All About the 6.5
Be Nice and pay attention
REF: DieselHub - The Diesel Owners Resource
- The GM 6.5 Turbo Diesel has received an undeserved bad reputation during it’s lifespan. Unlike other diesel engines, the 6.5 does not tolerate abuse (or lack of maintenance) well. The 6.5 is NOT a “Disaster” of an engine like some would have you believe...but it is important that you know it’s quirks and limitations.
- Educate yourself about the engine, and ask for help here *before* resorting to a mechanic - the 6.5 is one of the simplest diesel engines on the roads today, and if you have basic mechanics tools and aren’t afraid to get dirty, you can do most of your own maintenance and repairs easily.
PMD / FSD issues and failure
The 94 and up 6.5 comes equipt with an electronic DS4 Injection pump and a FSD. The FSD commonly fails due to their mount location (and heat) resulting in stalling, hesitation, and eventual no start/run.
The FSD ( Fuel Supply Driver ) is more commonly referred to as the PMD ( Pump Mounted Driver ). It’s a small black or gray box, about the size of a deck of cards mounted on the front of the injection pump. The injection pump itself is located under the intake manifold a single wiring harness plugged into it. The PMD module is the brain that tells the injection pump when to fire fuel into the cylinders. When it fails your truck won’t run. You need to relocate it.
A complete kit ( Cooler, wiring, and replacement
PMD ) to do this is about $500. The kit allows you to relocate the PMD / heat sink assembly elsewhere
under the hood, behind the front bumper or in front of the grille itself. Since the module and connector are
weatherproof you can mount it almost anywhere with good airflow and be protected from impact damage.
The fragile harmonic balancer on the 6.5
The harmonic balancer ( and sister pulley attached to it ) is an essential piece of equipment on the 6.5, and it’s also a part that commonly failed ( or failing ) without anyone knowing about it since it often doesn’t have any blatant symptoms except to the tuned ear.
The balancer is basically a big circular chunk of metal with a rubber ring in the middle. This ring should have no cracks, no missing chunks, or no deformations. The pulley that attaches to the harmonic balancer also has a rubber ring between the inner and outer races and even more often then the balancer itself, this rubber fails - often without much indication, often just a hairline crack and some noise.
A failed harmonic balancer assembly can BREAK YOUR CRANKSHAFT! The crankshaft of the 6.5 is a known weak spot of the engine and a failed harmonic balancer can (and will) eventually cause crankshaft failure. Symptoms of a failed pulley ranges from a loud clatter through the radiator ( audible clearly from the front of the truck when running ) to a “clunk” moments after the engine is shut off. Visibly, you may also see a shimmy on the pulley or harmonic balancer. If the pulley has failed you will often see impact marks on the stop-pins that prevent the inner and outer rings from freewheeling if the rubber fails - in extreme cases the inner ring actually impacts the bolt heads themselves leading to further damage. A circumference crack or giant missing pieces of the rubber rings on either the harmonic balancer itself or the pulley is indication that it has failed and requires immediate replacement.
Inspect carefully! Both the pulley and balancer itself are easily inspected from underneath the truck - simply look at the nose of the crankshaft and carefully inspect. If you are unsure, ask someone who is familiar with the 6.5 to inspect it for you!
What oil should I use?
Diesels require specially rated oils that can deal with the abuse that diesel engines dish out. Gasoline rated oils are not only of inadequate viscosity (thickness) for a diesel but also do not have the formulation to deal with the ash and other impurities that a diesel engine produces.
When changing the oil don’t be surprised to see that your brand new oil is jet-black the second you re-start your engine. This is common on most diesel engines due to the fact that the oil cooler typically holds a lot of oil that cannot be changed. Despite being black almost immediately the engine still benefits from the oil change. Change your oil regularly!
- every 5,000 miles/12 months under normal operating conditions
- every 2,500 miles/3 months under severe operating conditions ( periods of excessive idle, towing, frequent short trips, stop-and-go traffic, etc ).
| Vicosity | Temp | Notes |
|---|---|---|
| 30 | >32o | Recommended in consistently hot ambient temperatures. Not preferred in cooler temperatures, use 15W-40. |
| 15W-40 | >0o | Preferred viscosity within temperature range unless ambient temperature is consistently hot. |
| 10W-30 | <60o | Required cold weather viscosity. |
| Vicosity | >32o | 32o to 0o | >0o |
|---|---|---|---|
| 30 | not required | 2 Hours | 8 Hours |
| 15W-40 | not required | not required | 8 Hours |
| 10W-30 | not required | not required | 8 Hours |
A note about sizing block heater extension cords: Use 14 gauge or bigger! The block heater draws around 1000 watts. It should measure about 15 ohms when tested with a multi-meter. The cord should see less than a 2% voltage drop to be safe.
- Let's look at a 100' 14 gauge cord:
- Current = 1000 W / 120 V = 8.4 Amps
- Voltage drop = 8.4 amps * 0.258 = 2.2 volts
- % voltage drop = 2.2 V / 120 V = 1.8% voltage drop = OK
Dripping oil cooler lines
This is an extremely common 6.5 issue - the stock GM lines are junk and will leak often within a year of installation. Many aftermarket lines that are manufactured to be "direct replacements" are little better.
The factory lines are prone to splitting ( aside from their constant leaking ), but they also utilize an absolutely terrible connection method to the engine itself, relying on several small clips to hold the lines into the fittings on the engine block. These tiny clips corrode as the years pass, and if/when one of them corrodes and rusts to the point where it breaks, the clip can fall out, leaving nothing holding the line into the block fitting. Replacement of the factory lines is an essential modification for the serious 6.5 owner and there are a variety of aftermarket kits available ( For about $250 ) which use much stronger and durable braided lines. Several vendors sell these kits directly.
The Injection Pump and what should I do to protect it?
This Injection Pump (IP) was designed before the introduction of Ultra Low Sulphur Diesel (ULSD). ULSD has a much lower lubricity than the fuel that the 6.5 injection pump was designed for.
ULSD can cause premature wear (and eventual) failure of the IP at fairly significant cost. Some DS4 model injection pumps did have issues that caused failure. If you’re IP is still healthy - keep it that way. Use a quality lubricity additive and use it religiously with each and every fill up. Some people use simple 2-stroke oil, and others use brand name off-the-shelf additives. Lubricity Additive Study Results - Diesel Place : Chevrolet and GMC Diesel Truck Forums
What if the Lift Pump quits working?
The lift pump on the 6.5 is another essential piece of equipment, but also one that presents few (or sometimes no) symptoms if failed and can lead to issues. This pump draws fuel from the fuel tank and feeds the injection pump at about 7PSI.
The lift pump is mounted to the frame rail on the driver side of the truck. It is commonly mistaken for a fuel filter except if you look closely you will see two wires leading to it.If this pump fails, the problem is that often the truck will continue running, sometimes with no symptoms. The big issue with this is that the injection pump was not designed to “suck” fuel all the way from the fuel pump and as a result being forced to do so can result in IP problems down the road. When symptoms do present themselves, they are typically lack of power (fuel starvation) or difficult starts (Fuel draining back to tank from IP and not being primed at key-on). Occasionally the engine will not start at all as is common with an already worn or sick injection pump which no longer has the strength to “suck” from the tank itself. Often, by the time these symptoms present themselves the damage to your IP may already have been done.
Diagnosis is fairly easy - when you turn the key on (without starting the engine) you should be able to hear the pump run for a second or three. Secondly (and more determining) by opening the water-drain tap (located on the engine near the alternator) you should be able to obtain a steady flow of fuel if opened and the ignition key cycled If you open this tap but do NOT get a strong flow of fuel with the key cycled to the “ON” position, your lift pump may not be functional. Although it’s technically safe to drive in this condition, it’s not wise....get it fixed - your IP will thank you and you will potentially save yourself the hassle and expensive of an injection pump replacement down the road.