6.2-6.5 GM Diesel Engines - Aftermarket Mods

Running 6.2/6.5L GM Diesel engines in extreme conditions, like the ten percent grade in Fountain Hills, Arizona can push and, unfortunately, exceed the limits of a stock configuration. It can also take a chunk of money out of your pocket due to low fuel "economy".

Here's a look at some modifications that increase the power of these relatively inexpensive engines while improving fuel economy.

Some folks have a hill or a mountain pass they want to conquer with their RV but they discover that their engine temperature runs too high or that the speed runs too low. For me, it is a commercial application: dealing with mountain grades five days a week, 550 miles a day with a 24-foot cargo trailer full of parts and tires. Much of my story involves gathering baseline results and comparing that baseline to results after making improvements that paid dividends while running the daily parts delivery route. As a commercial route, I have Department of Transportation (DOT) hours of service to contend with. So a slow 6.5 diesel is a problem because I can run out of DOT driving hours! Fuel economy is also critical to the bottom line: one MPG is sometimes the difference between profit and loss.

We set the real life story in Arizona. It is not only a single mountain grade that I want to overcome, it is a lot of them and in Arizona where driving conditions tend toward the extreme. Extreme as in seven percent grades that go on for six miles and 121ºF temperatures that turn to ice on the same road in higher elevations. Elevations from 1,200 to 7,600 feet on my route. It is literally uphill both ways and will work any unloaded vehicle hard let alone towing. I am talking about non-stop mountain grades on Highway 87 from Phoenix, AZ to Payson and then on to the 260 to chug up to The Rim toping out at 7,600 feet, finally ending up in Springerville, Arizona.

The “Turn of AC to Prevent Overheating” signs are a caution to be seriously considered given Arizona’s high outdoor temperatures. However, after I made the cooling system improvements, I no longer needed to shut down my vehicle’s air conditioning to avoid overheating.

I started out with a bone stock eBay special 1993 GMC K2500 extended cab pickup with a 6.5 turbo diesel, mechanical injection, 4.10 rear end, 4L80E automatic transmission. The transmission was rebuilt and new injectors installed. High Output water pump, new condenser, and new radiator.

So I would start out in Fountain Hills and ‘coast’ into Phoenix with the nearly empty 24-foot cargo trailer ready to be loaded up with 50 to 100 tires, sometimes an engine, maybe a transmission or two, assorted car parts and batteries to be delivered to dealerships, mom and pop repair and tire shops. Frequently I had the trailer at the 8,500 pound limit with additional cargo in the bed of the truck. The first grade in Phoenix from I-10 to highway 202 in the carpool lane would slow me down at full throttle and the fan would already have been running on and off. The first part of highway 87, I could maintain 65 MPH. After the stoplight at the dump, usually red of course, I would be hard pressed to maintain just 62 MPH all the way to Fountain Hills. In fact, I would usually run in the 52 to 55 MPH range in a 65 zone when there was any noticeable grade. After Fountain Hills, however, the grades turn serious, resulting in top speeds of 33 MPH stock and 43 MPH with a Turbo Master and fuel turned up.

We also used a 28-foot cargo trailer that was eight feet high and full width. The results were very close to the 24-foot cargo trailer: both weigh slightly over 4,000 pounds empty.

We started our route sooner than expected as another driver quit due to blowing a $1,000 computer on his Dodge Diesel. This turned out to cause serious problems because we had a Kennedy Diesel low temp fan clutch and 20-inch steel fan on order. Sure enough, the second week we had a large load to pull. We used 4x4 low to make it up one grade as the cooling fan clutch and factory 19-inch fan wasn’t up to the task. A defective new radiator cap suddenly let the pressure out and shock cooled the block. It is well known for these engines that the driver’s side of the engine gets most of the coolant flow. We learned weeks later this had scuffed and cracked the number one piston. That week the low temp fan clutch and 20-inch fan arrived and, combined with a 180ºF single thermostat and high output water pump, maintained the temperature at or below 210ºF even in 110ºF degree weather. Oil changes occurred at 2500 Miles, a weekly event. The oil lab called saying that we needed to flush contaminates out of the system and fix the EGR. The engine did not have EGR and we knew we were in trouble. We were using a quart of oil every 500 miles working the engine this hard – a consumption rate that holds steady for all 6.5’s we used on this route.

Sure enough the third week the number one piston burned through the crack and popped the driver’s side valve cover gasket with blow-by. It blew a quart of oil all over the engine compartment.

So far we were getting seven MPG and a top speed on hills of 33 MPH. We had added a Turbo Master, 4-inch exhaust (no factory converter for this year) and turned up the fuel half a turn in the IP. The turbo was running near its choke point of 14 PSI and we had to turn down the Turbo Master to 12 PSI at low elevation because before it would creep up over 14 and beyond in higher elevations while blowing black smoke. The Turbo Master got results: 43 MPH top speed and the same fuel economy.

I never thought I would pull an engine from a vehicle but I had no choice. I ordered a surplus military 6.2 from Boyce Equipment with less than 30,000 miles on it. I pulled the heads and installed ARP head studs, 6.5 head gaskets and the retested 6.5 injectors. I kept the 6.2 injection pump and the 6.2 heads. The engine was clean enough to eat out of the valve covers. It was a relief when the truck started and ran without problems – three weeks later.

During those three weeks, we used several other vehicles to run the route. Speed limits are 65 MPH and 55 MPH in some of the twisty grade areas. A 2007 Trailblazer Super Sport with a 6.0L 395 HP 400 foot pounds torque V-8: It maintained the speed limits but at six MPG. It lasted a week before it turned the torque converter blue and put a bunch of transmission oil on the rear hatch. A 2010 Dodge 1500 with the 4.7L V8: speed limit at six MPG. For comparison sake, a 2008 Duramax was getting 8.5-9.5 MPG but on a different, steeper route with the same trailers.

The 6.2 got the same fuel economy as the 6.5, but, had more exhaust haze and lost three MPH around the 65 MPH top speed on a specific grade: timing and the smaller precups have a lot to do with these results. It ran three weeks (7,000 miles) before heartbreak. We hit a pregnant Elk at about 50 MPH with the brakes locked up. After being run over by the truck and trailer, the DOT guy didn’t want it. I bought the truck back from the insurance company and another 1993 (same year) Chevy that needed lots of help.

I swapped everything in the cab from the dash to the seats and carpet. Swapped the doors and bed. Eventually I swapped out the worn out 6.5 for the fresher engine and transmission from the wrecked truck. Both 6.5 trucks had the same feel. The engine would just fall over above 2,200 RPM. No matter what the fuel settings were, timing, boost PSI. 62 MPH at 2,200 RPM was peak power for pulling. They just would not go any faster foot-to-the-floor and would lose speed if you downshifted. As the grade increased for the mountain pulls, the automatic would shift for higher RPM and expected power and start slowing down. We dreaded the shifts and the quick loss of speed that followed them. The 6.5 would simply have an asthma attack over 2,200 RPM!

Pulling a trailer back from Lake Havasu with a headwind this became apparent. Just enough grade and wind to keep the engine at peak torque with the throttle floored and the TCC override switch on. (I do not recommend a TCC override switch with a factory converter.) We were at the sweet spot, 62 MPH at 2,200 RPM, with the throttle floored. It was apparent that something was badly affecting power over 2,200 RPM. I couldn’t get much over 62 MPH that day.

GM3 stock turbo versus A Team Turbo

Then I purchased a 1995 K2500 Suburban with a blown 6.5 engine, 3.73 rear ratio. It lost the number eight injector towing on the grades to Payson. This melted down the piston that locked up at TDC. It then shattered the skirt and the now loose rod beat four holes into the cylinder. Coolant and oil mixed and resulted in an impressive steam and smoke show that had people abandoning the vehicle like they were going to die. The turbo bearings were wiped out and the entire engine and oil cooler were scrap metal. Three-inch Banks exhaust and old tune chips were the only changes from stock. The Banks EGT gauge reported that everything was fine as the destruction went on.

I ordered another 6.2 military surplus engine and the A Team Turbo kit, sold by Walking J designs, along with a hot tune. The A Team Turbo is supposed to get better fuel economy and get rid of the falling over feel in the upper RPM, that is, the turbo is supposed to pull hard to redline. As I needed a turbo anyway, it was a cost effective decision to try it.

6.2L GM Diesel engine with A Team turbo

Suburban engine with A Team turbo

Soon after I got the Suburban running, I found myself going to the dump with the 28-foot cargo trailer. Pulling out from the dump in 110ºF weather, AC on high, I mashed the throttle to the floor to see what the A Team Turbo was worth. Remember that this is at the dump light to Fountain Hills section of Highway 87 for which 52-55 MPH was the low flat out speed with the 1993 pickup after turning up boost and fuel. At will, I was able to quickly get the Suburban up to 70 MPH and maintain 65 everywhere including mashing the throttle to get to 70 again on the grades to Fountain Hills. The engine would pull hard all the way to redline.

The 2,200 RPM Asthma Attack was Gone

This first run showed me some remaining problems, ruined catalytic converter from the antifreeze and the oil it ate, the three-inch exhaust was too small, and algae/bugs in the fuel that were plugging the tank sock and a lift pump that had just failed. In fact, the engine would not start after I took the fuel cap off and released the tank pressure. I had to change the lift pump at the fuel station in order to continue my journey!

After I fixed these problems, we took the Suburban to Payson and back with a 4,000-pound hydraulic dump trailer in tow. The fixes were a four-inch exhaust, new fuel tank and cleaned fuel system, Walbro FRC-10 lift pump, no tank sock, and a new six-position selectable tune chip. We did this with Tom Buddy who wrote a six-switch selectable tune for the 1995 OBDI computer. There were three people in the Suburban. We reflashed the chip in Payson with tune changes and at the end of the trip came up with the following conclusions.

The 6.2 Military precups limit boost to 17 PSI unaffected by fuel or timing. The 6.5 turbo precups can get over 20 PSI of boost from this turbo. The new top speed was 55 MPH compared to the 43 MPH of the 1993 pickup. At 55 MPH, I only asked for more power twice: a big improvement over chugging along with the flashers on like the pickup had done.

We could downshift with confidence and have power till redline.

10.4 MPG versus the 7 MPG of the pickup.

The cooling fan ran a lot less. Even in 32ºF weather, the cooling fan would have been screaming on the pickup on every hill. It only kicked in twice each way with the Suburban and the thermostat stayed at or below 210ºF for the entire trip.

We tuned the smoke to be a light haze, no worse than a 6.2 NA engine.

Even with the 6.2 precups limitation, there is a lot more power to be had with the A Team Turbo with different precups. The results we experienced point out the GMx turbos run out of air when you need it the most: working hard on a grade towing. GM3 turbos turn the three MPG of wasted fuel economy into heat, slow you down and lock in the cooling fan. Not only that, the three MPG improvement we realized with the A Team turbo means that a relatively inexpensive 6.5 diesel can closely compete with modern diesels relative to fuel economy and, important for our application, give us room in DOT hours of service to finish the day because of the improved top speed. The changes are the difference for us between profit and loss.

Example of Savings

Running the A Team Turbo over the GM3 stock turbo resulted in a savings of nearly $85 per day based on 550-mile days. This means that we paid off our $1,500 investment for the turbo, tune and full exhaust in just under 10,000 miles or 18 days of driving.

Daily Drive on the GM3 Stock Turbo at 7.5 MPG
73.33 GAL / $302.86 per day

Daily Drive on the A Team Turbo at 10.4 MPG
52.88 GAL / $218.41 per day