6.5L GM Diesel Review

Summarizing our Experiences with our GMC C3500HD Project and the 6.5L Turbodiesel

The 6.5 diesel isn't everyone's favorite - especially when you consider General Motor's far superior 6.6L Duramax, which was introduced in 2001. However, the engine isn't nearly as bad as some would suggest (not to say it doesn't have its inherent flaws). After all, the engine was and is used extensively in U.S. military applications. While modern technology and the hunger for more power has made the 6.5L somewhat obsolete, it has had its place. Here's a summary of our experiences with the shop's 6.5L Detroit diesel after a year of ownership.

The truck we acquired was a 1995 C3500HD; not your standard C3500 model, but rather a class 4 chassis cab with a 15,000 GVWR. It is equipped with a 195 horsepower version of the 6.5L diesel, a NV4500 5 speed manual transmission, and a Dana 80 rear end stuffed with a relatively short 4.63 : 1 gearset. Everyone who drove the truck had the same impression...it was absolutely gutless. After a series of diagnostics, we determined there was nothing wrong with our 60,000 mile engine from a performance perspective. Seeking answers, we tipped the scales - to a whopping 8,700 lbs unladen, without a driver, no gear on board, and only a 1/4 tank of fuel. Factoring in the chains, binders, tools, straps, shackles, and a collection of trailer hitches, she cruises down the highway at well over 9,000 lbs now. The performance characteristics of this platform become much more justified after this finding; the truck is just plain hefty.

The biggest problem we found with the 6.5L, which was a universal consensus, is that the engine lacks any sort of low end grunt. This seems to give the false impression that the engine makes no power. RPM is everything with this engine and it pulls best in the 2,000 RPM to 2,500 RPM range, which is substantiated by the fact that the engine produces its peak rated torque at 2,000 RPM. Even with its extremely low gearset, there's no lugging the engine.

If you happen to grab 4th gear before revving out to 2,700 RPM, you'll ultimately struggle to keep the engine in its sweet spot; and it's quite a burden while towing. Then again, once you get used to the powerband it's much easier to tow with this engine. The 6.5L's powerband is the opposite of a 12 valve Cummins or 6.9L/7.3L IDI diesel; those engines produce gobs of torque on the low end, but power falls off sharply at the higher end of the engine's operating range. The 6.5L GM will pull strong all the way to 3,000 RPM. Keep in mind that we're dealing with a 6.5L turbodiesel and not a naturally aspirated version of the engine, which was offered for many years.

Fuel mileage has not been all that great, but we have little to compare the truck to considering its heft. The GM/Detroit motors have historically been conservative on fuel however our 30 something gallon tank seems to drain quickly, even with a light foot. As previously mentioned the truck weighs 8,700 lbs with a 1/4 tank of diesel, no driver, and empty tool boxes; so there's that. The less than stellar fuel economy of this truck is largely do to its rear end ratio - 60 mph translates into roughly 2,500 RPM in overdrive.

Towing and hauling with the 6.5L has revealed more about its performance characteristics. The largest payload we've loaded on the bed weighed in at almost exactly 5,000 lbs. The truck performed flawlessly, and while the weight was noticeable we expected a much greater impact on driveability. In reality, there was plenty of power and the load went down the highway at 55 mph like it wasn't even there.

When we finally hitched the truck up to a trailer, engine performance was impacted considerably and the inherent limitations of this engine quickly became well known. The truck is a flatlander; as long as you keep it on level ground, there's no issues to report. It does not get up to speed quickly by any means, but it does so with reasonable effort and maintains speed without excessive accelerator input. Find a nice, steep grade and you can't downshift fast enough to keep the truck moving. The greatest load we towed was in the 9,000 lbs range, which included a 2,500 lb trailer and 6,200 lb tractor (plus chains, binders, gears, and two deep cycle batteries for our winch). The truck handled nicely but the engine definitely struggled to get up to speed, especially in the taller gears. With a 9,000 lb trailer this truck cruises down the highway at nearly 18,000 lbs GCWR.

We acquired the truck with only 60,000 miles, and thus hoped to confront limited problems with such a low mileage truck. However, we encountered the following problems early on:

Engine overheating - For the record, our engine never technically overheated (that we know of), but it did run hot and the engine coolant temperature was erratic (BOTH coolant temperature sensors were changed to rule out a sending unit issue). At highway cruising speed (55 - 60 mph) with no payload the ECT would climb just over 210° F and quickly drop to the 200° range, repeating the cycle every 10 to 20 seconds or so. Occasionally, the ECT would spike to roughly 215° and maintain for some time before dropping back into the normal range. This occurred regardless of ambient temperature, and we first noticed it on a cool 60 degree (F) morning cruising at 55 mph.

After performing a coolant flush, full cooling system service, and steam cleaning the radiator, the problem disappeared and the engine runs somewhat consistently in the 200 - 205° F range. We also converted to an ELC - no maintenance for 300,000 miles unless the new thermostat decides to stick.

Lift pump failures - The lift pump went out shortly after the truck was purchased. It ran fine, but started to buck/jerk during heavy acceleration. A pressure test revealed there was no fuel pressure at idle. For good measure, the lift pump and oil pressure switch (OPS) were replaced. 750 miles later, the lift pump failed again and was replaced. Fingers crossed the previous pump failure was a fluke. We're using the factory ACDelco fuel pump.

Slow engine crank - The engine seemed to start fine when cold but had a hot start issue. It would crank for 20 seconds or more before starting when you tried to restart the engine near operating temperature. The engine typically had to rest for an hour or more before it would restart normally. Our initial instinct was a PMD issue, but after replacing and relocating the PMD the problem persisted. Two new batteries later, things were better, but not great. The engine seemed to be cranking slow, so after a few quick tests the starter was replaced; problem solved (mostly). Like all diesel engines, the 6.5L likes to spin fast in order to start rapidly. After these repairs, the engine will cold start on a 30° F morning in mere seconds. We plugged the old PMD back in to fuel our curiosity - sure enough, it also contributed to the hard start issue. Compound problems are always difficult to diagnose with certainty.

NV4500 transmission growling - This is an unresolved issue with this the truck. The transmission makes an intense "growling" noise in neutral and in 1st and 2nd gear at idle speed. The issue disappears with a slight increase in engine speed. That is, a small increase in RPM makes the noise disappear. This is not the typical sound associated with a throwout bearing but is more reminiscent an intense gear rollover noise from a single mass flywheel setup. We've pawned it off as just that and have not pulled the transmission (yet). At idle, the transmission exhibits a distinct roar; at 800 RPM, there is no noise. Transmission shifts tight and there are no additional issues.

Leaking thermostat housing - At the time of publishing we've removed and resealed the thermostat housing three times. The first time it was installed with a gasket and silicon on the non-adhesive side of the gasket; it leaked terribly. On the second attempt, no gasket was used and an alternative "high temp" RTV silicon was used; after one week it began leaking fluid worse than before. On the third attempt, a different RTV silicon was used and silicon was applied to both sides of the gasket; it started leaking about three weeks later. We're going to switch back to our original RTV silicon and replace the thermostat housing.

Working on a 6.5L diesel is not particularly cumbersome. There's actually quite a bit of room in the engine bay and the various sensors/components are somewhat self-explanatory. Water temp sensor? No problem. Glow plugs? Remove the passenger side inner fender (easy) and the rest are well within reach. Turbo removal? They were kind enough not to put it against the firewall in the engine valley. Get to know this engine and repairs are not particularly difficult. However, it would be convenient if the fuel filter housing was moved up front, rather than positioned at the rear of the engine.

Summary - Would we buy another 6.5L Detroit?

The 6.5L wouldn't be our first choice for a tow rig. The engine's torque characteristics on the low end are not particularly impressive and the engine seems to favor higher engine speeds. This is no surprise considering the 6.5L is a big bore, short stroke engine. We feel like this engine is fragile, almost as though we have to baby it and are afraid to beat on it from time to time. If we had no towing needs the GM diesel would be a contender, however it just seems hard to believe that this engine produces its rated 400 lb-ft of torque at full load. Our little 6.9L IDI will run circles around this truck with under 350 lb-ft of torque. Once again, this truck's heft puts it at a huge disadvantage. If we were to purchase another 6.5L in the future, it would be as a 1/2 ton model and the goal would be to have a fuel sipping parts runner.



Ours is straight piped. It sounds mean, very distinct exhaust note. Reminds you of a 10 wheeler cruising down the highway.

Low oil capacity. With only 7 quarts of engine oil (8 for 1999+ MY) it seems like a burdensome task to the keep the engine oil cool, even with an oil cooler. Comparable engines of this size typically have a 10 quart + oil capacity.

Inexpensive truck to get into, there were lots produced and as an added bonus you can get a 1/2 ton model.

Tedious service schedule and short service life of many components. Dampers, for example, should be replaced at the 100,000 mile mark; a failed damper results in a total loss.

Parts are readily available, easy to find, and not particularly expensive.

Engine runs on the hot side, is slow to cool down following rigorous duty.

Like most diesels of yesteryears, it gets the job done.

Whose idea was it to run the lift pump off of the oil pressure switch?

Easy to work on, plenty of access to engine components.

Cannot lug engine. At all. Engine speed is critical when towing. With a manual transmission, the 3rd to 4th gear step feels extreme if you don't rev the engine out.