A rebuilt diesel engine is a huge investment, and the owner-operator or fleet manager expects big returns. But when the engine is consuming engine oil at an alarming rate during its first thousand miles in use, the owner may question if there is a problem with the engine. Many people don’t know that there is a proper technique to break-in a diesel engine, and not following these guidelines can cause long-term oil consumption issues. For the repair technician, identifying oil consumption claims is difficult at best. Following recommended investigative and troubleshooting procedures will save time and money and a repeat of the same problem.
Piston ring seating is a shared responsibility of the rebuilder and the operator in the initial run-in and proper operation of the engine. Excessive idling after rebuilding, low operating temperatures, operation with low or no load, incorrect engine oil or dipstick calibration, as well as air compressor or turbo problems, can cause the complaint of oil consumption.
Why does a diesel engine need to be broken-in?
Although current technology provides the means of manufacturing engine parts with unimaginable precision, the manufacturer still falls far short of achieving the near perfect fit that a proper break-in will provide. “Break-in,” for the most part, is the allowance of the machined cylinder and ring surfaces to conform to each other’s shape during engine operation. This conforming or “mating” of ring and cylinder surfaces is the ultimate goal of a proper break- in. Mating these two specific parts will produce a very tight seal in each cylinder.
How does oil consumption happen during break-in?
A tight seal is very important because it prevents the escape of unburned fuel and pressurized gasses into the crankcase, while further preventing crankcase oil from entering the cylinder above the top compression ring. During break-in, a small amount of compression blow-by, oil-fuel dilution, and oil consumption will be experienced. This is perfectly normal and quite common in new engines.
Although acceptable at first it is imperative that these undesirable attributes be as close to zero as possible after break-in has been completed. Although the others are important, blow-by is the primary reason the ring and cylinder wall interface must fit together so tightly. Measure crankcase “blow-by” or pressure with a manometer, available from your tool supplier. Specifications for crankcase pressure are listed in your service manual.
How does a diesel engine get broken-in?
In order for break-in to occur, a fair amount of heat, friction and resulting wear will have to take place before the compression rings will have “mated” with the cylinder walls. When the rings and cylinder wall are new, a modest amount of heat is created merely from the friction of the new rings passing over the freshly honed cylinder wall.
While the heat from friction is significant, the real heat is created from combustion of fuel in the cylinder. When the fuel is burned, gasses are produced that expand and heat all of the cylinder parts. If enough fuel is introduced, the resulting combustion can create gasses that expand so much they will actually expand the cylinder wall and the compression rings. It is important to understand this because expanding these parts places additional pressure on them, which creates more friction and correspondingly more heat. Too much heat will create an unseating effect.
How does oil glazing affect oil consumption in diesel engines?
The engine oil, lubricating the cylinder walls, will flash burn when it contacts the very hot rings. The burned oil will leave a hard, enamel like residue on the cylinder wall, commonly known as oil glazing. When the rings are permitted to operate under such high temperatures, oil glazing of the cylinder can happen very quickly. Once this glaze builds up, the only repair is a labor-intensive process that requires disassembling the engine and re-honing the effected cylinders.
Oil glazing is a problem because it is typically not distributed evenly in the cylinder, and the spaces that exist between the ring and cylinder wall are either still there or new larger ones are created. Oil glazing is typically thicker towards the top of the cylinder, and it builds up in the areas where heating is the greatest. The glaze has very smooth and friction-free properties that do not allow it to be scraped away by the rings. This inhibits further metal-to-metal wear between the cylinder wall and rings, preventing further mating of ring and cylinder. Thus, those small gaps between ring and cylinder surface will never seal. These spaces will then allow pressurized gasses and unburned fuel to escape into the crankcase, while allowing oil from the crankcase to enter the cylinder above the top compression ring.
Will running the engine at idle or under no load stop oil consumption during break-in?
This is bad too. It can create a similar condition to glazing. The rings need to expand a little during this initial break-in period, just not so much that they overheat and flash the engine oil. The engine needs to be moderately loaded in order to break in correctly. Running the engine under very light or no load prevents the oil film placed on the cylinder wall from being scraped away by the expanding compression rings. The rings will instead “hydroplane” or ride over the deposited oil film, allowing it to be exposed to the cylinder combustion. The oil film will then partially burn on the cylinder leaving a residue that will build up and oxidize over time. Eventually this leaves a hard deposit on the cylinder wall that is very similar to the glaze left from flash burning.
What will happen if the proper break-in process is not followed?
A caution to those just running the engine as a normal daily driver (without some loading) and especially those who love to idle their vehicles, expect some VERY extended break-in periods (up to 40,000 miles on some engines). Expect oil consumption forever due to oil glazing. The rings never really seat well if they cannot expand from the dynamics and heat that a load produces. Expect poor mileage due to the passing of compression and combustion gasses around the compression rings. Additionally, expect to see increased bearing wear and engine wear due to the fuel passing the rings diluting the engine oil.
What’s the best way to break-in an engine?
Heavy loading and light loading can cause some major problems. Moderate loading is the key to a proper break in for the first 1000 miles. It permits the loose-fitting piston rings to expand into the cylinder walls allowing them to perform double duty: First, scraping oil off the cylinder wall, and second, to create friction that will promote wearing the two surfaces to each other’s proportions. Furthermore, moderate loading will allow the rings to get hot but not to the point where it will flash the lubricating oil supplied to the cylinder walls.
Once the rings and cylinder have "mated," they will have worn away a considerable amount of their roughness. They will wear slower than they did when they were new. This reduced wear rate indicates the end of break-in, and a decrease in oil consumption should be obvious to the vehicle owner or operator. Furthermore, blow-by and fuel dilution should also be reduced but may not be so obviously evident.
What if I have plasma-faced rings?
Be aware that engines employing plasma-faced ring technology will take a longer time to break-in. These rings tend to wear far slower than chromium-plated rings. The plasma ring’s hardness allows it to wear the cylinder wall in a more aggressive manner while only polishing the ring surface. Eventually the cylinder wall wears to the shape of the ring and subsequent cylinder wear evolves to a polishing process. This extended process drastically improves the sealing potential of the cylinder, which will correspondingly reduce blow-by and the amount of physical wear on these components. Therefore, we can safely say that the plasma faced ring / Ni Resist insert combination greatly extends engine life. Unfortunately, the price of this better seal is a longer break-in period.
How long does it take for an engine to break-in?
Outside of the rings being hard as rocks and just taking their own sweet time to mate to the cylinder bores, the greatest factor is how the engine is broken in. Most engines will be broken-in after running for some time, but some ways of breaking-in an engine are far superior to others as they are more likely to produce low blow-by and near zero oil consumption.
Therefore, we will lay out some recommended DOs well as definite DON’Ts:
- DON'T run the engine hard for the first 50 to 100 miles. It is recommended that the engine be operated around the torque peak (1500 to 1800 RPM) in high gear. This loads the engine verygently andallows the internal parts to "get acquainted" without any extreme forces.
- DON'T let the engine idle for more than five minutes at any one time during the first 100 miles, even in traffic.Remember those loose-fitting rings, and possible fuel-oil dilution that were noted above? (Fuel dilution is very common when diesels idle, even with well broken-in engines.) Well, if that fuel is allowed to contact the main and rod bearings during break in (not really good at any time), you might be looking at an engine that will always consume some oil and one that may not produce power or mileage as expected. In the first few miles of break-in, the bearings are mating to the crank, rods, etc. It is imperative during this time that the lubrication qualities of the oil remain robust. Fuel in the oil will reduce its ability to absorb shock and float the rotating parts in their bearings. Contact between bearings and journals will occur more frequently which will result in additional friction wear. This will ultimately reduce the tight tolerances between the bearings and journals. What was originally a tight fit will be sloppy and will never be able to mate properly.
- DO drive the engine at varying RPMs and speeds until about 1000 miles. The idea is to alternately heat and cool the rings under varying RPMs. Manual transmission-equipped trucks are the best for this as they typically employ engine compression to slow the vehicle during normal operation, this constantly allows for varied RPMs.
- DO put a load on the engine at around 1000miles and get the engine hot! Diesels are designed to work, and in many cases, they operate best under a load. Baptize your engine with a nice "initiation load," to introduce it to hard work. Keep the revs up and make sure the coolant temps rise. Hooking up your trailer and finding some hills to pull works great for this. After the 1000-mile pull, just drive it normally, always making sure to let the engine get up to normal operating temps (no 1-mile trips to store). Towing is ok but remember to not overload and to monitor your gauges carefully erring on the side of caution. Under these conditions, most diesels completely break-in between 10-15,000 miles, and you can tell that point from mileage gains. One may also notice that the "symphony" of the engine also changes slightly at this point.
If you enjoyed this article, subscribe to our email list to avoid missing out on any of the technical information about diesel engines we regularly share and stay informed on the latest news from AFA Industries.