Diesel engines do not have conventional spark plugs. Those strange elements that from one day to the next made our grandparents' gasoline cars stop working. However, what they do carry are other elements called heaters or glow plugs, whose faults can cause our diesel car ends up working very badly when cold or that it directly stops starting. Other names by which they are known are preheaters o glow plugs.
Nowadays, practically all diesel engines have a glow plug warning light. And those who do not have it, at least turn on the light of engine failure, to indicate that something is wrong. When the ignition is activated, all the lights come on for three or four seconds and then go off, indicating that everything is correct to start the engine. If during normal driving of the vehicle the warning light comes on, does not go off after starting or stays blinking, possibly the car has a fault in one or more heaters (normally diesel engines have a preheater for each cylinder).
Note that some models only warn of the fault for 20 or 30 seconds and then turn off so as not to interfere with normal driving. Other models may also display a warning on the on-board computer screen, with a text similar to: "check glow plugs".
What are glow plugs used for?
The mission of these small elements is facilitate the cold start of the enginesdiesel. In addition, they may also be required for a short period of time after start-up, in order to achieve stable engine operation and limit polluting emissions.
In reality, they are nothing more than metal pieces that glow when the contact is activated. Thanks to this, they help to heat the air and combustion chambers the motor. In the engines of direct injection They are not as necessary as in the old pre-chamber ones, but they are still needed in very adverse weather conditions.
How to know if the diesel heaters fail?
This breakdown is especially noticeable in winter, although in summer it also affects the correct starting of the car. The main indicator of fault is the warning light for the preheating plugs. Although this light will not always work, if the heaters have only lost part of their capabilities.
Therefore, to know if you have one or more heaters out of action, you should pay attention to symptoms such as:
- that from time to time the engine has a harder time starting
- the light takes longer than necessary to turn off
- the exhaust pipe expels a gray or white smoke that previously did not broadcast
- After starting the engine, there are several knocking, rattling sounds and the engine seems to misfire. That is, unstable operation until it heats up.
How to change the glow plugs?
As with many auto mechanic jobs, replacing diesel glow plugs can be easy task or hell. It depends a lot on the car model and the type of motor what a mount Normally they do not have more than a thread to the motor and an electrical connection, but the difficulty lies in more or less easily accessible them through the engine compartment.
They usually go located in the buttvery close to the injectors and they are easily located thanks to the electrical connection they have on their heads. It is convenient to pay attention since the motor usually has some indentation or notch to be able to insert the key and unscrew them without problems. Of course, you will have to have new ones glow plugs suitable for your model. In Amazon you can enter your car so that it tells you which ones are compatible.
Like most of the elements that are threaded in the cylinder head, it is necessary to treat them with extreme delicacy. The steps to follow are those:
- The first thing is to remove the engine cover, which is normally screwed or held by tabs.
- Disconnect the car battery.
- Then you have to locate the electrical connection of the heaters. Normally it is a bridge, which is nothing more than a cable with several washers that are attached to each of the heaters. To remove it, you just have to release the hooks with a wrench or screwdriver, as required by the model.
- Once the heaters are uncovered, proceed to carefully loosen them. If they resist, it will always be helpful to spray abundantly with alojalotodo, let it act and insist by turning and giving small dry taps.
- Important notice: you must not go overboard, because you would not be the first to break the heater and force the cylinder head to be disassembled to remove the thread that has remained inside.
- Once loose, you must fit the new heater soon so that dirt does not enter the engine. Asqueeze it with a torque wrench with the torque indicated by the manufacturer (normally between 10-20 Nm or a little more).
- Reconnect the jumper or wire that supplies power to each heater.
- Plug the battery back in.
How to locate the faulty engine heater?
If we are not going to change the four diesel glow plugs, we will have to locate where the fault is. For this we can proceed in several ways, being necessary (or not) to remove them from the engine. Diesel glow plugs normally receive current through the positive pole of the head (thread on one end), while the negative ground is made by the body that is threaded to the engine.
taking out the heaters
- Removing the glow plugs from the engine and jump-starting it with jump starter clips. To do it safely follow these steps:
• Place the clamps on the battery: the black cable on the negative terminal and the red cable on the positive terminal. That the metal parts of the tweezers do not touch each other, or yourself, or that they conduct electricity.
• Place the black (negative) clamp on the threads of the heater body.
• Hook the red (positive) on the thread of the heater head, at one end.
• Count the time it takes for the other end of the heater to glow. If it oscillates between 4 and 6 seconds, the heater is in perfect condition. When it takes longer, it is that you start to fail. If it exceeds 15 seconds or directly never heats up, it means that the heater is bad. - Once you have them out it is also convenient check them with a multimeter or tester:
• Set the multimeter to continuity. For them you just have to turn the rotary knob to the position marked with the symbol of a loudspeaker and/or an arrow with a vertical line.
• Place the black (negative) lead of the multimeter on the threaded body and the positive (red) lead on the heater head, which is the threaded end.
• If the multimeter beeps, it means that there is continuity and the spark plug still works. Although it is better to test connecting it to the battery because that is where you see if it heats up fast enough.
Without removing the heaters from the engine
- Without removing them from the motor, pass current to the heater and measure the intensity (in amps) that each one consumes. For this, an ammeter must be inserted in series that admits a good number of amps or we will melt it. The heater that consumes the least (or that shows zero directly) will be the most damaged.
- Without removing them from the motor, measuring the internal resistance that it has between the positive pole and the ground. To do this we simply have to unplug the electrical connection and insert the multimeter between the positive pole and the ground of the motor itself. A heater normally has between 1 and 3 ohms of resistance, if we get a very high resistance reading the heater is shorted and needs to be replaced.
If your engine heaters are located in an accessible position, the method of removing them and connecting them to the battery is the most recommended for its reliability. Unfortunately, this is not always the case. In cases where you can't easily get to them, the method of measuring internal resistance is reliable enough to tell if they work. Also, it is safer because there is no risk of sparks or burns and it is quite comfortable.
Types of diesel heater failure
Notably the usual fault is that the resistance of the heater breaks. In these cases it will not consume current, it will have almost infinite resistance (measurement Ω of the multimeter or tester) and it will not work at all.
Other times it will only partially break, short-circuit, or shunt slightly. In which case it will continue to work although it will heat much less. So it will not become incandescent or it will take a long time. So it will be more difficult to detect with the multimeter and the on-board computer will not always be able to detect it. To avoid this, it is always advisable to follow the car's maintenance plan and change them when necessary.
How much does it cost to change heaters or glow plugs?
Unit price of glow plug depends a lot on the model, but approximately they start from 15 euros to about 50 euros, for tourism-type vehicles. In the workshop, the labor to be applied also varies enormously, from 1 hour or less for simple cars, up to 3-4 hours depending on the elements that need to be disassembled. There are engines that require removing the filter box, the alternator, the EGR valve and a long etc of organs, before dismantling the glow plugs.
In general it is not an expensive fault, and the good thing is that in a modern car it's hard for us to stay put for this reason. The bad thing is that precisely for this reason, many drivers are postponing the repair more than necessary. Something that you should not fall into either because it can cause breakdowns in other parts of the vehicle.
When to change diesel glow plugs?
The most common is that diesel heaters have to be changed every 120.000km. Although this figure is not fixed, most manufacturers, who sell these components to brands, recommend this mileage as the right time to change them. Nevertheless, they can also deteriorate prematurely due to a defect in its assembly or manufacture.
As in almost any product, car brands manage to achieve very different levels of reliability. Diesel heaters are not the most common failures (except for age), but they are less likely to fail prematurely if they belong to a more reliable model. In the next video You can see the OCU's 2020 Reliability and Satisfaction survey. You will see that there is more of a striking contrast between number of breakdowns and price.
Like many of the parts found in a car in groups (such as spark plugs of a gasoline engine), it suits change them all at the same time when doing this operation. If you don't do it like that, decompensation could be caused inside the engine because of the difference between the new ones and the old ones. Where this imbalance would be most noticeable is when starting, at which time some cylinders would burn badly and others well.
If before 120.000 km the car has problems starting cold, and you don't have battery problems or starter, it is quite possible that the heaters will need to be replaced ahead of time. As we have already told you, this number of kilometers is only a recommendation and It can vary a lot depending on the use that has been given to the engine..
For example, diesel heaters they wear more when doing many short trips. 120.000 km in which the car has been started 4.800 times is not the same as that same mileage having only been started 600 times. That is the difference between making trips with an average of 25 km (typical to go to work) and an average of 200 km.
It also usually happens that, being very resistant pieces never stop workingbut let them do it less effectively. This will cause you to have problems starting up, but you always end up getting it. Although the characteristic white or gray smoke that we have mentioned before comes out. If you see that this is the case, it is better that you change them even if you are still able to start, leaving the engine in that state can cause other breakdowns or decrease its useful life.