Boost Gauge: Essential for Every TDI Engine (Articles)

TDI engines rely on turbocharger pressure.
In the event of a failure of the turbocharging system, one can roughly expect the performance to be similar to that of an SDI engine – possibly even less, because the long intake passages of the TDI engine (including the air filter and, potentially, a non-driven turbocharger acting as a flow restriction) can worsen the gas exchange compared to an SDI, and therefore the fuel injection quantity of a non-turbocharged TDI will likely be even lower than that of an SDI.

Given the somewhat complex design of the turbocharger pressure control system, with its turbocharger, solenoid valve, engine control unit (ECU), and various hoses and cables (and based on the experiences of countless TDI drivers), it seems almost naive to expect that the system will function flawlessly for the entire lifespan of a car.
In forums dedicated to TDI issues, you often come across descriptions of power loss, the causes of which are frequently related to the turbocharger pressure regulation system.

If the engine suddenly loses power while driving, there are initially several possible causes.
In the worst-case scenario, the turbocharger may already have broken down, because most (1.9-liter) TDIs only switch to limp mode if the target boost pressure is exceeded by more than 300 mbar for more than about 6 seconds continuously! For example, if the variable turbine geometry (VTG) gets stuck in the "closed" position during a full-throttle acceleration to over 4000 rpm in 2nd gear, the engine control unit (ECU) initially doesn't care how high the boost pressure rises: limp mode only activates after the aforementioned approximately 6 seconds. It's hard to imagine that the turbocharger can withstand that for so long...
With a boost pressure gauge, you at least have the chance to catch a glimpse of the boost pressure "going beyond the limit" and to ease off the accelerator pedal in time.

However, even with less dramatic defects resulting in a loss of performance, an LDA (Lambda-regulated air mass control) can provide a partial diagnosis at a glance: If the boost pressure is within the normal range, the cause of the problem is almost always to be found elsewhere, for example, in the mass airflow sensor (MAF).

Even this simple DIY quick check is not possible for the average TDI driver, because no TDI comes with an LDA from the factory.
And so, standard TDIs that are lacking power may already need to be taken to a workshop, even if it's just for a simple check of the turbocharger pressure.
If the workshop has the necessary expertise and equipment, a diagnostic tool will be connected, and during a test drive, there is hope that the fault will reappear.
If the error is kind enough to reappear, the measurement values must be saved precisely at that moment and then analyzed before a conclusion can be drawn about whether the boost pressure control is the cause.
Interestingly, the information provided by standard diagnostic devices, despite all the high-tech involved, is often less informative than that provided by a simple LDA (Lambda Digital Analyzer). The LDA displays the boost pressure over practically any time period, allowing for analysis of control processes, etc., while standard diagnostic devices only provide individual "data snapshots" and, furthermore, even rapid changes like overshoots can be partially electronically filtered out.

While logging measurement data with the versatile VAGCOM tool allows for the detection of temporal sequences, even then, a targeted diagnosis still relies on the occurrence of the error itself.
Measurements of sporadic errors are only likely to be captured in extremely rare circumstances – unless you have a laptop constantly connected and passengers who are willing and able to continuously log the presumably relevant data blocks until the error, despite being "trapped," reveals itself again.

Furthermore, reading measurement values via the OBD port requires a significant amount of electronics between the engine and the display.
A mechanical LDA, on the other hand, is almost Stone Age primitive and therefore, in case of doubt, more reliable, requiring no additional effort or costs after installation , and it is constantly in operation . And if the car is acting up, a quick glance from the corner of your eye is often enough for an initial partial diagnosis.

But wait, there's a fault memory, and its contents should be at least as informative as meticulous observation of an LDA (Local Differential Analyzer), even after just a single instance of performance loss!
In a theoretical ideal scenario, yes, but in practice, no, because most error detection thresholds are set very generously to prevent the memory from constantly being filled, even in vehicles with high mileage and corresponding signs of aging and wear on various components.

Therefore, the error memory only contains a useful indication, such as "Intake manifold pressure control differential" or "Forced induction pressure control limit exceeded," in a few cases of (smooth) power loss. This typically occurs when the engine control unit (ECU) has switched to a reduced-power emergency mode due to a consistently and significantly excessive boost pressure.
Otherwise, in cases of insufficient performance – including situations with low boost pressure – the error memory usually doesn't contain any useful entries; unless a simple electrical defect is detected, such as a disconnected connector or a short circuit at the boost pressure solenoid valve.
Furthermore, no measured values are stored in the error memory. Therefore, based on an error entry, it is not possible to determine the "severity" of the problem, i.e., whether the measured value is only just within the range defined as an error, or whether the relevant subsystem has completely failed.

So, it remains the case: As long as defects are not immediately apparent upon opening the hood (while, of course, one must not be blind ), most cases of power loss can only be attributed to the areas "charging system" or "other" through extensive reading of measured values during a test drive, without using an on-board diagnostics (OBD) system.

Even if the TDI owner is lucky, meaning the workshop promptly fixes everything and the problem even cooperates, the costs for troubleshooting, just to determine if the boost pressure is correct, often already reach a level that, in retrospect, would have been at least equal to the purchase price of a turbocharger.

Normally, the "workshop effect" will inevitably kick in.
As long as a diagnostic device is connected, the car runs perfectly, but in everyday use, one problem follows another.
It's possible that you might have to take the car to the workshop x times, and each time you'll pay for a fruitless troubleshooting process – costs that could often be avoided if an on-board diagnostic (OBD) system were available.

Sometimes, in such situations, a new turbocharger is installed preventatively (which can be quite expensive), of course, without necessarily fixing the underlying problem .

Occasionally, boost pressure problems that stubbornly resist any conceivable repair to the boost system (and, of course, incur significant costs) can be traced back to the often-overlooked aspects of the "boost pressure" control loop: the pressure sensor itself, as well as its electrical connections and hose connections.
If the sensor provides erratic readings, the engine control unit (ECU) cannot properly regulate the boost pressure. When reading the boost pressure, you will also see the nonsensical measured values displayed, overlaid by the desperate control attempts of the ECU.
An LDA (Lambda-Sonde) can also provide clarity here: if the displayed value and the read pressure (at least temporarily) are noticeably far apart, the sensor range must first be corrected before considering conventional repairs - which are often no longer necessary .


Nevertheless, in its wisdom, the VAG group deemed it unnecessary to equip TDIs with an LDA.
But why ??


The official answer would likely be that the turbocharging system is so well-developed that it doesn't require constant monitoring by the driver, who can instead focus on the traffic.
Here, one might ask why, for example, a full-fledged engine thermometer is installed to monitor the coolant temperature, instead of a simple warning light, even though, based on general experience, the cooling system in TDIs is significantly more reliable than the turbocharger pressure regulation system.

According to the author, the main reasons for not including LDA as standard equipment are:
-> Production cost savings of a few euros per vehicle.
-> Avoiding potential negative reviews due to "overly complex" instrumentation or confusion caused by the driver who is already stressed by traffic.
-> The deliberate design that completely shields the engine compartment from the driver's view is intended to stifle any DIY aspirations from the outset, forcing drivers to take their cars to workshops for even the smallest issues. This is because workshops are perceived to possess the necessary expertise (which appears inaccessible to the driver), and knowledge is power.
(Some workshops don't know much - that's okay... )
-> Given this subtle position of power held by the workshop and the customer's own helplessness, the average TDI driver should gratefully and willingly accept any information provided by the workshop - that's how you easily please customers .
-> This gives workshops complete freedom in deciding on the repair strategy: Depending on financial constraints or greed, and based on the estimated level of understanding of the customer, unnecessary work can be performed first. With "more valuable" customers, the workshop can choose to address the identified problem directly.
"Minor fluctuations in boost pressure and unusual, seemingly unpredictable behavior such as overboost could be detected by the driver via the LDA system, leading to annoying and often unprofitable customer inquiries for dealerships and workshops."
-> Workshops should have the option to charge for even a simple reading of the boost pressure - although, of course, using a high-tech diagnostic device makes a much greater impression (and is more likely to loosen the purse strings of the average customer ), than if the mechanic only glances at the LDA (Low-Pressure Sensor) while driving.

The likely consequence is that every TDI driver with even the slightest technical interest will install or have installed an LDA in their vehicle .

A general rule of thumb for the required pressure range of the display:
Up to the standard power output of 130 horsepower (without turbocharger tuning), a boost pressure of 1.5 bar is sufficient. Above that, a pressure of approximately 1.8 to 2.0 bar is more appropriate to clearly read any overboost conditions during turbocharger adjustment.

When selecting the instrument further, the suction range displayed initially seems illogical, because a diesel engine typically doesn't have a throttle valve.
However, even in TDI engines, blockages in the intake manifold can occur, and early detection of these blockages can explain a lack of power and, in some cases, even prevent expensive damage.

Why?

The speed of the turbocharger increases with the ratio between the exhaust pressure and the intake pressure.
In high-altitude environments with low atmospheric pressure, the turbocharger would therefore over-rev if it were to deliver the usual full boost pressure (which is measured as an absolute value by the engine control unit).
To prevent this, the MSG monitors not only the boost pressure but also the ambient pressure and, if necessary, reduces the target boost pressure.
Up to moderate altitudes, the full output pressure is required from the loader (when operating at full load).

If there are blockages on the intake side of the turbocharger (due to sucked-in leaves, completely dirty air filters, etc.), the turbocharger intake pressure can drop far below the ambient pressure while the engine is running. The engine control unit (ECU) doesn't detect this because it measures the ambient pressure inside its own housing instead of at the turbocharger inlet. Despite this "air starvation," the ECU controls the turbocharger in such a way that it delivers or attempts to deliver the usual boost pressure.
The charger can become overloaded and fail .
If the impeller shaft breaks, the engine can subsequently suck oil from the turbocharger supply and burn it. This can cause the engine to rapidly accelerate, potentially leading to catastrophic failure such as a connecting rod breaking or valve damage, without responding to throttle reduction or ignition shut-off.
Therefore, a major engine failure is potentially possible due to just a few leaves being sucked in - not to mention the hazard potential of a continuously running engine .
. . . therefore, a very meaningful system design, which has probably already helped several workshops to obtain profitable turbocharger replacement orders. And the best part: a normal TDI driver has practically no chance of recognizing this design flaw, for which he ultimately pays .

In a carburetor with a venturi, blockages in the intake area will reveal themselves through an "unusual vacuum" during acceleration and/or idle.
If the driver notices this sign in time, correctly interprets it, and as a consequence, drives cautiously with reduced turbo pressure until the obstruction is cleared, he can reliably prevent damage to the turbocharger.
Without a suction area on the LDA, he would likely be more concerned about the slow pressure build-up and might even try a few full-load tests to investigate the problem further -> possible consequences, see above.

In certain TDIs, an LDA with a suction range can also enable monitoring of the non-return valve - when the pressure behind the valve is tapped.
When the engine is switched off, the pressure behind the closed valve temporarily drops significantly into the suction range. If this drop occurs, the valve will not close properly.
If a vacuum or low pressure is constantly indicated at idle or under light load, another potential cause for a performance-reducing restriction could be an incomplete opening of the bypass valve.

The Euro 4 TDI generation actually has a throttle valve that allows for more precise control of the EGR rate, and can be set to a higher level compared to just using an EGR valve alone.
In these vehicles, the flap is constantly being moved within a varying load range. If it is closed too much, a significant vacuum may occur.
The potential resulting engine roughness is difficult to diagnose without a vacuum range indicator - with a vacuum-based LDA, a simple glance is sufficient .


Where is the pressure tapped off for the LDA?

In older TDIs with a turbocharger pressure sensor located in the engine control unit (ECU) or as a separate component (1Z, AFN, and others), a small hose runs to it, branching off somewhere between the intercooler and the intake manifold from the intake pipe. This hose can be easily tapped into using a T-connector for the LDA (Lambda Probe Adapter).
This generation of devices usually doesn't have a bypass valve that could be monitored by LDA.

In newer engines (such as ALH, ASV, and PD engines, etc.), the pressure sensor is directly integrated into the intake manifold, so they no longer have a factory-installed hose connection to the intake manifold.
At least their bypass/AGR unit offers a slightly more complex way of tapping into it via one of the unused threaded ports on the side. These ports are conveniently located behind the bypass valve, meaning that tapping into them also allows for the monitoring of the valve itself.

To do this, the through-hole must be extended to reach the suction channel.
To avoid damaging the threads while drilling, try inserting a piece of cable insulation or a very thin piece of tubing into the threaded hole and drilling through it.
Collect the drilling chips in the suction pipe either during the work or remove them thoroughly afterwards; removing the AGR unit may be helpful in this process.
Then, drill through a suitable screw or threaded piece (usually M5) lengthwise and make the outer surface suitable for connecting a hose.
Then, screw the screw into the drilled-through hole.
Tip: Applying a small amount of thread locker to the outer threads (instead of a sealing washer) will secure the screw and provide a perfect seal at the same time.
-> See attachment. The nozzle was soldered together from an old spark plug connector (the type that screws onto the high-voltage terminal), a brass threaded piece with a hole drilled through it lengthwise, and a brass nut.

Other compatible parts are also available for purchase, see below.
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Flickering LDA pointer - what to do?

Pressure pulsations in the intake manifold can travel through the hose and reach the LDA (Lambda Probe), potentially causing its needle to swing wildly and audibly, especially if the connection point is close to the EGR valve, the hose to the LDA is relatively short and thick, and the LDA is an undamped model (instruments with damping have small amounts of silicone grease in the pointer mechanism, but even this does not provide complete protection against swinging needles under strong pulsations).

As a DIY solution, combinations of constrictions (narrowing sections) in the LDA tube, along with damping volumes located behind them on the LDA side, can be used. This helps to filter out rapid pressure fluctuations.

The damping volume function can already be fulfilled by the hose section between the throttle and the LDA. The damping characteristics are fundamentally determined by the ratio of the flow resistance of the (intake manifold side) throttle and the size of the damping volume.
If the system is excessively damped, not only will pointer jitter be filtered out, but rapid pressure fluctuations (e.g., during overshoots) will also be either only indicated or even completely suppressed.

In long and thin LDA probes (typically used with rear-mounted engines), pulsations can often be reduced to such an extent without special throttling that the LDA displays a completely stable reading.

Simple universal fuel line filters, installed just before the LDA (Low-Pressure Fuel Accumulator), can also effectively stabilize fluctuating readings.
Regarding the principle of operation, a role reversal occurs: Compared to the relatively large internal volume of the filter, the normal LDA hose between the intake manifold and the filter acts as a throttle (instead of acting as a damping volume, as is usually the case).

To optimally dampen the LDA (Lambda Dynamic Airflow) sensor so that the reading is stable without excessive oscillation, while still responding quickly to changes in boost pressure, one should consider using a restrictor sleeve placed at the intake-side end of the LDA hose.

Advantage of the principle: In long tubes, not only wall friction acts as damping, but also the inertia of the air column, whose direction of movement must be repeatedly reversed for each pressure fluctuation transmitted to the LDA.
(This creates an acoustic low-pass filter with an attenuation of 12 dB per octave, whose cutoff frequency can be calculated using the formula for a Helmholtz resonator.)
The length of the baffle tube allows for very precise adjustment of the cutoff frequency and damping characteristics.

To create a throttle, for example, a hollow tube, a piece of cable insulation, or something similar can be inserted into the hose leading to the LDA (Laser Doppler Anemometer), and the hose can be constricted with wire or a similar material to fix the throttle in place from the inside.
Instead of crimping the LDA hose, you can also slide a short piece of tubing of the appropriate thickness tightly onto the throttle (or several pieces stacked on top of each other, possibly warming them up and/or lubricating them slightly) so that the whole thing fits tightly into the hose leading to the LDA -> see attachment.
If the throttle body has sufficient strength, it can also be used as a connection between the intake manifold and the actual LDA hose.

When sizing the throttle at the engine-side end of the LDA hose, you can initially use the following rules of thumb:
-> Inner diameter approximately 1 mm, but no more than 1/3 of the LDA tube diameter.
-> Neck length approximately 10 to 15 cm for a 1-meter LDA tube with a 4 mm inner diameter.
-> The required length of the restrictor changes inversely proportionally to the length and inner diameter of the LDA tube; that is, for a 2-meter tube for the LDA, a restrictor of only about 6-7 cm is sufficient.
The theoretical cutoff frequency for such throttle-choke combinations is approximately 40 Hz, which corresponds to 1200 rpm in a 4-cylinder engine. However, the frictional resistance of the air within the choke already has a damping effect at lower engine speeds.

If the LDA is overdamped with this configuration, the throttling is reduced in steps of approximately 10% until the LDA responds almost as quickly as it did without damping, but the pointer still does not oscillate.

The effect of foam pieces and similar materials placed inside the LDA tube can only be dosed relatively coarsely compared to tube restrictors, which means that achieving satisfactory damping of an LDA (without excessive damping) using this method usually takes significantly longer.
Furthermore, they offer poorer damping of rapid pressure fluctuations compared to long tubes.


Regarding the selection of the installation location for the display and the details of the hose routing to it (avoid kinks!), no specific instructions can be provided within this context -> improvisation is key.
You can find ideas here:
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Electronic alternative for vehicles with MFA:
http://shop.dieselschrauber.de/polarfis-advanced-mfa-p-323.php