Understanding Induction Systems on Naturally Aspirated & Turbo-Equipped Engines

You have heard it said before that the internal combustion engine is basically an air pump. The internal combustion engine has never been 100% efficient, but recent technology has improved the engine on a major basis with the addition of intake runner control systems, variable valve timing systems and turbo and turbo-equipped engines.

Before we look at these systems, let’s go back and review the fundamentals. The first rule is to determine what type electronic fuel injection (EFI) system the engine is using. The most popular system is the mass air flow (MAF) equipped engines.

For example, let’s say during a cruise condition the MAF is reporting 56 grams per second on an 8 cylinder engine. The powertrain control module (PCM) would assume that each cylinder on its intake stroke would pull 7 grams per second. This formula would only apply to engines with equal length intake runners sometimes referred to as tuned intake runners on a tuned port engine.

Each intake runner has the same length and inside diameter. A clogged converter on Bank 2 would impede those cylinders’ ability to breathe and would cause negative fuel trim values on that bank, whereas the bank with the good converter will be drawing the major volume of air and its fuel trim numbers will go double digit positive (see Figure 1). 

Some late model GM and Ford systems are using a hybrid type EFI system whereas both a MAF and MAP sensor are used. To check if the engine is breathing properly and the MAF sensor is working properly, we can use the calculated load value on the scan tool. For those of you who live in elevation below 1,000 feet above sea level you should see a minimum 80% calculated load at 5,000 rpm at wide open throttle (WOT) conditions. Another method is to simply multiply the liter size of the engine times 40.

For example, a 4.0 liter engine should pull a minimum 160 grams per second at WOT and 5,000 rpm. When capturing scan data during this test, be sure to lock the transmission in low gear to avoid getting a speeding ticket and by all means use the record mode on the scan tool and keep your eyes on the road. The “times 40” rule applies to engines with fixed valve timing. On engines where both the intake and exhaust cam timing is varied, these engines breathe better, so multiply the liter size of the engine times 50. These formulas will only apply to naturally aspirated engines.

Some auto makers are on the turbo band wagon to improve the volumetric efficiency of the engine and thus increase the horsepower and torque rating of the engine. On these turbo-equipped engines I have taken the liter size of the engines times 80 at 5,000 rpm during WOT conditions. For example a 2.0 liter 4 cylinder engine at WOT conditions at 5,000 rpm will pull upwards of 160 grams per second. Later on we will be monitoring the turbo boost sensor on these engines. 

MAF EFI systems are very sensitive to vacuum leaks and or false air leaks. Positive fuel trim values at idle when manifold vacuum is strongest while normal fuel trim values under road load conditions point to a vacuum leak. Negative fuel trim values at idle coupled with double digit positive fuel trim values under road load conditions point to an air measurement error which usually indicates a bad MAF sensor.

All manufacturers except Chrysler have gone to the MAF-type EFI systems.

The Speed Density-type EFI systems used on Chrysler systems rely on MAP sensor inputs, rpm and throttle angle to calculate the load. Anything that creates low manifold vacuum will shift the MAP values high and create a rich condition and cause negative fuel trim values under road load conditions, such as a restricted exhaust or retarded valve timing. The Speed Density EFI systems have the quickest throttle response while the MAF-type systems have the edge on fuel economy and emission control.

A combination of both type systems is known as hybrid-type EFI systems that use both the MAF sensor combined with a MAP sensor. GM first used this system in 1986 on its 3800 V-6 engine. This type of approach is being used on most late model GM and Ford systems. GM systems use the minimum of 80% calculated load at WOT and 5,000 rpm to determine the engine’s ability to breathe instead of using the MAF grams per second formula we talked about earlier.

GM hybrid EFI systems do not use the MAF input to calculate the load but instead uses the MAP input, rpm and throttle angle to calculate the load. In the mid-1990s, Ford and Chrysler began using Split Intake systems. What this allows is to improve the engine low load performance and enhance the engine breathing ability at higher engine rpms. The first engine we will talk about is the 3.8L and the 4.0L Split Port engines as well as the 5.4L Triton engine.

Split Port/Triton engines

Tuned intake systems have been around for at least 20 years. This means that all intake runners have the same length and inside diameter. In the late 1990s, Ford released engines with dual intake runners on the 3.8L split port engines and the 4.0L engines as well as the 5.4L Triton engine.

During normal low road load conditions a longer intake runner was used. As the air flow entered the open intake valve on the piston’s intake stroke, good manifold vacuum was sustained. When the intake valve closed the air flow abruptly stopped and actually began a bounce back in the opposite direction. If the larger intake runner was at the proper length, by the time the piston came back around to its intake stroke the bounce back of air flow actually increased the engine’s ability to breathe during normal road load conditions.

When the throttle was increased and the engine rpm reached higher rpm the PCM would energize an electric motor that was connected to butterflies that would open and direct air to the short intake runners which reduces intake restriction and increases the engine’s ability to breath at higher rpms (see Figure 2).

Lets’ look at a case study from a 5.4L Ford F-150 with a lack of power complaint. This engine is equipped with an intake manifold runner control system. We all know that a lack of power complaint can be due to lack of enrichment, a clogged exhaust and or retarded valve timing. The technician had checked for good enrichment previously and found it was OK. 

Having prior knowledge that a previous shop had replaced the intake runner control unit, he then took the truck for a test drive and recorded the throttle position sensor (TPS), rpm and intake manifold runner control functions (see Figure 3).

On the left side of the screen the TPS is closed and the engine is at idle. Note that the position of the intake manifold runner control (IMRC) indicates open. Note that when the throttle is commanded open and rpm increases the IMRC goes from open to closed. This appears to be the exact opposite of how the system was supposed to operate. The technician removed the intake manifold to discover the flapper doors were in the open position for the short intake runners when they should have been closed.

A careful inspection revealed that the rods connecting the IMRC motor to the flapper doors were reversed.

Let’s now take a look at data after the repair (see Figure 4). Note that at idle the IMRC is in the closed position. As the throttle is opened and rpm increases, note that the position of the IMRC went from closed to open. The lack of power problem was diagnosed properly. The short intake runners reduce the air intake restriction and thus improve the volumetric efficiency of the engine. There is a feed back signal to the PCM from the IMRC as to the position of flapper doors and likely would have been detected by using the key on engine off (KOEO) self test.

The technician had the advantage that the problem occurred after a previous shop had replaced the IMRC unit (see Figure 5).

Turbo-equipped engines are getting more popular and more powerful. For one example, Honda offers a 1.5L turbo engine that cranks out 174 hp @ 6,000 rpm with 16.5 psi of turbo boost. Twenty-five percent of the vehicles built since 2014 are turbo-equipped engines. The early turbo-equipped engines were plagued with a turbo lag symptom, whereas during low rpm conditions when exhaust flow was low the turbine speed was low resulting in little to no boost.

Recent engineering changes have improved that somewhat. Looking at the innards of a turbo, we have the turbo impellers and the compressor connected by a shaft (see Figure 6).  The exhaust gases spool up the turbo impellers which in turn rev up the compressor. The compressor draws in fresh air and forces pressurized air into the charge air cooler then on into the intake system. This pressurized air generates a lot of heat. The air is pumped into the air charge cooler where it is cooled and now more condensed. Any air leak between the compressor and the air intake will result in an under boost condition and a lack of power complaint. These turbos spin in excess of 100,000 rpm.

An oil pressure supply line keeps the bronze bearings lubricated. A lack of oil pressure to these bearings will destroy the unit. Customers who ignore oil change intervals or use the wrong oil will ultimately pay the price. The American Petroleum Institute (API) and the Society of Automotive Engineers (SAE) have designated turbo-approved oil to prevent the oil from coking up from turbo heat and clogging the oil supply to these critical bearings. GF5 is the new designation of oil that prevents the oil thermal breakdown.

In addition, synthetic-based oil has superior flow characteristics and better protection against thermal breakdown. Oil pressure is known to leak into the compressor chamber causing oil consumption. If a visual inspection on the compressor side of the turbo unit indicates oil or a film of oil this should be conclusive to a worn out turbo unit.

Cheap or clogged air filters create an under boost condition. The wastegates are designed to dump off excessive boost under high rpm conditions. The wastegates are normally spring loaded closed and opened or controlled by boost pressure. Most modern day turbo-equipped engines have a limited boost of around 16 psi. We will be monitoring this boost value with the scan tool later on. Note Figure 7 where the PCM duty cycle controls the boost solenoid.

As the duty cycle command increases, the solenoid bleeds off the boost pressure to the atmosphere. When the max boost pressure is reached, the PCM turns off the boost pressure control solenoid and boost pressure opens the wastegate to dump off boost. The PCM uses inputs such as MAP, IAT, ECT and knock sensor inputs to more accurately control boost pressure.

One of the early complaints on turbo equipped engines was a audible whooshing sound whenever the throttle was released after acceleration. This sound came from the excessive boost pressure that had nowhere to go and be dumped off. The engineers have solved that by incorporating a by pass valve.

In Figure 8 is an example of a 3.5L Ford V-6 Twin Turbo Ecoboost engine. Note that there are separate turbos, one for each bank. The idea here is to improve throttle response while limiting turbo lag. The wastegate control solenoid is duty cycle controlled by the PCM. The higher the duty cycles, the more boost pressure is bled off from the wastegate and boost pressure increases. When the PCM turns off the boost control solenoid, excessive boost pressure overcomes the spring in the wastegate and the wastegate opens dumping off boost pressure.

A two stage Mighty Vac would be needed to ensure the wastegate can hold vacuum and then switching to pressure to confirm that the wastegate opened. There are some common reasons for under boost (note Figure 9). There are also common reasons for over boost (see Figure 10).  

There are 2.0L Ford turbo engines wherein the wastegate is normally open and closed with vacuum (see Figure 11). This system allows for more accurate wastegate control under all engine operating loads. Note that there is an external vacuum pump to supply a vacuum to the wastegate and to the brake booster. Since turbo-equipped engines don’t generate vacuum under boost conditions, many systems with a brake booster will have an external vacuum pump.

The by pass valves are designed to dump off boost pressure once the throttle is released and are normally closed and opened with vacuum. 

Figure 12 shows an intake manifold from a 1.4L GM engine with the vacuum reservoir attached. The vacuum is used to open the by pass valve during decel conditions to dump off boost pressure. The by pass valve solenoid is controlled by the PCM to open and direct vacuum to the by pass valve. This vacuum signal opens the by pass valve and dumps off the boost pressure. These vacuum valves are known to leak thus causing an under boost condition. The PCM electrically monitors the control circuit to the by pass solenoid.

Figure 12A shows a turbo unit from a GM 1.4L engine. The only serviceable unit is the wastegate control solenoid. Figure 13 shows the vacuum control to the by pass valve. Figure 14 shows an exploded view of a by pass valve. The PCM will ground-side control the solenoid to send vacuum to the by pass valve to open it to dump turbo boost.

Let’s look at a normal turbo-equipped engine during WOT acceleration and monitor for good boost (note Figure 15). Note the APP and TPS indicate WOT. The top parameter indicates the amount of boost which indicates 26 psi which is 11 psi above atmospheric. In another example, let’s look at an over-boost condition in Figure 16. The APP is requesting WOT. Note to the left of the cursor we momentarily got WOT from the TPS. Note that the boost pressure sensor indicates 38.5 psi which is 23.5 psi above atmospheric pressure. Note the MAP value reads 12.3 psi above atmosphere. The PCM electrically backed off the throttle from a over-boost condition. The problem here was a faulty boost sensor.

Let’s look at a lack of power complaint on a 2014 Ford 2.0L with a P0299 under-boost code. The first thing to look up is the code set criteria. The PCM must see more than 4 psi difference between the actual pressure and the desired boost pressure for more than 5 seconds. This sounds as if it is a very reliable code. We talked about this engine briefly before.

As you may recall, the wastegate is controlled by vacuum and is spring loaded open and closed by vacuum. The PCM controls the wastegate control solenoid with a ground side controlled duty cycle signal. Refer to Figure 11. Let’s capture some scan data from this problem vehicle. Note the recorded scan data in Figure 17. The APP is requesting 98% throttle.

The MAP is indicating 21 psi or 6 psi above atmospheric. The rpm is at 4,100 rpm. The turbo charger boost sensor reads 21 psi. The desired turbo boost value is 31 psi. There is a 10 psi difference between actual boost pressure and desired boost pressure. The PCM has ramped up the duty cycle command to the wastegate control solenoid in an effort to increase boost. There are no intake air leaks and the air filter is not clogged.

The first thing to suspect is a wastegate that may not be sealing or a worn out turbo. To check the wastegate, the turbo unit must be removed. The wastegate is spring loaded opened and closed with vacuum from the wastegate control solenoid. Applying vacuum to the wastegate with a Mighty Vac unit indicated that the wastegate would not fully close. The wastegate is not serviceable, which means that the entire turbo unit needs to be replaced.

Let’s look at the scan data with the new turbo unit in Figure 18. The APP is requesting 98% throttle. The rpm is at 5,800. The turbo charge boost sensor (TCBP) indicates 27 psi. The desired turbo boost pressure is nearly the same at 26 psi. Note the duty cycle command to the wastegate control solenoid at 58%.

Well, that’s it for this round. In closing, thank you for your commitment to this industry.    ■