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6.5L Diesel Engine Discuss the 6.5 GM diesel engine & associated components. Automatic transmission questions & problems belong in the 4L80/85 - 4L60E - 6L90 Transmission Forum is the premier Duramax Diesel Forum on the internet. Registered Users do not see the above ads.
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Old 08-13-2005, 12:42 AM   #1 (permalink)
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Reference Material: How to buy a truck

So, you wanna buy a truck eh?

So, what’s it gonna be? Crew cab, long bed? Are you towing, or ‘round town-ing?

Let’s dive into this issue a bit deeper. So many have been asking about “should I buy this truck or not?” The answer is, we don’t know. We’ve never seen it. We’ve never rode in it. We don’t know what you wanna do with it.

Only you can stop forest fires. And only you can stop from buying the wrong truck.

So let’s examine a few things here: Why do you want a diesel truck?

Here are a few questions and answers to ponder here:

I want an engine that will last a long time. Diesels have a history of being a heavier built engine. An engine that was designed to put up with the riggers of towing, abuse, and need for power. Diesel engines provide more torque. They are not fast turning engines like their gasoline counterparts. Put in fewer revolutions, they put out much more power.

How does a GM 6.5 diesel stack up to the other diesels in the market? Well, you are comparing apples to artichokes. We’ve got to go through some history about this engine. The 6.2 was the original engine design. Detriot Diesel was the original designer of this engine. It was an IDI (Indirect Injection) engine. It was one of the first “smokeless” diesels. Ran smooth, did the workhorse stuff that needed to be done. They weren’t set up to be dragsters. It was a light duty diesel engine.

As time went on, people wanted more and more from their diesel engines. Improvements came along to help meet the demand. These improvements came at a cost. It moved up from being a 6.2 to a 6.5, a 6.5 turbo. We are talking light years ahead of the original design parameters here. Power increased amazingly, but it was still the original engine design for the most part, just slightly bigger bore with a turbo.

When the 6.5 turbo came on the scene, all of sudden, cooling issues arose. The original cooling system was not adequate enough to handle a turbo powered upsized engine. These issues would continue on until 97.

The next thing on the list was GM forged a new trail in the diesel world, by making it computer controlled. A new injection pump that would have a computer making the timing, fuel metering, and all the functions that usually were handled mechanically, were now computerized. This all began in the 94 model year. Some a step ahead, but then again, a step behind. There were issues with this new design that have been revised over the years. Now, with a few modifications to your system, the reliability factor increases tremendously.

The engine serves it’s purpose well, but there isn’t much fudge factor left on it. It’s near it’s maximum limits as it comes off the factory line.

So, back to the original question: how does this engine stack up? Well compared to a Cummins, or a Powerstroke, it’s not in the same league. But, Cummins and Powerstrokes come at a much higher price tag. That price tag is front end, and back end (repairs) Both of those engines are strong contenders in the diesel market. But should you need to repair either of these engines, be ready to spend some money. If repairs be necessary on a 6.5, the parts are usually available, AND at a reasonable cost.

The 6.5 seems to get a bad rap from the “other guys.” Truth is, it’s still a decent engine, but it all depends upon how you treat it. If you are strictly looking for 0-60 times in the quarter mile, and making all sorts of improvements in power, the stock design doesn’t allow much “extra” for you to play with unless you do some serious modifications.

So, what are you going to be using this truck for?

Here is the question that you need to answer: Are you using this truck for towing, or around town? Towing, you’re going to require (depending upon the load) a 4.10 rear. If you are going around town, you can get away with something like a 3.73. The difference is in what speed the engine is turning. The faster the RPM’s the more fuel it will burn. Diesel engines are not as forgiving as gas engines on their RPM’s. The faster you turn, the more fuel you burn.

You’ll need a 4.10 to be able to haul loads and tow. This will increase your fuel expense, but it makes the engine fair much better.

Now, if you are limiting yourself to only around town driving, over the road, no towing, then something like a 3.73 would do you better. This will increase your MPG’s. No need in burning extra fuel if you don’t need to pull something behind you that will lag the engine.

How can I tell what rear end I have?
Check the RPO codes. That’s the sticker with all those letters on it that don’t mean anything. Check out RPO codes to see what it has.

Is this a good price to pay for this truck? Well, you’ll have to make that determination yourself. The answer depends upon good research, and being honest with yourself. There is a lot you can research first to see if the asking price is a good one or not. Remember, “asking price.” The other factor, is can you really afford it? Take into consideration your fuel expense. With today’s fuel prices very unstable, you may be spending a lot more than you anticipated. Sometimes you may be spending more in fuel per month than you are spending on the payment to own it.

What are things to look for to see if there are any problems? Well, take a look through the FAQ thread, read up on what seems to be common issues with these trucks. If you go to other forums, guess what you’ll see problems. No matter what type, make, model, year, anything, you’ll find problems. Every make has them. The question is, can you live with them? Are they in your scope to handle if needed to be repaired, or is that something you’ll have to “farm out” to a mechanic to get repaired?

It is hard to make a determination online as to weather or not a truck will give you good service or not. You can check to see if it already has. Get the vehicle’s VIN number and post it for someone to run the warranty history on it. You’ll be able to see what all has been done under the vehicle’s warranty period.

What are the “big ticket” items to look out for when buying? One of the most expensive parts that could fail would be the injection pump. Depending upon the year that you buy will depend upon how you can tell if there is injection pump issues or not. With the electronic injection pumps, they will have codes stored in memory of the computer. These are not something you want to “skip past.” The Service Engine (SES) light on means a whole host of other things nowadays as compared to the few codes the computers used to store. Instead of about 90 codes, they have hundreds of codes. All of which will rat out any issues. Please note, that the SES light not being on, doesn’t mean that there isn’t a code in the computer, it just doesn’t mean that there is one that is active at the moment.

One last thing to note, if you test drive, start the engine a second time. That second time may light up the SES light. How? This is what I didn’t understand, because it happened to me on my purchase. When you pull the battery power to the computer, the codes get cleared. Sometimes the SES light won’t come on until the second time you start your engine. Just do it, trust me. It could mean an injection pump.

For more information on computer codes, check out the FAQ link. There are two different generations of computers, OBD-I, and ODB-II. There are two different methods of checking these systems. The first only requires a paperclip, the second requires a code reader (more expensive than a paperclip).

I pulled the oil dipstick, and the oil was black? Welcome to the world of diesel! If you change your oil in a diesel engine, it will turn black, rather quickly. Sometimes before the Wait to Start light goes off just after changing it. Well, maybe not that fast. Oil color is not a very scientific way of telling if the engine is okay or not. In most diesel engine applications, oil would be sent for analysis. This will give you a breakdown of all the things floating around in that nice dark mixture. If the engine oil is milky, well, that's not good.

Could you name some things to check quickly before purchasing?

Check your radiator hose just after cold startup. If this hose gets hard just after starting up, and the engine is still cold, you may have some internal issues allowing clyinder pressure to enter your coolant system. This spells trouble.

Oil Fill. Open the oil fill and see if you hear any unusual noises, smoke comes out, or if there is a huge abount of air blowing out of it.

Smoke check. See if you have any colored smoke at startup. White smoke on a cold start up may indicate a glow plug issue. Black smoke while on the road under hard acceleration may indicate turbo, or lack of air issues in coming to the engine. A lot of white smoke while running could spell some issues.

Check turbo operation. See if the wastegate actuator rod is sucked up into the actuator with engine running at idle. See also if you can move it. It should be almost impossible to move. If it's not working properly, it's pretty easy fix usually. But something to look at first since it seems to be a weak link in the system. Read the turbo link first and see what all the possibilites are. Could be cheap, but could be more.

Lift pump check. Open the water drain valve on the thermostat housing. Then you can check for proper lift pump operation.

There are whole host of other things that you could check. Not everyone would be excited about doing extreme things like dropping the oil pan to check the lower end out. Doing this before a purchase may be extreme, but it could be a good thing to check.
Well, there will be other things added to this thread as time goes on. If you don’t find the exact answer to your question here, feel free to do a search. Many have asked about purchasing before, so don’t feel like you are alone here. If you don’t see the specific answer to your question, feel free to begin a thread, and we’ll do our best to help you out.

**Please note that this thread is meant for reference purposes. Begin a new thread if you have any questions that would pertain to your particular situation. If you see any missing material here, feel free to post and add more information for others to see.

Look here-> GM 6.5 Reference Material & FAQ's <-Look here

Diagnostic Links & Common Failures: Turbo System * Lift Pump/OPS * PMD * Stalling

°Please include your vehicle details in your signature line. Did you read the FAQ's and search before posting?
*Bus 1, rest in peace. Bone stock, now just bones. 4.10 rear 10/13 mpg, 182,000 miles with a set of MCI motor coach seats.
*Currently driving an "Canadian Camo" Snow White 2001 5.7L Chevy Express.
*Bus 2, 99 7.3L PSD.

Last edited by knkreb; 03-01-2008 at 11:36 PM.
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Old 08-15-2005, 10:00 AM   #2 (permalink)
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This is a very nice and well laid-out summary of what to look for (and expect) when buying a 6.5 turbo.

Well done!!!



"If they ever come for your guns, be sure to use them..." - Unknown

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Old 08-15-2005, 10:39 AM   #3 (permalink)
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Great Job!
1994 Z71 Blazer / remote pmd#7 resister Heath unit / 3" down pipe 4" pipe no CAT/ K&N cold air intake / 97'cooling system with fan & clutch / "S" 6.5 / Autometer gauges EGT pre-turbo, Boost, & trans temp / 10k Pot Boost fooler / Home Made TM / high idle switch / Optic Bump / Removed Vac-Pump

2004 GMC D-MaX LB7 - Stock
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Old 08-15-2005, 11:57 PM   #4 (permalink)
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Hey, just remember, this ain't a one man show here. If you have additional thoughts about similar subject matter that would be helpful to others, feel free to post it in the proper thread, or begin a new one. We can reference them in the FAQ's to reduce the double posting, and repeative newbie questions.
Look here-> GM 6.5 Reference Material & FAQ's <-Look here

Diagnostic Links & Common Failures: Turbo System * Lift Pump/OPS * PMD * Stalling

°Please include your vehicle details in your signature line. Did you read the FAQ's and search before posting?
*Bus 1, rest in peace. Bone stock, now just bones. 4.10 rear 10/13 mpg, 182,000 miles with a set of MCI motor coach seats.
*Currently driving an "Canadian Camo" Snow White 2001 5.7L Chevy Express.
*Bus 2, 99 7.3L PSD.
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