turbo torque turbo vehicle fault diagnosis 100x100

I just fitted the turbo & it's still blowing smoke!

Probably the most frequent question I've been asked over many years and more often than not by mechanics because in fairness they may rarely repair a turbocharged vehicle...


Unfortunately in most cases where mechanics aren’t familiar with turbocharged vehicles, analytics is limited to, "the turbo is leaking oil so it must be the turbo!” The dilemma is although the turbo was indeed leaking oil it is usually due to some other factor causing it to leak or inflict damage to it. Don't resolve the actual primary source of the problem and you will be back to where you started, oil leakage from the turbo and/or blue smoke after you have replaced it coupled with total frustration as to what to do next. I'm sure most mechanic's initial response is, "I'm going to exterminate the dude who rebuilt this turbo!”

I've spent many hours, months, years on the phone going through a check list of tests due to the turbocharger being the most misunderstood, victimised and cursed component on any vehicle when in fact it is one of the most reliable. Most of the tests listed below won't be found in any workshop manual as turbo fault diagnosis is not taught at trade school, it’s learnt from experience, theorised during many a sleepless night, and tested over the years. In most cases, one of the tests below will find the cause of your smoke quandary unless it falls under the category, “other possibilities”.


What's a Piston Ring Seal Anyway?

Briefly, most turbochargers use piston ring seals1 & when installed there is a specified minimum / maximum measureable installed gap2 as per the turbo manufacturer's specifications to allow for expansion caused by heat. In addition to the seals the machining design on the rear of the turbine wheel shaft3 & front collars are intended to throw any oil in the vicinity of the seals back towards the centre of the bearing housing as it spins, loosely termed as an "oil slinger".

If there is fault causing oil to be forced or build-up against these piston ring seals they will indeed leak as technically their not oil seals, their primary purpose is to prevent boost on the inlet side & exhaust gases on the turbine side from entering into the bearing housing, the reversal of what is commonly presumed.

Technically a turbocharger fitted on a good engine with correct oil pressure, adequate oil drain and well designed breather systems etcetera, could operate with no seals installed at all and would not leak.

Pressurised oil enters the bearing housing and stabilises the turbine shaft. After the oil passes through the bearings it falls into a cavity at the bottom of the bearing housing making it's way back to the sump via an oil drain system, the flow rate back to the sump is governed solely by gravity.

A common statement I hear from many customers; "My turbo must have faulty seals" is not in fact a realistic possibility.


Turbine Seal Sketch

Turbocharger Lubrication System:


The following image displays the manner in which oil flows to the turbocharger from the engine oil supply under pressure then returns to the sump with gravity, it is then scavenged up by the oil pump and the cycle keeps repeating itself whilst the engine is running. (Engines vary from the below image but the principle is the same for most!)


Turbocharged Engine Sketch


  • Allow time for any oil residue in the inlet or exhaust to burn away.
  • Remove the oil filler cap & check for fumes.
  • Check the oil level.
  • Check the air filter, intake pipes, clamps.
  • Check the breather hoses, (filter and canister if fitted).
  • Spark plug test.
  • Check the oil quality, type, was it changed?
  • Check the oil drain for any restrictions.
  • Sump pressure test via removal of oil drain.
  • Sump pressure test under boost conditions.
  • Compression and leak-down test.
  • Check oil flow &/or pressure at idle and high rpm.
  • Other possibilities.
  • The turbocharger.



1. Unburnt oil in the inlet or exhaust system / condensation:

1. Unburnt oil in the inlet or exhaust system / condensation:

Quite often after a turbocharger is fitted back onto a vehicle the installer will start the engine, observe smoke from the exhaust, turn off the engine then ring me. The turbocharger was removed because it was leaking oil so naturally there will be puddles of oil in either the inlet system, exhaust system or both. Sometimes there may be as much as half a muffler full of oil and this will take quite some time to dissipate before smoke stops being emitted from the exhaust system therefore allow plenty of time for smoke to clear.

On more than one occasion I've found the problem was as simple or silly as condensation on a coldish day. It's important to remember oil burns bluish/white when it leaks from the inlet side and is partially burnt in the combustion chamber, conversely oil burns whitish/blue when it leaks out of the exhaust side onto a hot exhaust, fuel burns black and condensation white. Also condensation will evaporate into the air very quickly where oil or fuel will linger around or be blown into the distance by wind. Oil quite often is also accompanied with a pungent smell.

2. Valve cover smoke test:

2. Valve cover smoke test:

Very simple yet very reliable provided the engine oil isn't well overdue for an oil change and lost all of its viscosity. While the engine is running at operating temperature, with rag in hand, undo the oil filler cap carefully so as to not get splashed with oil. It is best to tilt your head level with the valve cover and check if there are any signs of visible fumes being emitted whilst giving the engine a few light revs. You may feel some pressure which is usually normal on some vehicles but no signs of fumes or smoke whatsoever which is generally the signature of engine blow-by which is oil making its way into the combustion chamber, partially burning then returning back as smoke out through the open oil filler via the timing cover.

Blow-by will cause pressure in the sump restricting the oil being returned from the turbocharger, a bit like putting your thumb over the oil drain pipe. Oil enters the turbo under engine oil pressure; however after it has left the turbo bearings there is no oil pressure at all on its journey back to the sump. If there is any sump pressure, no matter how minuscule, it will prevent oil returning to the sump and force it out of the turbo through the piston ring seal(s).

Pressure in the sump increases as boost increases & the smoke from the valve cover also increases but you can only test this under boost with the aid of a chassis dyno although having said that I have to admit I have tested vehicles on the road by installing a very long breather hose into the passenger side window. Anyway on the dyno under full boost it is better to remove the breather hose to try and view any fumes rather than the oil filler cap to avoid oil being splashed around the engine bay.


Turbo Oil Drain Restriction Due To Blow-By

4. Blocked air filter or intake pipe(s) / intake hose(s):

4. Blocked air filter or intake pipe(s) / intake hose(s):

Any restriction at the intake side of an engine such as a heavily clogged air filter will result in oil leaking from the inlet side of the turbo. The compressor wheel, whilst spinning to generate boost will attempt to source large volumes of air from wherever possible, if it cannot acquire enough air via the intake system, it will tend to pull oil from within itself through the front piston ring seal.

It may also drag oil from the breather hose pending its location.

If there is a severe blockage it may also cause thrust bearing wear allowing excessive end-float from a pressure imbalance leading to a severe turbocharger failure.

I have seen "soft" intake hoses collapsing under higher RPM as the inner walls of the hose pull inwards, touch and act as a seal.

If leaking oil puddles in the bottom of the compressor cover to the point where the compressor wheel blades are "scooping" oil, it will tend to pull the compressor wheel downwards wearing the bearings and other turbo components, again causing severe turbocharger failure.

Should there be a leak in the air cleaner assembly, split hose or loose pipe due to an unfastened clip or clamp, dust will be drawn into the intake system, damage the compressor wheel and score the cylinder bores of the engine requiring both an engine and turbocharger rebuild.

Blocked Air Filter

5. Blocked or kinked breather hose(s), blocked PCV valve:

5. Blocked or kinked breather hose(s), blocked PCV valve:

All engines must breathe as the action of the pistons travelling up and down act as an inverted air compressor on the sump side of the pistons which must be vented. Check the breather connections aren't blocked, hose's are not kinked or softened and any breather filter, canister or air filter assembly is free from oil, dust, dirt or debris. Breather system designs vary from vehicle to vehicle but the principle of removing pressure is a common denominator.

On many “extreme” race vehicles it’s common for a vacuum pump to be fitted to the lower part of the engine block or upper sump area, pumping and directing any pressure generally into the exhaust system.

A regular customer popped in with his twin turbo Toyota Soarer emitting smoke profusely from the exhaust system. He had fitted a "high performance catch-can" with an oil bleeder to his vehicle the night before and presumed he had installed it incorrectly. We checked the vehicle on the chassis dyno and sure enough clouds of smoke were bucketing out of the exhaust. It didn't take long at all to diagnose the problem; the original Soarer breather hose is 1 inch in diameter and the catch-can kit was fitted in line with the original, as per the instructions. We discovered the 1” hose entering the catch-can and a 5/16 of an inch fitting and hose exiting it thus the engine couldn't breathe properly, sump pressure was introduced because of the design of the catch-can and consequently oil leaked from both piston ring seals of the turbocharger resulting in smoke.

We had a flood a phone calls shortly thereafter by Soarer owners with the same problem due to the breather kit being promoted as a “must have “on the Soarer Club forums. We just quickly explained how to solve their problem over the phone and the problem soon went away.

Sump Pressure Caused By Stuck PCV Valve

6. Spark Plug test: (Consider doing Test # 11 whislt the plugs are removed)

6. Spark Plug test: (Consider doing Test # 11 whislt the plugs are removed)

Reading spark plugs can give some very detailed signs about your engine condition, individual cylinders, tuning etc. Any oil fowled plug(s) will indicate probable blow-by via the engine piston rings or valve guides consequently causing your turbocharger to leak oil. A damaged spark plug(s) can cause impact damage to the turbine wheel causing imbalance and eventual turbocharger failure. Remove your spark plugs and check them against this Spark Plug Diagnostics Chart, located here which will open in a popup window.

7. Engine oil quality:

 7. Engine oil quality:

Replace a turbo, replace the oil & filter. If the oil is rarely replaced, it soon loses its viscosity due to the heat it acquires when cooling the turbocharger. Once it loses viscosity it has the characteristics of water which will result in a possible variety of problems. I have solved many a "smoking" vehicle after a turbo replacement by replacing the new "cheap" oil, if it was changed at all, with turbo quality oil and correct grade. Also on a turbocharged engine, synthetic oils need to be changed more frequently rather than every 20,000 Km or more as advertised.

Occasionally a customer will use a particular oil in his vehicle from the day it was first purchased, change the brand & the car emits smoke via the exhaust, put the original brand back in & it stops; rare but it has happened to me a handful of times and I have no idea why! One case I clearly remember was a vehicle which had always used Penrite HPR30, the owner for some reason supplied Castrol GTX, we changed the oil and the vehicle smoked profusely, we replaced the Castrol with Penrite and the vehicle stopped smoking almost instantly; go figure! This is in no way a bad reflection on Castrol as the reverse has also occurred.

8. Oil drain restriction: silastic, kinked hose, carbonised pipes:

8. Oil drain restriction: silastic, kinked hose, carbonised pipes:

Many a turbo will leak oil or fail due to carbonisation caused by lack of oil changes and/or repeated hot shut downs which I will cover in more detail elsewhere since carbon build-up in the turbo is removed during the reconditioning process and not related to installation. However if the turbocharger was heavily carbonised it's very likely the oil supply and oil drain pipes were also carbonised, some totally. What is carbon? Think of it as lethal, heat treated, hardened, black kryptonite which if not completely removed will destroy your turbocharger. It may be relatively easy, very difficult , or virtually impossible to remove pending how long the lack of maintenance and/or hot shuts downs continued for.

Soaking in a cold or hot acidic* bath over night, preferably sonic, should soften the carbon although sometimes it only softens a small amount at each end in extreme cases. Digging with wire, tapping with a dolly, applying heat, drilling, an industrial jack hammer or TNT may also assist. The oil feed pipe can sometimes be near impossible to clean properly if at all as they are smaller in diameter, around 1/4" in most cases and can be long with many bends. The oil supply is probably best replaced in most cases. Oil drain pipes generally are much larger in diameter and a tad easier to clean but not always. Anyway the point is both pipes have to be completely cleaned or replaced with no doubt of cleanliness attached whatsoever!!!

Be cautious of some vehicles, particular some front wheel drives, where the vehicle manufacturer, not caring or not knowing, ran the oil supply pipe directly along the top of the exhaust manifold to insure carbonisation occurs, In these cases the turbo maybe fairly clear of carbon but destroyed due to no oil yet the oil supply pipe maybe partially or totally blocked. Fit a new turbo without cleaning the pipe and the new turbo will fail immediately.

*DO NOT fool around with acid unless you follow all the safety precautions, have all the required protective clothing, proper ventilation, stored in a safe place etc. In fact it’s best left to professionals.


9. Oil drain removal test for sump pressure:

9. Oil drain removal test for sump pressure:

Another way to test for sump pressure is to remove the oil drain pipe / hose if easily accessible. Requires two buckets and a hoist is handy if available. Whilst the engine is running allow the oil to drain into a bucket, not too much of course to starve the engine of oil, then switch buckets and pour the engine oil back into the running engine via the oil filler.

Continue to do this for about 15 mins to test if the smoke being emitted from the exhaust starts to reduce. Since we have eliminated the sump from the equation, should the smoke reduce you now know the problem lay with sump pressure & then analyse why. This method is only good for a vehicle emitting smoke at idle as we are not introducing boost, some vehicles only emit smoke at idle, some only under boost and some under both conditions which requires a different type of test.

10. Sump pressure test under boost conditions:

10. Sump pressure test under boost conditions:

This test is best conducted on a chasis dyno but can also be conducted on the road with a passenger. Using a good quality vacuum hose, slip it over & cable tie it to the dip-stick tube, (remove dip-stick first :) ). Insert the other end onto a good quality, vacuum / pressure gauge which displays small increments of pressure and vacuum. Run the car up on the dyno, or if on the road, pass the gauge through a gap in the bonnet then through the passenger window. Obtain full boost whilst watching the gauge and the indicator arm should never travel into the positive side of zero. Any reading, no matter how small above zero will point to sump pressure causing the turbo to leak oil.

*Insure the oil drain pipe/hose in properly connected when using this method as a test!


Sump Pressure Tests

11. Compression & leak-down tests:

11. Compression & leak-down tests:

Most mechanics would have completed a compression or better still a leak down test well prior to the above tests I have already mentioned, however I have found them to sometimes give a false positive i.e. all is ok when in fact all maybe not ok. If the turbo was leaking oil profusely then it's likely all cylinders will have a fair degree of oil in them. Under compression or leak down test the oil will tend to disperse outwards then downward the skirts of the pistons and effectively act as a seal between the engine piston rings and bore consequently indicating compression is good which may not be the case, (unless you have a damaged piston). However I do rely on a leak down test to test valves and guides with a fair degree of confidence. Oil leakage through valve guides into the combustion chamber will also cause smoke and most mechanics will presume it’s once again the turbocharger at fault.

12. Too much or not enough oil pressure / flow:

12. Too much or not enough oil pressure / flow:

Not very common and occurs mainly due to lack of maintenance in the case of a sticky oil pressure relief valve or as a result of high pressure oil pumps in race applications. More pressure, more volume; too much oil will flood the bearing housing as it can't drain quick enough back into the sump and consequently oil will leak from the turbo piston ring seal(s). Obviously removal and cleaning or replacing the oil pump relief valve should solve that problem. Conversely not enough oil pressure will cause a turbo to self destruct rather than leak. Oil pressure can be lost elsewhere in an engine for any number of reasons causing low oil pressure at the turbo as well. Too test whether there is too much or not enough oil flow follow the directions in bold below.

In the case of the race engine where the owner requires higher oil pressure for other reasons a restrictor can be placed in the line to the turbo however this must be done with much care as not enough oil will obviously cause the turbo to fail in a major way. In order to fit a restrictor you first need to source the required information; "the flow per minute of oil for the turbocharger from the turbo manufacturer and measure this amount out of the turbo oil drain pipe". Start with larger inside diameter restrictors and gradually decrease in size until you find the correct size restrictor that meets the turbo manufacturers specifications at the correct given engine rpm.

If oil pressure needs to be checked it's best done at the oil entry to the turbocharger and will require a fitting to be made for the oil pressure gauge which will vary pending the brand of turbo.

Turbo Oil Pressure and Flow Test

13: Other possibilities:

13: Other possibilities:

Some vehicles have their own quirks from the manufacturer such as the oil drain pipes being located only a smidgen above the engine oil level as mentioned earlier.

Another which caught out a mechanic mate of mine and a very good mechanic at that, his son has won Bathurst a few times, was on a Volvo which had an oil strainer inside the sump on the oil drain which was blocked; I rebuilt the turbo twice and finally, yet reluctantly gave him a new turbo as I couldn't find any problem with the original after which the actual fault in the sump was finally found.

On a RB30ET engine you have to be careful the moulded bend in the oil drain hose is not kinked and the bend is above the oil return hole in the sump otherwise it will puddle in the bend of the hose and cause the turbo to leak.

Hardened valve stem seals, fairly common with older vehicles, will allow oil into the combustion chamber causing smoke and causing the mechanic to think it is the turbo only to find removal and repair of the turbo didn’t solve the problem. Some cylinder head experts reuse the old valve stem seals and as a consequence they prematurely harden or even pop off.

I've had the wrong dipstick fitted to one vehicle causing it to be always over filled with oil hence measuring the amount of oil when draining it from a vehicle is a good practise before removing the turbocharger.

A Mazda was towed to our premises from a rotary specialist, the vehicle was smoking at idle after an engine and turbo rebuild, we found the oil squirter lever for the pistons cable tied & feeding too much oil causing the smoke.

Another rotary specialist, another turbo rebuild causing smoke. After removal of the oil drain we found the oil splash tray inside the engine pushed up hard against the oil drain hole, we bent the splash tray back and problem solved.

Same customer above fitted a new T4 turbo to a rotary, it started leaking oil due to high oil pressure, the factory Hitachi turbo oil feed pipe has a built in oil restrictor to compensate for high oil pressure which the Garrett T4 retrofit doesn't have.

Yet another RB30ET was towed in after a turbo refit and we found the diaphragm on the automatic transmission was leaking oil which was attached to the inlet manifold via a very long pipe; when the inlet manifold saw vacuum, transmission fluid would leak upstream into it causing smoke.

A Pulsar was towed in this time not for smoke but it wasn't making any boost, the Nissan specialist mechanic replaced the air filter but didn't remove the zip pull cardboard packaging from the bottom side of it, no air = no boost.

I sold a new turbo to a workshop who retrofitted it to a Holden 202; they drove the vehicle around to our premises because of smoke bellowing out the exhaust. I popped the bonnet and found the turbo fitted in a vertical launch position similar to a space shuttle prior to take-off at NASA. Oil was leaking straight out the rear seal into the exhaust.

Another Holden 202, carburetted, used for speedway with a new turbo, new engine and a high pressure oil pump fitted. Since it was running very rich I initially thought it maybe blow-by causing the problem, (polished bore from too much fuel), so I advised him to disconnect the oil drain & drain the oil into buckets as per test # 9 above. He rang back and said that didn't help, he also said he found it easier to run a hose from the oil drain uphill into the valve cover oil filler. I then explained why an apple falls out of a tree and oil won't flow up hill. It turned out to be both, glazed bore and too much oil pressure.

I wish I had a dollar for every time I was told ”I just rebuilt the engine it can’t be blow-by” as I’m looking at smoke from the removed oil filler cap, (See Test 2), hovering around the ceiling of the building and an air cleaner assembly full of oil from the breather hose.

Patched up used cars, I’m not even going to go there as I won’t live long enough to finish this paragraph if I do!

The list goes on and on, too many to mention and the possibilities probably infinite. I could certainly embarrass some well know performance mechanics and probably should as prior to having a vehicle towed to me for diagnosis I would usually receive a scathing phone call by the mechanic who has assured me it’s definitely the turbocharger as everything else has been thoroughly checked and in return rarely even a thank you when the actual fault was diagnosed, fixed and towed back.

However I'm human as well and not impervious to err, “people who live in glass houses ...” Anyway I'm drifting off topic a tad as I reminisce whist reaching for an indigestion tablet when the main point of this long paragraph is to try and encourage you to think outside the box rather than jump to the obvious conclusion being, "the turbo is leaking oil so it must be the turbo" as mentioned somewhere above. In addition, these days you can search the internet also for any paranormal activities common to a specific vehicle which hopefully will solve your problem before removing the turbocharger a second time.

14: The Turbocharger:

14: The Turbocharger:

Although extremely unlikely it is possible that the turbocharger wasn't properly micro-metered when it was rebuilt. The piston ring seals, centre housing seal hub, turbine seal hub, thrust assembly, end float and radial play etcetera all have minimum and maximum tolerances which require measuring which is standard procedure when rebuilding a turbocharger. I personally balance and micro-meter every turbocharger before assembly and have done so for many years and guarantee no turbocharger leaves our premises with incorrect specifications or is out of balance.

In addition to this I have the necessary equipment to test and double check a core as an assembled unit as added insurance.


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