Eliminating Performance Roadblocks on a '94 to '98 5.9L Cummins Diesel

While reading a Driving Line story about 5.9-second diesel drag trucks, another story caught my attention. It was an excellent tutorial on how to get more performance and power from a mid-90s Cummins diesel engine. We'll give a brief overview here of the highlights and then link to the story, which includes photos that illustrate exactly what is being described.

Don't you hate it when something is explained but you have no clue how to do it because you don't know exactly what you are looking for. This article shows and tells, and is worth bookmarking.

The article by Mike McGlothlin, published last summer, is titled Performance Roadblocks of the '94 to '98 5.9L Cummins. This particular Cummins hit the road in '94-'98 Dodge Ram three-quarter and one ton trucks. 

The article opens by noting how durable the Cummins engines of this period were. 500,000 miles is not unusual at all with a few performance modifications. The factory setup was somewhat conservative for this engine, however, so get get the most from it McGlothlin zeroes in on the various ways this superb engine could be improved.

Roadblock 1: The P-pump
This is a Bosch P7100 mechanical injection pump, a "little engine" on the engine. Because of its factory setup the P-pump is more of a limiter than contributor of power. Hence, the starting point for more power is to move this roadblock.

The goal is to modify the air-fuel ratio. To do this, adjust the AFC (air-fuel control) by adjusting the pre-boost screw. (The article includes photos so you know where to find it.)

The second adjustment is to tweak the star wheel that sits on top of the AFC housing. By gingerly rotating the star wheel toward the passenger side you will increase the engine's power. If you go too far you will have black smoke that you do not want, so tweak accordingly.

The third adjustment again focuses on the AFC. Bring the AFC forward as far as it will go. The author writes, "Combining a full-forward AFC with the aforementioned star wheel rotation, it’s not uncommon to see a 60hp and 200 lb-ft gain."

His fourth trick is to have you remove the P-pump's fuel plate. To do this you will have to remove the AFC housing. These four moves alone will make a difference and cost nothing to implement. He then turns to a couple of "cheap" maneuvers.

His "Cheapie Fueling Fix #1" is to pick up and install a $15 Mack rack plug. This alone will add up to 35 hp to your truck. His "Cheapie Fueling Fix #2" is a timing adjustment.  

Roadblock 2: RPM
The second Roadblock, McGlothlin explains, has to do with RPM, or lack thereof. "If you perform all (or even some) of the fueling mods listed above, your power band will remain limited until you address the factory governor springs." The solution he says is to get a 3,000 RPM governor spring kit.

Roadblock 3: Lack of Boost
The adjustments you have made so far should have added 100 hp and 250 ft-lb. of torque. But the with factory turbo untouched, you're missing out on the full potential this engine offers.

His first suggestion is to install a boost elbow. This is a $25 item that enables youto fine-tune boost. It comes with precautions, though. 

Roadblock 4: Factory Torque Converter or Clutch
Another weak link to consider addressing, especially after you have removed all the other roadblocks to performance.

If this kind of thing interests you, but you have a different kind of engine, be sure to check out the forums (listed in the right hand column of this blog) where a lot of smart and experienced people gather to share ideas and help their fellow diesel friends.

You will find the Driving Line article here.