The legendary Cummins 12v, also known as the 5.9L 6BT, is widely regarded as the best diesel engine ever. The 12v was released by Cummins in 1984 primarily for farm equipment. In 1989, Cummins and Dodge partnered with the 12v launching in Dodge /Ram’s 2500 & 3500 trucks. It is the big brother to the 3.9L Cummins 4BT, which is essentially a 12v 6BT minus 2 cylinders.

The 6BT Cummins 12v ran from 1989 until 1998 when it was phased out for the 5.9L Cummins 24v, also known as the ISB 5.9. The inline-6 cylinder 12v diesel produced 160-215hp and 400-440lb-ft of torque.

Over its 10yr lifespan, the 6BT went through a number of revisions. In 1994, the VE44 rotary injection pump was replaced with the P7100 “P-pump”. Additionally, in 1991.5 an air-to-air intercooler was added. The engine went through three turbocharger changes, from the Holset H1C to the Holset WH1C, to the Holset HX35.

5.9L Cummins 12v Engine Problems

Killer Dowel Pin (KDP)
Heater Grid Failure
Throttle Position Sensor Failure
P7100 Overflow Valve
Transmission Power Limitation

1. Killer Dowel Pin – Cummins 12v

Despite the brutal reliability of the 5.9 Cummins 12v, it does have one fatal flaw. The 12v has a dowel pin that sits on the front of the engine. The pin is responsible for ensuring proper alignment of the timing cover, and is made of steel.

Engines experience “heat cycles” which is the basic heating and cooling of the engine. When your drive your truck, the engine heats up, and then you turn it off and it cools down. All metal is subject to expansion and contraction from heat cycles. With respect to the dowel pin, it expands under heat and contracts in cold. Over time, this can cause the pin to become loose. Combine that with the constant vibration created by a running engine, and then pin is prone to falling out of its hole.

When the pin falls out of the hole, there are a number of things that can happen:

Best case: it falls through the engine, hits nothing, and lands in the oil pan, causing no harm or damage. Unfortunately this is the least likely and least common result.
Bad case: pin contacts the cam gear, gets sucked into the injection pump and crank gears. Timing gets thrown off, the pistons collide with the valves, likely trashing both the head and block.
Worst case: the pin hits the gears, lands in the timing gear housing, and viciously blows a hole in the crankcase. Oil gushes out of the engine and your pistons and other internal engine components self destruct from a lack of lubrication.

Killer Dowel Pin Prevention

95% of the time, the KDP results in catastrophic engine damage, requiring a new engine. If you’re lucky, nothing bad happens. Unfortunately, there are no symptoms or warning signs to let you know before this will happen. It just happens randomly and ruins the engine instantaneously.

The commonality of this problem is debated, but it’s common enough that everyone recommends you fix it. Fortunately, preventing this problem is as simple as installing a tab over the dowel pin that will hold it in place and prevent it from falling off.

The prevention kits are about $50. Install is slightly more tricky, since it requires the removal of various parts on the front of the engine. Whether you can DIY it or have to pay for install, we 100% recommend this fix for all 5.9L Cummins 12v…and for any ISB 5.9 24’s.

KDP Prevention Kit install guide: https://www.pavementsucks.com/threads/kdp-tab-install-step-by-step-with-pics.70419/

2. Cummins Heater Grid Failure

Unlike gas engines, diesels do not use spark plugs because diesel fuel only requires heat to ignite whereas gasoline needs spark. Ford PowerStroke and Chevy Duramax engines use glow plugs. Each cylinder has its own glow plug which is essentially a metal rod that sticks into the cylinder and heats each one individually.

Cummins engines however use a heater grid instead of glow plugs. The heater grid sits within the intake manifold. Air coming through the intake passes over the heater grid, where it is then delivered to each cylinder.

Over time, the heater grid is known to either outright fail or become clogged and dirty to the point that it no longer effectively heats the air. The heater grid only kicks on when the temperatures are below 59 degrees, so symptoms are likely only to occur in cold weather.

Cummins Heater Grid Failure Symptoms

Hard start or no start in the cold
Engine dies while idling
No heat coming from around the intake manifold

While the heater grid is mostly a cold start aid, it also helps reduce emissions and smoke dumping on start-up. Warmer air burns fuel more efficiently which in turn reduces emissions. Since the heater grid somewhat restricts airflow, it is not uncommon for people to delete the whole unit in high power applications. Although, there is pretty much no power or performance benefit to deleting it on a stock lightly modified Cummins.

3. Throttle Position Sensor (TPS) Failure

The throttle position sensor measures how far down you’ve pressed the accelerator pedal and relays that information to the mass airflow (MAF) sensor. Based on how far down the pedal is pressed, the MAF sensor either opens or closes the throttle body to let in the right amount of air. Essentially, it balances out your air to fuel ratios.

When the TPS sensor goes bad it sends bad readings to the MAF sensor. The end result is your engine either ends up getting too much air or too little air for the amount of fuel being delivered to the cylinders. This impacts air to fuel ratios which have a material impact on overall performance and driveability.

On the 5.9 12v, the TPS sensor is known to get clogged up and eventually begin failing. Over time the sensor itself picks up plenty of dust, dirt, and other materials which make it ineffective. Additionally, rusted or damaged wiring can also cause the TPS to malfunction.

Cummins Throttle Position Sensor Failure Symptoms

Erratic acceleration or RPM movement
Rough idling
Engine dies while running
Overall poor engine performance
Possible check engine light

Luckily, a bad TPS sensor is a very easy replacement. Unfortunately, it is about $230 for the OEM part which is a tad expensive for a small sensor. There are some more favorably prices aftermarket options on the market, but they are generally going to be less reliable.

94-98 TPS Replacement Part
89-93 TPS Replacement Part – Aftermarket

4. P7100 Overflow Valve

The P7100 injection pump (aka P-pump or inline pump) is a boss. Because it is mechanical and not electronically controlled, it is capable of handling up to 600rwhp without needing upgrading. Additionally, a few changes can be made by hand to make some additional power for free. All in all this is a super desirable injection pump and is known to be highly reliable.

A few things are known to happen with the OEM Bosch overflow valve:

The spring either breaks or loses its force
The seat erodes which causes the spring to lose force
Overflow valve leaks

Each one of these known issues results in reduced fuel pressure. When fuel pressure decreases, overall engine performance decreases as the cylinders do not get an adequate amount of fuel.

P7100 Bad Overflow Valve Symptoms

Hard starting
Poor acceleration and performance, loss of power
Low fuel pressure
Cylinder misfires
Rough idling

The best replacement option for a bad P7100 overflow valve is replacing the valve with a Tork Tek adjustable overflow valve. It allows for fuel pressure to be adjusted without messing with springs, it fixes leaking, seat erosion, etc.

5. Transmission Power Limitations

This is less of a common problem and more of a heads-up for anyone looking to mod their 5.9 12v. Because you can make about 100hp for $0 on these diesels, it’s very tempting to do so.

However, the transmission options for the 6BT will hold back performance capabilities without adeqaute upgrades. The 6BT was offered with three transmission options:

Chrysler 47RH (4-speed auto)
Getrag G360 (5-speed manual)
New Venture NV4500 (5-speed manual)

The 47RH auto is the least desirable with respect to added power. The Getrag and New Ventures are more favorable to added power but still require upgrades once you try to power past the +100hp mark. At lighter power levels, an upgraded clutch will do, but as you start to look to push past 300rwhp, you will need to further modify the transmissions to hold the power.

5.9L 12v Cummins 6BT Reliability

The Cummins 12v/6BT is widely regarded as the best and most reliable diesel engine ever produced. These engines are extremely simplistic, operating with mostly mechanical components rather than complicated electronic ones. The simplicity allows for massive power potential in conjunction with ultimate reliability.

The engine and its components such as the P7100 P-pump will easily make it to the 500,000 mile mark with adequate maintenance. These engines are even known to break the 500k mile mark while pushing 500rwhp+.

The only potentially threatening problem with these engines is the Killer Dowel Pin. We highly recommend installing a KDP protection kit to prevent the possibility of blowing your whole engine. Outside of this engine, everything that breaks is small and easy to repair. The lack of electronics makes the 12v highly reliable and easy to work on.

With its insane power potential and relentless reliability, it’s no wonder this is considered one of the best diesels ever made.