Starting problems and glow plug system issues in TDI engines with VP37/VP44 injection pumps (Articles)

General information about starting problems:

In perfectly functioning TDIs, the starter motor engages and disengages within a maximum of 1 second, whether the engine is cold or warm. Organs start up smoothly without much fuss and run immediately at a stable idle speed.
A faulty glow plug system in TDIs typically only becomes noticeable during starts when the engine temperature is below approximately 5°C (see section 2). Sure, here is the translation of the text from German to English:

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Cold start and/or warm start problems, even with a functioning glow plug system, can essentially have the following causes and can be resolved by experienced DIY enthusiasts as follows.

Air or blockages in the fuel system:
-> Check/repair lines and filters, and if necessary, refuel the car.
When dealing with VP engines during starting, observe the transparent hose leading to the injection pump: Constant air bubbles significantly hinder or even prevent starting! A single, free-floating bubble up to approximately 1 cm in length is not critical.
Specialty 1: A clogged fuel tank ventilation system can create a vacuum between the tank and the fuel pump during operation, which can draw air into the system (at the hose clamps and similar weak points), which in turn can lead to starting problems and/or a lack of power.
Test: Start without the fuel cap. If the engine starts well, then the problem has been identified.
Specialty 2: Air leaks into the system through porous fuel lines between the injectors and the fuel pump, which must then be "bled out" before the next start.
Typical symptom: The longer the engine sits, the longer it takes to start, and the starting problems sometimes vary depending on the incline (slope, gradient, angle) of the parking surface.

Immobilizer active (engine turns off approximately 1 second after starting).
-> Try other authorized keys; if necessary, have the key repaired or properly programmed.

Defects in the engine temperature sensor area:
-> Check with VCDS (formerly VAG-COM), and if necessary, replace it or repair any damaged cables or connectors.

The check valve for the distribution pump is not opening properly.
-> Measure the supply voltage and resistance (approximately 8 ohms). If these are okay, check for internal connection problems, if necessary. Remove imperfections.

Difficulty in operation or electrical faults in the quantity setting device of the VP 37 (often in conjunction with error memory entries):
-> make it common or replace the pump, or alternatively... Repair cables.

Other electrical or electronic defects in the area of the Electronic Control Unit (ECU) or wiring harness, relay 109, etc.:
-> Inspect and replace / repair.

Starting quantity specified is too low:
-> It is calculated based on temperature and may, depending on the software and the tolerances of the temperature sensor, be slightly lower than the actual value, especially during cold or warm start-up. It can be increased using VCDS (VAG-COM).

Low starter motor speed despite a fully charged battery:
-> If the engine starts immediately when jump-starting or rolling, first verschandeln all the corroded contacts between the battery and the starter.
If crimp connections, cable kinks, etc., become hot quickly while playing the organ, replace these cables.
If all the cables are in good condition, replacing the sensor might improve the situation.
However, when the material is quickly pulled or rolled in, a (slightly) higher compaction level is achieved, which can mask poor compression values. Then, of course, a change of personnel will not bring about any significant success.

Weak battery (noticeable by a tired starter motor and, for example, a very dim interior lighting).
"If a current of approximately 0.1 A is measured with an ammeter between a disconnected battery terminal and the battery terminal of a parked vehicle, starting problems may occur after only a few days, and even sooner with higher fault currents. -> Search for and eliminate faults in the vehicle's electrical system."
-> If the charging voltage at the battery is correct (around 14 volts) with the engine running, check the acid density, etc., if necessary. Replace the battery.
-> If the voltage is below 13.5 volts, check the cables between the alternator (LiMa) and the battery for kinks, breaks, corrosion, etc., and repair them if necessary.
-> If the cables are okay, check/repair/replace the alternator, including the regulator and charging control.

Incorrect setting of the VP 37 pump (start of injection):
-> See the relevant article.


Quantity setting mechanism adjusted too lean (only for VP 37).
Using VAG-COM, measure the idle fuel quantity. If it is above approximately 6 mg with a warm engine and no electrical consumers or climate control turned on, and/or if the engine exhibits noticeably weak performance with slight acceleration in the lower RPM and load range, you can try adjusting the fuel mixture slightly richer (up to just before the point where it starts to stumble). Refer to the 'technical article on the fuel mixture control/RWG' and the error database 'DIY solution for stumbling issues with VP 37 engines'.

Engine control malfunction (e.g., crankshaft timing belt pulley loosened, camshaft timing incorrectly adjusted, errors during timing belt replacement), sometimes in conjunction with incorrect pump adjustment:
-> Have it corrected or repaired.

Compression loss / Notch wear:
-> For a preliminary check, see 'Used Car Tips'; consider performing a proper compression test, and carry out repairs as needed based on the results.

Other defects or excessive wear in the area of the pump and injectors; heavy deposits in the injection system due to poor fuel / biodiesel, etc.:
-> Replace or verschandeln the affected parts.

Fuel with a cetane number that is too low, and fuel gelling due to excessively low outside temperatures.
-> to drain / to tap (including...) (pump) and fill up with the "correct" diesel fuel.


In some vehicle models, warm start problems occur frequently, even though the engine starts perfectly well when cold.
The cause is usually a deliberately limited initial injection quantity (due to emission regulations), which is only sufficient for a spontaneous start if the starter motor speed largely corresponds to the vehicle's initial state.
An increase in the amount of warm-up fuel injected at a lower starter motor speed, for example, due to declining battery performance, is not implemented (again, for emissions reasons).
In addition, the wear and tear on the injection hydraulics due to usage causes a problem: the (un-injected) leakage increases, especially at low speeds – i.e., when playing sustained notes.
For emissions reasons, the engine control unit is often configured so that fuel injection is inhibited when the engine is hot and operating below a certain minimum speed. A worn-out starter motor and/or a weak battery can reliably prevent the engine from starting because the required minimum speed cannot be reached.

To confirm the diagnosis, you can try driving the warmed-up engine at approximately 30 km/h in 5th gear. Let the engine idle: in the cases mentioned above, it starts perfectly fine.
The same applies when you simulate a cold engine for the MSG. You can disconnect the water temperature sensor connector (depending on the year of manufacture and vehicle model, it may have a 2-pin or 4-pin connector), but doing so will result in an error code being logged.

As more comfortable solutions, a series resistor can be inserted into the circuit of the EDC sensor (in the case of 4-pin sensors: the one with the higher resistance values) to lower the reported temperature.
This results in a higher starting torque for the engine, which improves its starting performance (see attachment).

However, these kinds of tricks only address the symptoms.
The best approach to eliminating exclusive warm-start problems is to increase the starter speed by...
-> Overhaul of the starter motor (replace worn carbon brushes, renew grease filling, etc.), if necessary. Exchange.
-> Check the alternator charging voltage / Replace the alternator / the voltage regulator.
-> Replacing an aging battery.
-> Check the positive and ground cables between the alternator, battery, and starter motor. Replacing corroded cables: Only flawless cables can fully charge the battery and provide the starter with the maximum starting power!

For VP 37 engines, adjusting the injection start timing close to the upper limit of the basic setting tolerance range can slightly improve the warm-start behavior.


The preheating system in the TDI :

The glow plug system essentially consists of the battery, the glow plug relay, a glow plug fuse, the glow plugs themselves, and the wiring connecting these components. The glow plug relay is controlled by the engine control unit (ECU).

The decision of whether to initiate the preheating process is made by the engine control unit (ECU) based on the engine temperature reported by the coolant temperature sensor at startup. If this sensor provides incorrect readings, it can lead to starting problems, even though the actual glow plug system is functioning correctly.
Normally, the glow plugs only activate when the engine temperature is below approximately 5°C when the ignition is turned on. Additionally, after the engine starts, the glow plugs remain active for a few minutes to improve exhaust emissions and reduce combustion noise (knocking). Afterglow can also occur with higher starting temperatures, even when preheating is not used.
Because the indicator light doesn't illuminate, it's difficult to tell whether or not the device is still glowing, and for how long. Only the clicking of a relay a few minutes after a cold or warm start reveals to the attentive driver when the glow plugs are switched off.
In most engines, afterglow is interrupted as long as the engine speed is above 2500 rpm.


For the optimal function of the power-consuming preheating system, there must be no resistance due to corrosion or other factors anywhere in the circuit (battery – preheating relay – glow plugs). The system will be largely disabled if only 0.2 ohms of resistance develops somewhere between the battery and the glow plugs – a level of resistance that is hardly noticeable or insignificant for most other electrical components in the vehicle.
The glow plug fuse often causes these problems, and it's also well-hidden in some vehicles. For example, in a 1997 Golf 3, it's located on the top of the central electrical unit as an oversized flat fuse, making it difficult to reach, while the owner's manual refers to the splash guard in the engine compartment.
In vehicles with a fuse box located on the battery, the glow plug fuse is usually located there as well.

The current flowing through a healthy, cold TDI glow plug (Bosch GSK2 and similar types) is initially 15 amps or more, which corresponds to a heating power of approximately 200 watts and quickly brings the glow plug into the red-hot range. To prevent it from burning out during the afterglow phase, its resistance increases with temperature due to its design, and eventually, only about 5 amps flow through each bulb.

A realistic functional test of a glow plug can actually only be determined based on its visible glowing behavior or its current draw. A multimeter must have a 20-ampere measurement range for this purpose.

If such a measuring device is not available, a preliminary measurement can be performed by routing the current from a headlight bulb (approximately 60 watts) from the positive terminal of the battery through each spark plug to ground. If the lamp only glows weakly or not at all at one spark plug, while it glows brightly between the positive terminal of the battery and ground, then the spark plug in question is defective. However, during the lamp test, the spark plug remains almost cold, so that faults in the upper temperature range may go undetected. This, of course, applies even more so to a standard continuity test using a test lamp or a resistance measurement.

For the glow plugs of the water heater, an acoustic test with the engine off is usually sufficient: Disconnect the connector and connect the glow plug to the positive terminal of a battery using a cable. The heating power of a functioning glow plug will instantly cause the surrounding coolant to boil, which can be heard as a hissing sound. If there is no hissing sound despite a fully charged battery, and the test cable does not become noticeably warm anywhere, the glow plug is likely defective. Before undertaking a complex replacement (draining the water, etc.), However, in cases of potential leakage issues, additional checks should be performed using an ohmmeter (cold resistance should be around 1 Ohm) or a test lamp.

According to general experience, spark plugs do not last for the entire lifespan of an engine.
Faulty spark plugs are most likely to become noticeable during cold starts in winter.
The engine is difficult to start and/or runs roughly for the first few seconds.
Often, white to bluish exhaust fumes are emitted.

Unfortunately, such symptoms are not only caused by faulty glow plugs, but any disruption to the electrical current flowing to the glow plugs (see above) can lead to similar issues. Furthermore, there are also causes outside of the glow plug system to consider, such as those mentioned in the 1st paragraph. Part of the listed.
Despite this, when dealing with cold start problems, spark plugs are often replaced, and if that doesn't fix the issue, a lengthy and often random troubleshooting process often begins.

If you are experiencing cold start problems despite the glow plugs appearing to be functional, the voltage available at the glow plugs during the preheating process should still be measured. If the glow plug connectors are disconnected, small resistance points often remain undetected, which can still disable the preheating system - see above.
Measuring the voltage at the terminals used to be straightforward with the older, exposed screw terminals, but it has become practically impossible with the newer plastic connector housings, unless you create specific openings to access the live components. Instead, one can only probe the cables leading to the connector block and measure the voltage against ground there, hoping that there are no interruptions or transition resistances between the measurement point and the spark plug.

The current consumption, and thus the functionality, of most of the preheating system can be roughly estimated by measuring the voltage drop, for example, on a test lead that is plugged into the two wide terminals of the preheating relay socket instead of the preheating relay itself. "Anyone interested in DIY projects should consider building such a circuit (or a suitable test resistor) as a precaution, because it allows for tests independent of the glow plug control unit (MSG), which may not activate the glow plugs at all in the summer or when the engine is warm – or, if it does, only for a very short time, making it difficult to take accurate measurements."

The test lead should definitely be fitted with robust connectors and quickly plugged into the relay socket at the beginning of the measurement (don't be alarmed by the initial spark!), so that the initial current of around 60 amps in a 4-cylinder engine doesn't cause overheating issues. After a few seconds, the normal preheating current flow is still approximately 30 amperes, which, for example, in a 1-meter-long section of wire with a 1 mm² copper cross-section, causes a voltage difference of about 0.6 volts, with a slowly decreasing trend.
To ensure the measurements are taken under reasonably realistic conditions, the voltage drop in the test lead should not exceed 2 volts at 30 amps, which would correspond to approximately 3 meters of wire with a 1 mm² cross-sectional area. The wire should not be thinner than 0.75 mm², as this cross-sectional area already heats up noticeably during testing of a functioning glow plug system.

Measurements are ideally taken at the ends of the test lead, such as at the connectors.
Since the location of the glow plug relay socket is often inconvenient for measurements, the ends of the test lead can be equipped with (very thin) measurement branches, which also allow for measurements to be taken remotely.

If the current flow is too low despite functional spark plugs and a fully charged battery, the problem area should be located within the wiring. As a guideline, when a test cable is connected, you should measure approximately 12 volts from the battery to the fault location relative to ground, and a significantly lower voltage beyond the fault. This usually allows you to quickly pinpoint the problem. If the remaining current is still a few amperes, a faulty point with a significant voltage drop can also be identified by a noticeable local heating.

Since the glow plug relay is bypassed during the wiring harness test, it should be checked or replaced as a precaution if there are any glow plug problems. Even if it gets hot during a short pre-glow cycle, it may still close, but it will have an increased resistance between the load and output terminals. Possibly, only the switching contacts are dirty, and cleaning them might resolve the issue.


When attempting to remove spark plugs, users may experience a rude awakening if they (as is often the case) have been neglected for several years. Despite the low torque specification of approximately 15 Nm for installation, the plugs often require considerable force to remove. The threads may be difficult to turn along their entire length, and occasionally, the aluminum threads in the cylinder head may become stripped, meaning the spark plug cannot be removed at all.
In some cases, the candle portion between the hexagonal head and the threads has simply sheared off.

The cause is the recessed position of the spark plug and cylinder head-side threaded holes in the cylinder head.
Over time, water, dust, and other particles can seep into the air gap above (which is not protected by any seal, as the sealing surface is located on the combustion chamber side) and accumulate there. Depending on the chemical properties of the materials involved, candle threads and cylinder head threads can corrode, which can sometimes act like a very strong screw lock.
Sand grains and other debris in the aforementioned gap can become lodged when the candle is unscrewed, making the job even more difficult.
Finally, dirt and debris that accumulates during the removal of the spark plug can fall through the spark plug hole into the cylinder and, during the next engine run, potentially cause problems. cause further damage.

To prevent at least the latter from happening, the area around all the candles should be cleaned before lighting them. Applying a thin layer of oil around the spark plug using a brush helps to remove loose debris or adhere it to the cylinder head, preventing it from falling into the spark plug well.
In addition, you should let a few drops of rust remover (or, in less severe cases, motor oil) seep into each gap. While this method only occasionally makes the job easier, the oil causes any dirt that accumulates in the air gap to adhere to the spark plug body or the top of the cylinder head bore, preventing it from trickling down into the cylinder when the spark plug is removed.


If the space around the wick is already full of debris, it should be cleaned out as much as possible before removing the wick, for example, using a thin wire with a small hook at the end (e.g., made from a fine spiral spring).

How do you remove seized, defective spark plugs when the commonly recommended socket wrench is not sufficient, or gets stuck and risks breaking the spark plugs, even after the engine has been warmed up and a generous amount of rust remover has been applied to the threads?
Then, instead of further complicating things, one should create a clear workspace, for example, to accommodate socket wrenches, by disconnecting interfering fuel injection lines, etc. (while protecting the openings of nozzles and lines from dirt and avoiding bending any fuel injection lines!).
Spark plug socket set with 10mm square drive: For some pipe wrenches, a 10 x 11 socket head fits perfectly into a 14mm ring wrench, socket wrench, etc. Sometimes, small burrs or imperfections need to be filed away for a proper fit. When using a lever, always attach a tool that fits snugly onto the socket wrench and provides a long lever arm.

To prevent the aforementioned corrosion on the candle threads or to stop its further progression, it is advisable to remove even intact candles shortly after acquiring the vehicle and to lubricate their threads, for example, with copper paste, before reinstalling them.
If you want to be thorough, you can also seal the joint superficially with some silicone sealant or similar material. Of course, you shouldn't use any materials that would harden, otherwise you'll create a problem for yourself the next time you need to replace the candles...

The candles should also not be screwed in too loosely, otherwise the exhaust gases can bypass the seal and push soot into the candle thread, which can, over time, cause it to become so stuck that, when attempting to unscrew it, the top part of the candle will shear off .

If the glow plugs (which have been cleaned of soot and other residues) on used spark plugs appear corroded or melted, the cause is usually a malfunction of the fuel injectors. Then, it is recommended to check or replace the relevant nozzles to prevent the newly installed glow plugs from developing similar damage to the old ones in the near future.