Diesel a no-go? First, check the glow plugs

Hey there, time traveller!
This article was published 28/12/2002 (8361 days ago), so information in it may no longer be current.

QUESTION — The temperature dropped to -20C a couple days ago and my 1995 VW Jetta diesel didn’t start.

I have a feeling the glow plugs need to be changed, but to do that I need to get to the garage. What would be the best way to warm the engine in the meantime and get her going? I’ve left a flashlight on under the hood while I’m at work. Will this help, and if not do you have any suggestions? Also, do you know if this is a typical occurrence with this type of engine and are there any preventative maintenance suggestions to prevent this problem happening again?

ANSWER — The problem with your Jetta could be faulty glow plugs, but there are also many other possible causes of it not starting. Let’s look at the glow plugs first. When the engine computer turns on the glow plug relay, battery voltage is fed to the glow plugs. The glow plugs turn red hot and heat the air in the combustion chamber of the engine. There is one glow plug for each cylinder. If a glow plug fails, that cylinder may not get hot enough during cranking to ignite the diesel fuel when it is injected. Diesel engines rely on the heat of the compressed air in the cylinders to start combustion, but extra heat from the glow plugs or another source is needed during cold start-up.

It is unusual for all glow plugs to fail at once unless the relay controlling them sticks in the “on” position. Glow plugs are cycled on and off every few seconds during start up to prevent them from burning out. If they stayed powered up too long, the glow plugs overheat and fail. Is this normal? It doesn’t happen often, but it can happen with any diesel engine.

Heating the intake air will help the engine start if glow plugs have failed. Placing a 100-watt light bulb under the hood and placing a blanket over the hood and grill to keep the heat in will help, but be sure to place the light where it cannot touch plastic or rubber parts or a fire could start. Charging the battery so the engine will crank as quickly as possible will also help the engine start easier. Don’t use starting fluids such as ether, because this can cause expensive internal damage to the engine. If it still won’t start, you are better off having it towed to the repair shop rather than risk a burned-out starter or other engine damage.

Other faults could, such as a malfunctioning fuel supply pump or engine computer, could also prevent the engine from starting. This type of problem is difficult to diagnose without doing a few on-vehicle tests, but I hope this helps.

QUESTION — The front headlights on our 1999 Dodge RAM 1500 perform like they are possessed by some demon. At times, they both come on and at times, they don’t. The daytime running light only works on the left in spite of the fact that we replaced the right headlamp. After a carwash the headlights will come on only after a delay that could last five minutes or up to 30 minutes, but eventually both, or one, do light. Is this a harness problem or a cable dampness problem? I ask this because if the humidity is very high it also effects the performance of the headlamps. During heavy rain or heavy snow they perform intermittently then eventually, both remain until more rain or snow happens.

ANSWER — The daytime running lights on your truck use a module that operates a relay to ground both left and right lamps at the same time. If only one lamp is not working, then the problem must be on the power supply to the lamp instead of the ground. Each headlamp gets power from a separate fuse in the underhood electrical centre (this is common on many newer vehicles), so a bad fuse could make one headlamp not work.

To stop both headlamps from working, the power supply for both headlamps must be faulty. Battery power supplies the voltage to both headlamp fuses as well as many others. Because other systems do work, the problem could then be traced down to inside the electrical centre. A bad wiring connection, broken wire, or corrosion could be the culprit and moisture can affect it.

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sk.sympatico.ca