Tech Tip: Poor Throttle Response on VW's Diesel Engine
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Tech Tip: Poor Throttle Response on VW’s Diesel Engine

Condition: Poor throttle response on 1.9L TDI equipped vehicle. Models affected include 1999-2003 Golf and Jetta, 2001-’03 Jetta Wagon and 1998-2003 New Beetle (all ALH, 1.9L TDI engines).

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Models affected: 1999-2003 Golf and Jetta, 2001-’03 Jetta Wagon and 1998-2003 New Beetle (all ALH, 1.9L TDI)
Condition: Poor throttle response on 1.9L TDI equipped vehicle.

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Service: Perform steps 1 through 9; if any of these steps reveals a problem, repair or adjust as necessary and perform a road test to determine whether the customer concern has been resolved prior to continuing on to step 10.
All steps should be documented using VAS 5051 / 5052 printouts.

1. Inspect the Engine Control Module (ECM) ground (GND) connection for
corrosion, poor or no connection.

2. Check start of injection pump timing (see Repair Group: Diesel direct injection system, servicing). figure 1

Tip: An adjustment to the upper range is advised.

3. Check the snow screen (if applicable), located below the left front fender in the air duct (see Figure 1, 3), and clean as necessary. Inspect the air cleaner element (Figure 1, 1) for proper installation.
If the air cleaner element is deformed due to improper installation, this may cause the Mass Air-Flow (MAF) sensor to fail. Replace the air cleaner element as necessary.

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4. Check the integrity of all vacuum hoses and connections, and repair as needed.

5. Check the Exhaust Gas Recirculation (EGR) valve and intake manifold air flap – integrated in the EGR valve.
– Remove the intake hose (see Figure 2, 1) at the EGR valve.
– Remove the vacuum hose from the EGR valve, Figure 2, 2.
– Connect the hand vacuum pump (VAG 1390) to the EGR valve, Figure 2, 2.
– Apply vacuum and make sure that the air flap opens and closes freely. See Figure 3, arrow.

If the flap does not open and close freely, replace the EGR valve/flap.

6. Check the correct function of the MAF sensor and EGR valve including the solenoid valve (see Repair Group for procedure and operation ranges).figure 2

Tip: Both systems work closely together. If one system malfunctions, it will directly affect the function of the other. Therefore, it is important to follow the procedure carefully.

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In addition, the MAF sensor must be full-load tested. (See Repair Group 01, On-Board Diagnostic (OBD)).
If MAF sensor readings are out of tolerance (even if no MIL on condition or DTC is present), before replacing any parts (MAF sensor, -G70- or EGR solenoid valve -N18-), ensure the wiring is OK and electrical connections are clean and functioning properly.

If the wiring is OK and the electrical connections are clean and functioning properly, replace the MAF sensor.

If the MAF sensor failed the full-load test, and after replacement of the MAF sensor, full-load value still cannot be attained, check the turbocharger (see appropriate Repair Group).

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7. Inspect and verify that the crankcase ventilation system is functioning to OEM specifications.

8. Check and adjust the engine oil level to proper level if necessary. DO NOT overfill!

9. Check the fuel quality; if the condition occurred suddenly, fuel quality may be inferior. If fuel is suspected to be of poor quality, drain and refill fuel with known-good quality fuel.Figure 3

10. Check for carbon buildup.In most cases, performing steps 1 through 9 will restore throttle response.

In some isolated cases, if all previous items are functioning according to specifications and the vehicle still exhibits poor throttle response, it may be necessary to physically inspect the EGR valve and intake manifold as (in higher mileage vehicles) there may be excessive carbon buildup (greater than 10 mm) in the intake manifold and/or EGR valve.

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Tip: While some carbon buildup is normal and not a concern, excessive carbon buildup (greater than 10 mm) is related to fuel quality and soft driving behavior (operation at low speeds and short driving distances, under extremely cold and damp operating conditions).

To inspect for carbon buildup, remove the EGR valve and inspect and record the depth of carbon in the valve and intake manifold.

If the carbon layer in the EGR valve and/or intake valve is greater than 10 mm (more than 50-60% clogged), replace the EGR valve and/or intake manifold.

If the EGR cooler is clogged more than 50-60% in its flow area, replace the EGR cooler.

If new parts are not available, or if it is more economical, the intake manifold and EGR cooler can be cleaned. Tip: Prior to cleaning, the intake manifold and EGR cooler must be removed from the engine to ensure that no carbon particles enter the engine.

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Courtesy of Mitchell 1.

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