There comes a time in the life of a turbocharger when it no longer performs like it once did. The problem, of course, is figuring out when it’s the turbo and when it’s something else.
A turbo’s basic purpose is to boost power, so when an otherwise healthy engine suddenly becomes anemic, it often (but not always) means turbo trouble. Some of the symptoms that may indicate a sick turbo include a loss of power, loss of boost pressure, turbo noise, increased oil consumption and oil fouled spark plugs, or excessive exhaust smoking on diesel engines.
The easiest way to diagnose a weak turbo is to observe the vacuum/boost gauge or boost indicator light. If it doesn’t show normal boost at full throttle (typically 9 to 14 psi for many OEM turbo systems), better get the toolbox. Be warned, however, that excessive backpressure (often due to a clogged catalytic converter) can also prevent the turbo from developing its normal boost pressure.
Watching the Wastegate
Observe the operation of the wastegate. If the wastegate does not move at all, overboost and detonation is the usual complaint. But if it is hanging up and does not close fully, it will bleed off exhaust pressure from the turbine wheel and prevent the turbocharger from developing full boost. Try disconnecting the wastegate and working the linkage by hand to check for binding and proper closure.
On vehicles that lack turbo instrumentation, you can get an indication of boost pressure by hooking up a vacuum/pressure gauge to the intake manifold. Most such gauges read vacuum in inches of mercury (Hg) and show pressure in psi. The gauge must be teed into a manifold vacuum line, not a “ported” vacuum connection on the throttle body.
Make sure the emergency brake is on and the transmission is in neutral or park, then rev the engine up several times by snapping the throttle wide open. Your gauge should go from vacuum to boost as rpm increases. Factory boost setting may range from as little as 9 pounds to as much as 14 pounds, so compare your readings to whatever specs match your engine.
If the turbo fails to deliver any boost, or the boost pressure is more than a couple of pounds below what it should be, note the vacuum reading at idle. A late-model engine with no vacuum leaks or unusual exhaust restriction should show about 16 to 22 inches of vacuum at idle. A lower vacuum reading means you either have excessive backpressure in the exhaust system, an air leak somewhere in the intake system below the throttle (check the turbocharger hoses for leaks or loose connections), or a vacuum leak in one of the many hoses or accessories that tap off the manifold.
Assuming the engine shows normal vacuum readings, but the turbo shows low or no boost, the next step is to find out what is wrong with the turbo. One of the most common causes of poor turbo performance is bad shaft bearings, which often results in rubbing or binding between the compressor and turbine wheels and their housings.
Turbos spin at speeds that are often in excess of 100,000 rpm. If there is excessive play in the shaft bearings, the shaft will wobble allowing the wheels to scrape against their housings. The added friction prevents the turbo from spooling up normally, reducing the turbo’s speed and effective boost delivery.
The best way to check for bearing problems or contact between the housing and turbo wheels is to remove the intake or exhaust plumbing from the turbo (whichever is easier) for a peek inside. Any sign of scraping on either the turbo blades or the housing means the bearings are shot and the turbo needs to be replaced or rebuilt. You should also spin the turbo by hand to feel for any roughness or binding. If it doesn’t turn freely, repairs are in order.
Other problems to watch for are cracks, nicks or chips in the turbo wheel blades, and/or “eroded” (worn) blades. A turbo is a very delicately balanced assembly. Most automotive turbocharger wheel assemblies are balanced to within 0.001 oz.! A broken blade, worn blades or even nicked blades can be enough to throw the assembly out of balance.
The turbo may still spin freely, but the imbalance will prevent it from reaching maximum rpm and eventually pound the shaft bearings out-of-round. There’s no way you can rebalance the rotating assembly, so if there is any indication of damage, the damaged parts must be replaced.
To check bearing clearances, there’s usually no need to remove or disassemble the turbo. A dial indicator can be positioned against the shaft hub to check for bearing free play. Up and down shaft movement should be no more than 0.003” to 0.006”. End play should be 0.001” to 0.003”. This test may not reveal excessive clearances, however, if the turbocharger has varnished or coked bearings.
An alternate technique for checking bearing clearances is to use a dial indicator with an offset probe that can be inserted into the turbo center housing through the oil supply or return port. Positioning the tip of the indicator against the shaft will allow you to measure free play while the shaft is turned. This technique will also allow you to peek inside the housing to check for fried oil on the shaft or housing.
Another thing to check for is the presence of oil in either the compressor or exhaust housing. Shaft seals at both ends of the wheel shaft normally keep the oil where it belongs. But if either seals goes bad, oil can be pulled out of the bearing housing. Oil leaks are more common on the compressor side because that side of the turbo is constantly exposed to intake vacuum.
Oil leaks can also be caused by residue plugging up the oil return line. If oil is present in both housings, but the turbo shows no signs of binding, wobble or rubbing, remove the oil return line and check it for obstructions by passing a stiff wire through it.
The Post Mortem
If the turbo is bad, you should always tear it down regardless of whether you’re going to replace it or rebuild it. You need to determine the cause of death so the problem doesn’t repeat itself. The first thing to check is the condition of the shaft bearings. The bearings are usually destroyed by oil contamination, oil starvation or oil coking.
Heavy grooving or pitting of the bearing surface usually indicates dirty oil, possibly due to inadequate oil filtration. Check for a plugged oil filter. Once an oil filter becomes clogged and its bypass valve opens, unfiltered oil flows to the turbo and the rest of the engine.
If the inside of the bearing housing resembles the bottom of a frying pan and is coated with black crusty deposits, oil coking was the cause of failure. The bearings are oil cooled, and during normal operation temperatures, don’t get hot enough to cause oil coking.
But when the engine is shut off, temperatures can rise to 600° to 700° F inside the housing as the turbo undergoes a period of heat soak. The oil oxidizes and forms coke deposits in the housing that then act like an abrasive to wear the bearings. Using a high temperature “turbo” oil or synthetic oil, installing an auxiliary oil cooler, and changing the oil every 3,000 miles can avoid oil breakdown and coking problems.
In water-cooled turbos, coking is less of a problem provided the oil is changed regularly and you use a quality motor oil. So if you find an accumulation of black crud inside the housing, better check the coolant hoses for a kink or restriction.
Melted or glazed bearings with metal transfer onto the shaft point to oil starvation.
Check for a low oil level, oil leaks or a restriction between the turbo and engine, or low oil pressure (30 psi is the recommended minimum for most turbos). Oil starvation when the engine is shut off can be avoided by letting the engine idle for a minute or so after a hard run so the turbo has time to spool back down.
A bearing that’s been pounded out-of-round, cracked or broken is usually the result of an out-of-balance shaft and wheel assembly, or excessive bearing clearances.
The next thing to note is the condition of the compressor and turbine wheels. Cracks can be the result of metal fatigue, but nicked, bent or damaged blades can only be caused by foreign objects. If the compressor blades are badly eroded, unfiltered air has been entering the intake system. Check for a missing, torn or poor-fitting air filter.
Lastly, check the condition of the housings. If cracked, badly corroded, warped or scored, replacement will be necessary. Beware of cheap replacement housings made of ordinary cast iron. The exhaust housing should be of nickel alloy to resist high temperature operation.
Installing a new or remanufactured unit is probably the safest way to repair a sick turbo. Not only do you avoid the pitfalls of do-it-yourself rebuilding, but you also get a warranty.
If you do decide to overhaul the turbo yourself, one of the most difficult aspects of the job is getting the inside of the center housing spotlessly clean. Ordinary engine degreaser won’t remove the baked-on oil deposits, but spray-on gasket remover will. Another way to clean the housing is to boil it out in a hot solvent or caustic tank, jet washer or thermal oven.
One thing the experts DO NOT recommend doing is having the center housing sand blasted. Besides the possibility of leaving sand residue inside the housing where it could later damage the bearings, sand blasting changes the surface texture, which can lead to lubrication problems.
The bearing surfaces must be highly polished, not rough, so sand blasting is out. Glass beading is okay provided a fine bead is used and the housing is thoroughly cleaned afterward.
The shaft itself can be cleaned in solvent. If the shaft bearing surfaces are not perfectly smooth, round and blemish-free, plan on replacing the shaft and turbine wheel.
Dirty turbo wheels can be cleaned with solvent and a wire brush. Avoid using abrasives and don’t sand blast. Do not attempt to straighten a bent wheel blade. Doing so will only weaken it further, causing the wheel to explode when the turbo hits high rpm.
A bent blade upsets the aerodynamics of the wheel, reducing pumping efficiency, so always replace any wheel that doesn’t have perfect blades.
If the compressor or turbine wheels are damaged, they must be replaced. There are a few experts who claim they can successfully repair damaged wheels by welding, but for the average repair shop, this type of precision welding is out of the question especially when you consider the “downside” risks of what might happen if a patched compressor wheel were to fail and spew shrapnel into your engine. Rebalancing a repaired turbo wheel also requires special balancing equipment.
Worn turbine shafts can sometimes be repaired by grinding the shaft journal down and installing an oversized shaft bearing, or by building up the worn shaft journal with hard chrome and remachining it back to its original dimensions. Regrinding and using an oversized bearing is cheaper than hard chroming, which if not done correctly, will flake off and cause a repeat failure.
If the lip of either turbo wheel has been rubbing against the seal area on the center housing, you may find a groove worn into the housing. If this is the case, plan on buying a new center housing. In many instances, it is cheaper to buy a new “turbo cartridge” (the center housing with both wheels and shaft preassembled) than it is to try to rebuild a damaged unit.
When reassembling the turbo, cleanliness is absolutely essential. Any dirt, grit or unremoved crud can become the cause of a future relapse.
The shaft bearings must be lubricated with assembly lube, and checked for proper fit and alignment. New seals are another must, as is torquing the compressor wheel to specs. Once the unit is together, spin the wheels to make sure the shaft turns freely and nothing rubs. A final check of clearances with the dial indicator is advised before reinstalling the unit on the vehicle.
Some of the things that should always be done after rebuilding or replacing a turbo include changing the oil and oil filter, checking the air filtration system for leaks or obstructions, and priming the turbo oil supply before starting the engine. Cranking the engine for 10 to 15 seconds with the ignition coil disabled will pump oil to the turbo. Observe these simple precautions and you’ll have a successful repair.