You wake up early on a winter morning and head out to your truck to go to work. It's been getting a bit tougher and tougher to start as the weather gets cool, but, today, with the temperature approaching zero Fahrenheit, the truck just cranks and cranks and never fires. Chances are, your glow plug system is failing.
Glow plug troubleshooting and repair is fairly easy and not too expensive. Taking your truck to the dealer for this service can cost well over a thousand dollars while you should be able to do the work yourself in a few hours for less than two hundred dollars.
THEORY OF OPERATION
Unlike gasoline engines that use spark plugs to operate, diesel engines operate by compressing a fuel and air mixture. When a gas is compressed its temperature increases. The extremely high compression ratios of diesel engines raise the heat of the fuel-air mixture to the point of ignition through pressure alone. However, at very low temperatures this compression isn't enough to cause the fuel to burn. This is where the glow plugs come in.
The glow plugs are heating elements in each cylinder, which provide a very hot point that raises the temperature of the fuel-air mixture to the point of ignition. If one or two of your glow plugs have failed you might notice your truck is a bit hard to start on cold mornings. If more than that have failed it might become impossible to get your truck going.
Troubleshooting of the glow plug system is fairly straightforward. All you will need is a multimeter to measure resistance and voltage and, possibly, depending on the design of the probes on your meter, some straight pins or something similar to test the glow plugs themselves. You will also need a couple of pieces of jumper wire about two to three feet long, preferably with alligator clips on each end.
The first thing to test on the system is the glow plug relay. This relay is a common point of failure. The glow plug relay is located on top of the engine right behind the fuel filter housing and slightly to the passenger side. If there are two relays located there it will be the relay toward the back of the vehicle with large yellow and brown wires leading to and from it.
To test the relay you want to check the voltage at both large terminal posts right after turning on the truck. The glow plug relay should activate for 10 seconds or more, no matter what the temperature, longer when it is colder. You might wish to have someone turn the key to start while you have the meter on the terminals. Place the ground probe on a bare piece of metal somewhere on the engine and the source probe on each terminal.
Both terminals should read around 12 volts. If one reads that and the other reads zero, you next want to check if the control module is bad or if it is the relay itself. (It most likely will be the relay. I, personally, have had two relays fail in my truck in just over 100k miles but never a problem with the PCM.)
To test directly for function of the relay and make sure the problem isn't the control module you will need to apply battery voltage across the two small terminals on the top of the relay. At this point, if the relay is good, you should have 12 volts across both terminals. If the relay fails both of these tests you will have to replace the glow plug relay.
But, don't put your multimeter away just yet. While you're under the hood you might as well test all of your glow plugs. On the inside edge of each valve cover there is a wire connection with nine wires. These wires connect to the fuel injectors and glow plugs. Carefully remove the wire connecter by pressing on the latch and carefully prying the connector apart with a small screwdriver. The glow plug wires are the two furthest forward and two furthest back.
Set your multimeter to test resistance and, using a straight pin or something similar if necessary, test the resistance across these four connections. Keep your ground probe attached to a piece of bare metal on the block or the negative battery terminal. Measuring each of these connections should have a resistance between 0.6 and 2.0 ohms. If any of them measure infinite resistance they have failed and will need to be replaced.
Be sure and check both sides.
If any of your glow plugs have failed, you now have a decision to make. If it's just one or two on one side you might decide just to replace the ones that have failed. If you have some on each side that have failed it's probably a good idea to go ahead and replace them all while you have everything opened up.
For the replacement of these parts any good, basic tool set will do. You may need to have both SAE and metric tools, however, as there may be a mixture on this engine. A torque wrench is recommended but if you're careful not to under or over-tighten the appropriate parts you may be able to get by without that.
You can buy OEM glow plugs at almost any parts store and should be able to save as much as 75% over the cost from your local ford dealer.
Before starting any of the next repairs you should remove the negative battery cable from the batteries. This will prevent any accidents caused by working with hot wires.
Replacement of the glow plug relay is quite straightforward, just be certain to pay attention to which wires go to which terminals and don't get them crossed up. Remove the four wires on top of the relay. The relay is held to the engine with two nuts. Remove the two nuts and lift out the relay. Put the replacement relay in, replace the nuts holding it in place and reattach the wires. And you're all done.
If you didn't find any failed glow plugs when you tested them your truck should be ready to go. Just reattach the battery cables and she should start right up.
If you need to replace the glow plugs, things get a little bit tougher, but not too bad. You want to start by removing some of the pieces over the top of the valve covers. You'll need to remove the turbo tubes and, on the drivers side, the breather parts. It might be easier if you were to move some of the other wires and hoses out of the way but it isn't absolutely necessary.
If you reattached the wires leading to the valve covers go ahead and disconnect those.
Remove the valve covers. Be careful when doing this as the gasket is reusable rubber and has the wiring connections integrated into it.
To remove the glow plugs you'll want to use an 8mm deep socket. Remove the wire from the top of the glow plug and slide your socket down and unscrew the glow plug. It's a lot easier to pull the glow plug out, once it's loose, by slipping a short length of rubber tubing over the top and simply lifting it out. Be careful at this point, it is unlikely but if the glow plug is adequately burnt and damaged the tip could break off in the cylinder. If this happens the head will need to be removed to get the part out before the engine can be started.
When you put the glow plug back in you should torque it to 14 ft-lbs with your torque wrench. Once you've replaced the glow plugs that are needed replace the valve cover and torque the valve cover bolts to 8 ft-lbs.
After you've replaced the hoses and tubes you've removed your truck should start fine.
Troubleshooting and replacing the glow plugs on your diesel truck can save you over $1000 and it is very satisfying to perform such work yourself.