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Cummins 855 failure

Cummins 855 failure

Cummins 855 failure

I've been working on a stationary pump engine that has been giving us problems for 5 years. It is a 1978 Cummins NT855 F1  for fire pumps. The engines it has ~ 2K hours on it, most of that time is from exercising 30 min per week pumping at the given rate of the system.  
The problem we are having is that the # 5 piston is over heating  with in the cylinder causing scuffing on the cylinder wall and or the crown is being melted to the point of seizing the engine. The cooling system seems to be drawing off the heat of the engine in general, the temp. gage (tested) is running in the range of 180 degrees This engine has been rebuilt now for the 4th time. New cylinder kit, (piston, rings, liner) new main bearings and the rod bearings through out.
Each time we get about 200 hrs on it before failure. The engine has been rebuilt by Cummins trained personnel from 2 different companies, and no one can give us a definitive  reason for the cause of the failures.

Can any one try to shed some light on what the causes might be?

RE: Cummins 855 failure

Not that familar with Cummins engines, but does the engine have piston cooling oil spray and is the nozzle for this cylinder plugged, damaged, or otherwise ineffective?

RE: Cummins 855 failure

when I've seen the top of the crown melt, it has always been due to overfueling - check for injector problems.


RE: Cummins 855 failure

Could the cam lobe for the #5 exhaust be scrubbed?

RE: Cummins 855 failure

The NT855-F1 was built from the standard block casting but it was re configured somewhat different to meet the NFPA standards for the time.

The oil gallery on the right side where the oil nozzles are, was made with no feed lines open,  there is no oil spray from nozzles cooling the pistons from below.  Due to the fact that the engine was for fire pump application it was built to run in extreme conditions and no oil is used to cool the skirts

We have a total of 3 engines side by side  built the same way, each having the same configuration and running fine.   

RE: Cummins 855 failure

Ivymike this a good point. The injectors have not been changed out.  We were told that they were bench tested and showed to be OK.

For what injectors cost we are having new installed this time around.

Potteryshard   Another good point.  The cam lobs have not been checked at this time. I'll pull a cover and check this out.

RE: Cummins 855 failure

Those engines are fussy as far as the settings for the injectors, and are easily goofed up by people that don't understand how to do it. Especially if the marks on the pulley are not correct, usually because it wasn't timed correctly. Timing is the injectors is also not an easy deal. So how is number 6?  Something else to consider is did someone goof on the cam grind for that injector?  

RE: Cummins 855 failure

Does the engine run rough?  First I'd suspect the #5 injector, but not over fueling, rather not atomizing the fuel and spraying raw fuel onto the cylinder wall washing down the lubrication, unless of course it was the following:

I DID have a 1978 Cummins 855 once upon a time in an automotive application and I DID have some pistons that looked exactly like that but it was because the driver had monkeyed around with the injector pump and over-fueled it so much that it literally melted itself.  Only a couple of the pistons looked like that.  But you should have heard the stories up and down the road about how well it pulled (it was distinctively marked so it was easy to determine that it was my truck being bragged on).  Has anyone monkeyed around with the pump on this one and "turned it up?"

Is this a turbo'd engine?  And if so, is it a 300HP version or a 350HP?  Someone may have tried to make a 300HP run like a 350 by turning up the pump.  The timing and advance is different between the two and you can't get from one to the other by just 'turning up' the pump.



RE: Cummins 855 failure

The photo is a typical thermal failure. The piston temp has increased causing expansion to the point where it no longer fits the bore, resulting in lubrication failure resulting in more heat generation rsulting in . . . . etc.
The root cause is often overfuelling or late timing but can be lack of lubrication or lack of cooling (oil squirter blockage) etc.

Engineering is the art of creating things you need, from things you can get.

RE: Cummins 855 failure

Thanks for the input everyone. This forum is invaluable for taking a problem and looking at it from many points of view and coming up with good solutions.

Dicer, early on it comes back to me that the first person to work on the engine did go through and adjust the timing with the shims under the cover of the cam followers and a gage on top. Now weather these were set correctly is a good question.  The standard NT855 is so common that the parts guys often don't see the F1 at the end of the model number. This makes a bit of a different build. That's something to check.
 I have pulled the follower cover on  5&6  and find every thing to be in good shape. The cam is not worn, the rollers are tight and in good working order.

RMW The engine runs smooth right up to the point of  failure. At that time I have heard it start to drop in RPM slightly and have a bit of a subtle clatter to it, not real bad but noticeable. I am assuming that the piston is starting to swell and tighten up at that time.   

 The fuel pump has been pulled and rebuilt, actually adding a few leeks that were not there in the first place. These engines are turbocharged and rated for 255 HP @ 1750 RPM  and actually run at 1600 RPM once the oil pressure gets up to 65-70 PSI.
 Some times I wonder if the constant speed is a detriment to them.

RE: Cummins 855 failure

>>>The cooling system seems to be drawing off the heat of the engine in general, ... <<<

That does not rule out localized overheating, perhaps from casting sand or flaws in the water jacket.  

Careful mapping of the block with an infrared thermometer might reveal an anomaly around #5.


Mike Halloran
Pembroke Pines, FL, USA

RE: Cummins 855 failure

Its not just a side cover deal, also very critical adjusting the rockers. A loose injector setting or very tight will cause problems.  

RE: Cummins 855 failure

Yes  I agree there is much more than just popping the cover to have a "look see". I thought I'd spare the details of  using the dial indicator on the top side to get the timing settings and  adjusting the rockers for intake & exhaust & injector clearances.
Thanks for the reminder.  

RE: Cummins 855 failure

I had an 855 used in military service that had the inside of the skirt oiled by a drilled connecting rod and a drilled crankshaft. #5 had failed as well. This was a rather low HP engine as the truck was governed at 45 MPH. I've seen #5 fail on a lot of 6 cyl. Cats as well.

RE: Cummins 855 failure

I wonder if the water pump is pumping enough water to cool that much HP.  Automotive diesels, for which this engine has (had) by far the most usage, in the day and time that this engine was among the top dogs typically ran between 1800 and 2100 rpm (if the pump wasn't turned up), plus they typically had a good charge of ram air due to vehicle motion into the radiator to aid the engine fan induced air flow.  Unless your fire pump designer recognized that the cooling had to be upsized to account for reduced water flow and reduced convection to the radiator, you may be having a cooling problem in that area of the engine.  

I used to know my way around that engine pretty good, but it has been over 20 years since I have been the proud owner of a "come apart."   When mine came apart, other than the one time I mentioned due to the driver turning up the pump, it was usually due to a cooling system failure, usually that little chicken neck shaped piece of hose on the side opposite the fuel pump.  I can't remember what it went to, just that when it failed - and it was famous in the industry for failing, in those days it was a ~$15-20,000 ding.  And that was when that much money was a big chunk of an engineer's annual salary.  Bought several new "come apart's" over the years.  Wouldn't have one today.


RE: Cummins 855 failure

rmw  thanks for the suggestion, It is interesting how the cooling system has been modified to adapt it to a stationary application.  On this set up the radiator has been replaced with a water cooled heat exchanger. The engine has glycol in it as usual but it circulates through the heat exchanger with a water jacket  drawing off the heat. The exchanger has been replaced a while back due to leakage, and it is clean from debris  upon inspection and the thermostat is being replaced on this round of rebuild.

 I see how some blockage in the engine coolant ports could be problematic in drawing off the heat causing the build up of heat in one cylinder. I'll need to look deeper.

Thanks for the continued input.

RE: Cummins 855 failure

Speaking of glycol, what is the ratio?  The higher the percentage of glycol, the lower the heat transfer capability.  Some people believe (I am certainly not one of them) that if a little glycol is good, a lot is better.  I knew one fellow that ran 100% glycol in his engine.  Ugghhh.

As the concentration of glycol goes up, the viscosity and density of the fluid changes, and with respect to pumping it(relates to flow through the engine) and soaking up heat with it, the higher the concentration goes, the worse the stuff gets with respect to carrying off engine heat.

What kind of ambient conditions is this engine having to exist in?

I just ran a case where the glycol solution at a certain low ambient temperature became so viscous at the concentration that the case was based on that it could not flow into the engine fast enough to blend with the warmer fluid in the engine with the pumps present.


RE: Cummins 855 failure

I have herd that the coolant gets viscous when cold but never experienced it.
In the past I have gotten the old green coolant from our motor pool. It was a premixed 50/50 (tested to confirm).  Now on this motor I've been told that the " yellowish universal coolant" is all that is available, so at this time that is what is going in. It will be mixed on site to 50/50 blend. Personally I would like to stick to the green, but we'll give the new stuff a try.

The engines are in a clean  heated block building with fresh air vents measuring  48 in X 120 in,. the louvers open when the engines start and or when the ambient gets in the 90s during the summer.  The engines have block heaters (oil & coolant  non circulated at rest) on them keeping them at about 120 F for a quick start with out warm up.

RE: Cummins 855 failure

Do you have coolant filters?  This discourse is bringing back lots of (mostly bad) memories.  There was a problem with coolants in Cummins (and maybe others) back about the time I was a proud owner called "Green Goo".  I think that term will Google.  I can't remember all the details except that the stuff was bad and that there were measures, simple measures to keep it out of your engines.  The most effective measure was a coolant filter that had a charge of a "Green Goo" fighting chemical and all that was needed was to change the filter regularly.

ON the other hand, if you got the "Green Goo", that stuff was thick (different than cold viscous EG or PG) and nasty and would plug up cooling ports in the engine.

Another thing that the maintenance of the chemical make up of the coolant accomplished was to minimize electrosys of the cylinder liners.  Now that is a problem that Cummins were famous for in the day.


RE: Cummins 855 failure

Thanks rmw.  
The green goo has not been a problem in this case. We do use a coolant filter with the DCS additive and it seems to work quite well keeping deposits from building up. That is a good point to watch for on further inspections.   

I found some photos of the inside of the block and the piston on the engine we have been working on but at a different episode.    

 Dose any body see something we've over looked?

I have seen pitting on the sidewalls of the liners even after just 2 years, it was most noticeable where the coolant port in the block emptied into the liner cavity. Small pitting was there in a line on each of the liners that have been removed.

RE: Cummins 855 failure

is the liner cavitated thru so that coolant enters the cylinder?

also what did the injectors test show? or did you replace/exchange all of them.

RE: Cummins 855 failure

No in each case the cavitations were noticeable but not deep enough to penetrate the liner.

In regards to the injectors, the last break down they were sent out by the firm that was called in to do the work on engine and it was reported back to us that the injectors were fine. This time I told them to change them all.

It looked to me on the posted photo, that the spray pattern was directed to one side and it moved the soften metal to that side.  It is the only thing I can think of that would make the mound of material.

RE: Cummins 855 failure

I was originally going to say "definitely injector torching" when I saw the photo, but after looking at the pic more I convinced myself it was foreign object damage.  Looks a bit like you dropped a valve & it printed sideways in the piston before shaking loose and getting "digested".


RE: Cummins 855 failure

looking at it again (and since you haven't mentioned FOD), I will go back to "injector torching" as my explanation - perhaps the injector tip cracked between 2 orifices?

RE: Cummins 855 failure

Injector torching due to a cracked tip sounds like a good call on this piston.  There was a case on one of these where some of the piston material was imprinted with head as it came up on the return stroke.  No serious damage..

Thanks every on for your insightful input. After 100 hours of run time I plan to pull the head an see what condition the parts are in. It may be the case of a slowly developing problem or a case of  quick failure at approximately  200 hours  each time.
Run time will tell.................

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