If you've noticed your truck struggling to fire up on cold mornings, it's probably time to look at your 99 7.3 powerstroke glow plugs. There is nothing quite as frustrating as heading out to your rig at 5:00 AM, turning the key, and hearing that slow, labored crank that leads to absolutely nothing but a cloud of white smoke. We've all been there. The 7.3 Liter Powerstroke is widely considered one of the most reliable diesel engines ever built, but even a legend has its weaknesses. In this case, it's the cold-start system.
Unlike modern common-rail diesels that can almost start on a prayer in freezing temperatures, the old-school 7.3 relies heavily on its glow plugs to get the combustion chambers hot enough to ignite the fuel. If one or two of those plugs go south, you might get a rough idle and some smoke. If four or five fail, you're basically driving a very heavy lawn ornament until the sun comes out.
Why Cold Starts Are Killing Your Battery
When your 99 7.3 powerstroke glow plugs aren't doing their job, you end up overcompensating with your starter and batteries. You know the drill: you cycle the key three or four times, hoping to build up some residual heat, and then you crank it for twenty seconds straight. This puts a massive amount of strain on your starter motor and drains your batteries faster than a leak in a bucket.
Eventually, the friction and the heat from the glow plugs (if they were working) should be enough to get it to pop off, but by then, you've probably shaved months off the life of your expensive AGM batteries. It's a domino effect. Fixing the glow plugs isn't just about getting the truck to start; it's about protecting the rest of your electrical system.
How to Tell if the Plugs are Actually the Problem
Before you go tearing into the engine, you need to make sure it's actually the plugs. Sometimes, the issue is actually the Glow Plug Relay (GPR). The relay sits right on top of the engine, and it's responsible for sending the high-amperage current to the plugs. If the relay is toast, none of your 99 7.3 powerstroke glow plugs will get power, no matter how new they are.
A quick way to check this is with a simple test light or a multimeter. With the engine cold and the key turned to the "on" position (don't start it!), check for voltage on the large output post of the relay. If you have power there but the truck still won't start, the problem is likely deeper in—meaning the plugs themselves or the wiring harness under the valve covers.
If you want to be really thorough, you can test the plugs individually from the outside. You don't even have to take the valve covers off yet. Find the large 9-pin connector going into the valve cover gasket. The two outer pins on each end are for the glow plugs. Use an ohmmeter to check the resistance between those pins and a good ground. If you see "infinite" resistance or anything over 2 ohms, that specific plug is dead.
The Danger of Cheap Parts
I can't stress this enough: do not buy the cheapest 99 7.3 powerstroke glow plugs you find on the internet. There are plenty of "no-name" brands that cost half of what the OEM parts do, but they are a nightmare waiting to happen.
Cheap glow plugs are notorious for "swelling." When they fail, the tips can expand or even break off inside the cylinder head. If the tip swells, you won't be able to pull it out through the hole when you try to replace it next time. If it breaks off while the engine is running, you're looking at a complete engine teardown to fix the damage to the piston and valves.
Stick with Motorcraft ZD-11 plugs. They are the gold standard for the 99-03 7.3 engines. They are designed to withstand the heat and pressure of these engines without failing in a way that destroys your head. It's worth the extra twenty or thirty bucks for the peace of mind.
Diving Into the Replacement Process
Replacing the 99 7.3 powerstroke glow plugs is a bit of a project, mostly because they are located underneath the valve covers. This isn't like a spark plug change on an old Chevy small block where everything is sitting right there in the open.
You'll need to remove the intake piping, the intercooler tubes (since it's a '99), and then all those tiny bolts holding the valve covers on. The passenger side is usually pretty easy, but the driver's side can be a bit of a squeeze near the firewall and the brake booster.
Once you get the covers off, you'll see the injectors and the glow plugs tucked in beside them. Pro tip: Use a piece of small-diameter fuel hose to help get the old plugs out and the new ones started. Once you loosen the plug with a socket, slip the hose over the end of the plug to spin it out without dropping it into the oily abyss of the cylinder head. Do the same thing to start the new ones so you don't accidentally cross-thread them.
While You're Under the Valve Cover
Since you've already gone through the trouble of removing the valve covers to get to your 99 7.3 powerstroke glow plugs, you really should check the Under Valve Cover Harness (UVCH). These harnesses are famous for "the 50-cent mod" or simply failing because the plastic clips get brittle and the vibrations of the diesel engine cause them to unplug themselves.
Check the pins for any signs of melting or charring. If the plastic looks burnt, replace the harness and the gasket. It's much cheaper to do it now than to put everything back together only to have a fuel injector stop firing two weeks later because the harness finally gave up the ghost.
Also, it's a good time to check your torque on the rocker arm bolts and the injector hold-down bolts. These tend to get a little loose over a couple of hundred thousand miles. Just a quick check can prevent a major headache down the road.
The "Buzz Test" and Final Steps
After you've swapped out all eight 99 7.3 powerstroke glow plugs and put the valve covers back on, don't just slam the hood and head out. If you have access to a scan tool (like an OBDLink with the Forscan app), run a "Buzz Test." This will fire the injectors while the engine is off, letting you know that you plugged everything back in correctly and that your harness is seated properly.
When you finally go for that first start, don't be surprised if it's a little grumpy for a second as the air works its way out of the oil rails (if you messed with the injectors at all), but generally, if you only did glow plugs, it should fire right up. You'll immediately notice how much smoother the engine sounds in those first few minutes of idling. No more "romping," no more clouds of smoke that annoy the neighbors, just that steady, rhythmic 7.3 clatter we all love.
Keeping Everything Running Smooth
In the end, maintaining your 99 7.3 powerstroke glow plugs is just part of the deal when you own one of these trucks. It's a small price to pay for an engine that can easily go 400,000 miles if you treat it right.
If you live in a particularly cold climate, you might also want to consider an upgraded Glow Plug Relay, like the Stancor or Western units. They are much beefier than the stock Ford ones and can handle the high-draw cycles of a long winter much better. Combine that with a good set of ZD-11s, and your truck will be ready to work, no matter how low the mercury drops.
It's one of those weekend projects that really pays off the first time a blizzard hits and your truck starts on the first try while everyone else is out there with jumper cables. Take your time, don't drop anything down the pushrod holes, and use the right parts. Your Powerstroke will thank you for it.