Solving Early Duramax Cooling Issues With Late-Model Parts

In life, some decisions can be difficult. At the beginning of 2022, I decided to sell my 2007.5 LMM Duramax and purchase my father’s 2004.5 LLY Duramax. It’s always been my favorite body style and the fact that it does not have a DPF system that could fail at any point were some of the factors that prompted my decision. Two of my biggest hesitations were: going backward in the power department and experiencing firsthand some of the cooling system issues the LLY is plagued with. Luckily, both of them can be remedied. For now, to help with engine-oil cooling, I will be getting help via L5P OE parts.

Since my father purchased the truck new, I have been involved with all of the maintenance, repairs, and upgrades since then. With that, there is no doubt in my mind this truck has a lot of life left in it and will be reliable for a long time to come. This truck is currently my daily driver and it’s used to haul our 32-foot toy-hauler during the summer.

Getting new head gaskets and ARP head studs installed. Nothing flashy in the engine bay, but it works.

Many of my trips are to leave the 100-plus-degree heat of Phoenix, in favor of cooler weather. Towing at max capacity in conditions like this can be challenging for a lot of trucks, let alone one that has a hard time keeping engine temps at a reasonable level. Now that I’m back in my favorite truck, it’s time for some upgrades and tests.

The Test Subject

Before getting into the oil cooler swap and our results, here’s some information about the truck and trailer used for this test. My truck currently has 257,000 miles on the original engine, transmission, fuel injectors, injection pump, and turbo. Last year, I had the head gaskets replaced and ARP head studs installed due to the passenger-side gasket leaking coolant.

Upgrades include an Airaid intake, AFE turbo inlet, PPE downpipe, MBRP cat-back exhaust, GM driver-side exhaust manifold upgrade, FASS 100 gph lift pump, larger transmission cooler, Mag-Hytec transmission pan, Edge CTS3 Insight, and EFI Live tuning. The parts used are a very common recipe for a diesel truck.

My trailer is a 2018 Pacific Coachworks Blaze’n, triple axle bumper pull that weighs 9,700 pounds, dry. While testing, the camper’s fuel and water tanks were empty, it only had some basic tools in the storage compartment, and there were bedding and kitchen utensils inside.

Prepping For Data Collection

Within the last year or so, I’ve seen more and more companies offering the L5P engine-oil coolers in kit form as an upgrade for the older Duramax engines. This oil cooler is factory installed on the 2020 through 2022 model years. Comparing the specs of the early Duramax oil coolers to the newer models is great, but I wanted real-world testing. Speaking of specs, the OE oil cooler is a 10-stack plate design whereas the L5P cooler is a 19-stack plate design. It’s almost double the capacity.

Factory LLY oil cooler on the left and L5P oil cooler on the right. With a size difference like this, I am hoping for good results.

My plan is to do back-to-back towing tests, comparing the coolant and oil temperatures between the two oil coolers towing the same trailer during the same ambient temperature.

The first step was to add oil temperature reading to my Edge Insight. To do this I used the Edge EAS Competition Kit which allows me to monitor EGTs, lift pump pressure, and engine oil temperature. I used the provided temperature sensor installed into a fitting on the factory oil cooler housing which was the easiest location to install the sensor. This location gives me the fluid temperature after it passes through the oil cooler. Keep that in mind as you read further.

The next step was to source the parts needed to make the swap. The parts are sold individually from GM and not as a kit. However, you can find all of the parts needed in one kit from many online diesel stores. To complete my “kit” I also ordered a PPE High-Efficiency Oil Filter and a Max-Flow fitting from the DMAX Store that will provide an oil temperature reading in the same location as the OE cooler.

The parts used for this upgrade.

The installation is much more than I’m capable of taking on, so the team over at performed the work. I wanted to know how the old oil cooler was functioning. So they separated the housing and there was no sign of any corrosion or restriction that would hinder coolant flow. That’s what happens when you service coolant frequently.

Stock Oil Cooler Function

Prior to the first trip with the trailer, I drove around town for a few days monitoring coolant and oil temps. Since the ’01 through ’16 Duramax trucks don’t provide an oil temperature reading, it was nice to see this new and useful information. Depending on driving conditions and speed, my coolant would fluctuate from 180 to 195 degrees and oil would average 25 degrees higher than the coolant.

It was now time to see how hot things would get. With the trailer in tow, my plan was to head north, out of Phoenix (1,086 feet above sea level) to the rest area at Sunset Point (3,400 feet above sea level). It is a steep, 30-mile climb and a test for any truck. It was 84 degrees when I left town, so it wasn’t terribly hot. I headed north on I-17 from Pinnacle Peak Road while watching my monitor as much as the road. My coolant temp fluctuated a bit from 203 to 214 degrees which is normal. I paid close attention to see where my oil temp was going and it continued to climb at a steady rate. At the point it reached 250 degrees, I was hoping it would stop climbing. Then it hit 260 degrees.

I was now watching my monitor more than the road ahead of me while continuing up a gradual climb. It then topped out at 274 degrees, much hotter than I expected. This was also only 12 miles into my drive as I approached the exit for Anthem, Arizona. I continued on another 16 miles and exited at Black Canyon City to let things cool down before heading back. In total, I had only climbed approximately 500 feet in elevation. I still had another 9 miles to go before climbing the grade to Sunset Point. But with the oil temperature this high, I wasn’t comfortable pushing it further. On the drive home I wondered if GM intentionally did not monitor the oil temp on the Duramax for this reason.

Engine Oil Temperature (EOT) readings with the stock oil cooler in place just before exiting the highway to head back to Phoenix.

L5P Oil Cooler Function

With the new cooler installed, I drove the truck for a couple of days to watch the temperature while in stop-and-go traffic and unloaded freeway cruising. I was now seeing oil temperature staying about 10 to 15 degrees higher than my coolant temperature. This larger oil cooler dropped oil temps right away, which I was pleased to see and couldn’t wait to see how it performed while towing.

Temps shown above were at freeway cruising speeds after about 30 minutes. The original oil cooler is on the left, and the L5P oil cooler is on the right.

I hooked up the trailer and once again headed north, on the I-17 in identical weather as the previous trip. I also took the same route with no changes in trailer weight and used the same 15W-40 semi-synthetic engine oil. As I began the gradual climb north, I saw the oil temperature rising again. Eventually, it peaked at 251 degrees for a minute or so, then dropped back down into the mid-240s.

That’s a substantial temperature drop over the original oil cooler and proves this larger cooler is more efficient. There was a side effect, however. Since the oil cooler is larger, both fluids spend more time in it and have more of an effect on each other. I saw my coolant temperature on this tow was slightly higher than on my first trip. It ranged from 210 to 219 degrees and my fan clutch was engaged for most of the drive. I recorded video while towing with both oil coolers in place, and while headed north as well as south back into town.

Takeaways And Thoughts

At the time of testing, diesel fuel was $5.49 per gallon. I was happy to do the testing for my own peace of mind and for those of you wondering if this is a worthwhile upgrade. If you drive a 2001 through 2007 Duramax, I would highly recommend it, especially for those that tow or live in a hot climate. It will also work on the 2011 through 2019 Duramax.

Even if you’re not having issues keeping things cool, you’ll know that oil temps are lower than before. That means less chance of your engine oil breaking down and increasing the longevity of the bottom end. I also chose the PPE oil filter for improved filtration. It’s not required, as you can use an OE-type of filter for the L5P engine.

Both trips north for this test were done climbing mild grades. That said, there’s no doubt that longer, steeper climbs will be too much for the engine’s cooling system. So, stay tuned for part two for the next upgrade in hopes of shedding more heat, to keep me on the road and not the side of it.

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About the author

Gary Maschner

Gary has been working in a diesel shop since 2009 and he continually expands his knowledge of diesel repair and performance. His free time is filled with his boys’ ice hockey, camping and enjoying the Arizona back country in their RZR.
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