This is a series of articles outlining common, mostly relatively simple to diagnose and repair causes of hard and no start conditions that can affect IDIs. There are many different things that can cause it, including but not limited to: glow plugs, fuel system leaks, electrical problems. Each article will focus on a single topic.
PART 1: GLOW PLUG SYSTEM
For those of you who may be new to owning a diesel, glow plugs are individual heating elements inserted into each cylinder that are energized (quickly heated up) before you start the motor. They have nothing to do with the running condition of the engine after it's running, unlike spark plugs on a gasoline engine, so if they are failing, you can have a no-start or hard-start condition due to the lack of heat required to start a diesel engine, due to burned out glow plugs or an otherwise inoperatable glow plug electrical system.
Be sure you have BOTH batteries installed, fully charged, and not weak/worn out, since glow plugs pull a LOT of power. If your starter motor turns over slowly, charge or replace your batteries before proceeding since that can also cause poor starting. The glow plugs will drain weak batteries, leaving you with low amperage for the starter motor after the glow plugs are deenergized.
If you have an early (83-84) motor, you might want to skip the section about block heater, since many people report that using the block heater on those early year motor castings can cause a crack in the block at the heating element's port, resulting in a coolant leak. Instead, you may want to buy and install an in-line or magnetic pan style engine heater.
BLOCK HEATER
The easiest way to diagnose glow plugs is to use the factory block heater. Typically the original cord is going to be rotten beyond use after 25-35+ years, so you will most likely need to buy a new one. Luckily, they only cost about $20 and simply plug in to the block heater's element on the passenger side of the block, just above the starter motor.
The connector that plugs into the block heater element looks like this:
A couple replacement cord part numbers/links:
Kat's 28400 (Link: Advance Auto)
Zerostart 3600006 (Link: O'Reilly)
Run the block heater for an hour or two (longer if it's well below freezing in very frigid climates) and then try starting the engine. If it starts easily or relatively easy after heating the engine with the block heater, odds are you have a problem in your glow plug system.
If your WTS (wait to start) light comes on but quickly goes off on a completely cold engine, that's a pretty good indicator that you probably have many bad glow plugs. If it doesn't come on at all, you have either bad glow plug plug solenoid or controller.
Also, be sure a previous owner didn't install a manual glow plug button somewhere, bypassing the automatic controller. The solenoid will have an extra oddball single wire running to it from a momentary switch in the cab, if that's the case. Don't manually run the plugs longer than about 8-10 seconds or you risk burning them out. The less long, the better. The warmer it is, the less long you should run them. On a completely warmed up engine, you typically don't need them at all, hence why a block heater works to start a "cold" engine without working glow plugs.
GLOW PLUG SOLENOID
The first thing to check is the glow plug solenoid. On 1983-1986 vans, it's located on the driver's side above/behind the headlight, inside the engine compartment. (The identical looking solenoid located on the passenger side is for the starter, not glow plugs). It should make a loud, audible CLICK on when the key is turned on and the GPs are activated. It should CLICK off when the WTS light goes off as well. You should have +12V on both of the large terminals if it's working properly with the key turned. You may try hitting it with the plastic handle of a screwdriver to see if it helps it work, since they can get weak/sticky with age. They aren't very expensive (About $25-30) if it does need replacement.
It appears that all years of IDIs use the same glow plug solenoid, regardless of the controller type (discussed later on in this article).
Standard Motor Products # RY175
GLOW PLUG CONTROLLER
Model years 1983-1986 6.9L engines have a different glow plug controller and solenoid setup than 1987 6.9L and 1988-1994 7.3L vehicles. (1987 6.9's are referred to as "7.3 style").
The glow plug controller on 83-86 IDIs is located on the back side of the engine. They are rather buried and hidden on pickups due to the firewall, but can be easily located and accessed on vans.
Many people report that this early style of glow plug controller is prone to getting stuck on, causing the glow plugs to burn out from being heated for too long. It appears that the original design is electromagnetic, due to the large cylinder on the top. New aftermarket controllers available today are solid state, which probably remove the danger of that occurring. They are a bit expensive at around $100-$150, but a set of glow plugs isn't cheap either (costs about the same), and are much more difficult to replace than the controller.
1987 6.9L and 1988-94 7.3L have an updated, solid state style controller from the factory with the solenoid mounted on top of the controller. The solenoid alone can be replaced if it goes bad, since it's much cheaper than the complete controller assembly. On NA engines, it's located behind the air cleaner (back side of the motor) and looks like this.
Motorcraft # DY1128
If you find that your glow plug solenoid and controllers are working properly, you may need to replace the glow plugs. Be sure to replace all 8 as a set, and ONLY use Motorcraft or Beru brand plugs in your IDI! Other brands may swell up, get stuck and/or break off in the cylinder heads when they are removed in the future. That would mean you have to tear apart your engine to get out the broken bits.. so it's not worth trying to save a few dollars. 1983-86 6.9L use blade connector plugs (ZD-1-A), while 1987 6.9L and 1988-94 7.3 use "bullet" connector plugs (ZD9):
