Written By, Mike McGlothlin
It’s the engine that more than doubled Cummins’ share of the Class 8 market in just eight years’ time. It was extremely durable, powerful, and returned best-in-class fuel economy. It’s called the Cummins 855 Big Cam, and it dominated the heavy-duty diesel segment in the 1970s and 1980s. But, far from perfect, this 14.0L inline-six isn’t without its shortcomings. Many of them have surfaced in recent years due to age, but others have existed since the 855 (Big Cam and Small Cam) was still relatively new. From NVH and father time-induced oil leaks, PT pump failure to liner pitting, and the problematic low-flow cooling system they came with, there were problems and solutions to address. We’ll cover all of them in detail here—and then wrap up each summary with the best solution(s) for this time-tested workhorse.
HIGHLIGHTS
Most Common Problems:
*Broken Piston Cooling Nozzle
*Cam Lobe Wear
*Liner Pitting
*Low-Flow Cooling (Overheating)
*Cold Start Issues
*Reduced Drain Flow (RDF) Fuel Injectors
Most Expensive Problem:
*Cam Lobe Wear
Less-Frequent Headaches:
*Step Timing Control
*Oil Leaks
*Random Stalling
*Failed PT Pump
Problem #1: Cam Lobe Wear
Premature camshaft lobe wear wasn’t the entire genesis behind Cummins’ redesign of the 
855 to include a larger, 2.5-inch diameter camshaft for 1977 (i.e. the “Big Cam”), but it likely had something to do with it. As an engine utilizing Cummins’ PT (Pressure-Time) fuel system, specific lobes on the camshaft are dedicated to actuating the fuel injectors. This, in addition to performing its traditional role of operating the valvetrain, can contribute to cam wear. Lobe wear is especially common in the Cummins 855 when the overhead falls out of spec, or isn’t set properly.
What Happens
In both Small Cam and Big Cam 855’s (although it’s more frequently experienced on the
former), eroded and damaged camshaft lobes are common, with camshaft replacement often being required long before the engine needs to be overhauled. Typically, it’s caused by friction between the cam followers and the lobe they ride on. As we alluded to above, an improperly set overhead (or one that is way out of spec) is a leading culprit. Incorrect valve spring pressure (sometimes due to worn springs) or a lack of lubrication between the follower and cam lobe are common contributors.
Solution(s)
Complete camshaft replacement, along with all-new followers, is the only real remedy for this problem—and it’s both a bit laborious as well as tedious. Not only do you have to gain access to the camshaft, but you’ll need the appropriate thickness cam follower housing shims, and you’ll have to reset the overhead when finished—something that typically requires an experienced mechanic or PT fuel system expert. If the push rods pass inspection they can be reused, so long as they’re reinstalled in the same order they were in when they were pulled from the engine. Due to being 30-plus years removed from production, aftermarket camshafts are the only “new” option at this point.
Problem #2: Liner Pitting
Every Cummins 855 is equipped with wet cylinder liners. And, while this plays a key role
in the engine’s million-mile durability, it doesn’t mean the 855 is bulletproof. As with any heavy-duty diesel engine, improper installation during overhaul, cooling system and maintenance neglect, and even time can contribute to liner failure. In particular, liner pitting is common on the Cummins 855. If it goes unnoticed or is ignored, stress risers will eventually lead to pitting on the outboard (engine block) side of the liner. It’s all downhill from there.
What Happens
Liner pitting initiates with the presence of additional heat, oftentimes due to an increase in 
horsepower. However, the leading cause is neglect. To keep the 855’s cooling system on the up-and-up, its antifreeze must have the appropriate additives and corrosion inhibitors present. When coolant additive tests aren’t performed at the specified intervals and the quality of the coolant is allowed to degrade, protection against erosion, corrosion, and even cavitation is lost. Once liner pitting protrudes into a cylinder, a myriad of issues can unfold, including rapid coolant loss, mixing of coolant and engine oil, elevated engine coolant temperature, and even a pressurized cooling system.
Solution(s)
Avoidance is the best recipe for liner pitting, and it starts with proper, timely cooling system maintenance. Test the coolant at regular intervals and always make sure the proper additives are present. Liner issues can also be averted by ensuring the engine doesn’t see excessive coolant temperature (we’ll note here that engine overheating can be a telltale sign of a liner problem). When it comes to liner replacement, always source parts from a reputable aftermarket source, check for proper protrusion, and begin anew with fresh antifreeze—complete with the correct additives—present in the cooling system.
Problem #3: Step Timing Control (STC)
This particular issue only effects engines beginning with the New Big Cam IV designation, the versions of the 855 that were produced from 1987-on. These 855’s featured Cummins’ temperamental Step Timing Control (STC) system. A fairly complex arrangement, some of its key components consist of the fuel injectors, an oil control valve, and a plumbing and check valve. The overall goal with the STC system was to be able to fine-tune the Cummins 855’s injection timing. Specifically, the engine maker was focused on reducing white smoke emissions on cold starts.
What Happens
Because it’s a timing system that relies on hydraulic actuation, the condition of the engine oil is paramount. In addition to clean engine oil, the STC system is dependent on precise valve operation and all of its mechanical controls functioning in perfect working order to effectively vary injection timing on-the-fly. If just one of many components fails to work properly, a loss of power, excessive EGT, increased smoke, and rough operating conditions can result. Tampering with the oil control valve or its associated plumbing is a highly common source of STC-related issues, too. The oil control valve must be properly calibrated for a specific volume and pressure.
Solution(s)
Beyond proper oil control valve operation and verifying the functionality and installation of its associated plumbing, careful inspection of the STC system mechanism is a requirement—not a suggestion—in order to keep the 855 humming along problem-free. The path to a solution involves enlisting an STC expert to diagnose and ultimately fine-tune your New Big Cam IV (or later 88 Big Cam IV). In this day and age, this can be an arduous task, as most of the seasoned Cummins 855 technicians and mechanics are no longer in the work force.
Problem #4: Low-Flow Cooling (Overheating)
The low-flow cooling system was implemented beginning with Big Cam IV engines (1984). Fairly complex, it featured a dual loop cooling system where one circuit was dedicated to circulating coolant through the engine block and the other was used for the radiator, aftercooler, and oil cooler. A single radiator utilizing a two-pass core allowed coolant to enter at the bottom, work its way up to the top of the tank, cross over to the other side, and then back to the outlet on the bottom. A divider was present in the bottom tank to keep the two cooling loops separate. This divider must 1) be present, and 2) seal properly, or engine overheating will occur.
What Happens
One major problem with the low-flow cooling system is that any restriction in one of the two cooling loops results in the other seeing more coolant flow. For example, plugged radiator tubes reduce coolant flow, forcing the coolant present in the block to remain there longer and eventually become super heated. Another common source of overheating with the low-flow system comes from modifying, missing, or removed upper water rail O-rings. Overheating can crack cylinder heads, warp valves, and even damage cylinder liners. There is no way around it, the low-flow cooling system in a Cummins 855 Big Cam must be well maintained to avoid overheating issues.
Solution(s)
The best way to keep the low-flow cooling system happy is to make certain that the radiator remains clean, the thermostats are in good working order, and it always pays to confirm that a Big Cam IV-specific water pump is being run. Note that the Big Cam IV water pump looks identical to the Big Cam 3’s but utilizes a smaller pulley and incorporates a vent in the housing to purge air from the cooling system. The upside to having the low-flow cooling system is that when it’s in proper working order it performs as intended—and by that we mean improved fuel efficiency over earlier Big Cam models.
Problem #5: Reduced Drain Flow (RDF) Fuel Injectors
As with so many other heavy-duty diesel engine manufacturers, Cummins was eventually forced to begin working toward meeting more stringent emission standards with the 855. Following the introduction of Step Timing Control in 1987 on the New Big Cam IV, Cummins moved forward with further plans to lower tailpipe emissions. Specifically, the company was intent on making the Big Cam compliant with the tighter particulate matter (PM) standard that was set to go into effect in 1988. To get there, Cummins introduced reduced drain flow (RDF) fuel injectors.
What Went Wrong
The RDF units, in conjunction with the STC system, were designed to maximize combustion efficiency across the board. From cold-start performance to max load at low rpm, the goal was to clean up the 855. This was accomplished by increasing peak injection pressure (from roughly 11,000-psi to 14,000-psi, in-cylinder) and, in turn, returning less fuel back to the tank. In other words, it was a concept that called for a more efficient use of fuel. The problem was that the rest of the fuel system wasn’t really up to the challenge. Under the added strain of producing higher pressure, injector reliability suffered.
Solution(s)
Not only do the problems associated with the STC system listed above apply, but injector flow and return issues are added to the mix on the 88 Big Cam IV. As with the aforementioned remedies, it pays big dividends to have an experienced 855 mechanic available (such as the veteran wrencher at Stevens Truck Power, shown here). Setting the overhead, the injector train, and a proper understanding of the STC system is paramount in keeping these old-school power plants in operation. It’s no surprise that this was the last version of the 855 Big Cam Cummins ever produced. Electronic control would soon take over, debuting in the form of the 855’s replacement, the Cummins N14 Engine.
Problem #6: Cold Start Issues
Cold start issues like long-cranking and excessive smoke on start-up are rampant with the Cummins 855. While this is par for the course on mechanical injection diesel engines with relatively low compression ratios (as low as 14:1 in many versions of the 855), it was a problem Cummins was well aware of and worked tirelessly to improve. And it’s part of the reason the Step Timing Control, low-flow cooling system, and even the RDF injectors were introduced. To aid the cold-start process, some owner operators even installed ether injection systems.
Causes
Aside from the inherent hard-starting scenarios that come with a low compression diesel engine, an incorrectly set overhead or injector train can result in a myriad of cold-start issues. This leads to imprecise injection timing and often allows raw fuel to leave the exhaust system in the form of white smoke. In colder climates, cold-start issues are only amplified further. Low compression diesels are notorious hard-starters in frigid conditions, when the engine can’t build up enough heat to ignite the fuel that’s present in-cylinder on its own.
Solution(s)
First and foremost, it’s crucial that the overhead on a Cummins 855 is set correctly. To limit white smoke emissions at start-up and when cold, the injectors must be set properly as well. And because low engine oil temp and coolant temp only serve to intensify cold-start problems, the use of a block heater is highly recommended for any Cummins 855 that’s expected to work in cooler climates. Oil immersion heaters have also long-been a helpful way to shorten cranking time and reduce cold-start emissions. We’d pass on the ether button idea unless it’s absolutely warranted.
Problem #7: Random Stalling
Nothing creates a white-knuckle ride quicker than an engine that dies going down the highway. And few issues present more of a hassle than an engine that continuously stalls at idle. You won’t find random engine stalling in the top five problems associated with the Cummins’ 855, but it’s frequent enough that it made our list. With it, you get the obvious out-of-the-blue shutdown, but also a drastic and noticeable loss in power when the engine is running, and oftentimes you’re left with an engine that won’t fire back up.
What (Often) Happens
As is the case with any mechanical injection diesel, sudden engine shut down can typically be traced back to a lack of fuel somewhere in the system. The most common cause of stalling on an 855 stems from a plugged fuel tank vent, which eventually starves the PT pump of its low-pressure fuel supply. When the fuel tank vent is blocked, most 855’s will run fine for 10 to 15 minutes, then die suddenly and not restart until after a short cooling period. In worst case scenarios, the fuel tank can collapse due to the excessive pressure.
Solution(s)
By locating the fuel tank vent and its respective line, it can typically be cleaned using compressed air, brake clean, or a combination of both. Corroded lines should always be replaced. In some dual fuel tank applications (common on old Freightliners, for example), a clogged splitter valve can cease to function properly if the tank vent is plugged up. In these cases, fuel won’t be equally drawn from both tanks and engine stalling is the eventual outcome. An easy, old-timer’s remedy for this is to install a crossover line between tanks for a simple, gravity-feed fix.
Problem #8: Failed PT Pump
Finding a failed PT pump on a Cummins 855 isn’t extremely common as these mechanical injection units are well-built and engineered to last. That said, with many of them more than 40 years old, plenty have bitten the dust, and others are on their second or third rebuild. When the PT pump heads south, drivers typically notice hard-starting (especially when warm), excessive smoke, engine stalling, rpm surging, and a drop in fuel economy. Seepage from the pump’s housing, governor cover, solenoid stem, or throttle shaft usually means it’s time for an overhaul. In the rare case when a PT unit fails internally, metal-on-metal knocking is audible.
What Fails & What Causes It
The Cummins 855 was built at a time when even low sulfur diesel (LSD) fuel had yet to be implemented at American filling stations. Fast-forward through the arrival of LSD and you arrive at today’s ultra low sulfur diesel (ULSD). The lack of lubricity present in today’s ULSD is said to kill a lot of the old PT pumps, with many typically only going six to 10 years between rebuilds. Common failure points within the PT pump include throttle shaft seals (followed by the pump sucking air), a bad AFC (air fuel control) diaphragm, or the low pressure gerotor pump located at the back of the PT unit checking out.
Solution(s)
Avoiding PT pump failure begins with proper fuel filter intervals and only quality diesel fuel being poured into the tank(s). Fuel additives that add lubricity to on-road ULSD are highly recommended. When it is time for a pump rebuild, make sure you choose a reputable injection shop that’s familiar with the PT unit. The cheapest option is seldom the best here so take it from us: fork over the cash to have an experience professional overhaul your pump. If you decide to go with a new unit, buy from a distinguished remanufacturer in the industry.
Problem #9: Broken Piston Cooling Nozzle
Piston cooling nozzles are vital in keeping engine oil present on the underside of the piston to keep its operating temp from skyrocketing. All heavy-duty diesel engines have them, not to mention that many engines also have integrated oil galleys within the underside of the piston to improve cooling even further. Unfortunately, when a piston cooling nozzle fails there is usually no dead giveaway that you’ve lost one—especially if you’re not watching oil pressure closely. It’s often a failure that goes unnoticed until the affected cylinder experiences a super-heated piston and incurs damage.
What Happens
The plastic piston cooling nozzles employed in the Cummins 855 are known to become brittle with age, and they’re notorious for cracking—many times even breaking off and falling into the oil pan. Key symptoms include increased engine oil temp and a sudden, instant drop in oil pressure. It’s important to note here that oil pressure won’t fall to 0 psi. The general rule of thumb is that a blown piston cooling nozzle results in a 10 to 15-psi loss in oil pressure. Again, unless you’re keeping constant, accurate tabs on overall engine oil pressure, it can be hard to notice the sudden, marginal loss that takes place when a piston cooling nozzle fails.
Solution(s)
If it’s caught before any engine damage occurs, replace the problematic piston cooling nozzle with an OEM equivalent. But while you’re in there, it’s always a good insurance measure to replace all of them. The good news here is that you don’t have to pull the oil pan on the Cummins 855 to access the piston cooling nozzles. They can be accessed externally. Their installation calls for coating the O-rings of the new units with fresh engine oil and orienting the nozzle’s spray pattern optimally (i.e. make sure they’re pointing the same direction as the original units were). After that, simply install the retaining bolt and torque to spec.
Problem #10: Oil Leaks
Surprise, surprise, old diesel engines leak oil. Old age, vibration and excessive NVH, thousands of heat cycles… it all takes its toll over time and leads to what is all but inevitable. And, not unlike so many other old-school, ultra-reliable engines from yesteryear (the CAT 1693 and later the 3406, Detroit Diesel’s two-stroke engines and later its Series 60, and even the Mack E6 come to mind), the Cummins 855 isn’t getting any younger. In fact, many were built more than four decades ago.
Where Oil Leaks Occur
On the Cummins 855, it’s commonplace to find an oil leak at the rocker boxes due to worn, eroded, and/or shrunken gaskets. It’s also frequent to discover a leak at the valve cover gaskets. Additional areas of the engine that often spring leaks are bad oil cooler seals, failed (rubber) O-rings on the rocker box shaft plugs, and loss of fluid from the accessory drive pulley seal. Of course, like any engine, front and main crankshaft seals are known to allow oil to escape with age, miles, and extended use.
Solution(s)
The only tangible way to solve oil leaks is to start with fresh seals, gaskets, or O-rings. Granted, this can turn into a lot of work if your repair forces you to remove several sealed components, but it’s also why various high-quality reseal kits and comprehensive upper and lower gasket sets are available in the aftermarket. It goes without saying that a complete engine reseal is all but required in the event of an overhaul. Last but not least, it’s always worth examining the crankcase breather. Any blockage or clog here will cause excessive crankcase pressure to build, which oftentimes forces oil past worn gaskets, O-rings, and seals.
