Glow plugs are electrical heater that extended down into the cylinder a short distance.They do not function as spark plugs do, as they do not ignite the fuel. Glow plugs just heat up the area where the fuel is injected so the compression can do the rest. In Tdi engines, the glow plugs are only really necessary to help start the engine at temperatures well below 50 degrees Fahrenheit. Due to the two stage injectors of the Tdi’s through 2003, and the complicated injectors of the PD & CR diesels, the fuel ignites fairly easily. Most engines will start without the glow plugs working even down to 40 F, but may have to crank longer to warm up the combustion chambers. Below that, they are necessary.
With the first Tdi Passat of 1996, glow plugs started having problems. In a Tdi, the glow plugs also turn on anytime the injection mixture goes rich in order to increase combustion efficiency and reduce emissions. The wires in the glow plug harness are a borderline diameter considering that a glow plug fires up at about 23-25 Amps, then settles in to 9-11 Amps when hot. The sudden high amperage is less when the glow plugs come on while running, because they are already at combustion chamber temperature. Harnesses also fail commonly right at the glow plug connector. Up through 2003, the glow plugs are installed at an angle to clear the injector. With the PD motors, the injectors moved to inside the head, and the harness and plugs are perpendicular. This translates as less side strain when removing and installing the harness, and less likely to get damaged.
The harness up through 2001 had two wires that each split for two glow plugs. The glow plug relay confirms the identical current on each branch to the engine control unit. If there is a difference, the ECU turns on the glow plug indicator light on the dash and sets a code, or DTC. These older systems were sometimes not correct with the monitoring function. Even with a new harness and new glow plugs, sometimes one would get the code and the light. At Karmakanix, we learned the the two wires could be joined together to eliminate the issue, but it also eliminates the monitoring function. That was a trade off we had to make so a driver would be alerted if some other problem occurred that would also turn on the glow plug light and set a code.
With 2002, a four wire monitoring system was used. It still had all the problems of early glow plug and harness failure, but at least the monitoring function did not lie. However, when a code arose that referred to a particular plug, the cylinder it named was usually not correct, the factory started counting from the number 4 cylinder as number 1 glow plug, etc. And sometimes had a pattern all its own, making diagnostics tricky. With the Pump Duse engine series, in 2005, the glow plugs finally went to a 7 volt system that radically prolonged the life of a glow plug.
Starting in 2009, the Common Rail engines use a glow plug that also doubles as a pressure sensor. The tip slides up and down slightly with each combustion cycle of the cylinder. One can see the running compression of the engine in the Measuring Value Blocks (MVB) using VAG COM or a factory scan tool, very useful for diagnosis failing engines. Of course, that compression rises radically under power with boost. The Engine Control Unit (ECU) actually monitors the rise curve of the compression and plans the precise injection pulse sequence for the best combustion each and every firing cycle.
There are specifications and recommendations for checking and replacing glow plugs. Karmakanix see it a little differently. We perform the resistance tests for the plugs, then the plugs and harness together, while we wiggle the harness. That way loose connections show up. If one glow plug gives up, and the rest are near or over 80,000 miles or so, we replace the set. That way the customer is not likely to have another one failing every few months.
We have seen a number of glow plugs frozen in the cylinder heads. Usually this results from them not being tightened correctly or cross threaded. When loose, carbon builds up in the threads and lock the glow plugs in place. About half the time, we can get them out using a heat blocker compound, a small torch to heat the head around the glow plug, and a pyrometer to check the heating progress. Once the head gets to 250 *F, the plug may come out. Further heating could damage the head or headgasket. About half the time, we have to remove the cylinder head and have a fancy shop remove the remains of the glow plug with a laser. We do have an expandable rethreader to remake the glow plug threads in the head if they are cross threaded or damaged. Glow plugs really should be tightened with a tiny torque wrench.
If a glow plug shatters, it has no where to go that isn’t damaging to something. It beats around inside the combustion chamber for a few, then exits through the turbocharger. The turbo always gets damaged. Sometimes the turbo still functions, but can be noisy, and may die soon. Sometimes the turbo dies right now. Because of this possibility, we remove and inspect all 4 glow plugs any time we find a bad one, and sometimes just to see them if they are around 100,000 miles old. When a glow plug is cracking, one can usually only see the cracks when the glow plug is warming up. Another good reason to just replace all four glow plugs if they are old.
Quick Clue: NEVER use starting fluid in a modern diesel motor. The most common result is that it can shatter glow plugs and even break piston rings. If your diesel won’t start, fix it. Do not risk destroying it.