Introduced in 2001 as a joint venture between GM and Isuzu, the Duramax V-8 was the General’s bid to one-up Ford and Chrysler’s newest generation of powerful and efficient diesels. This move was not voluntary on GM’s part, as competitor Chrysler had just purchased their previous engine supplier, Detroit Diesel. GM was eager to develop and speed to market the 6.6-liter LB7 Duramax, which produced 235 horsepower. Later versions including the LLY, LBZ, LMM and LML would use more sophisticated turbos and engine controls, ultimately increasing both horsepower and torque by about 80 percent from 2001 to 2011.
1. Test-drive the truck; this provides the first indication of whether the turbo is on the way out. Pay close attention to how the turbo acts. If it takes longer than before to make power or produce the audible whine you’re used to hearing, then it may be going bad. Also listen to the pitch of the whine. If it sounds quieter or lower-pitched than it normally does, you may have a problem. It may help to have a second identical truck on hand against which to compare.
2. Check the turbo boost. If your truck doesn’t already have a boost gauge, check boost using a store-bought vacuum or boost gauge and the appropriate fitting for the pressure test port on the manifold plenum. If you can’t find the appropriate fitting, check boost at one of the vacuum ports. With the gauge connected, open the throttle and check the peak boost. All models should check in at 20 pounds per square inch (psi) of boost maximum, with the rpm at about 1,800 for 2003 to 2004 LB7 motors and 1,600 for all others.
3. Squirt the tube connections with soapy water and watch for bubbles. If your boost reads low, the turbo might not be at fault, so check for boost leaks first. If you see bubbles around any of the couplings or anywhere on the intercooler, found in front of the radiator, then you have a boost leak and not a bad turbo. Check all of the fittings from the turbo to the intercooler and back to the engine intake from the intercooler.
4. Watch the wastegate actuator rod and actuator on the turbo, which applies only to LB7 models without a variable-geometry turbo. The wastegate is a spring-loaded diaphragm that opens a valve to bypass exhaust around the turbo at maximum boost. Watch the wastegate actuator rod; it should move very quickly to open the bypass at the turbo’s maximum boost point.
5. Pay attention to the actuator on the variable-geometry turbo housing. Instead of a wastegate, variable-geometry turbos use variable-pitch vanes to modulate turbo boost. The vanes are close together at low revolutions per minute (rpm), increasing exhaust gas velocity and helping the turbo to spool up faster. An actuator pushes the vanes open at high rpm for maximum flow. From idle, the vanes’ actuator rod should move linearly with engine speed to open the vanes and then begin to move back to close them and limit boost at 20 psi.
6. Shut the engine down and quickly remove the tube that connects the air filter to the turbo inlet. Reach into the turbo inlet and spin the blades using a plastic ballpoint pen. The compressor wheel should spin with minimal resistance and keep spinning for a few seconds afterward. If the compressor wheel doesn’t spin freely or immediately grinds to a halt, then the turbo bearings are shot. This procedure is best performed with the turbo as hot as possible as metal expands with heat, so allowing the turbo to cool may not give you an accurate gauge of its condition.