Friday, July 18, 2008

Project 6.5 Diesel Suburban Renewal

When the 6.5s were built, there really was no horsepower race and 190 HP was, well, “not bad”. Compared to the Duramax, 190 HP doesn’t measure up and, as we all know, power really does matter. To make things worse, the 6.5 problems have not been limited to less-than-desirable power; they have also had reliability issues – as we experienced for ourselves in the course of this rebuild. Perhaps, in a quest for more power and reliability, you have searched some of the big name diesel upgrade companies only to conclude that there are not any products out there that will mollify the frustrations you have experienced with your 6.5. There are, however, solutions that will make these trucks perform like they should – at a price that can’t be beat.
maxxTORQUE, seeking to get the word out about the hidden value of these vehicles, hooked up with Heath Diesel Power (HDP – and asked them to build up a common 6.5L truck. Bill Heath, the owner, was delighted to participate noting that “the 6.5 is much-maligned due to the problems experienced with them beginning with the introduction of the 1994 models that featured the Stanadyne fuel injection system.”
Trucks were either running screwy or conking out. Dealers were ill-equipped to offer effective solutions. In many cases, trucks that experienced failure of only the pump mounted driver (PMD) module – we’ll address this item soon – were subjected to the standard GM diagnosis flow chart that suggested replacement of the entire fuel injection pump, including a fresh 6.5 PMD. The result of the repair was a truck that might run well for a year or so, only to have the same issue resurface. It did not take too many “incidents” like these for the 6.5s reputation to tailspin.
The good news about a 6.5 with a reputation for being less-than-reliable is that there are straightforward and relatively inexpensive steps that will transform the vehicle into a fully reliable and powerful vehicle. And, because of the bad rap, owners can be on the road with an upgraded, reliable and powerful diesel truck for under $10,000. When you consider the price of a new rig, it is easy to see why many of us have opted for a 6.5 and avoided those always-fun-to-make $500-plus monthly payments.
Unfortunately, 6.5 owners are often unaware of the upgrades available to transform a “frustration” into a reliable and powerful work truck. A few times up a steep grade in a stock 6.5 with a trailer in tow and you might find yourself thinking, I should have mortgaged the house, as late-model trucks pass you by. The answer, however, isn’t dipping into your children’s college fund – that 6.5 has all the reliability you need and plenty of power: the answer is just a matter of helping her find it. The truth is, after a weekend’s worth of bolt-on upgrades, you will not only power up those hills faster and more efficiently, you will get to enjoy the looks on the faces of other diesel owners as you keep pace with their high-dollar rigs.
To help you envision the potential of a 6.5, Heath Diesel completed a build-up on a stock 1994 GMC 2500 4x4 Suburban. Before the build-up, the Sub was all original, including the 6.5L engine. Talk about a great vehicle to put a diesel in! Not only can you hitch up a 10,000 pound trailer, you can do it with the entire family onboard and still have room for more. Our project Suburban had 185,000 miles on it and ran great. She just needed a little push in the right direction to unleash some hidden abilities.
6.5 Diesel PMD Strikes
The guys at Heath had a pretty good idea of the kind of power our project Sub would deliver on the chassis dyno; however, they suggested that before any dyno testing, we ought to drive our Sub in order to gain some perspective on her abilities and personality. Doing so would allow us to be better able to appreciate the value of the upgrades. As it turned out, our time on the road proved to be valuable indeed. We took turns behind the wheel, sharing our impressions of the vehicle. We were impressed with how quiet and smooth she ran and appreciated the great utility of the Suburban.
Out of nowhere, the engine stopped – just stopped – cold. Not quite as abruptly, we rolled to a stop along the roadside realizing we had – most likely – just experienced a real-world 6.5 PMD failure. After a few tries, the engine started up like nothing had ever been wrong. If we hadn’t known about the diesel PMD issue the 6.5s tend to suffer, we would have been scratching our heads wondering what had caused the stall. Fortunately, it happened on a quiet country road on a cool Washington State day and not on some rush hour-crowded freeway... at night... in the rain. So Murphy was only half-right.
The Heath guys suggested – and we agreed – that before any attempt at dyno pulls, and in order to assure a good base line performance, we should go ahead with the installation of the 6.5 PMD Isolator system. They also felt that we should check fuel 6.5 lift pump operation in order to make sure it was actually running when the engine ran and, finally, check the TDC Offset (base timing).
The total cost of these upgrades ran $696; in terms of time the installations cost us an hour and a half.
  • 6.5 diesel PMD Isolator System for reliability $549 | 45 minutes
  • Heavy-Duty Fuel 6.5 Lift Pump / Oil Press Switch $109 / $38 | 45 minutes

The 6.5 Diesel PMD Isolator System relocates the 6.5 PMD away from the heat of the engine.

6.5 PMD-Isolator System

This is the first and most important step in assuring sound and reliable performance with the 6.5. Factory-mounted on the driver-side of the fuel injection pump, the diesel PMD has a problem with overheating. When the box operates at temperatures exceeding 140ºF it will fail. While the primary symptom is complete engine shutdown, like we experienced, other symptoms range from poor fuel economy to rough running and occasional starting difficulty. Fortunately, the solution to this issue is as simple as installing the 6.5 diesel PMD Isolator system that relocates the diesel PMD away from the heat of the engine. This easy installation can be accomplished in your driveway. The system is bolted to the front skid plate, just below the radiator on 4x4 models and on the lower flange of the front bumper on 2x4 models.
The 6.5 PMD Isolator system comes complete from Heath, ready to install. It is designed for easy driveway installation, requires only basic hand tools and includes a seven-year, no-nonsense warranty.

Heavy-Duty Fuel 6.5 Lift Pump and Oil Press Switch

Upgrading the fuel lifter pump to Heath’s Heavy-Duty pump assures adequate fuel supply pressure and flow and is considered a must for any Max-E-Tork equipped 6.5. The Heavy-Duty fuel 6.5 lift pump is a direct replacement for the original. Heath suggested that the engine-mounted oil pressure switch be replaced at the same time. These switches suffer when they are forced to operate an old, worn out lift pump. A worn lift pump places a greater amp load on the oil pressure switch, which has a detrimental affect on its internal contacts. We installed both at the same time in order to avoid problems down the line. Last before the baseline testing, we checked the TDC offset to make sure it was within factory specs – it was, at minus 0.53.

The Heavy-Duty pump assures adequate fuel supply pressure and flow. 

The Dyno – Performance Baseline

After completing these upgrades, it was time to see how our Sub measured up on the dyno. The chassis dyno operator secured our Suburban onto the model 248 Dynojet dynamometer and we did four pulls. The pulls confirmed our Suburban was performing as we had expected. She put out an average of 148 HP, a fairly typical power-number for a bone stock 6.5 and not exactly the level of power necessary in a work truck. With our baseline numbers in hand, it was back to HDP to get on with the planned upgrades. It was up to Bill and the rest of the team to breath new life into our 6.5.
Before making any power modifications, Heath Diesel recommended assessing the relative condition of the pistons, rings and mating cylinder walls by looking at crankcase blow-by volume at the oil fill tube. “Blow-by” refers to combustion gasses that leak past the seal that has been worn down by the friction caused by pistons and rings contacting the cylinder wall. The greater the wear on the seals, the greater the volume of blow-by gasses. This condition can also occur when the engine has overheated and a piston has scuffed its cylinder wall. When this occurs, the rings cannot affect a good seal against the damaged cylinder wall surface. The easy, do-it-yourself test consists of getting the vehicle up to operating temperature and opening the oil filler cap to view any combustion vapor blow-by rising out of the oil fill tube.
Once the Suburban was warm and with the vehicle at idle RPM, the Heath mechanic twisted off the cap. We observed occasional, very light wisps of vapor coming from the oil fill pipe on the Suburban, suggesting that the cylinders were in good shape and that our 6.5 was a good candidate for upgrades. A healthy 6.5 will put out little or no visible vapor. On the other hand, if the blow-by looks like “Old Faithful” it indicate the need for an overhaul. Performance upgrades certainly will not ‘heal’ a sick engine.
We had already talked over our desire to transform our Suburban into a stand-up towing rig (we admit, we also wanted her to be able to hold her own in a sprint race, should the opportunity arise). Based on our input to Heath, they recommended the following:
  • Electronic Filter Harness Upgrade $58 | 15 minutes
  • Super-Flow Four-Inch 6.5 Diesel Exhaust System $449 | Four hours
  • AFE Air Filter Cartridge $77 | 15 minutes
  • Hi-Flow 6.5 Diesel Exhaust Crossover Pipe $135 | 45 minutes
  • Turbo-Master Boost Controller $109 | 45 minutes
  • Max-E-Tork Computer PROM $329 | Five minutes
  • High-Output 6.5 Fuel Injectors $479 | Six hours
  • Duraterm Glow Plugs $139 | No additional time when done with HO 6.5 Fuel Injectors
  • Extra Heavy-Duty cooling system upgrade $409 | Three to Four Hours

Cost of Upgrades (no labor included) $2,890.00
Purchase Price of Suburban $5,300.00
Total Cost of Upgraded Suburban $8,190.00

The Electronic Filter Harness

The 1994-95 models use an external electronic filter on the injection pump to filter out electronic frequency interference (EFI). When the filter experiences years of heat cycles, it often fails to do an adequate job.
The main symptom, known as a fish bite misfire, is a sudden and pronounced “miss” – usually during acceleration. Other symptoms include setting off codes 17 or 18 (or both). In some cases, you may also experience difficult cold starts. Heath recommends replacing the harness at five year intervals.

Super-Flow Four-Inch 6.5 Diesel Exhaust System and AFE Air Filter Cartridge

Next, we installed the Super-Flow Four-Inch exhaust system and two-piece mandrel bent crossover pipe. Heath has experimented with every conceivable 6.5 diesel exhaust system configuration and found that the design of the Super-Flow is exactly what the engine likes. It features a high-velocity three-inch mandrel-bent turbo down-pipe. The balance of the system is four-inch mandrel-bent, heavy wall tubing that features a large, efficient muffler available in two different choices of sound attenuation.
The Hi-Flow crossover pipe is a two-piece design which allows a perfect fit to the engine.
For all 6.5 turbo diesels, letting the motor breath is key to producing power and keeping 6.5 diesel exhaust temperatures under control, the 6.5 is no exception. The following two steps allow the GM 6.5 turbo to get as much air as it needs:
Remove the factory air box and take out the snorkel located in the fender; and,
Replace the factory air filter with a much higher flow rate Advanced Flow Engineering (AFE) replacement.
The factory 6.5 turbo (model GM-4) on the 1994-95 6.5 is adequate for most performance upgrades. For better towing performance, Heath uses and recommends the GM-8 as supplied on the ’98 and newer engines: it provides an improved exhaust flow and better compressor section performance. Our GM-4 was in good shape and we decided to keep costs down by leaving it in place.

Upgrades to the 6.5 diesel exhaust system of the Suburban include the Super-Flow Four-Inch Exhaust and the Hi-Flow Exhaust Crossover.

6.5 Turbo Master Boost Controller

GM used a vacuum-controlled waste gate on the 6.5 to control boost pressure. This system is prone to failure in more than one area:
  1. The vacuum pump can fail;
  2. Vacuum lines can become brittle and crack; but the most common issue is:
  3. The control solenoid sticking. 
Three areas to fail and a limited max boost capability meant something needed to be done. HDP resolved this issue by eliminating the vacuum controlled system altogether, substituting a fully adjustable, mechanical waste gate control, the 6.5 Turbo Master. The Turbo Master combines with the turbocharger’s waste gate valve to form a smooth working, fully adjustable turbine inlet-pressure relief mechanism. Controlling and limiting turbine inlet pressure (drive pressure) affects control of turbocharger shaft speed and consequently, boost output pressure. In our case, the 6.5 Turbo Master was adjusted to 14 to 15 PSI boost at full throttle in order to assure correct airflow to the engine.


The next upgrade performed was the installation of the Max-E-Tork PROM (programmed, read-only memory) inside the Suburban’s computer. This was a very important part of the “Suburban Renewal” equation and another area of 6.5 engine control in which Heath excels. All Max-E-Tork programming is unique including proprietary fuel delivery and start of injection timing curves. Whether for towing or performance use, each “chip” is designed specifically to deliver outstanding diesel combustion efficiency. When combustion efficiency is improved, two very good things happen: stronger power output and greater utilization of fuel (better fuel economy).
Heath’s Max-E-Tork is offered in two distinctly different programs; HPT, a highly refined towing program and HPP with programming better suited to a performance-oriented driving style. These programs, along with other variations in upgrades, allows HDP to perfectly accommodate a wide variety of applications and uses. We thoroughly discussed our intentions for the Suburban with Heath before making a decision on which way to go: because our plans for the Sub leaned in the direction of heavy towing, we opted for the HPT series Max-E-Tork. This chip is designed for heavy work such as towing or hauling at higher combined gross vehicle weights (over about 13,000 pounds). The HPT series is created to deliver strong pulling power with a cool, clear exhaust – a Heath trademark. The new Heath PROM replaces the stock PROM located inside the Power Control Module (PCM) behind the glove box. The Max-E-Tork HPT series is rated to deliver an additional 45 HP and 105 foot-pounds of torque while simultaneously improving towing fuel economy. An important characteristic of the HPT programming is its ability to produce 6.5 diesel exhaust that is virtually smoke-free. The absence of smoke combined and low exhaust temperature are critically important characteristics for any truck that tows. While the results of this upgrade were significant, the effort involved was not. In fact, this was the easiest of the day’s modifications and took only five minutes to complete!

High-Output 6.5 Fuel Injectors and Duraterm Glow Plugs

While Heath typically recommends against using their High Output 6.5 fuel injectors in engines that tow heavy loads (more than 13,000 CGVW) we decided – since we were using the HPT series Max-E-Tork – that the high output 6.5 fuel injectors would complement the PROM upgrade nicely. The combination delivers easily managed and controlled full-throttle uphill towing exhaust temperatures. Heath says that the combination will typically remain within the recommended maximum EGT of 1200ºF pre-turbo, on long, hard, heavy pulls. For those vehicles accustomed to towing at more than 14,000 CGVW, Heath recommends the use of its standard output injectors along with the HPT Max-E-Tork. Also, as a preventative maintenance tip, HDP highly recommends replacing 6.5 fuel injectors every 90,000 to 100,000 miles. Old injectors not only rob your engine of power and fuel economy, they can also cause extreme piston temperatures, which leads to failure. The injector kit includes the latest Viton rubber fuel return hoses as well as 6.5 turbo mount and oil return gaskets. Since this Suburban lives in a colder climate, we also decided to put in a set of Bosch Duraterm glow plugs. These differ from the factory 60Gs by getting hotter during the computer-allotted glow time to assist initial starts in colder climates.


Cooling System Upgrade

The cooling system used in the 6.5 vehicles, is only barely adequate for towing – when it is in perfect condition. A normal accumulation of bugs, dirt, weed seed and leaves will dramatically reduce any cooling system’s efficiency. Heath tells us that, while GM put the “do-not-exceed” red warning mark over at the end of the temperature gauge dial (way past ruined engine), the real-world maximum do-not-exceed operating temperature is 210ºF. The 210ºF mark is found at mid-scale on the temperature gauge and this is where the red mark should have been placed. When the engine is operated at temperatures higher than 210ºF, there is a real possibility of damage to the engine – don’t do it.
If the cooling system will not control temperature at or below 210ºF – under whatever conditions your driving demands – there is a problem that must be corrected in order to avoid damage from overheating. Always, the first step is to thoroughly clean the radiator and air conditioner condenser core of any accumulation of bugs and crud. If, after the system has been cleaned, there is still and issue with temperature control, it may be time for some system upgrades.
In the case of our Sub, since we plan some heavy, summertime towing, we opted for Heath’s Extra Heavy-Duty cooling system upgrade. In addition to superior cooling performance, the package replaced our high mileage water pump, fan clutch and thermostat with new components – good preventative maintenance. This upgrade consists of the latest, high output water pump, the Duramax fan clutch, Duramax 21-inch, nine blade composite fan, correct GM thermostat and all the necessary gaskets. Since our Sub’s radiator was in questionable condition, it was replaced with a new GM unit during this upgrade process.

TDC Offset

Now that the bolt on upgrades were completed, there was one last thing Heath Diesel recommended to optimize the tune-up. They wanted to reset the TDC Offset to the minus 1.94 they like best. They pointed out that so long as the TDC is set somewhere in the range of minus 0.25 to minus 1.94, the engine runs OK. The Max-E-Tork programming, however, is designed to work best with the TDC offset at the minus 1.94 setting. When adjusted to the recommended offset, the Max-E-Tork equipped engine will deliver an improved combustion efficiency for better power and optimum fuel economy. The mechanic plugged in the Tech 2 scanner and performed this procedure. Your local GM mechanic can perform this procedure for you in about 20 to 30 minutes with his Tech 1 or Tech 2 scanner.
From start to finish it took Heath Diesel one very busy day to complete all of the upgrades, but it is nothing you and a friend can’t accomplish in a weekend. In all, we estimated the total amount of labor involved in these upgrades to be in the neighborhood of 17 hours.
It was now time to get the refined work truck back on the road. When we first drove our Suburban before the upgrades it had only mediocre power. It ran well enough, but was short on power and, in our opinion, would not be suitable for towing anything more than a very light trailer. It slowpoked across intersections and lazily crawled up to speed. We noticed that the throttle had seemed to be useless above about 60 to 70%: nothing much happened beyond that point.
Now, however, with all our planned upgrades, we have a vehicle that delivers a spirited performance. A performance that would put a scare into a factory late-model diesel. From around town, to down the highway and up steep grades, this Suburban now runs great: it pulls from start to finish on the throttle with no weak spots. It makes full throttle upshifts at 3500 RPM and wastes no time in pegging the speedometer. After spending some quality time with our project Sub, we can confidently say that she has been transformed into the reliable and powerful work truck we imagined she would be after the upgrades.

Dyno Follow-Up

She felt great but we still needed to head back to the dyno to see what the Suburban’s numbers would be. On the second trip to the dyno, after running eight dyno pulls, we averaged 208HP at the wheels (about 270-280 HP at the engine, according to a Dynojet tech). That’s more than a 40% increase in horsepower over our initial baseline of 148HP! Still, everyone who drove the Sub before the upgrades and then after sensed that the improvement was far greater than the dyno indicated. Bill Heath offered this explanation: the dyno measures only full throttle performance, not real-world, part-throttle power and torque – such as you feel in day-to-day engine operation.
The torque output curve changed substantially from the original. While the original curve peaks sharply then falls dramatically as the RPM rises, the new curve peaks at a higher level and maintains a flatter curve, representing a comparatively stronger torque output as RPM increases. This means more area under the torque curve, which translates to much stronger engine output in the normal engine operation range.
As great as a 40% improvement in horsepower is, the engine actually feels far better than the dyno numbers suggest.
Everyone witnessing our Sub’s dyno pulls was impressed with the absence of black smoke out of the tail pipe. The 6.5 diesel exhaust was uncharacteristically clear and not at all what you expect from a diesel.
We are all amazed at the improvements we see in our Suburban, she is genuinely fun to drive and we do not cower when sitting at a stop light next to a late model diesel. In fact, we find ourselves driving much more aggressively than before. For a big, 6,800 pound vehicle, this thing really scoots! However, there is more to the upgrades than improved power, reliability and fuel economy. We now have a vehicle that we can call a work truck without a stutter.

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Comments (2)add comment

Chad H said: 

Re: 6.5 Diesel Suburban Renewal
I was thinking of buying a 98 gmc dually with a 6.5 turbo diesel that has 138000 miles on it. I was reading your article on the suburban build up and was wondering,did GM improve any of these reliability and power issues on the later year models? Say after 98 and up? Thanks.

December 03, 2009
bheath said: 
Click the following link for Bill Heath's (heath Diesel) response to the above question...

1 comment:

  1. 1998 would be an obd2 controlled engine. I do not believe the heath chip mentioned here will work in that engine. Otherwise the rest of these upgrades should work. I would recommend an intercooler on any 6.5 you plan to boost higher than stock pressures.