Trouble

When investigating running problems on a turbocharged engine, you need to remember that there are two categories of problems that can arise. The first category includes those types or problems that can happen to any engine, whether turbocharged or not. Turbo engines can still have problems with spark plugs, plug wires, coils, ignition control boxes, EFI computers, timing chains, water pumps, fan belts, alternators, throwout bearings, cam bearings, and . . the picture is obvious. With regard to these problems, a turbo engine is no different from a normally aspirated engine. Today's attitude toward service and repair of the turbocharged performance car generally leads to the somewhat ridiculous/comical response of, ''Whatever the problem, it's that damn turbo's fault.' Fixes for general engine problems can be sought elsewhere and are not within the scope of this book.

The second category is the malfunction of a component in the turbocharger system, or a problem caused by a malfunctioning turbo system, This chapter offers a guide to isolating and recognizing these problems. Also, at the end of the chapter, you'll find a troubleshooting guide which offers a lot of information. Study it carefully and eliminate the simple things first.

Inspecting the Engine for Turbo-Induced Damage

When you encounter any problem that even remotely hints at possible engine damage, it is best to check it out pronto. Get proof that the engine is undamaged, or focus on fixing it. Worrisome signs are rough running at idle, loss of power, or bluish-gray or white smoke issuing from the tailpipe. Excessive puffing of oil vapor from the valve cover or crankcase breather is also cause for concern.

Fig. 15-1. Worst case scenarios are never pretty, and the fallout from a malfunctioning turbo system is no exception.

The proper method of checkout is a leakdown test, which indicates the condition of individual compression rings, intake and exhaust valves, and the head gasket, and the presence of cracks in the block or cylinder head. This is done by pressurising the combustion chamber and observing the amount of leakage and where the leaks are. The amount of leakage is measured by regulating the compressed air going into the chamber to a convenient number. One hundred psi is the most useful pressure, as the pressure remaining in the chamber is the percentage seal of the chamber. The location of leaks can be determined by listening at the tailpipe for exhaust valve leaks, at the air filter for intake valve leaks, and through the oil filler cap for blow-by past the rings. Damage to the head gasket or cracks that intersect the water jacket will show up as bubbles in the cooling system,

The leakdown must be done on a warm engine, with both valves closed and the piston at top dead center. Judgment of the measured numbers is somewhere in this area:

Very good

Serviceable

OK but impaired

88 or less  Fix it

The leakdown check is superior to the old compression check in a variety of ways. The condition of the battery and starter motor don't matter. Valve lash variance won't matter. Cam timing doesn't count.

Fig. 15-2. The leakdown check is the most sophisticated test yet devised for determining the integrity of the combustion chamber. The regulator controls pressure to the cylinder. Gauge 1 indicates that pressure. Gauge 2 indicates pressure remaining in the cylinder after all leakage. With the source regulated to 100 psi, gauge 2 reads the percentage seal of the chamber.

Cylinder head gasket sealing can easily be checked by a chemical process that identifies traces of exhaust gas products that find their way to the coolant. Check the parts store for the product.

The area around the combustion chamber is just about the limit for turbo-induced engine damage. It is extremely unlikely that any other damage can be even remotely related to the turbo.

Fig. 15-3. The air/fuel ratio meter is an indispensable testing and troubleshooting tool

Inspecting the Turbo System for Malfunction

Will not start. The turbo can cause a starting problem only if the problem is related to an air leak in the system. This is even limited to EFI cars equipped with an air-mass flow sensor and to draw-through carb systems. An air leak in the presence of a mass flow sensor will rob the sensor of some of its signal, creating a lean condition on start-up. It's a similar deal for a draw- through carbureted system. Frequently the flowmeter is responsible for turning the fuel pump on. Thus, a large leak can often appear as a fuel pump failure. A speed density EFI, which employs no air-mass sensor, cannot fail to start due to a turbo problem, as air leaks are of no consequence. A draw-through carb system can have one additional problem: trying to get a rich cold-start mixture through a mass of cold metal. Not too bad in Yuma in August, but Duluth in December will rule out going anywhere. This is not a turbo problem, but a design problemreason enough not to build a draw-through piece in the first place.

Finding a vacuum leak is a standard troubleshooting procedure. The same technique applies when a turbo is present, except for leakage upstream of the throttle. Leaks upstream must be huge to affect starting. Look for disconnected hoses, big cracks in hoses, tubes dislodged, and items of that magnitude.

Poor idle quality. Less significant leaks than those generally associated with hard starting can upset idle quality. Idle air/fuel ratio will always be a critical adjustment. Consult the proper instruments and adjust accordingly. These leaks will likely be downstream of the throttle.

Misfires. The turbo can create two conditions in which the engine will misfire: a lean condition and a requirement for higher voltage to spark off the denser mixture in the combustion chamber. The turbo can occasionally cause an EFI-equipped car to suffer a lean spot at or near atmospheric pressure in the intake manifold. This is brought about by the fact that the turbo will actually be pumping pressure up from, say, 15 inches of vacuum to maybe 10 inches. To keep the vehicle from accelerating, the throttle position must be reduced slightly, thus reducing the throttle position sensor's signal to the EFI computer. This reduced signal will slow down the fuel flow for any given airflow, producing a lean condition.

Any misfires at full throttle induced by a lean condition are serious and must be dealt with prior to operating at that boost level again. A lack of fuel raises

chamber temperatures dramatically. Heat is the cause of detonation, which is the nemesis of high performance. Don't be too slow to fix any lean conditions.

Lean running conditions can easily be detected by some of the portable oxygen sensors. A requirement for increased voltage to the spark plugs is some-times encountered and is due to the fact that the air/fuel mixture in the combustion chamber is actually an electrical resistor. The more air and fuel pumped into the chamber by the turbo, the greater the resistance; hence, the greater the voltage required to drive the spark across the plug gap. This problem is readily helped or cured by adding voltage to the system and/or by installing new spark plugs.

Power loss. Troubleshooting power loss should be centered around inspection and optimization of boost pressure, ignition timing, air/fuel ratio, throttle opening, and tailpipe back pressure. Except for throttle angle, which is self-explanatory, these items are all covered elsewhere in this chapter.

Excessive boost pressure. Over boost is worrisome. Since the wastegate is charged with boost-control responsibility, it is certainly the first item to investigate when overboost is encountered. Several facets of the wastegate are subject to failure:

Signal line. The wastegate can malfunction if it fails to receive a proper sig-naL The signal line can get clogged, or it can develop a leak. Check out both possibilities. Also, check the fittings at both ends of the signal line.

Actuator. Virtually the only part of an actuator subject to failure is the internal diaphragm. On an integral actuator, the simplest test is to blow into the signal port, The signal port should be a complete dead end. Any sign of leakage is evidence of the problem and requires replacing the actuator This same test can be used on remote wastegates, except that pressure must be applied to the atmospheric side of the diaphragm. The valve side of the diaphragm is almost always designed for a small amount of leakage around the valve stem; thus, testing from the valve side will measure stem guide leak as well as the damaged diaphragm.

Valve. The wastegate valve can become jammed and refuse to open, or become otherwise dislodged. This requires removal and disassembly of the wastegate valve mechanism to determine the cause and the fix.

Flow. The owner of a homemade turbo system must know that the flow capability of the wastegate is up to the demands. This can also afflict kit makers on occasion. Matching these flow requirements is a design problem, not a troubleshooting problem. If all else checks out and the wastegate strokes properly when given a pressure signal, investigate its size relative to the application.

Tailpipe. The tailpipe can frequently cause an overboost problem. Often, the wastegate depends on an increment, of back pressure in the tailpipe to function properly. This is particularly true with integral wastegates. The problem can be further aggravated by the OEM's tendency to use smaller-than-reasonable turbos. These factors can combine to cause overboost when something in the pipe fails and reduces back pressure. Wouldn't it be fun to have a rust hole in the muffler of your expensive turbo car cause an overboost problem that leads to engine failure? No wonder Yankees park their performance cars in winter. It could be argued that some ought to park them regardless.

Exhaust housing. If a homemade or aftermarket turbo system exhibits over-boost but the tailpipe and wastegate are known to be in order, turbine speed may be too high for overall engine/turbo conditions. This means that the turbo exhaust housing is too small, thus overspeeding the turbine and making too much boost. The answer is to increase the A/R ratio of the exhaust housing, slowing the turbine, which in turn reduces the tendency to overboost.

Low or Sluggish Boost

Turbo. Several aspects of the turbo can cause low or sluggish boost response. Most of the causes are applicable to either a misbehaving new setup or an old system with a new problem.

Size. If the turbo is too big, certainly the response will be sluggish. It is possible to get the turbo so large that it does not produce any boost at all, because exhaust gas from the engine is insufficient to power it. Although this is highly unlikely, it is almost equally unlikely that the optimum size turbo was selected on the first try. The fix is generally to reduce the size of the exhaust housing.

Exhaust leaks. Large exhaust gas leaks before the turbine can contribute to sluggish response. Leaks this large will not only be audible, they will be obnoxious. Unless a hole is found that you can stick a pencil through, don't expect the exhaust leak to fix a response problem.

Compressor nut. The compressor retainer nut, if loose, will allow the shaft to spin inside the compressor wheel Access to the turbine wheel is necessary to anchor the shaft while tightening the compressor retainer nut. These nuts are generally tightened to about 25 in.-lb of torque. This can be approximated by tightening the nut until it touches the compressor wheel and then an additional quarter turn. When tightening a compressor nut, it is important not to permit any side load to reach the turbine shaft. This eliminates the possibility of bending the shaft with the torque wrench.

No air filter. Damage to a compressor wheel can reduce boost. Operating without an air filter will eventually cause the compressor wheel to erode to the point that it can no longer pump air. When the eroding process is occurring, the compressor wheel will lose its efficiency, causing the air temperature to rise, which in turn can lead to detonation problems.

Wastegate. A mechanical problem that keeps the wastegate from closing properly will create a large exhaust leak around the turbo, producing sluggish low-speed response.

Fig. 15-4. One way to tighten the compressor retaining nut. Using a T handle will eliminate bending loads in the turbine shaft.

A failed wastegate valve will seldom keep the turbo from producing about the normal amount of boost, but it will take a lot more revs to reach that normal amount. If, for example, the wastegate valve seizes at the position it reaches to control maximum boost, the system must produce enough revs just to overcome the leak before producing any boost.

Tailpipe. Any failure in the tailpipe that creates a blockage for the exhaust gases will tend to produce a higher boost threshold and/or less maximum boost. Check the pressure in the pipe upstream of any possible blockage. In general, back pressure greater than 10 psi will cause almost a complete loss of boost. Back pressure greater than 2 psi is undesirable under any circumstances, even if not of a magnitude to cause loss of absolute boost pressure.

Air filter. An air filter that is too small or too dirty will keep the system from functioning up to expectations. This condition will also create the bad side-effect of raising intake temperature.

Compressor inlet hoses. Almost always, the air filter or airflow meter will be connected to the turbo compressor inlet by flexible hose of some sort. If the filter or flowmeter is restrictive, it is possible for the vacuum thus created to collapse the connecting hoses. Usually the symptom of collapsing hoses is a sudden loss of all boost. The forces on large hoses from small pressure differences can be deceptively large.

Misfires. Any misfire while under boost will be caused by a failure to ignite the mixture or by an air/fuel mixture too lean to burn. Failure to ignite the mixture can be a bad plug, wire, coil, or all those stack ignition problems. If the ignition checks out properly, then the problem will be found with the air/fuel ratio.

Bogging. A distinct type of full-throttle malfunction is an overly rich air/ fuel-ratio-induced bog. This is manifested in a loss of power at full throttle, often accompanied by black smoke from the tailpipe.

Another frequent cause of bogging, with similar full-throttle feel, is an over-active ignition retard. A failing knock sensor can induce the same symptoms. A dangerous side effect of retarded ignition is a dramatic rise in exhaust gas temperature. Exhaust manifold and/or turbine damage can result from retarded

timing.

Detonation. The audible metallic pinging sound of detonation is a clear signal that the engine's life is threatened. Every effort must be focused on ridding a system of detonation problems. The wide variety of detonation causes can prove lengthy to troubleshoot, but a turbo engine that pings under boost must be considered a pending serious expense. In general, all detonation problems will stem from one of the six items discussed in the following paragraphs. Their likelihood as the source of the problem is approximately the same as the order in which they are listed.

Octane. A fuel's octane rating is a measure of its resistance to spontaneous combustion, or detonation. The greater the octane, the greater the resistance. Fuel quality is relatively consistent, but it is advisable when quality is suspect to change brands.

Ignition tuning. Improper ignition timing is rarely a system failure but, rather, an adjustment error. A check of both static and maximum advance will virtually always uncover any discrepancy in the ignition system. The knock-sensor-coTitrolled ignition timing retard can be subject to many types of failure, one of which is failure to recognise knock and do something about it.

Should a knock-sensor system failure be suspected, consult the service manual for the unit or, in OEM applications, for the vehicle.

Lean air/fuel ratio. A lean running condition will promote detonation, because a lesser quantity of fuel, when vaporized, will absorb less heat. Thus a lean mixture increases heat, the root cause of detonation. A turbo engine offers the freedom to run slightly richer mixtures than with a normally aspirated engine, permitting the extra fuel to act a bit like a liquid intercooler. Call that an OEM intercooler.

Exhaust gas back pressure. A very small turbine, blockage in the exhaust manifold, or some form of restriction in the tailpipe will cause an increase in the system back pressure. Back pressure keeps the burned, hot gas in the combustion chamber. A failure of any sort that increases back pressure seriously aggravates the detonation characteristics of an engine.

Intercooler. An intercooler strongly affects the detonation threshold of the turbo engine. Anything that conies along and compromises the intercoolers efficiency will lower the detonation threshold. Other than removing the obvious newspaper stuck in front of the intercooler, the only periodic service needed is to clean out the internal oil film that accumulates in normal use. The oil film will noticeably decrease the efficiency of the intercooler

Ambient heat. There are days when nothing works right, and ambient heat certainly contributes to some of these. Higher-boost-pressure turbo systems usually operate somewhere near the detonation threshold and can easily cross over to the dark side when the ambient temperature takes a turn for the worse. David Hobbs, one of the more able and literate racers, once suggested that turbocharged race cars were so sensitive, he could feel a power loss when the sun came out from behind a cloud. Engineering around the seasonal and daily changes of ambient temperature is not within the scope of this book.

And Furthermore . . .

What is detonation, and why is it so destructive?

Detonation is the spontaneous combustion of the air/fuel mixture ahead of the flame frontcombustion by explosion rather than controlled burning. It occurs after the combustion process has started and Is usually located in the area last to burn. As the flame front advances across the chamber, the pressureand thus the temperaturein the remaining unburned mixture rises. If the autoignition temperature is exceeded, this remaining mixture explodes. The audible ping is the explosion's shock wave.

Detonation is extremely destructive. This is a result of temperatures that can reach 18000F in the center of the explosion. The pressure spikes caused by the explosion can reach several thousand psi, and pressure rise is rapid enough to be considered an impact load. These temperatures and pressures are almost ten times higher than those accompanying controlled combustion.

No metals in existence today, no forged pistons, and no special head gaskets can withstand sustained detonation. Virtually nothing can withstand sustained detonation. Consider also that at 6000 rpm, fifty explosions can occur in each combustion chamber in one second. Thus:

** RULE: If you ever hear a ping, you lift your foot.