If you drive a diesel truck in the Dayton area, you know what a January morning feels like when it's 10 degrees and you need to get to work. You turn the key, wait for the glow plug indicator to cycle off, and hit the starter. A healthy diesel fires within a few seconds. A diesel with a failing glow plug or grid heater? It cranks and cranks, maybe catches and runs rough for a minute, or doesn't start at all.
Fall is the time to check your diesel's cold-start system — before you're standing in a frozen driveway at 6am wondering whether to call a tow truck or try one more time.
How Diesel Cold Starting Works
Diesel engines don't have spark plugs. They ignite fuel through compression — the piston compresses air in the cylinder until it's hot enough to ignite diesel fuel when it's injected. This works well when the engine is warm, but in cold weather, the cylinder walls, pistons, and incoming air are all cold. The compressed air may not reach ignition temperature without help.
That's where glow plugs come in. Glow plugs are small electric heating elements — one per cylinder — that heat the combustion chamber before and during starting. When you turn the key on a diesel, the "wait to start" light on the dashboard indicates the glow plugs are heating. Once they reach operating temperature (usually 3-10 seconds depending on ambient temperature), the light goes out and you crank the engine.
The Cummins exception: The 6.7L Cummins uses a different approach — an intake grid heater that heats incoming air before it enters the cylinders, rather than glow plugs that heat each cylinder individually. The concept is the same (heat the intake charge), but the components and failure modes are different.
5 Signs Your Glow Plugs Are Failing
1. Extended Crank Time in Cold Weather
The most obvious symptom. Your diesel takes noticeably longer to start when it's cold. Instead of catching within 2-3 seconds of cranking, it spins for 5, 10, or 15 seconds before firing. As temperatures drop further, the problem gets worse.
What's happening: One or more glow plugs aren't heating properly, so one or more cylinders aren't reaching ignition temperature. The engine eventually starts on the cylinders that are firing, but it takes longer.
2. Rough Idle After Cold Start
Your diesel starts, but it runs rough for the first 30-60 seconds — misfiring, shaking, uneven idle. It smooths out once the engine warms up.
What's happening: The cylinders with failed glow plugs aren't firing consistently until the engine heat alone is sufficient for combustion. Once the engine reaches operating temperature, the glow plugs aren't needed anymore and the rough running stops.
3. White or Gray Exhaust Smoke on Startup
A puff of white smoke on a cold start is normal for any diesel. Heavy, sustained white smoke that lingers for several minutes is not.
What's happening: Unburned or partially burned diesel fuel is passing through the cylinders that aren't igniting properly and exiting through the exhaust. The white smoke is essentially raw fuel vapor. Once the engine warms up and all cylinders fire, the smoke clears.
4. The "Wait to Start" Light Behaves Differently
On most diesel trucks, the wait-to-start light illuminates for a few seconds when you turn the key, then goes out. If the light doesn't come on at all, comes on but goes out immediately, or stays on much longer than usual, there may be an issue with the glow plug system or its controller.
What's happening: The glow plug control module monitors the system and may detect a fault. On some trucks, a glow plug circuit failure triggers a check engine light rather than a change in the wait-to-start indicator.
5. Check Engine Light with Glow Plug Codes
Modern diesel trucks monitor glow plug circuit resistance. If a glow plug fails or its circuit has high resistance, the engine computer logs a fault code and may illuminate the check engine light.
Common codes: P0380 (glow plug circuit malfunction), P0381 (glow plug indicator circuit), P0382 (glow plug circuit B), and individual cylinder glow plug codes (P0670-P0683 depending on the engine).
Cummins Grid Heater: Different System, Similar Symptoms
The 6.7L Cummins doesn't use traditional glow plugs. Instead, it uses an intake grid heater — an electrically heated element in the intake manifold that warms incoming air before it enters the cylinders.
The grid heater is powered by a relay, and that relay is the most common failure point. When the relay fails, the grid heater doesn't activate, and the engine behaves exactly like a gas truck with failed glow plugs would — extended cranking, rough start, white smoke.
The good news: The grid heater relay is a relatively inexpensive part. The bad news: if you don't know to check it, you might spend time and money diagnosing other causes.
At Murphy's, we test the grid heater circuit as part of any cold-start diagnosis on a Cummins.
Why Fall Is the Time to Check
Glow plugs can fail gradually — they may still work in moderate cold (40°F) but fail to heat adequately in severe cold (0°F). If you wait until January to discover the problem, you're dealing with a no-start situation in the worst possible conditions.
A pre-winter glow plug check takes about 30 minutes. We measure the resistance of each glow plug (or test the grid heater circuit on a Cummins) and identify any that are out of spec. If we find a failed plug, we can replace it while the weather is still mild and our schedule is more flexible.
Ohio winters are no joke for diesel trucks. Dayton averages about 25 days per winter with temperatures at or below freezing. Extended cold snaps in January and February regularly push into the single digits. A diesel truck with a failing glow plug system in those conditions is unreliable at best and stranded at worst.
How Long Do Glow Plugs Last?
Glow plugs are a wear item, but they last a long time — typically 80,000 to 100,000 miles under normal conditions. Some last longer, some fail sooner. Factors that accelerate wear:
- Frequent short trips in cold weather (more cold starts = more heating cycles)
- High-mileage engines with blow-by (combustion gases degrade the plug tips)
- Voltage irregularities from a weak battery or alternator
When one glow plug fails, the others are typically near the end of their life too. We usually recommend replacing all of them at once rather than chasing individual failures.
Schedule a Pre-Winter Diesel Check
Don't wait for the first hard freeze. Murphy's Autocare can test your diesel's cold-start system at either location — glow plugs, grid heater, batteries, and starting circuit.
- Beavercreek: 937-426-2886 — 3815 Dayton-Xenia Rd
- Centerville: 937-433-0140 — 100 N Main St
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