Pre-97 most 6.5’s were equipped with a low-flow water pump and a single-thermostat setup. In 97 and newer 6.5’s the water pump was upgraded to a high-flow model and dual thermostats were added to combat overheating issues.

The radiator plays a critical part in cooling your 6.5. Commonly crud built up between the radiator and AC condenser (as well as the oil and transmission cooler) contributes to heat build up. Cleaning the rad is important on a 6.5, and if it’s in poor condition internally you need to replace it .

A properly operating fan is also essential - the stock fan clutch often doesn’t engage until 200-205 degrees, almost too late, but still adequate so long as you cooling system is up to the task of quickly cooling off the engine once it engages. The cooling fan should be blatantly obvious when it engages fully - a load roar (which varies with engine RPM) will be evident. You may notice the roar when you start your engine cold and drive away - this is normal, but it should disengage within a mile or so until it’s built in thermostat calls for it to re-engage. Under normal driving conditions with the AC off (and normal temperatures) you may never hear the fan come on, but as temperatures approach 200-205 (210 in some cases) the fan SHOULD be coming on, and you SHOULD hear it. The effect on engine temperature should be immediately evident on the gauge, and the temperature should not continue to rise..and ideally, should begin to fall rapidly.

If you have overheating issues (and anything over 210 should have you worried!) you should pay particular attention to your cooling system. If towing with your 6.5 this is all the more important. If you can’t maintain 210F or less (even towing) your cooling system isn’t working properly - inspect and repair.

Commonly accepted maximum temperatures for 6.5 equipped vehicles:

• Engine coolant NEVER over 210.
• Transmission (4L80E) NEVER over 240.
• EGT (Pyrometer) NEVER over 1200 degrees

This CDR valve is a maintenance item that is commonly overlooked or neglected. [ It is neglected because it is a bit pricey! Normally $75-$100 ]It should be replaced every 30K miles as recommended. 6.2/6.5 diesels need to run a slight vacuum in the crankcase and this valve regulates that. It installs in minutes and it gives you a chance to look at the associated hoses and connections..

This valve is located on the right hand valve cover (passenger side) and has a hose connection back to the turbo inlet. If you have excessive oil around your turbo, it is probably a sticking CDR valve. If this valve fails, you will see oil in your turbo and intake manifold and higher oil consumption. Likewise excessive external engine oil leaks due to pressurizing the crankcase.

AC Delco #CV-910 / GM #25097699 Rock Auto / Standard #BF-41 / Standard Motor or Federated #V335 / Echlin/NAPA #CRB 29445 / Wells #PCV387 / Carquest #76-2713 / Airtex #6P1256

However, there are a few easy modifications.

Exhaust - Do it yourself 4” turbo-to-tailpipe systems will help your 6.5 immensely. The stock system is very restrictive and really holds back the 6.5 and replacing it with a 4” system is often the most drastic change (per dollar) you can spend aside from an ECM swap. You will gain horsepower, MPG, and lower your EGT’s by about 200 degrees!

#9 resistor in the PMD - Inside the plug of the PMD itself is a small resistor that calibrates the fuel flow from the factory. Typically the stock is a #4 or #5 resistor, but replacing it with a #9 resistor will add *slightly* more fuel and if driven gently can even help your fuel mileage. The value of this resistor is only read every 50 Keystarts (unless prompted to read instantly with a GM Tech-2 scanner) so the result may not be immediately evident. Don’t expect drastic results, the effect is minimal at best.

ECM re-flash/Replacement - The “King” of all horsepower modifications. New "power" ECM’s will be calculated with many changes (including increased fuel flow) that will add more power to your truck, but the price can be steep and your engine should be in tip-top mechanical condition before asking it to perform even harder. Unless you are towing or really NEED this extra power, ask yourself if it’s really worth it.

All three of these things can easily add up to $1000 or more, so many people opt to simply go with some of the lesser expensive options above first and work upwards from there...if necessary.

If you have the stock (factory) air filter and intake system you may not be able to actually hear the turbocharger, but that doesn't mean it's not working. Gauges will help you see exactly what your boost system is doing. The maximum boost levels for the 6.5 should NEVER exceed 15PSI. The 6.5 has signifigantly higher compression ratios versus many other diesel engines and as such requires lower boost in order to achieve the same result. Anything more than 15PSI is only risking premature head gasket failure!

What is a “Turbo Master”?

A “Turbo Master’ is effectively a mechanical replacement for the vacuum (and computer) operated wastegate on the turbocharger.

Typically people will install a turbo master for two reasons:

1. The stock vacuum pump has failed leading to non-operation of the wastegate, and as a result, the engine is puking black smoke everywhere.
2. They (incorrectly) think that turning up the boost to 15PSI will make more power.

#1 is a worthy reason - rather then replace the vacuum pump some people opt to simply remove it, as well as the vacuum solenoid on the turbo and simply go with a manual setup. If you build the turbomaster yourself this can offer significant financial benefit and some people may suggest it adds a bit of extra horsepower as the engine no longer has to drive the vacuum pump.

However, it’s not essential - simply replacing the portion of the vacuum system that has failed (be it the pump, the vacuum solenoid, or the vacuum pot) will restore the function of the stock vacuum system as well as the protections that it offers over a mechanical system that the ECM has no ability to control.

#2 is a complete fallacy! You will gain VERY LITTLE (if any) horsepower by exceeding the boost figures that the engine requires in it’s stock setup. More boost does NOT equal more power - more FUEL (which in turn will then require more boost) equals more power, but simply adding more boost without the fuel part of the equation does nothing. To the contrary, it can potentially be harmful as the engine will be forced to breathe hotter air due to increased IAT (Intake Air Temperature) numbers.

If you want more fuel you need to step in to the ECM reflash as discussed earlier in this FAQ. Only THEN should you consider adding more boost, but if you have a stock wastegate setup the reflash will automatically take care of that anyways by simply commanding the stock wastegate setup to behave differently and make more boost. No "Turbomaster" needed!

If you tow heavy with a 6.5 the GAUGES ARE ESSENTIAL! Pyrometer, engine oil temperature and transmission temperature gauges! NO EXCEPTIONS!

A more free-flowing exhaust will help control EGT’s which are commonly an issue when heavy hauling. Your cooling system must also be in tip-top condition otherwise you WILL (!!!) overheat towing with a 6.5.

A differential gear ratio will also drastically effect your towing ability. 6.5 equipped trucks with 3.42 ratios are very limited in what they can to tow, with the 3.73 ratio being better, but the 4.11 ratio being the best for towing. If you are planning to tow a heavy trailer, you want the 4.11 ratio, although for moderate or occasional towing the 3.73 ratio provides a good mix between fuel efficiency (when not towing) versus towing ability but will reduce your capacity by a few thousand pounds.

When towing ensure that your braking system (on both the truck and trailer) is working perfectly.

Things to do to keep your 6.5 Detroit Diesel alive

• Relocate your PMD to a cooler location. DONE
• Inspect your harmonic balancer and pulley. If showing signs of failure ( or impending failure ), REPLACE ASAP! DONE
• Check the lift pump, make sure your IP isn’t sucking itself to death.
• Ensure your IP has adequate lubricity - using an additive is essential! DONE
• Your glow-plug system needs to be top notch. DONE
• Inspect ( and eventually replace ) your oil cooler lines.
• Pay attention to temperatures, most importantly coolant temps. DONE
• Consider an aftermarket 4” exhaust, especially if you tow.
• If towing heavy, GAUGES GAUGES GAUGES!
• If you see black smoke on a 6.5, something on your boost system is broken, fix it ASAP.
• Be careful who you let work on your 6.5 - a lot of mechanics are clueless about this engine but will be happy to take your money while they experiment and learn on it. DONE