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Your Diesel Questions Asked and Answered

You’ve Got Questions, We’ve Got Answers

Bruce W. Smith
Jan 7, 2019
Photographers: Bruce W. Smith

’50s Diesel Retrofit

QUESTION: Where can I find an adapter to link a 2.8L Cummins crate diesel engine with a Muncie SM420 four-speed manual transmission? The gearbox is a rebuilt military Con-Diesel version (4SXM) from the mid-’70s. I’m building a ’50 GMC 1-ton with a custom rollback and want to stay with the old-style transmission, so the interior stays vintage and the truck retains a PTO to operate the bed’s hydraulics.
M. Weber
via email
Photo 2/6   |   Mating Cummins’ R2.8L turbodiesel engines to old four-speed manual transmissions, like this Con-Diesel Muncie SM420 being installed on a standalone application, is becoming easier and easier as companies continue to expand their adapter offerings to fit a wide range of applications.
ANSWER: Excellent engine/transmission combination for a retro-mod truck: old-school meets high-tech. The Cummins Repower R2.8L turbodiesel is growing in popularity because it really is a turnkey diesel conversion and well suited for retrofitting old iron like yours. That engine purrs like a baby kitten and doesn’t smoke at all. You also chose one of the stoutest GM four-speed manual transmissions ever made, according to many historians. The military version has the lowest compound-low gear ratio of all the GM truck transmissions at 7.05:1, which works well with the application you’re undertaking. It’s also popular among conversion specialists, as is the R2.8, so the adapter, clutch, and related parts are readily available. Two shops that specialize in the R2.8 conversion parts (including the adapters you need) are Advance Adapters (advanceadapters.com) and Axis Industries USA (axisindustriesusa.com). Advance’s clutch/flywheel assembly is PN 712500M and the R2.8L GM adapter kit you’d need is PN 712594-A. (Advance sources its R2.8 adapters from Axis Industries.)

Air In Fuel System

QUESTION: I’ve been driving my ’00 GM ¾-ton pickup every day since she was new. I wore out the 5.7L V-8 gasoline engine and transplanted a customized ’86 6.2L diesel with a Banks Power turbocharger. It’s been running great since the diesel swap—until a few weeks ago when it started to get harder to start when cold, and it idles rough and surges while sitting at stop lights. I know those are usually signs of an air leak in the fuel system. But for the life of me, I can’t find the leak. Any suggestions?
Warren Spears
via email
Photo 3/6   |   An easy way to track down an air leak in an old GM pickup’s 6.2L diesel fuel system is to lightly pressurize the fuel tank so the air leak becomes a fuel leak, which is far easier to spot. Most air leaks are the result of loose or bad clamps between the tank(s) and injection pump.
ANSWER: Yes, surging, rough idle, and hard initial cold start are all common symptoms of air getting into the fuel system somewhere between the tank and injection pump. No diesel behaves well when air gets into the fuel. A little trick to force an air leak to show itself is to apply a little air pressure to the fuel tank, then inspect the fuel line from the tank to the engine to see bubbles that identify a leak. First, replace the return line at the injection pump with a length of clear ¼-inch hose so you can see the fuel (and any air bubbles that might be traveling through the system). If air is present, add some pressure to the fuel tank. Under normal operating conditions, fuel is drawn to the engine by the lift pump. However, to expose an air leak, the line must be under pressure, not vacuum. Find an extra fuel filler cap that can be sacrificed. Create a “pressure cap” by drilling into the cap and installing a standard Schrader tire valve. Use a tire pressure gauge and air compressor to bring the fuel tank’s pressure to no more than 10 psi. Then inspect the fuel line all the way to the injection pump. The air leak will now become a fuel leak that’s easily spotted. Quite often the air leak is the result of some hose/tube clamp being loose so air is sucked into the system. Once the source of the leak is found, fix it. Let off the air pressure using the valve cap. Remove the pressure cap and replace it with the original fuel cap. Then replace the clear plastic tube you used on the return line with the proper fuel line.

4L80E Lost OD

QUESTION: The transmission on my ’93 Silverado K3500 no longer shifts into Overdrive or downshifts into a passing gear. The unit upshifts from First to Third with no problems. There’s just no torque-converter lockup, engine light, or diagnostic trouble codes from the ECM. I replaced the throttle-position sensor and set it to the proper voltage range (.5V to 4.7V) per the shop manual. I also checked the fluid, and it’s a good pink color with no burnt smell. The fluid level O.K., and the filter was replaced a month before Overdrive started failing.
Wayne Wilson
via the Internet
ANSWER: The 4L80E four-speed automatic is a totally electronic transmission with eight solenoids controlling shifts and other functions. Solenoids were problematic, especially on early units, so it's possible either the torque-converter clutch (TCC) solenoid that applies and releases fluid pressure to activate the converter lockup clutch failed, or the transmission-control module (TCM) that communicates with the ECM is bad. Or, there is a bad connection to the wiring harness. There is also a transmission-fluid temperature sensor that shuts down Overdrive if the sensor fails or it senses the fluid getting too hot. Additionally, there are two speed sensors in the early 4L80E: input-speed sensor (ISS) and vehicle-speed sensor (VSS). The ISS is located behind the bellhousing, and the VSS is located just in front of the extension housing. A faulty engine-speed sensor (ESS) can also cause those shift issues. A good transmission shop can use a Tech II tool and do the diagnostics to track down which of the above is the real culprit. If those check out, it could be a bad torque converter or leakage around the TCC sleeve bore, necessitating a rebuild of the valvebody. Aden McDonnell at Automatic Transmission Specialists (autotransservice.com) in Livingston, Montana, does a lot of automatic rebuilds. “This could be a number of issues, but without diagnostic trouble codes, we are just guessing in the dark,” he says. “A common problem is a skewed transmission temperature sensor reading abnormally high, which prevents Overdrive and lockup. We’ve also seen the bad connectors, fluid leaking through connectors, and just bad wiring going into the plug.” Aden says there are replacement pigtails readily available for the 4L80E to replace the old wiring.

Stuttering 6.7L

QUESTION: I have a ’16 Ram 2500 with 51,000 mostly highway, non-towing miles on it. I installed a cold-air intake, and shortly after that the engine began shaking a little under hard acceleration. I took it to the dealer, which said my diesel particulate filter isn’t regenerating all the way, which is causing the problem. So, they forced a full regeneration, and now the truck goes into regen about every 100 miles. And, the engine stuttering under hard acceleration is still there. Is this right? I'm at a loss.
Bobby K
via email
ANSWER: Common issues with some aftermarket cold-air intake systems are: mass airflow and intake-air temperature sensors are not in the exact same position as the stock sensors, the installer doesn’t get the two sensors reinstalled correctly, or the connectors aren’t tightened during installation. Having the MAF and IAT sensors function properly is critical to the 6.7L’s performance. The MAF is the closest to the turbocharger, and if the CAI manufacturer didn’t get its position exactly right or the sensor is damaged in some way during removal/installation, it will cause performance issues. If it’s plugged in and working but the position in the CAI tube is off (not sitting deep enough, or too deep in the air path), the MAF is not measuring the correct amount of air for the amount of fuel under a certain load. But no diagnostic trouble code will be triggered because the engine’s computer sees the MAF is working. Also, double-check the MAF sensor to make sure it is reinstalled in the correct position. There’s an arrow on it showing the direction of airflow. We’ve seen them installed a quarter or half turn off. If the sensors are good and installed correctly, pull the CAI off and reinstall the stock piece. Alternatively, call a manufacturer representative to get his or her input. The regen issue is one you should have the dealer keep looking into, as the truck is under warranty, but also understand regens between 100 to 300 miles are not uncommon. The frequency of regens has a lot to do with quality of fuel, driving style, and the amount of time the engine idles. Lugging the engine and idling a lot will increase regen cycles, as the soot builds up quicker in the DPF under those conditions.

Wagons, Ho!

QUESTION: My son and I have a ’46 Dodge Power Wagon we are restoring and repowering. Our goal is to slip a 5.9L Cummins into it and get it on the road. We have a line on a good engine and NV5600 six-speed manual transmission pulled out of a totaled ’04 Ram 2500, but there’s no wiring harness for the engine. Before we make the leap, we wanted to see which hurdles face us in terms of wiring it in. Do we need to find a full wiring harness from a Ram of that same year? We don’t have many resources out here in this part of Nebraska.
Sherman Mills
via email
Photo 4/6   |   When it comes to wiring a newer Cummins I-6 diesel engine into a classic Power Wagon or any other standalone application, the simplest approach is to use a custom wiring harness like those now offered by GOS Performance for any ’03-to-’09 B-Series powerplant.
ANSWER: Swapping 24-valve 5.9L Cummins engines into vehicles has become a lot easier over the past few years, with companies offering components for a wide variety of transplants into Fords, Jeeps, and classic rigs like yours. When it comes to wiring that diesel into your Power Wagon, the simplest approach is to contact GOS Performance (800-620-4467). The company developed a new wiring harness for any ’03-to-’09 B-Series Cummins transplanted into a “standalone” application. The year, make, and model of the vehicle it’s going into doesn’t matter; the harness literally needs three wires hooked up to start and operate the engine. “The advantage of our new wiring harness is a lot of customers source used engines, and they either don’t come with a harness or the harness is destroyed,” says owner Aden McDonnell. “Our new harness solves those issues. You just plug it onto the various sensors of the engine, provide power and ground, and fire it up.” McDonnell says GOS has Cummins harness setups for ’03-to-’09 models and is beta testing wiring for ’10-to-present 6.7Ls. The cost of the new harnesses ranges from $925 to $1,195, depending on the application.


QUESTION: The “Change Fuel Filter” indicator lit up while I was towing an equipment trailer over the Sierras with my ’15 GM pickup, which has about 70,000 miles on it. A couple minutes later, the engine lost power, leaving me stranded. The dealer says the CP4 injection pump failed for some inexplicable reason. The $8,500 in repairs to replace the fuel pump and injectors and flush the fuel system is covered under warranty. Are there any upgrades that can be done to ensure this doesn’t happen again when the truck is out of warranty?
Rick L.
via the Internet
Photo 5/6   |   Bosch CP4.2 fuel pumps flow less volume than their CP3 predecessor, which underscores the need for using additives that increase the lubricity in today’s ultra-low-sulfur diesel fuel. Low fuel lubricity is the common cause for these pumps’ premature failure and costly repairs.
ANSWER: CP4.2 injection-pump failures happen, just as failures happen to any part that has exceptionally tight working tolerances under high loads. Such failures are usually the result of poor fuel quality, low lubricity in the fuel, contamination, or the result of cavitation from running out of fuel. The injection pump essentially grinds itself to failure. When a CP4.2 pump “grenades,” the metal debris from its internals finds its way through the fuel-pressure regulator and into the rails and injectors, and back into the tank. That pump works extraordinarily hard pulling fuel from the tank and supplying the high rail pressures for the injectors to work. One of the common upgrades to ease the minds of those who have fears of CP4.2 pump problems is to install a pressure regulator that does a better job filtering the fuel before it reaches the rails and injectors. Exergy Performance offers an inlet metering valve for the ’11-to-’16 6.6L Duramax LML product called the System Saver (PN E05-10505), which filters down to 25 microns using a double-layer stainless mesh, whereas the stock single-mesh screen filters just 80 microns. The upgraded fuel-control actuator (FCA) assembly that is part of the CP4.2 doesn’t prevent fuel pump failure. It only mitigates the damage caused by metal debris circulating through the entire fuel system before the engine shuts down. The company claims the finer stainless-steel screen of its FCA traps more pump debris and contaminants sooner, plugging and shutting down the engine before extensive damage is done to the rest of fuel-delivery system—but it does help keep the repair costs down compared to staying with the stock FCA. Beyond that, you can take some of the workload off the CP4.2 by installing an aftermarket lift pump/filter system, which helps ensure the fuel getting to the pump is as clean as possible at the same time. Keep in mind: Contamination of fuel and the lack of proper lubricity in ultra-low-sulfur diesel are the common killers of CP4 fuel pumps. A CP4 uses a lower volume of fuel than its CP3 predecessor, so its internals are getting less lubrication. The way to be proactive in protecting a CP4.2-equipped diesel from an early demise is being diligent on using fuel additives that add lubricity with every fill-up. Ideally, you want diesel fuel with a lubricity that meets the minimum ASTM standard of 520 microns High Frequency Reciprocating Rig (wear scar) rating. HFRR is currently the internationally accepted, standardized method to evaluate fluids for lubricating ability. This is a very important number when it comes to diesels running ultra-low-sulfur fuels that don’t have very good lubricating properties without dosing with an additive. The larger the number, the poorer the lubricity. The poorer the lubricity, the quicker the road to pump failure. If you can, use a fuel additive that lowers the HFRR of the fuel into the 300s. Additives such as Opti-Lube’s XPD or XL have shown some of the highest lubricity improvements in comparative testing.

Hard Ride

QUESTION: I just purchased a ’15 Ford F-250 with 20-inch LT tires to use as my daily driver. It only has 69,000 miles on it. The ride is starting to annoy me because I feel every little bump in the road at highway speeds. I can put a few hundred pounds of weight in the bed and it’s much better. I keep the tires aired to specification. Do you have any suggestions on aftermarket products that will help smooth out the ride?
Willy T.
via email
Photo 6/6   |   Ride quality starts with tire inflation. New owners of heavy-duty diesel trucks keep their rigs’ stock tires inflated to the maximum as noted on the vehicle’s door placard. Those pressures are ideal for maximum load-carrying capacity. However, many owners lower inflation pressures to the mid-40s for empty or non-towing driving conditions, softening the unloaded ride. It’s important to remember, however, to reinflate the tires to their recommended pressures when towing or hauling.
ANSWER: Over-inflated tires are the problem. The cold inflation “specifications” on the sidewalls are only for that tire’s maximum load-carrying capacity and do not relate to the vehicle they are supporting. The tire-inflation specifications in the vehicle owner’s manual and on the placard on the driver doorframe indicate the maximum inflation pressure for the tires as related to the truck’s maximum towing/hauling capacities, and those pressures are usually lower than the values indicated on the tire’s sidewall. It’s not uncommon for tire pressures for heavy-duty trucks to be 65 psi or higher, which is ideal for towing or hauling heavy loads. But, as you discovered, it’s a little uncomfortable for the occupants. We have heard of Super Duty owners who set tire pressures at 45 psi in front and 40 psi in the rear, which softens the ride considerably without any apparent compromises in handling. If you go that route, make sure you reinflate the tires to the vehicle’s placard inflation settings when towing or hauling. You’ll also need to have a diesel technician (or use a programmer) reset the tire-pressure monitoring system alarm level to reflect the lower air pressures or you’ll have a warning indicating “low tire pressure.” Another thing to keep in mind is the shorter the sidewall between the rim and the ground, the harsher the ride. That’s why 18-inch wheels provide a more pleasant ride than 20s if the overall tire height remains the same, because there’s more rubber to help absorb encounters with potholes, expansion joints, and the like. The tire sidewall is actually the first stage of any vehicle’s suspension.