Tech Q&A Part #3

Light-Duty Diesel?

Hello,

I have not purchased a rig yet, hence my search for information. I wonder if you have tackled the following question before.

I was recently warded off of a purchase of a diesel rig due to my projected use of the vehicle. I live in Jackson, WY, and would use the rig between half and 3/4 of the time for trips of 20 minutes or less. The remainder of the use is why I want a large, crew-cab diesel. I’m planning trips of 3-9 hours, with necessity for 4×4 and steep, off-road (not technical) access to the National Forest. I do not need to tow anything.

Is the 50-75 percent around town use enough to defeat the purpose of owning a diesel, which I would choose for fuel economy, power for long trips and off-road access? I have been told the sort trips cause condensation build up and carbon deposit build up that severely hamper diesel engine performance.

Thanks so much for your time.

Hamilton Smith
Jackson, WY

Hi Hamilton,

Actually, there is a little truth in what you’re heard. However, most of the armchair discussion about light-duty diesel use developed 20-30 or more years ago. Modern high-pressure common-rail consumer diesels are not nearly as negatively affected by light-duty driving. So, I’d say get what you really want.

For the older diesels, their much lower pressure fuel injection systems didn’t atomize diesel fuel nearly as well as the newer systems do. As a result, complete combustion was difficult to obtain when the truck was lightly driven. This meant more soot was being created in combustion areas, on piston rings and on injector nozzles. All of this degraded engine performance slowly over time. Excess soot could be dealt with if the truck was occasionally used for towing, where higher combustion temperatures and fuel flows would help to keep the affected areas clean.

Otherwise, read what you can about cold weather operation. It can get cold there in Jackson during the winter. The engine block heater is your diesel’s best friend when temperatures dip below 0F. Use fresh commercial diesel fuel and keep fuel treatments handy if you experience any fuel related problems. I’ve been dealing with diesels and winters since the mid 1980s, and our seasonal temperatures, here in Montana, are similar to what you experience in Jackson. Good luck.

Glowing a 1983 GMC Caballero?

I own a 1983 GMC Caballero that is equipped with GM’s notorious 5.7L diesel engine. I’ve owned this vehicle since the late 1980s and have enjoyed the fuel economy and dependability—till now.

What is giving me fits is the glow plug system. Glow plug operation is erratic at times, and sometimes it won’t glow the glow plugs at all when the engine really needs glow to start. I replaced the glow controller, but the problem remains. While doing some other electrical maintenance under the hood, I noticed that if the electrical connector on the alternator is unplugged, the glow system seems to operate normally. The alternator load tests fine, and there’s not a problem with battery voltage or recharge.

I’m at my wit’s end. Local diesel mechanics and everything I’ve read online hasn’t helped. I realize this is an older and somewhat rare diesel, but I’m grasping at straws here. Please let me know how to solve this problem. Thank you.

Arthur Bennet
Salem, OR

Thanks for writing Arthur,

The 1980s Caballero was GMC’s answer to the Chevy El Camino—same sheet metal and chassis, with trim being the major difference. The diesel Caballeros are extremely rare today. In 1982, the Caballero was produced using the G-body platform as the A platform switched to front-wheel drive. The GMC’s optional 5.7L V-8 diesel engine and 3-speed automatic produced an EPA rated 22-33 mpg.

By 1982-83, most of the problems with the 5.7L diesel engine had been resolved, but the damage to its reputation had taken its toll, and it was quickly forgotten by the industry. That said, and as you’ve discovered, the later 5.7s have performed well for their owners.

Now the glow system… The early 1980s 5.7L diesels used a thermo-resistive glow system very similar to that used in the 1982-84 GM 6.2L diesel engines. Internal to the glow controller was a series of thermistors (thermally heating resistors) that heated with electrical current flow. As these devices heated, they would activate a series of thermal switches that controlled the cycling of the glow plug system. When they worked, they worked fine, but they could also produce frustrating and hard to diagnose series of glow-related problems. In addition to the glow controller’s use of thermistors, it also received a signal from the vehicle alternator, which senses that the engine is running and that glow cycles were no longer needed.

So, if the alternator fails in just the right way, the voltage output will be normal, the alternator will load test normally, and there will be no outward appearance of an alternator problem. However, the glow system could still be affected. An alternator replacement will resolve the issue if unplugging the electrical connector from the alternator allows the glow controller to function. Sounds to me like this describes your situation.

Long-term, I’d suggest replacing your existing glow controller system with that used in the 1985-93 6.2L/6.5L diesel engines. The newer controller is half the cost and is twice as reliable. A little rewiring is all that’s required. Good luck.

LT265/75R16 vs LT245/75R16 Tire Issue on Chevy/GMC 2500HD?

I stumbled across something you’d written back in 2005 about tire sizes and powertrain warranties, and wondered if GM’s position had changed with the passage of time. Can you tell me what the latest information is for this issue? My truck is a 2006 equipped with the Duramax/Allison…

Thanks,

Bruce Nothdurft
Via Email

Hi Bruce,

Back in 2005, Duramax/Allison owners were surprised to learn that installing larger-than-stock tires could invalidate their truck’s powertrain warranty. The single rear 2500HD trucks were equipped with LT245/75R16 size tires, and we learned that installing 265s could cause a warranty denial. In addition to GM’s position, tire retailers were refusing to install any non-stock tire sizes as a result. Our local Costco even refused to sell me 265 Michelins for my truck due to GM’s policy.

All this was a result of warranty claims for Allison transmission failures. GM knew, dealers knew and even owners knew that Allison failures were far more likely to be due to abuse from aftermarket power adders than a simple upsize in tires. Restricting tire size helped GM keep more control over aftermarket modifications, but that effort certainly didn’t eliminate them.

Your email got me curious as well, wondering whether the official policy had changed through the years. Was the policy in effect for the 2015 model year? To learn more, I called our local dealer and spoke to the service manager about tire sizes and GM’s policy. Turns out, nothing has changed in the past 10 years. I was told that GM still insists that owners maintain the factory original tire size, and that there are dealer directives backing up policy enforcement. The good news is that the newest trucks have more and nicer tire/wheel options, especially with larger wheel rim diameters available.

All this said, your 2006 has been out of warranty for a while, so it’s no longer an issue with GM warranty coverage. Run what you want, with the confidence that tire size won’t harm your Allison – assuming you upgrade the computer with aftermarket programming to reflect the new tire diameter and to prevent a problem with the ABS braking system. I’ve been running 265s on my truck for more than 70,000 miles without an issue, and a lot of those miles were with a trailer in tow. Good luck.

If you like the Q&A series, here’s the previous post in the series: Tech Q&A Part #2