Diesel engines light their fuel with the heat of compressed air. On a cold engine, the cylinder walls soak up that heat, and glow plugs make up the difference by heating the combustion chamber before and just after startup. When they fail, the engine cranks longer, runs rough and smokes white until it warms up. RDI Power diagnoses and replaces glow plugs at our Brooksville shop.
Which diesels use glow plugs
| Engine | Cold-start aid |
|---|---|
| Ford Power Stroke 7.3, 6.0, 6.4 and 6.7 | Glow plugs |
| Ford 3.2 Power Stroke (Transit) | Glow plugs |
| GM Duramax, every generation | Glow plugs |
| Mercedes Sprinter diesels | Glow plugs |
| Ram 1500 3.0 EcoDiesel | Glow plugs |
| Cummins 5.9 and 6.7 in Ram heavy-duty trucks | Intake grid heater |
The old claim that newer trucks do not have glow plugs is wrong. Modern Power Stroke and Duramax engines rely on them, and many run them after startup as well to cut smoke and noise while the engine warms. Cummins engines use a grid heater in the intake instead, which has its own failure points; see our Cummins service page.
Glow plug symptoms
- Long cranking before the engine fires, worst on cool mornings
- White smoke and a rough idle for the first minutes after a cold start
- A glow plug warning or check engine light with glow plug codes
- A wait-to-start light that stays on oddly long, or not at all
- Misfire that clears up once the engine is warm
In Central Florida, a single failed plug can go unnoticed for months because the engine starts on ambient heat. It shows up on the first cool morning of winter, or as smoke complaints.
How we test glow plugs
- Scan for codes from the glow plug controller and engine computer.
- Resistance or current draw test on each plug to find the dead ones.
- Controller or relay test. The 7.3 uses a glow plug relay; the 6.0, 6.4 and Duramax engines use control modules that fail on their own.
- Harness inspection. On the 7.3 and 6.0, the under-valve-cover harness feeds the plugs and can fail inside the engine.
We also confirm the plugs are the cause. Long cranking can come from low battery voltage or a fuel problem; see starter repair if the engine cranks slowly.
Removal is the risky part
Glow plugs sit in hot cylinder heads for years, and carbon can seize them in place. Sprinter engines in particular are known for plugs that seize and resist removal. Forcing them is how a glow plug job turns into a cylinder head job. We work them out with the engine at the right temperature, penetrating oil and controlled torque, and if a plug breaks, we extract it without removing the head where the design allows.
Glow plug repairs
- Replacing failed plugs, or the full set on high-mileage engines
- Replacing a failed controller, module or relay
- Repairing or replacing glow plug harnesses, including under-valve-cover harnesses
- Extracting broken plugs when necessary
- Clearing codes and confirming operation on a cold start
Platform detail lives on our Power Stroke, Duramax and Sprinter van pages.
If your diesel takes longer to fire or smokes white on cold mornings, bring it in for a glow plug test before the first cold snap.