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Glow plugs quality evaluation
A guide to glow plugs quality evaluation. How to test and compare glow plugs.

The demand of cheaper spare parts is always high, leaving the well known path of oem brands lead the customer into a wildernass plenty of great opportunity but also full of dangerous products. We seriously think there are some manufacturers which really deserve attention, and looking at the datas collected by our tests there are manufacturers with a lot of potential and with very good quality products. On the other hand, however, there are also many manufacturers with awful products. Too many times we see distributors leaving the spare parts market after having problems with such horrible products. Most of the times it ends with the customer going back to the oem brand without really exploring and testing what the spare parts market can really offer in terms of quality, service and prices.

This small guide is intended to drive a customer choosing the right quality glow plug, this guide is not intended to be part of an advertising campaign, so we will not speak about what our company do and what we don't do, we'll just expose some very important requirement a well made glow plug should match. With a very few tools anyone will be able to check and compare products on the market building up his own opinion.

Point 1: THE TUBE

Materials used in a glow plug manufacturing make the big difference in terms of price. The most common material used for big part of components is iron, with the exception of the heating tube. The tube must be made with nichel-crome alloy known to the world with the term of Inconel 601 or Nicrofer, or just Alloy 601. This alloy is commonly used for its resistance to corrosion, carbon deposit and temperature. The glow plug's tube works in the combustion chamber or pre-chamber in contact with high temperature and exausted gas, using cheaper materials usually ends with a huge carbon deposit forming on the glow plug's tube after a few months making the glow plug extraction really difficult. The tube is great part of the glow plug components cost, using a cheap tube usually ends up with a very cheap glow plug.

Point 2: THE BODY

The body has a very important role when designing a glow plug. It holds the heating tube and the junction between the body and the tube is crucial. The heating element is embedded in the body with an hydraulic press, the pression should be carefully checked, using too much force can internally damage the glow plug, using low force can have the glow plug fired out the engine head. Also tolerance is very important to limit the difference between the minimum and maximum force needed, limiting the waste and having a better quality product. As usual a large tolerance body is cheaper then a tight tolerance body. Also the dimension is very important, thread, diameters, conus angle, everything must be exactly like the oem glow plug, if not there can be problems fitting the glow plug in the engine. The body is usually coated with a chemical-treatment to resist salty mist avoiding rust. A cheaper tratment can be money saving but can expose the body to oxydation.<br>


Filaments are crucial. The resistor is the hearth of the glow plug, a good filament is essential to make a good glow plug. It's not of great importance having a double filament or a single filament glow plug, a good glow plug can be made with both technologies, but it's very important to have a good manufacturer using the right machines and alloys. But how can we test a glow plug? Professional equipments to check glow plugs have optical pyrometers ( istruments able of getting object's temperature by infrared sensor) and can draw a diagram temperature - current, time. This is what we call characterisation, and it's the glow plug's fingerprint. Professional equipments are quite expensive and necessary for a producer, but not for customers. There are a couple of test anyone can do to check the quality of a glow plug, using a clock, a 12V or 24V battery and two cables with clips. The main feature we should look at are:

TIMING: using the cables connect positive wire to the upper terminal, and negative wire to the body, connect wires to the battery, immediatly the glow plug start to glow (watch out, do not hold the glow plug with your hands). Start the clock in the same time you connect the last cable to the battery and stop the clock when the glow plug'tip is red-hot. The time must match the glow plug specific heating time declared on the catalogue. Usually it should be < 8 seconds, many manufacturer can go around 5-6 seconds, but from our experience a time < 8 seconds is usually enough for almost any application. There are 5V, 6V 6,5V ecc.. out on the market, they are called QGS (old japanese) or ISS (new european) glow plugs, are low voltage glow plugs pushed with 12V for a few seconds to allow a very fast heating (2 seconds), do not light on this kind of glow plugs with a battery and two cables, you will probably end up burning out them all.

HEATING QUALITY: Watch the glow plug while heating up. It must glow starting from the tip, then spreading slowly in body direction untill the glow plug's temperature get to the higher point. If a new glow plug starts heating from the body or from the middle of the tube you're probably looking at a bad quality glow plug. The injector's stream points in an area between the tip and 8 mm from the glow plug's tip.

AUTO REGULATION: A glow plug for vehicles from 1990 on is usually heated for longer time, up to 180 seconds at 13V-14V for anti pollution purpose, so it is very important the resistor is made with one or two auto regulating filaments (it can be monofilament or double filament glow plug). To check it by yourself just heat the glow plug and keep it heated with a battery for a longer time, up to 5 minutes. Sadly you can't have the waranty it doesn't burn out at 13V-14V using just a battery, you should use a power supplier (alternate current is fine if you can't get a stabilized one).


Lifespan is the most important feature and strictly connected to heating time. Usually the hard point is having a fast glow plug with long lifespan, on the contrary is very easy to have a long life but a very slow glow plug. When the heating time is under 8 seconds, likeley between 5 and 7 seconds, lifespan is the mirror of the glow plug's manufacturing quality. If the manufacturing process has some lacks it will probably produce a short life glow plug. Sadly the only way to test glow plug's lifespan is with professional equipment, it must be started at 11-12V then kept for minutes at 13-14V, cooled down to room temperature or less and then the cycle restarts again. This keep going on for thousands of cycles. If you have a timer and some electric equipment maybe you can craft a rudimental tester by yourself, you will be able to perform some comparative tests.


Latest glow plugs for direct injection engines usually fit in very small holes, the shape difference between the OEM glow plug and the competitos's glow plug should be very slight, expecially diameters. For example, a 5,2 mm tube diameter instead of the original OEM tube 5 mm diameter can cause the glow plug to knock in the engine hole. Having the right dimension means big investments in tools, and higher manufacturing cost.

Point 6: PATENTS

Oh yes, there are patents for glow plugs, especially for resistor materials. Infringing a patent is an easy way to do a decent product without investment, but can seriously put the manufacturer and the buyer in trouble. If you don't want to get in trouble check the glow plugs patents on the web or ask your supplier a declaration they're not infringing any patent or they're using their own patents, this can be of help in case of trouble. There are companies, big or small, investing in research and development of their own patents and alloys, buy from them.

What exposed are just some basic points to check for glow plug's quality. Glow plug design and manufacturing process are far more complicated and full of little tips which really can make the big difference, even between good quality products.

Bondani Cristiano
Technical Department

E Tecno 1 S.p.A. - via N. Bettoli 8 - 43038 - Sala Baganza (PR) - Italy - P.IVA IT 00584050348