This compressor wheel has been worn because of dirt ingested through the air intake. Since the wear is almost always uneven, imbalance and failure often result.
The turbo is the canary of the engine. It can reveal unseen problems early, just as the songbird would collapse in the presence of volatile methane when wary coal miners took one underground.
Because the turbo spins at up to 100,000 rpm, it is especially sensitive to proper lubrication. Keith Dewhirst, worldwide service engineering leader for Holset, maker of turbochargers for Cummins engines, says its bearing surfaces travel as far in one minute as a piston travels in an hour.
When the engine’s under load, the turbine, which picks up energy from the exhaust to spin the shaft, will run at 1,300 to 1,400 degrees, says Al Hertzog, director of training for Mack and Volvo. Since the bulk of the cooling, even in turbos with water-cooled housings, must be done by the oil, adequate oil flow is your biggest concern.
Experts insist a turbo will last indefinitely if you do everything right. “I’ve seen them go seven or eight years and a million miles,” Hertzog says. Here are preventive maintenance tips that can put your turbo into that long-life zone.
IDLE TO COOL THE TURBO. The worst time for the turbo is right after a hard run. Hertzog points out that the turbo shaft may still be spinning at more than 15,000 rpm. It takes much longer to slow than the engine and runs much hotter, but its oil comes from the oil pump, which shuts down a few seconds after you shut the engine off. This stops the flow of oil that cools and lubes the bearings.
To prevent problems at shutdown, says Hertzog: “Let the engine idle three minutes before you shut it off. If you do this, the turbo will slow and calm down. The temperature of the shaft and bearings will stabilize, too.” When a diesel engine idles, the exhaust cools off, cooling and slowing turbo parts.
According to the Detroit Diesel Turbocharger Technician’s Guide, “When an engine operating under full load is shut down without a short idle period, the heat of operation is not dissipated and all parts remain extremely hot. Oil films can be destroyed under this condition.”
Dewhirst extends the logic to startup. “Many modern engines fire up with short cranking times even at low temperatures,” he says. “So it’s just as important as ever to let the engine run at low idle for at least the first 20 to 30 seconds to ensure full oil flow has reached the turbo before going to high idle or on-load operation.”
Another lubrication problem can occur when a truck is towed: Air flows into the turbo and spins it, but with no oil supply, the bearings are often ruined. Hertzog advises installing a rain cap onto the exhaust pipe during towing.
/CHANGE OIL, OIL FILTERS AND AIR FILTERS. Oil change intervals are important for turbo health, says Greg Holderfield, director of technical support services for Volvo Trucks. “Soot-contaminated oil will reduce the life of turbo shaft bushings,” he says. Dewhirst adds, “In particular, correct oil grade and quality are vital to the turbocharger.”
Oil filter and air filter replacements come in a close second. “Turbos are more sensitive than most other engine parts to oil and filter changes,” says Dewhirst.
Air filters have a large effect on turbo life. First, according to Holderfield, “Unfiltered air can clog up passages and can easily erode impeller blade leading edges.”
Even worse, “The wearing away of the blades, if uneven, can induce a shaft motion, which will pound out and eventually fail the turbocharger shaft bearings,” says Thomas Miles, marketing manager at BorgWarner Turbo Systems. “Plugged or restricted air cleaner systems, resulting from poor maintenance procedures, will reduce air pressure and volume at the compressor air inlet and cause the turbocharger to lose performance. Restricting the air inlet reduces air flow to the engine and an overfueled condition results, causing excessive engine and exhaust temperatures and black smoke.”
Dewhirst says this is a serious problem. “A restricted air supply will also reduce the turbo boost pressure, which can lead to a lower engine air/fuel ratio and result in higher exhaust gas temperature. This can damage the turbine components.”
Note that with the latest engines, reduced air flow will result in reduced power rather than smoke because the electronic control module compensates for the lower air pressure by cutting fuel, Hertzog says.
“The increase in intake vacuum caused by a plugged air filter can cause an otherwise healthy turbo to leak oil into the intake air system,” Dewhirst says. This can clog the charge air cooler or other intake system parts. Dewhirst says the good news is that if you catch the problem before the turbo thrust bearing is damaged, simply replacing the clogged air cleaner with a new one will often eliminate the oil leakage.
CHECK CLAMPS, GASKETS, EXHAUST MANIFOLD. Hertzog points out that similar symptoms sometimes result from reduced inlet air pressure because of leaks in the air intake system. “Lack of back-pressure on the compressor in combination with 70 pounds of oil pressure like you get at full power can cause too much pressure drop and blow out the bearings,” he says. Miles points out that with a less severe air leak, the symptom may be oil loss through the compressor seals without an actual bearing failure.
This points to the need for regular inspections, Hertzog says. “Check all clamps, and make sure there are no gasket leaks,” he says. “Make sure the charge air cooler hoses are tight. Also, check the air intake hose where it goes into the compressor housing, as this fitting is under a vacuum.” If there were a leak there, dust would be sucked into the engine.
Miles suggests checking the exhaust manifold bolts. If loose, they could cause noise and low power. He says carefully disconnecting the turbo oil drain line and checking it for a restriction will help ensure continued proper turbo lubrication and cooling and will eliminate a cause of seal leakage.
MAINTAIN COOLANT. Oil temperature can be an issue if the oil cooler is not keeping oil temperature at normal levels because of scale deposits, says Dewhirst. This could become a progressive problem as high oil temperature accelerates oil breakdown, Holderfield says.
INSPECT PERIODICALLY. Don’t try to measure turbo wear unless you’re an experienced technician because you could easily damage the unit. “The progressive wear suffered by a turbocharger in normal use is so small as to be virtually unmeasurable, except using specialized equipment,” says Dewhirst.
Miles suggests removing the air inlet and checking visually “for wheel contact against the housing walls. This would indicate internal bearing damage from loss of oil, contaminated oil or imbalance.” Hertzog adds, “Look for signs of fin deterioration or distress on the housing.” He also recommends a thorough check for oil leakage from the bearings not caused by air flow issues.
Use light pressure to rotate the shaft. If it is stiff, there may be sludged oil or coking from overheating. Dewhirst recommends taking this step further by gently pushing the compressor up and down and side to side till the bearings stop it to see if it contacts the walls of the compressor housing. Too much finger pressure could temporarily bend the shaft and cause the blade tips to lightly touch the housing and bend, resulting in imbalance and failure. If there is no metal-to-metal contact and the turbo sounds normal, it’s likely to be in great shape.
Expert technicians can precisely measure turbo wear on doubtful units that show no obvious damage. This requires a good dial indicator and the ability to use it. You also need to get the specifications from the manufacturer’s manual or website.
Finally, be sure you’re measuring the right dimension. For example, to measure the basic bearing clearance on Macks, Hertzog says you “must remove the oil pressure line from above and put the dial indicator down through the feed hole and onto the shaft from above.” You then put one hand on either end to lift both ends up and down simultaneously to get the true reading.
“The most sensitive instrument most people own for detecting impending turbo failure is their ears,” Dewhirst says. “A turbo that has suffered bearing wear or blade damage resulting in imbalance will sound different – louder, with a sound that can be anywhere from a screech to a scream or a howl.” Note that any noise that does not change with both load and rpm is probably not caused by the turbo.
Wastegates, too, deserve checks. “A visual inspection of the external linkages to look for obvious signs of damage or excessive wear is the most effective thing an owner can do,” says Dewhirst.
If your turbo is still happily wheeling around, these routine maintenance and inspection practices can help extend its life. Keep it lubricated, give it time to cool, and you could see many years of faithful service.
REPLACING A TURBO TAKES SPECIAL CARE
Should you replace a blown turbo yourself? Opinions vary, but it’s clear the turbos on older models are much easier to do.
Holset’s Keith Dewhirst points out that there are six to eight major interfaces on every turbo. “Any of these can cause major problems if contamination gets in or leakage occurs,” he says. Also, on any water-cooled turbo, the coolant lines in and out must be replaced in their original positions.
Pre-lubrication is a big factor. “Fill it with oil till you see it purge to the top of the oil supply hole,” Hertzog says. “The oil will drain into a reservoir in the turbo housing.” This will supply the bearing till oil from the oil pump arrives.
Borg Warner’s Tom Miles offers these replacement suggestions: