Written By, Mike McGlothlin
Highlights:
The Bosch P7100 injection pump on the 12-valve 5.9L Cummins is highly reliable, but its factory overflow valve is a common failure point.
A failing overflow valve often leads to hard starts, loss of power, erratic idle, stalling, or poor drivability.
Internal failures typically stem from a weak spring or worn seat, both of which reduce fuel pressure to the injection pump.
Tork Teknology’s adjustable overflow valve solves these issues with a stronger spring, improved seat design, and consistent fuel supply pressure.
Upgrading to Tork Teknology’s valve restores performance, ensures reliable starting, and offers options for both stock and high-performance Cummins applications.
Most people don’t associate the P-pumped version of Cummins’ legendary 6BT engine with fuel system shortcomings. After all, this coveted version of the 12-valve diesel power plant makes use of the million-mile-reliable Bosch P7100 injection pump, right? Unfortunately, there is a small component on the return side of the P7100 that can make or break the performance potential of the 5.9L Cummins. It’s called the overflow valve and it’s notorious for supplying inadequate fuel pressure, resulting in hard starting, loss of power, and poor drivability. In this article, we’ll walk you through a typical overflow valve failure. Then, as we always do, we’ll show you the best solution.
Telltale Signs The Overflow Valve In Your 6BT Cummins’ P7100 Is Failing
- Hard starting
- Low rpm miss, or erratic idle
- Engine stalling at idle, when warm
- Reduced performance/low on power
- Dead-pedal feeling in the accelerator
- Poor overall drivability
What (And Where) Is The Overflow Valve?
Located on the return line banjo fitting at the front (forward-facing) side of the P7100, the
overflow valve regulates fuel pressure being supplied to the injection pump from the lift pump. Because it’s tasked with ensuring the P7100 receives proper fuel pressure—pressure that’s not only highly pressurized and sent to the injectors but that’s also necessary to cool the pump’s internal components—its functionality is vital to the overall health of the P7100. Unlike the rest of the components on the P7100, the overflow valve doesn’t enjoy million-mile durability. In fact, a factory unit can begin to fail in as little as 50,000 miles.
Symptoms Of A Bad Overflow Valve
A failing overflow valve can manifest itself in several ways. An engine that’s hard to start
tops the list, but a low-rpm miss and an erratic idle are common side effects of lower fuel pressure being delivered to the P7100. Of course, lower fuel supply pressure also results in reduced performance—and few things disturb a 6BT Cummins owner more than their engine being low on power. Poor overall drivability often accompanies the symptoms mentioned here, and some engines even die at idle (usually when warm). In rarer cases, owners will notice a dead-pedal type feeling in the accelerator.
What Fails?
Now to the crux of the matter: what fails inside the overflow valve. There are two primary points of failure within the overflow valve, but a weak or broken internal factory spring is the issue that’s most commonly experienced. When the OEM Bosch spring loses its spring force (typically due to stretching), the spring becomes lengthened. This results in lower fuel pressure being delivered to the P7100. Not surprisingly, this is also when owners begin to experience the telltale signs of overflow valve failure (mentioned above): hard starting, reduced power, a low-rpm miss, and/or poor drivability.
What Else Fails?
The seat. Or rather, what happens to the seat after repeatedly being pounded by the
internal check ball. After so many cycles (i.e. abuse), the check ball’s seat ends up eroding and rounding off—even though it’s supposed to be flat in order to function effectively. The breakdown of the factory overflow valve’s seat has been traced back to both a lack of material around the seat itself and lack of being treated to a proper finish after machining it. Once damaged, the seat becomes leaky and allows an inordinate amount of diesel fuel to backflow to the tank.
The Fix: Tork Teknology’s Adjustable Overflow Valve
With a goal of solving the internal spring and seat issues that plague the Bosch overflow
valve, Tork Teknology developed an overflow valve made with superior components. The Holland, Michigan company’s overflow valve is fully adjustable, utilizes a spring they say will never stretch, and delivers precise fuel supply pressure for optimum and predictable P7100 performance. For more than a decade, Tork Tek’s adjustable overflow valves have been the go-to for stock and modified 5.9L Cummins owners looking to cure their hard-starting and performance issues. The Tork Teknology overflow valve is a direct replacement piece for the factory one, and this particular version (intended for stock fuel systems) retails for less than $50.
A Better Spring
Obviously, the overflow valve’s inferior internal factory spring had to be improved upon. Tork Teknology addressed the problem by sourcing custom, premium springs for all of its adjustable overflow valves. In durability testing, its springs proved capable of surviving what amounts to a trillion duty cycles. Essentially, its service life is endless. Thanks to all the R&D the company and its supplier poured into the springs, they won’t stretch or break. And they also benefit from a redesigned seat and ball—one that places considerably less stress on the spring, so as to not fatigue it.
A Better Seat
Making sure the internal spring could live indefinitely called for a better seat and ball. Like
the OEM Bosch piece, Tork Teknology’s seat is machined and heat-treated. However, unlike Bosch, its seat is treated to a super finish after undergoing heat-treatment—which the company says is the difference maker. In addition to making the seat stronger and more wear-resistant, the super finish process leads to virtually zero leaks (vs. Bosch’s highly leaky seat). Tork Teknology states that there is so little leakage with its seat that it acts as an anti-siphon check valve.
Improved Fuel Supply Pressure
A high-mile factory overflow valve will typically provide between 14 to 18-psi worth of fuel
supply pressure to the P7100 at idle. With Tork Teknology’s adjustable stock replacement overflow valve in the mix (PN OFV010HP), fuel pressure jumps up to 25 to 26-psi at idle. While it may seem like a marginal pressure increase, it makes a huge difference on engines previously equipped with a worn-out factory overflow valve. The uptick in fuel pressure leads to a smoother idling engine (less NVH), a slight increase in idle rpm (30 to 50 rpm), and can even provide a noticeable off-idle increase in engine responsiveness.
Additional Perks
On top of making the P7100 happier by providing it adequate supply pressure (along with being more mechanically durable than the factory overflow valve), the Tork Teknology unit’s leak-proof design prevents backflow to the fuel tank. This means the fuel system will always be primed, offering you crisp, quick start-ups. And while on the subject of leak-proof, it’s worth mentioning that the Tork Teknology piece ships with the appropriate banjo washers (with flared lips) for the ultimate seal. Last but not least, Tork Teknology’s adjustable overflow valve is also compatible with the P7100 used on the 8.3L Cummins (6CT).
Stock And Performance Options
Because the use of the 6BT Cummins is so widespread, and also because so many people
modify them, Tork Teknology offers an adjustable overflow valve for every application. For factory horsepower ratings and engines with stock fuel systems, the overflow valve we’ve spent all our time talking about in this article is the right choice (OFV010HP). For those in need of higher supply pressure, OFV030HP (20-40 psi), OFV035HP (30-65 psi, preset at 45 psi), OFV040HP (for aftermarket AirDog or FASS systems with electric lift pumps) and OFV075HP (race only, 50-55 psi) overflow valves are available. No matter the application, there is a product that will ensure your 5.9L Cummins never suffers from hard-starting or lack of power again.
