If your 7.3 Power Stroke cranks longer than it used to on a cold morning, stumbles and blows white smoke until it warms up, or throws a glow plug circuit code, the glow plugs are the first place to look. The 7.3 uses eight glow plugs, one per cylinder, and they live under the valve covers alongside the under-valve-cover harness. That makes the job a little more involved than a gasser's glow or spark parts, but it's still a very doable driveway job with hand tools. Here's how to test them and replace them without snapping one off in the head.
What the glow plugs actually do
Glow plugs are heating elements that warm the combustion chamber so a cold diesel will light and run cleanly until it's up to temperature. When they fail, the affected cylinders don't fire well cold, so you get hard starts, a rough cold idle, and white unburnt-fuel smoke that clears as the engine warms. On the 7.3, glow plug problems are usually a mix of dead plugs, a tired glow plug relay, and worn under-valve-cover harness connectors, so plan to check all three.
Tools and parts you'll need
- A set of eight glow plugs (brands like Motorcraft, Beru, or a reputable OE-line supplier)
- New valve cover gaskets, since you're pulling the covers to get to the plugs
- A digital multimeter or ohmmeter for testing
- A deep socket that fits the glow plug and a short extension, plus a swivel for access
- A small amount of anti-seize for the threads
- A torque wrench that reads in the low range, because glow plugs are easy to over-torque and break
How to test before you buy eight plugs
1. Test resistance at each plug. With the engine cold and off, set your meter to ohms and measure from the glow plug terminal to a good ground. A healthy 7.3 glow plug reads a low resistance, typically a couple of ohms. An open circuit or a wildly high reading means that plug is dead. This tells you how many are actually bad.
2. Check the relay. The glow plug relay sends battery voltage to the plugs for a set time after key-on. If the relay isn't clicking and passing voltage, no glow plug will heat regardless of condition. Verify the relay energizes and passes voltage on key-on.
3. Inspect the UVCH. The under-valve-cover harness connectors get brittle and corroded. A perfectly good glow plug does nothing if the connector to it is burned or loose. Look at these while the covers are off.
Step by step replacement
1. Disconnect the batteries. You're working around the glow plug wiring and battery-voltage circuits, so pull the negative cables first.
2. Remove the valve covers. Clear anything in the way, unbolt the valve covers, and lift them off. Expect some oil. Set the covers aside with their old gaskets.
3. Disconnect the harness from the glow plugs. Carefully unclip the UVCH connector from each glow plug terminal. If a connector is burnt or crumbling, note it, because a harness that bad should be replaced while you're in here.
4. Remove the old glow plugs. With the correct deep socket, back each glow plug out slowly. If one feels stuck, don't force it hot or dry, work it back and forth gently. Snapping a glow plug tip off in the head turns a two-hour job into a machine-shop headache.
5. Compare and prep the new plugs. Set the new plugs next to the old ones to confirm they match. Put a light touch of anti-seize on the threads only, keeping it off the tip.
6. Install the new glow plugs to spec. Thread each in by hand first, then torque to the factory specification, which for a glow plug is a low value. Do not lean on them. Over-torquing is the number one way people damage a fresh glow plug or the threads. If you don't have the exact figure, snug them carefully to the light factory spec rather than guessing high.
7. Reconnect the harness. Clip the UVCH back onto each plug. Replace the harness now if the connectors were bad.
8. New valve cover gaskets and reassemble. Install fresh valve cover gaskets, set the covers, and torque the cover fasteners to spec. Reconnect the batteries.
9. Test the result. Cycle the key to let the plugs heat, then start it. A cold start should be quicker and cleaner, with far less white smoke. If it's still rough, revisit the relay and the harness, since those are as common as the plugs themselves.
Don't just throw plugs at it
The mistake owners make is replacing all eight glow plugs and still having a hard-start truck, because the real fault was the relay or the harness. Test first. If only two plugs are dead but the relay is weak, you fix nothing by ignoring the relay. Treat the glow system as a set: plugs, relay, and UVCH.
Parts you'll need
- Eight 7.3 glow plugs
- Valve cover gaskets
- Glow plug relay and UVCH if testing points to them
We group 7.3 ignition-assist and harness parts together so the plugs, relay, and harness for your truck are easy to line up. Start with the 7.3L Power Stroke diesel parts collection, then browse the glow plug category. Confirm fitment for your year before ordering.
FAQ
Are the 7.3 glow plugs under the valve covers? Yes. You remove the valve covers to reach the plugs and the under-valve-cover harness, which is why you replace the valve cover gaskets during the job.
How do I know which glow plugs are bad? Measure each plug's resistance with an ohmmeter. A dead plug reads open or very high, a good one reads a low couple of ohms.
Why is my 7.3 still hard to start after new glow plugs? Check the glow plug relay and the UVCH connectors. A weak relay or burnt harness keeps good plugs from heating.
Where we land
Glow plugs on a 7.3 are a satisfying job with a clear payoff: quicker, cleaner cold starts. Test before you buy, work the plugs out gently, torque the new ones lightly to spec, and check the relay and harness while the covers are off. Fix the whole glow system, not just the plugs, and the truck lights right off in the cold.
Shop 7.3L Power Stroke diesel parts at coredieselparts.com. Free shipping over $70, ships fast.
Related guides: - Hard-Starting Diesel: Glow Plugs and Cold-Start Diagnosis - 7.3 Power Stroke Glow Plug Relay: Symptoms and Replacement