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Tk8398 HalfDork
9/20/21 11:22 p.m.

I asked about the Ford 6.9/7.3 but now there are not any decent ones for sale, how about the GM 6.5?  I saw a 1/2 ton 4x4 one for sale for what might be a decent price.  From reading about them it seems like there are a number of issues, but also I imagine many of the bad ones have died by now.  Are they bad enough to avoid? 

ShawnG UltimaDork
9/20/21 11:46 p.m.

The GM 6.5 is a wheezing pile compared to a 7.3 Ford.

It will feel like you've left the parking brake on wherever you go.

The only thing they are better at than a gas 454 from the same era is fuel economy. That's about the only reason they sold them.

The Duramax is a far, far better engine.


rattfink81 Reader
9/21/21 12:26 a.m.

I worked on alot of the 6.2's and a few of the 6.5's in humvee's. They are fairly gutless but the only issues I saw were glow plugs mushrooming and the mechical injection failing. I believe most if the fuel pump failures were down to using jp fuel and not premixing oil or enough oil but that's not a problem you'll see. 

neverdone New Reader
9/21/21 6:13 a.m.

The only thing 6.5s are good for is dying in traffic due to fuel system issues.  I unfortunately had one, and will never buy another GM truck because of it.  Don't buy.. run away.

buzzboy Dork
9/21/21 7:31 a.m.

My old boss had a rental fleet of nine 6.5 suburbans. He was able to pick them up cheap because people thought they were garbage. Move the PMD and replace the harmonic balancer and you're good to go. I recall his lowest mileage rental having 200k and the highest having over 350k. He liked them for being super reliable and cheap. He also had two pickups with the 6.5 that he used as tow vehicles for his dunebuggy/golfcart fleet.

Cousin_Eddie (Forum Supporter)
Cousin_Eddie (Forum Supporter) Dork
9/21/21 7:47 a.m.

I was a mechanic for the Post Office for a good while (Fort Worth Texas VMF) and we had a fleet of two ton trucks with the 6.5 in them. I never saw one fail. The deepest I even went into one was injection pump. The mailmen beat those trucks like they owed them money and they just kept on clattering along year after year. 

They certainly lived up to expectations and more when it came to solid service.

Powar UltraDork
9/21/21 11:29 a.m.

I drove one and did silly E36 M3 with it for about 5 years before I moved on to my current tow vehicle. I relocated the PMD to behind the front bumper using an extension harness and heat sink, and replaced it once more after that IIRC. Replaced the harmonic damper with an OEM one, upgraded the crossover pipe, and installed DiamondEye exhaust... That was really all that I ever had to mess with that was diesel specific aside from filters and whatnot. It treated me great and I'd own another in a heartbeat. It towed wonderfully and got very fair fuel econ both loaded and unloaded. I didn't go into it blindly--- my previous Suburban was 6.2-powered, and it too was excellent until the trans started slipping. So yeah. I've bought both 6.2 and 6.5-powered Suburbans on purpose and liked them. I'll never understand the hate for them, as my firsthand experience after owning two was very positive.

This was my most questionable tow. It did fine from KY to DC so long as I kept the speeds below 65 or so.

pres589 (djronnebaum)
pres589 (djronnebaum) UltimaDork
9/21/21 11:32 a.m.

I think Curtis has some list of mods to do to these that makes them a lot better to own and drive.  Maybe he'll show up here with his grocery list.  

Opti Dork
9/21/21 12:25 p.m.

My friends into diesels tell me there junk. Unreliable and gutless.

People I know that own them tell me they are reliable once you address the PMD and balancer and they are gutless.

GM didnt build them like they do diesels now, as the max towing option. It was the fuel economy alternative to the best towing option (the 7.4)

The common theme seems to be they are gutless, Im told turbo ones are better than the NA which are REAL slow.

I support the GMT400 part of it. If its mostly a daily Id be looking for a half ton 5.7. If its mostly a tow rig Id be looking for a 454 2500.

FYI a 5.7 Vortec L31 makes 255 hp and 330 ft lbs. The 350 TBI makes 210/300 IIRC. The 6.5 makes 180/360. Id take either gasser over the 6.5. A TBi truck already feels pretty gutless, I wouldnt want slower.

No Time
No Time SuperDork
9/21/21 1:09 p.m.

I've had a few 6.2s and they weren't bad (1987 2500 and 1990 2500)

It's all about perspective and not comparing them to the newer DI turbo diesels. It may be that mine were all manual transmission (sm465), reg cab 4wd pickups with 3.73s, but they were peppy enough for what they were. They pushed snow well and were able to get low 20s empty and high teens for mpg when towing a 3500lb boat. Since they didn't have OD the fuel economy dropped off  once you got over 65mph.

The main issues I had were glow plug controllers and harmonic balancers. They would make  it to about 300k miles before snapping the crankshaft. If I knew back then about the balancer issues they'd probably still be running. 

The first year for the 6.5 turbo used a mechanical fuel pump so they don't have the PMD issues that the later ones had. 

bobzilla MegaDork
9/21/21 1:13 p.m.

In reply to Powar :

We had a few 6.2 trucks and 'burbs. They're great as long as you arent' expecting a land speed record.

tuna55 MegaDork
9/21/21 1:19 p.m.

Some of them had the 1-2-3-OD trans which is really neat. I have one in my truck now (behind a gas engine, same bellhousing pattern)


I think as far as addressing the gutlessness, you need only to give Mr. Banks a call: https://www.bankspower.com/c-25-products-turbo-upgrade-systems-and-components.html

No Time
No Time SuperDork
9/21/21 1:27 p.m.

In reply to Opti :

I think that is a common opinion among the diesel enthusiast crowd. They definitely won't hold up to being modified like a 5.9 or Powerstroke.

Start pushing a 6.5 to make more power than stock and you can quickly move into ticking time bomb territory before you reach power levels that are easily achieved with the 5.9 or 7.3 (direct injected).


Opti Dork
9/21/21 1:30 p.m.

In reply to tuna55 :

I couldnt find anything for the 6.5 on Banks website


tuna55 MegaDork
9/21/21 2:16 p.m.

In reply to Opti :

Opti Dork
9/21/21 3:05 p.m.

In reply to tuna55 :

I found it on their site its no longer for sale


81cpcamaro Dork
9/21/21 3:56 p.m.

I have owned a 7.3 IDI and a 6.5L TD, both were good running engines, just not up to the power of newer Diesels. My 6.5 was a 1993, so no PMD to worry about (1994-up). Since it was a 5-speed, it didn't even have a ECU at all. The previous owner had replaced the original engine with a Hum Vee take out. Sold it to a good friend so he could pull his RV trailer. Still going. 

As for towing, I have used 350s, 454s, 7.3L IDI, 6.5L TD and 8.1L Vortec. While the 6.5 wasn't fast by any means, it pulled better than the 350s ever did. 454 and 8.1 towed the best, just don't expect great mileage out of them. 6.5 would get into the 20s on the highway pretty easy.

bearmtnmartin (Forum Supporter)
bearmtnmartin (Forum Supporter) UltraDork
9/21/21 4:04 p.m.

I really like them. They are economical and reliable. And you do not pay the Cummins/powerstroke/duramax tax so they are quite affordable. I had one and just bought my son one. Replace or move the pmd, do all the deferred maintenance and don't touch anything else.

gearheadmb SuperDork
9/21/21 6:46 p.m.

Dad used to buy gm diesels. My first car was an 86 c3500 with a 6.2, 4 speed, and 4.10 gears. We had the pump turned up a little. Plenty of power, top speed just under 70 mph. We got about 200k miles out of it. Next he bought a 93 c3500 with the 6.5 w/ mechanical injection and a five speed. It got 20 mpg and was as dead nuts reliable as anything we ever owned. It was plenty happy pulling trailers, and we would haul as much as 6000 lbs. of stone in the bed. We put about 250000 miles on the truck, and the engine is still running around in a different body now. 

mdshaw HalfDork
9/21/21 8:55 p.m.

We had a '94 GMC Suburban with the 6.5 TD. The injection system was made by Stanadyne. I moved the PMD out of the engine valley out of the heat. Put it on a huge heat sink on the firewall.  It was very reliable after doing that but always carried a spare PMD. 
Had it for many years & towed our big boat. The cooling system is too small & had to watch the temps when towing in the summer on hills. I think in '98 GM gave it 2 coolant exists from the motor.  
Had to replace the harmonic balancer but that was about it in the 5-6 years we had it. Mileage was much better the gassers. If it has the 6.5TD then it has heavy duty everything else. Trans was a L80e, suspension & brakes were the 3/4 ton version. Was good but underpowered.  

drock25too Reader
9/21/21 10:20 p.m.

We used this '93 6.5 turbo diesel powered crew cab dually to tow this 24ft. trailer all over the country. While it was not the fastest hauler on the road, it only failed us once when the timing chain broke. Freak deal and the dealer replaced the engine under warranty. We used it a few more years after the engine change. Only reason he traded it in, was his wife wanted something newer. I went to the dealer the next day and it was already sold. 

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) MegaDork
9/22/21 10:24 a.m.

Dad had a 98 6.5TD.  Bulletproof for 235k with zero mods.

I suggest moving the PMD, big exhaust, and chip it.  Exhaust first.  The stock exhaust is awful.  Stock boost is around 14psi, and upgrading the exhaust frees up the turbine so much you might get 18-20psi after the swap.  You can buy a set of 40hp injectors for something like $200, and then all of those put together you can get 300hp and 550 tq.  You'll have to contend with a little coal rolling unless you do propane or upgrade the turbo.  I would have to dig deeper into refreshing my memory.  It's been 20+ years since I even drove one.

The thing you have to realize is that the factory ratings for gas have zero comparison to diesel. People look at 180 hp and 350 tq from a 6.5TD and try to compare it to 275hp and 350 tq from something like an LS.  Take a look at the actual torque curves.  The LS makes that torque at one RPM point and less everywhere else, while the 6.5TD makes the same basic torque from 2000-4500 rpms.  Since we have these pesky things called transmission ratios, the actual amount of torque that is being put to the ground as the transmission changes gears is far greater with the diesel despite it's "peak" torque being the same.  Also, the 6.5 diesel will do this at 24mpg while the LS will do it at 16mpg.

Dad's 6.5TD in stock form easily hauled 10k lbs worth of travel trailer up the rockies and never fell below the speed limit.  I had truckers on the CB asking me what mods I had done and the answer is "none."

The 6.5TD is also a great choice because they are still in production, so parts are plentiful.  The military thinks they're pretty reliable so they're still used in Hummers.  Performance parts are scarce, but because no one wants them, they're cheap.  Full set of upgraded injectors for $250, plug in modules were $175 last time I checked.

The bottom line is, it is a Detroit diesel.  It's made for abuse, so abuse it.

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) MegaDork
9/22/21 10:26 a.m.
neverdone said:

The only thing 6.5s are good for is dying in traffic due to fuel system issues.  I unfortunately had one, and will never buy another GM truck because of it.  Don't buy.. run away.

Sorry you had a lemon, but this is not the legacy of 6.5TDs.  Millions of them out there with an outstanding track record.

ShawnG UltimaDork
9/22/21 11:30 a.m.

I thought the Hummer switched to a Duramax when the H1 Alpha came out.


Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) MegaDork
9/22/21 2:27 p.m.

Civilian, yes (as in Hummer).  Military, (as in AM General) not that I have heard of.  I know that AM General is still making new 6.5s to put in the Army and Marines HMMWV.  From the wiki page:  

 but the 6.5 (6500 Optimizer) is still produced by AM General for the HMMWV.  

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