them day after like I have been and at this elevation.
I hope none of this will ever be an issue for you and that you can learn from what we have done to keep your own trucks rolling. Happy to answer questions.
Ken
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