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Your Diesel Questions Asked and Answered

You’ve Got Questions, We’ve Got Answers

Bruce W. Smith
Mar 7, 2019

Noisy Ford LSD

QUESTION: I recently started noticing a chatter coming from the rear differential of my ’09 Ford F-250 when making sharp turns. It isn’t locking up like it should in soft sand or in mud, so only one tire spins. I changed the gear oil, but that seems to have made things worse. I looked at the original window sticker that came with the truck and it lists the optional limited-slip differential. How long do they last and are they easy to rebuild?
Steven Laird
via email
Photo 2/7   |   Most limited-slip differentials require a special friction modifier for the clutch packs to work properly. Failure to use the manufacturer’s suggested additive in the proper amount can shorten the limited-slip’s life and make it noisy or nonfunctional.
ANSWER: The Ford Traction-Lok (not to be confused with the Dana Spicer Trac-Lok) is a common limited-slip differential found under the ’98-to-’10 Ford Super Duty running the 10.5-inch Visteon Sterling rear axle assembly. All clutch-type limited-slip units wear out because every time the truck makes a sharp turn or one tire spins, the clutch plates inside the packs on each side of the ring-gear assembly engage with the steels, causing wear. How long a limited-slip lasts depends on the types of conditions the truck is used in and how often the rear tires are searching for traction. Most regularly driven rigs should need the rear differential rebuilt after reaching 100,000 miles. Those that see a mixture of sand, dirt, gravel, and snow or are used for towing or off-pavement duties will probably need the limited-slip rebuilt before 60,000 miles. Yes, the Ford Traction-Lok can be easily rebuilt by anyone with a moderate amount of experience setting up differentials. It takes about four hours, and rebuild kits are plentiful. In fact, there are several detailed how-to articles on TruckTrend.com related to rebuilding limited-slip differentials, including the Ford Visteon Sterling LSD. But before you have the rearend pulled out, we have an important question: When you changed the gear lube, did you add the correct amount of friction modifier as required? If not, that differential requires 8 ounces of friction modifier, such as Yukon Gear & Axle’s Limited Slip Friction Additive or Red Line Oil’s Limited Slip Friction Modifier, so the frictions and steels don’t engage prematurely, causing chatter and premature wear. Add the modifier and then make half a dozen tight figure-eight maneuvers in both directions to see if the additive cures the chatter. After that, see if the limited-slip’s operation improves when both tires are on a loose surface and power is applied. By the way, there are some synthetic gear lubes that already contain friction modifiers. Be sure to check the labels and make sure they are compatible with Ford Traction-Lok. If the additive doesn’t work, it’s time to rebuild the old limited-slip differential.

Warranty Concerns

QUESTION: I have a ’17 Nissan Titan XD and considered installing a larger fuel tank per your suggestion in the January 2019 issue. I contacted Nissan USA with this question and was advised to talk to my dealer. After discussing this with my dealer, he told me that replacing a factory-installed part with an aftermarket part will void my warranty. Nissan’s warranty is eight years/unlimited mileage, so I don’t want to do anything to void it. Instead, I purchased a 25-gallon portable diesel-fuel container I can remove when not towing my camper.
Bedford Sorrell
via email
Photo 3/7   |   Installing an aftermarket part on a vehicle doesn’t automatically void the manufacturer’s warranty. The Magnusson-Moss Warranty Act of 1975 makes it illegal for automotive manufacturers (or their representatives, aka dealers) to void a vehicle’s warranty or even deny certain aspects of coverage under the warranty, simply because an aftermarket part is installed.
ANSWER: Installing an aftermarket replacement fuel tank shouldn’t automatically void your truck’s warranty. The Magnuson-Moss Warranty Act, issued by the Federal Trade Commission (FTC) in 1975, makes it illegal for automotive manufacturers (or their representatives, aka dealers) to void a vehicle’s warranty, or even deny certain aspects of coverage under the warranty, simply because an aftermarket part is installed. It’s like the dealer saying installing non-Nissan wiper blades, air filter, or engine oil will void the vehicle’s warranty. Not so; as the owner of the vehicle, you have the right to install/use any aftermarket item you desire. The caveat is, if that aftermarket part somehow causes or contributes to a failure you want covered under warranty, the dealer can deny your warranty claim—but only if it can prove the aftermarket part is the root of the problem. That’s a big ask when it comes to wiper blades, air filters, and fuel tanks. The well-known manufacturers of such tanks offer warranties on their products and ensure the tanks’ fuel and wiring connections are compatible with a stock cell that’s being replaced.

EPA Enforcement

QUESTION: From time to time, you guys mention the “legality” of us modifying our trucks in your magazine and how the EPA has stiff penalties for deleting smog hardware and using tunes. Are these just scare tactics or is there some truth in the statements? Does the government actually go after people who hot-rod their diesels?
Bart Bryant
via email
ANSWER: The special agents in the enforcement arm of the Environmental Protection Agency take their job seriously, and violations of the federal Clean Air Act that relate specifically to modifying/deleting the emissions components and software on pollution-controlled vehicles are constantly being investigated. If you want to see how serious they are when violators are caught, check out the “Clean Air Act Vehicle and Engine Enforcement Case Resolutions” page under the EPA Enforcement section: https://www.epa.gov/enforcement/clean-air-act-vehicle-and-engine-enforcement-case-resolutions. It gives a year-by-year rundown on settled cases. There are several diesel performance shops and aftermarket-product manufacturers listed in 2018 that paid fines and faced other penalties. Many other similar cases are still pending. (The EPA fine for each “tampering event” is now at $4,527). Of course, if the diesel-powered vehicle is being used specifically and/or exclusively for off-highway/off-road purposes, such as sled pulling, drag racing, or dyno challenges, there are no restrictions on what modifications can be made.

Spongy Ford Brakes

QUESTION: I have a ’93 Ford F-350 with a 7.3L IDI engine and 416,000 miles. The problem is the brake pedal feels soft and goes nearly to the floor before the truck stops. The master cylinder is rebuilt and the vacuum booster has been replaced, as have the brake pads and shoes. The brakes felt pretty good for a few-thousand miles—then they went back to being squishy. Any ideas what it might be?
Terry Housh
via email
Photo 4/7   |   A spongy or soft brake pedal on ’90s-era Ford pickups is possibly attributed to a worn rear antilock brake valve. The valve is bolted to the framerail directly under the floorboard on the driver side.
ANSWER: A spongy/soft brake pedal is normal on older Ford diesel trucks, and there are several options for getting the brake pedal to feel firmer. We suggest installing a new (not rebuilt) master cylinder. Then double-check the rear brake assemblies to ensure all the springs and nuts are in place and that the brakes are adjusted properly. Paul at Hydratech Braking Systems says, “As the rear shoes go out of adjustment, the pedal goes lower and lower and lower. So many people do rear brake jobs and don’t properly service the adjuster systems.” Shift your focus to the rear adjusters first. This is the easiest thing to check. If the adjusters are binding (not disassembled and greased before installation), then even “new” adjusters can be suspect. The Ford owner’s manual says to apply the emergency brake every time you park, but most people don’t. Automatic brake adjusters work, but setting the parking brake really gives them the actuation they need (besides the basic function that occurs when backing up).” Paul also notes that a high-mileage truck can suffer from brake rotor and drum runout, which creates heavy wear on the master cylinder’s pistons and cup seals as the brakes pulsate (chattering the pistons during braking). The wheel bearings are adjustable on the front and rear of this truck, which “can also induce bizarre braking problems of this nature,” Paul says. Also check the RABS (Ford’s rear antilock) valve bolted to the framerail directly under the floorboard on driver side. These are known to cause similar spongy brake pedals. Remove the big hex nut on the end of the valve and slip out the accumulator spring that’s under it. Hold a screwdriver against the piston and have someone fire up the truck and push hard on the brake pedal. If the piston pushes the screwdriver back out of the hole, then the dump valve is stuck open or leaking, which means the valve is bad and needs to be replaced. Another possibility is the calipers are flexing too much under pressure due to age-related fatigue. According to an old Ford technical service bulletin (TSB 96-25-19), the fix is to replace the stock 1 1/8-inch bore master cylinder with a 1¼-inch bore from a non-cruise-controlled ’97 F-350. If that doesn’t fix the soft brake issue, you can install a Ford “Zero Loss Brake Booster” (PN F5TZ-2005-CA), which is also found on the ’97 models. It’s known to give the brake pedal a much firmer feel along with crisper braking.

Leaking GM Axle Tube

QUESTION: I noticed the other day that I had an oil leak on the rear axle of my ’07 Chevrolet Silverado 2500HD. It strikes me as strange, because the leak is where the axleshaft housing meets the differential housing. The leak has me worried. Is this indicative of a major issue or is it something that can be easily fixed?
Mike Tolbert
via email
Photo 5/7   |   Top Tech Leaking Axle Tube
ANSWER: It’s not common for the pressed-in axle tube to start leaking, but if the housing has been stressed or seen hard off-road use, it can happen. It should be inspected more closely by a dealer or reputable off-road shop and repaired if deemed necessary. This isn’t a fix solved by slapping on a finger-load of JB Weld or running a thick bead of RTV sealant around the perimeter. “I have seen such leaks a few times over the years,” says Casey Castle, the shop manager and expert fabricator at Dunks Performance in Springfield, Oregon. “The only way to resolve the leak issue that I have found is welding the tube to the housing, which isn’t as simple as it sounds. It’s a steel tube and a cast-steel housing, so it's not a straightforward weld job. Also, there is oil between the axle tube and housing, and it can contaminate the weld. I have had to acid dip the entire housing to be able to properly weld one up—oil can get into the pores of the metal and cause all kinds of issues, as it is almost impossible to get it all out. Look at the plug welds on the housing to see if they show damage or leaks,” Casey says. We are pretty sure GM uses cast-steel centersections on those 10.5-inch 14-bolt rearends. If you’re not sure, use a grinder to test it. If you touch the grinder to the housing and short, dull, deep-red sparks fly, the housing is iron. If the sparks are long and bright orange/red, then it’s steel. Be careful to not over-weld it if you go to the tubes, as too much weld on the top of the tube will cause it to distort upward so your tires won’t be parallel and the top will be aiming inward. That’s why specialty shops put axlehousings in a jig before doing any welding—and they preheat the centersection to about 250 degrees before welding, because it will be slower to heat up than the thin-walled axle tube. After welding, they heat the centersection again and wrap it in a few fireproof welding blankets to cool overnight—just as one should do when welding on axle trusses. The idea is to keep the dissimilar parts heated as equally as you can, and to let the welded parts cool very slowly afterward to prevent cracking at the welds. Some welding experts suggest using 7018 stick or ER70-S6 wire if MIG welding. The Lincoln Welding website says this about the latter: “Choose an ER70S-6 wire for welding on plate that has mill scale or surface contaminants, since this wire incorporates the proper deoxidizer to combat these issues. A deoxidizer absorbs oxygen so that it vaporizes into the arc or forms as scale oxides. ER70S-6 is also better for creating a smooth transition from the weld to the base metal, also known as wash-in or tie-in. Better wash-in may be a requirement in applications subject to fatigue. ER70S-6 wire can provide better wetting at the weld toe when compared to an ER70S-3 wire.”

Repetitive Regens

QUESTION: My 6.6L Duramax–powered ’14 Chevrolet has gone into DPF regeneration seven times in the last 2,000 miles. It has never done it before. The truck just passed 49,200 miles. About every two weeks, the idle suddenly jumps from 600 rpm to 800 rpm and the fuel economy dives to 9 mpg. I took it to the dealership, which said there is nothing wrong—no diagnostic trouble codes or maintenance lights. Are all of these regens really normal?
Bernard Linkhauer
via email
ANSWER: Your truck is experiencing what’s known as “active regen,” which happens when the diesel particulate filter is clogged with soot and not flowing as freely as it should. Soot buildup occurs when the engine is not being worked very hard or spends time idling or poking along in city traffic. Active regens require fuel to be sprayed through the indirect fuel injector, located in the turbocharger’s downpipe, to raise exhaust temperature as high as 1,300 degrees to burn out the accumulated soot in the DPF. The ECM monitors several exhaust sensors, including one that measures backpressure, and extrapolates when the numbers indicate there’s approximately 42 grams of soot collected in the DPF, which is the amount engineers say is enough to reduce exhaust flow and hinder engine performance. Standard DPF regeneration is also triggered after 700 miles in the event an active regen hasn’t occurred during that mileage range. Diesel fuel is injected through the “ninth injector” directly into the exhaust. If regen happens and the truck is idling, the rpm will rise (and mpg, obviously, will drop). Have the dealership check the ninth injector. It has a tendency to clog up with carbon because it sits right in the flow of the exhaust as it leaves the turbo. GM sent a letter about this (Special Coverage Adjustment #14653) back in June 2015 and extended the warranty on the NOx sensor out to 10 years/120,000 miles. The notification says the indirect fuel injector “may have a condition where it becomes stuck in the closed position or can have low flow…” and dealers are to replace it with an “improved service part” at no charge to the customer. If the injector is not working properly, the regen will not burn the soot out of the DPF like it should, resulting in increased regen frequency as the ECM continues to try and burn off the soot still accumulated inside the DPF.

Cummins Ford F-750

QUESTION: I own a construction material retail store. We just bought a ’15 Ford F-750 flatbed, with a 6.7L Cummins engine and Allison automatic transmission and use it for deliveries. The major selling point of this truck is its load capacity, which is 17.5 tons—a huge increase from the Ram 3500, or even the 5500. I am concerned the stock "pickup" engine might not be enough to carry 35,000 pounds more than 200 miles through the hills of upstate New York (Cummins says the engine in the Ram pickups is identical to the powerplants used in the commercial Ford trucks.) Do you know of any companies that have modified the 6.7L Cummins in the commercial trucks? I want to fill the engine bay with a compound-turbocharger setup, bigger injectors, fuel, perhaps water-methanol injection, and a "work" ECM calibration. I am looking for low-rpm torque rather than top-end horsepower, and I’d like to find the right components for this work truck.
Pasquale Pilato
via email
ANSWER: Your medium-duty flatbed is one of the last for which Ford offered the ISB Cummins 6.7L engine option in its commercial trucks. Prior to the ’16 model year, F-650/F-750 buyers had the option of having a Cummins 6.7L turbodiesel installed with the Allison automatic. In 2015 (’16 model intro), Ford changed engine choices with the new trucks: either Ford’s 6.7L V-8 diesel, built at the company’s plant in Chihuahua, Mexico, or a V-10 gasoline engine built at its plant in Windsor, Ontario, Canada. When it comes to hot-rodding them, you are out of luck. Yes, that F-750 Cummins is a “de-tuned” version of the pickup engine, producing about 60 hp and 150 lb-ft less torque than the Ram 2500/3500 engine develops. Commercial diesels are “dyno certified,” while pickups are “chassis certified.” They also have different emissions requirements and are designed and tuned to be under high-load conditions all the time, while pickup engines are designed for much lighter demands. Part of that “tuning” process means the engine control modules Cummins uses in the commercial trucks are different than those used in the heavy-duty Ram pickups. We are told the pickup computers can’t be swapped into the commercial trucks, nor can any of the “tunes” for Ram pickups be used for the medium-duty rigs. Those companies that have cracked the ECM codes and written “tunes” for the pickups’ 6.7L aren’t inclined to spend the same amount of time and resources to do the same for the commercial side, simply because the demand isn’t there. The tight engine compartment of the medium-duty truck is also a concern (for fitting compound turbos and such). Will the “stock” Cummins in your F-750 be up to the task of carrying full loads of construction materials through the Adirondack, Catskill, and Appalachian Mountains? We think it will perform just fine. There’s plenty of torque to keep a heavy load moving along even on the steepest grades, and Ford put those trucks through a litany of real-world durability testing before they set the load limits.

Thirsty 6.7L Super Duty

QUESTION: I have a ’15 Ford F-350 with a 6.7L Power Stroke engine and GDP Tuning’s five-stage 200hp ECM calibration, 4-inch exhaust, S&B Filters air intake, and 35-inch tires. I’m wondering what is the best thing I can do to increase the truck’s fuel mileage from its current 12 mpg?
Victor Holm
via email
Photo 6/7   |   Taller, more aggressive tires look great and enhance traction. The trade-off is they can be attributed to a drop in a truck’s fuel economy because they increase rolling resistance, add more weight, change the effective gear ratio, and raise the vehicle, which creates more wind drag at speeds above 55 mph.
ANSWER: Horsepower isn’t free. It requires bigger shots of fuel. More fuel means less fuel economy. So if you are operating the truck with the tune set at higher power levels, your engine is burning more fuel. However, a multitude of other factors put a dent in unladen fuel economy. Bigger, heavier tires, like the 35s on your rig, require more power to accelerate and keep momentum up. The tire’s tread also affects mpg; going from a “street” tread design to all-terrain or traction/lug tread creates more rolling resistance. Taller tires also change axle gearing to a “higher” final-drive ratio much the same as going from, say 3.73 to 3.55. They also contribute to raising the truck a couple of inches, which changes its aerodynamic profile, causing more wind drag at speeds above 55 mph. More drag equals less fuel economy. Vehicle speed is a huge contributor to observed mpg. Driving at 70 mph instead of 55 can easily chop a pickup’s fuel economy by 3 to 4 mpg. Driving style also hits fuel economy. Custom ECM tunes can also affect mileage, as they change the stock fuel curves Ford engineers have spent thousands of hours designing to maximize that balance between power and mpg. All these factors have a cumulative effect on fuel economy. To partially offset the change with taller treads and get back to more factory-like fuel economy requires changing the axle gearing to 4.10, along with lightening up on the throttle.

Allison Tow/Haul Mode

QUESTION: I tow a fifth-wheel camper behind my ’13 2500HD GM pickup. We drove it from South Central Missouri to Breckenridge, Colorado, and back, and I used the transmission’s towing mode the entire trip. I hear it is in my best interest to keep the transmission locked in Fifth gear when towing our camper. My truck is all stock and will be that way for quite a while. What are your thoughts on this practice?
Danny Staples
via email
Photo 7/7   |   Keeping a GM pickup’s Allison 1000 six-speed automatic transmission in Tow/Haul mode and manually locked in Fifth gear is a good practice for maintaining low EGT while towing heavy trailers over relatively flat highway.
ANSWER: When trucks similar to yours tow trailers that weigh more than 7,500 pounds, we always recommend the transmission is locked in Fifth gear and the “Tow/Haul” mode is activated. The configuration makes the 6.6L Duramax LML engine and Allison 1000 six-speed automatic happy. As GOS Performance’s Aden McDonnell told us in a conversation about high EGT when pulling a heavy trailer: “Put the transmission in manual mode, tap it to Fifth gear, and watch the gauges. The EGT usually drops about 250 degrees, along with coolant and transmission temperatures.” Aden says: “Everybody thinks towing at a higher rpm in Fifth hurts fuel economy. It doesn’t. Staying on top of the turbocharger and using Fifth gear’s .71:1 ratio generally improves towing economy.” It also pays huge dividends to add gauges to monitor EGT, transmission, and coolant temperatures in real time. Watching how those temperatures rise and fall by changing the engine rpm through manual gear selection will help you pick the correct gear for a particular driving situation and minimize stress on the engine and transmission.

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