REVIEW: Everything Wrong With A GM 6.5 Detroit Diesel

What could possibly be wrong with the 6.5 GM-Detroit diesel?

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[shadymaint]

Most impressive thing I've seen out of a 6.5 was when we had a HMMWV engine run away. Some guys were messing with the injection pump trying to get more power out of it. When they fired it up it ran away and wouldn't shut down. Figured they would just let it blow up and put a new motor in it. Insulation in the transmission tunnel caught fire and it didn't take long for the whole thing to go up. Have to give the 6.5 credit. It never shut down until the aluminum intake manifold melted and got sucked into the motor.

[seansouth]

The 6.5 does have a cross over tube that feeds back up the bottom of the exhaust manifold and out through the turbos down pipe feeding the turbo from both banks

[victorrobison]

Rich, I used to maintain a fleet of 6.5 turbos in a fleet that belonged to a tree service. All of the diesel pick-ups were purchased used, with mileage that ranged from 189, 00 to 260, 000 miles. We would do a ring and bearing service, send the turbo out for bearings and seals, and check the heads. They always got the pump mounted driver relocated, and usually the board in the module connector was replaced also. This seemed to solve most of our fuel problems, but hard acceleration in the middle of an Arkansas summer was still going to produce a little smoke. The heads would crack in every cylinder between the exhaust and the intake valves, ( very fine cracks, cracks nonetheless )but whether we sent the heads out for repair or just re-installed them with new gaskets, the engines ran well. Two things to know is that the end of the push rod with the pink ring around it belongs at the top, and the thick fat clunky washer is the spacer between the camshaft timing sprocket and the end of the cam. To put the wrong end of the push rod up causes rocker arm failure at about 700 miles after re-assembly, and to leave out the cam spacer causes timing chain damage about 1, 000 miles after assembly. My service truck was a cable throttled 1991 3/4 Chevy (no PMD required) and was very reliable. The big weak spot on these trucks was the lift pump, and it would be folly to blame that on what turned out to be a pretty good engine.

[radradRbot]

The idea behind smaller valves giving more torque has to do with air speed. Smaller port means higher air velocity, so you actually get slightly more air at lower rpm. I used to tune motorcycles, and you could get more bottom end torque with a slightly smaller carburetor

[aarontuerk]

Ive had an lb7 duramax, 5.9 p-pump cummins but my favourite will always be my GM 6.5. its that crisp idi idle clatter that is ingrained in my ears for life.

[vikipapa]

On valve sizing in a nutshell: smaller valve and smaller intake: higher air velocity on lower rpm, but chokes out on high rpm, bigger valve and intake: slow initial (low rpm) air speed, but higher rpm it starts to "flow"

[MackB]

We did a lot of 6.2s and 6.5s for folks back in the day. Agree on all fronts! The 6.2 had big valve heads in higher GVW and smaller valves in lower GVW when it was introduced. The big valve heads were more prone to cracking and GM ended up with a smaller valve in everything. The big valve head did not have much material between the valves. I have one with about 300, 000 on it with big valve heads, it has never been overheated and it has not bothered. GMC offered a Banks Turbo 6.2 when you ordered a new truck. My uncle had one in ‘92 and it was better than the 6.5 on all fronts! Mr. Banks got it right on intake and exhaust. We saw broken cranks in 6.2s, they always had a cracked block that let the crank flex and you know the rest. Timing chains were a common problem on 6.2s if folks did not change their oil. If it was a truck we serviced, it had a two quart filter, 30W in summer and 15W-40 the rest of the year. We saw lots of them go over 200, 000, rust got them by then. Being a pre-chamber engine they would run at higher RPM. In a 2500 or3500with 4.56 gears they were singing all the time. This is hard to imagine now, but when the 4L80 came out it was not what folks needed. If the truck was trailering or working hard in overdrive things went south quick. The aftermarket did a great job fixing GMs transmission issues. As good as the Turbo 400 is, these diesels needed an overdrive. Great Video and Thanks for sharing! Merry Christmas !

[johnroethel]

Had a 1993 GMC one ton dually with a 6>5. Heads were changed when new cause of a recall. Truck went 778000 Miles before it went bang. Miss that truck to this day. Merry Christmas to all

[jeremykamel]

I worked on a LARGE fleet of generators with Yanmar and Kubota diesels. They ran at low loads for weeks on end while powering 40’ refrigerated can going cross country on the railroad. They carbon up and wouldn’t pass compression tests. So we’d run marvel mystery oil in them and run them at 100% of their rated load for 15-30 minutes.
95% of the time they’d clean up. Then when we pull the injectors to run compression we’d run the injectors through a sonic cleaner.

Oh, pro tip. If you can’t get injectors or glow plugs out due to carbon build up. Get it running and mist water into the intake. It’ll steam clean the upper cylinders and remove carbon deposits allowing the injectors and or glow plugs to come out.

[sixstringsid]

Do not turn the crank, it WILL break! The 18:1 pistons are no longer available and only came in .020 over sized. You don't want them as it will not want to start in your cold climate. Do not use marine injectors, there is no HP gain at all and they fire late so pump timing needs to be advanced a couple of degrees. If you want the block to live, short pour with hard block to the bottom of the core plugs. Remove all the stress risers, chamfer holes in block and clean up horrible machining. Run a main cap girdle and balance the rotating asm. Check the bearing halves for sizes as you might have one rod bearing std in the rod and a.013mm in the cap, same with the mains. The factory clearances are .0015. even at 200k. Also make sure you make cyl 7 and 8 at least a .0005 bigger. I have one apart right now. Need anymore info let me know.

[katiejameson]

Uh, ok, this is going to be a bit long but... I have worked on and owned a LOT of these 6.5 powered trucks and a few 6.2 trucks. These engines are very robust, we're built for fuel economy and longevity, though many say they are junk and I like to say, just like people who day Subarus are junk, you just need to do maintenance, especially preventative maintenance. A few of the best upgrades for these are the Duramax fan (sing heat kills these) and a Flight Systems PMD (purchased from Quadstar Tuning) with a relocation kit and a #5 resistor. Getting it down in the bumper where the heat sync can get fresh cold air is best. DO NOT continue to use Dexcool in these are that causes issues. There is a cross over pipe that links that drivers side exhaust manifold to the passenger so no, the turbo doesn't feed off only one side. Currently, Quadstar Tuning and Leroy Diesel make/sell some of the best products for these engine. John Faddis is the owner of Quadstar Tuning and actually makes tunes in house that will maximize the efficiency and longevity of these. With his help, 6.5s can now push over over 500hp. Also, there are no such thing are injectors that make more horsepower for a 6.5. It's all a myth!! The fuel is controlled by the injection pump. The marine injector is designed to run longer and more efficient at higher rpm

[rangerismine]

I have had a few military chevy pickups with the 6.2. The first things to do when buying one were 1. New balancer, 2. Check for a starter brace, 3. Change the oil cooler hoses. Besides that, I never had a problem with the 6.2. It eats used motor oil, kerosene, atf and diesel without issue, and moves the truck pretty good since I turned the fuel screw 1/4.

[iamthesargent]

One thing I didn't hear mentioned is that these blocks (6.2 and 6.5) LOVE to break starter bolts, and you have to use the specific GM type as a replacement after extracting the old ones. I love my 1989 6.2 J code engine except for that problem. From the story I was told by the original owner's fleet mechanic, it was on its second set of bolts before those also broke and they decided to auction it off. A few days in my driveway with a couple new creative words and it had new bolts along with the support bracket being tightened up correctly. I began having problems again a few weeks ago when i heard the starter grinding when starting and found the brace bolt rattled off and the main two bolts were starting to work loose. Checked the bolts for cracks and then tightened them down along with a replacement brace bolt and its been working fine again.

[DjWhitehead-rvki]

I think it's funny that he made the Video about a 6.5 but anybody who owns a 6.5 knows they have a crossover pipe so both sides feed the turbo. Nice to see 6.5 getting some attention in 2023 though

[RTDragonCommando]

We had a 2001 Chevy Express 3500 that had the PMD go on the highway. It shuddered, the engine light came on, and then it started accelerating on it's own and blowing a huge cloud of black smoke. Did the research when we got home, found out about the relocation kit, but my father decided he trusted the dealership more than me. So it went again less than a year later, also on the highway, and then finally got the relocation. Only other problems we had with that truck weren't engine related. Tail light wiring went bad on one side, and the diff exploded on the highway.

[DDescalchuk]

I have a soft spot in my heart for the ol Detroit diesel 6.5 have had a few of them and really enjoyed them. Was substantially more reliable then my lb7

[germanjesus]

I’ve owned and worked on hundreds of 6.5s and next to the 12 valve it’s the only other diesel I’d ever consider driving/owning

[pling]

YES! This is my favorite sounding diesel i've had a few of them throughout the years.

[skstibi]

I got rid of the DS4 pumps and went to mechanical DB2 on my 6.5 trucks. They don't make big power but a few tweaks and they perk up considerably and turn into REALLY nice running trucks.

[TheBrokenLife]

I think the turbo spinning in the wind is a combination of things. If you create a venturi of the exhaust by blowing air over it you're creating a low pressure behind the turbine. If you are also pushing air on the opposite side of the wheel you're creating a high pressure. Once those two get working together, it seems like it would be pretty easy to get the wheel to spin. I imagine this would also vary wildly on the machine in question and the conditions.