Most Hated: GM’s 6.2L and 6.5L Diesels

• October 25, 2019
• Story By
  Mike McGlothlin

They’ve been laughed at by diesel enthusiasts, avoided like the plague by diesel mechanics and put out to pasture by hundreds, if not thousands, of owners. We’re talking about the 6.2L and 6.5L oil-burners, the indirect injection V8’s that were produced by Detroit Diesel under the GM umbrella for nearly two decades. But exactly why are these engines so hated? Is it because newer, modern power plants put them to shame, the fact that they rattled, shook and smoked or the fact that they did in fact have a lot of problems? Sadly, all of the above is true, but that doesn’t mean these sloth-like, prehistoric diesels don’t have a few high marks worth mentioning.

Below, we’ll explore why the often-hated 6.2L and 6.5L engines get such a bad wrap. Pattern problems, injection system woes and a severe lack of power are all on the menu. But first we’ll start with the positives. While that list is short, it helps explain why these engines can still be found in use today (and are even still being manufactured, though not by GM). Be it in marine applications, the H1 Hummer, half-ton, ¾-ton or 1-ton pickups, old Suburbans, Blazers, vans or even school buses from yesteryear, they’re still hanging around.

The Good

Fuel Efficient, Readily Available and Easy to Swap

When working these engines, they definitely take their time getting up to speed and are in no way comparable to the 454ci gas engines available back in the 80s, but they returned much better fuel economy than the big-block. And unlike the offerings from Ford and Dodge of that era, the 6.2L and then 6.5L turbo diesels could be had in a half-ton truck. Their packaging is similar to that of a Chevy small block and they utilized the common 90-degree V8 GM bellhousing pattern. Better yet, GM built loads of these engines from ’82 to ‘00, and General Engine Products (a subsidiary of AM General) continues to produce them today, so if you’re on the hunt for one they can be found in either brand-new form or dirt cheap in the junk yard. Last but not least, thanks to their lengthy production run replacement parts (new and used) are easy to come by.

The Bad

The 240 lb-ft Wonder

The primary performance complaint about the 6.2L (and the 6.5L for that matter) is its general gutlessness. Though you’re bound to hear this gripe about the old GM’s in the year 2019, the age of 1,000 lb-ft factory diesels, they were still noticeably slouchy when they debuted in ’82, rated for just 130 hp at 3,600 rpm and 240 lb-ft at 2,000 rpm at the time. But then, they weren’t intended to be powerhouses either. For raw power and performance, the 454 gas job was the power plant to have. For fuel economy and light tow duty, the simple, mechanically injected 6.2L performed as intended and it’s not unheard of for half-ton pickups to see 25-mpg with one between the frame rails.

The 6.2L’s Only Path to 200 HP: Add A Turbo

Naturally aspirated throughout its production run, the cast-iron block, 2-valve cast-iron head 6.2L could be woken up via the addition of a turbocharger (Gale Banks Engineering even supplied factory turbocharger systems starting in 1989). Adding a turbo, along with performing the corresponding tweaks required on the Stanadyne DB2 injection pump to increase fueling, could pump up horsepower to a little over 200 at the flywheel (and 375 lb-ft of torque). A properly installed turbo kit on a 6.2L could edge out a factory 6.5L in the performance department, but keeping boost in check is vital to ensuring the head gaskets live.

The 6.5L (’92-‘00)

By increasing the cylinder bore to 4.06-inches from 3.98-inches and leaving the stroke the same (3.82-inches), the 6.5L was born and several versions of the new, 395ci V8 were offered, including both turbocharged and naturally aspirated mills. The most common 6.5L’s are the L56 and the L65, the turbocharged versions found in half-ton, ¾-ton and 1-ton pickups (note that due to their GVWR, engines in half-ton pickups were equipped with EGR and a catalytic converter). The L49 and L57 engines were naturally aspirated and RPO codes LQM and LQN were also offered. While the 6.5L (namely the L65 and L56) was an improvement over the 6.2L in terms of power output, it spent its production run lagging far behind the engine offerings available from Ford and Dodge (the 7.3L turbo IDI and Power Stroke, and the 5.9L Cummins, respectively).

DB2

From ’82 to ’93, the Stanadyne DB2 distributor style injection pump was employed on the 6.2L and 6.5L. This fully mechanical pump was known for its simplicity and reliability. Its major drawback was the lack of fueling potential (6,700-psi peak injection pressure vs. today’s 30,000-psi common-rail pumps). The DB2 is an opposed-plunger, solenoid-controlled rotary pump with an all-speed governor. There are roughly 100 parts in each unit, none of which are spring-loaded and there are no ball bearings or even internal gears. In fact, there are only four rotating pieces in the entire pump.

DS4

Beginning in 1994, the 6.5L was fitted with Stanadyne’s DS4 injection pump, an electronically controlled version of the previous DB2 that proved much less reliable (more on its issues below). As for performance, it outflowed the DB2 but could still only move a maximum of 80cc’s worth of fuel, which speaks volumes as to why both the 6.2L and 6.5L are unable to make much additional horsepower. By comparison, the Bosch VE—a similar style pump—aboard the ’89-’93 5.9L Cummins flowed 105cc’s and the inline P7100 flowed 125cc’s in OE trim. Mechanical failures of the DS4 point toward the pump’s being prone to cavitation.

PMD

Without a doubt, the biggest issue with the Stanadyne DS4 rests in its use of a pump mounted driver (PMD), the little black box with Stanadyne stamped in large white letters. Due to being fastened directly to the pump, the PMD is constantly subjected to vibration, engine heat and often overheats, which leads to the 6.5L’s notorious engine stalling issues. Through the year 2000, GM issued several updates for the DS4, most notably rebuild updates and different governors.

Sound Solution: Relocate The PMD

One of the best aftermarket fixes for the PMD problem involves relocating it somewhere other than the engine bay. Heath Diesel offers a complete, bolt-in relocation kit, which the company calls its PMD Isolator System. The package includes a new PMD already mounted to the supplied heat sink plate, and comes with a pre-installed resistor and an extension harness to move the module to the front skid plate. If you want the 6.5L to live as trouble free a life as possible, this mod isn’t a suggestion, it’s a requirement.

Prehistoric ECM’s

According to some knowledgeable 6.5L tinkerers, several of the DS4 pump’s issues can be traced back to the engine’s ECM. Back in the days of OBD-I and even at the outset of OBD-II the engine control modules aboard the 6.5L were extremely low-tech. On top of that, there are even missing pieces in the source code itself on the early versions of the ECM’s programming (yikes!).

Broken Cranks

Ready for the catastrophic news? The crankshafts in both the 6.2L and 6.5L are known to break and the blocks crack. In most cases, the end-game for the crank occurs due to the harmonic balancer separating (it’s also been said that the harmonic balancer should be replaced every 100,000 miles). As for the blocks, insufficient casting quality or a lack of meat is believed to be the primary culprit, with most crankcases splitting near the outer main bearing bolts, on up through the cylinders. As a general rule of thumb, a well cared for 6.2L or 6.5L isn’t expected to go much beyond 400,000 miles (be it due to the failures mentioned above or things getting too loose in-cylinder).

Oil Pressure Switch Failure

Another common headache on the 6.5L is caused by the failure of the oil pressure switch. Not a huge problem on most other engines, but in the case of the 6.5L the OPS serves two purposes. Not only is it a sending unit for the oil pressure gauge, but it’s also responsible for turning the lift pump on and off. When it goes out, the lift pump will fail to feed adequate supply pressure to the DS4, which needs to see 3-to-5-psi worth of fuel pressure in order to avoid hard starting, hesitation during acceleration and premature failure.

Prone to Overheating

The 6.5L’s tendency to run warm and even overheat was addressed by GM beginning in ’96 with the reworking of the engine’s cooling system. That model year, twin non bypass-blocking thermostats were integrated along with a higher volume water pump. For additional help, companies like SS Diesel Supply and Heath Diesel offer a larger (21-inch) fan and fan clutch upgrade to help keep water temps well within check.

Cracked Heads

High miles and exposure to excessive exhaust gas temperature (EGT) often results in cracked cylinder heads, namely between valves. To be fair, many cast-iron cylinder heads aboard diesel engines experience hairline cracks throughout their service life, even cracking between valves. However, if the cracks protrude into the valve seats it’s time to start over. Some attribute the 6.2L and 6.5L’s cylinder head cracking to poor casting quality.

Hard-Starts & Other Issues

Both the 6.2L and 6.5L are notorious hard-starters in cold weather. Bad glow plugs or a failed glow plug controller is usually to blame, but these indirect injection engines’ high compression ratios (as high as 21.5:1) certainly don’t make cranking them over an easy task. Another common failure point, this one reserved solely for turbocharged 6.5L’s, has to do with the wastegate system. The lines that feed the vacuum solenoid valve are known to leak and crack with age, and the vacuum pump that makes the system function is also infamous for seizing prematurely and destroying the serpentine belt when it checks out.