Winter is right around the corner. For us 6.0 and 7.3 owners, cold weather affects our engines more than any other modern era light duty diesel engine. The 6.0 and 7.3 are both tough as nails, but cold weather exposes weak batteries, tired glow plugs, and fuel system problems fast. An ounce of preparation goes a very long way for our HEUI-powered engines, but also for any diesel. Let’s discuss the most important things we can do to reduce any issues this winter.

 

1. Weak or Dead Batteries

Batteries don't like the cold. The cold reduces the chemical reaction needed, increases current resistance, long story short, batteries simply don't work as well in colder weather.

Symptoms: Slow crank, clicking relay, or no start.

Why it happens: Battery strength issues aside, thick oil and cold temps make these engines harder to turn over, and both batteries need to be strong for the glow plug cycle and starter draw.

Common fixes:

• Replace both batteries if one is weak (one poor battery will slowly kill a good one, always replace both).
• Clean and tighten battery terminals and grounds.
• Use a battery maintainer or plug-in battery tender if the truck sits for extended periods (or has any parasitic draws).

 

2. Glow Plug & Relay/Module Failures

In warm weather, failed glow plug systems may not cause any noticeable issues at all. But in temps around or below freezing, a failed glow plug system will cause hard starts, or even a no-start if the temps are low enough.

Symptoms: Long crank times, white smoke on startup, or rough idle for the first few minutes.

Why it happens: Weak glow plugs or a failed relay/module can’t preheat the combustion chambers.

6.0L Notes: The glow plug control module commonly fails — check for codes (P067x series).

7.3L Notes: The glow plug relay often burns out; test it for voltage drop.

Common fixes:

• Replace bad glow plugs (we highly recommend using Motorcraft).
• Replace the relay/module.
• Verify the system is actually cycling with a voltmeter or scan tool.

 

3. Fuel Issues: Gelling, Clogged Fuel Filters and Water 

Diesel fuel doesn't like cold either. It will begin to develop wax crystals at 30°F and will start to turn into gel at -10°F. Both of these issues will cause filter clogging issues, among other issues. Using additives will stop this from happening. However, most gas stations have additives already built into their diesel winter blends. Check the pump where you get your fuel. There should be a sticker telling you what temps their diesel is good to. If it's not enough or you're still worried, use a quality winter diesel additive. 

Symptoms: Starts then dies, surges, or won’t start at all.

Why it happens: Paraffin wax in #2 diesel thickens in the cold, clogging filters and lines.

Common fixes:

• Add a winter anti-gel (we like Opti-Lube).
• Replace the fuel filter more often.
• Drain the water separator regularly (under the fuel bowl on the 7.3, HFCM on the 6.0).
• Use winter-blend diesel.

 

4. Thick Oil / Slow High-Pressure Oil System

Cold oil is thicker and doesn't flow as well. Oil that flows well is important for any diesel but especially for the 7.3 and 6.0 since they rely on oil for the fuel injection system. Note: Using a different oil in the winter is somewhat controversial in the continental US as most of the country doesn't get cold enough to warrant it. Most experts only recommend running lower weather oils in temps that average below 15°F.

Symptoms: Long crank times or low ICP pressure before start.

Why it happens: Cold, thick oil delays high-pressure oil buildup needed for injector firing.

Common fixes:

• Run 5W-40 synthetic oil instead of 15W-40 in very cold climates. 
• Plug in your block heater (most engines have one).
• Check for HPOP leaks if cranking times are excessive even warm.

 

5. Block Heater or Cord Failure

Plugging in your diesel overnight, or whenever the temps get low can make a huge difference. Tip: use a plug-in timer to control your block heater. This way you can set it to start before you head out in the morning as opposed to it running/heating all night. How long it will need to run depends on where you live. Extreme cold climates may want to leave the heater on at all times, whereas warmer climates may only need it to run for 45mins or so, before engine start up. 

Symptoms: No improvement in start-up even when plugged in.

Why it happens: The element or cord corrodes or burns out over time.

Common tests and fixes:

• Test for resistance between the two power prongs on the plug (~10–15 ohms is typical).
• Replace damaged cords or elements.
• Always use a heavy-duty outdoor-rated extension cord.

 

6. Improper Coolant Used

Mainly, using too much or only water. There's a common misconception we often hear about coolant use: many feel pure water is better. Water does cool better than most pure coolant types, however coolant isn't only used to cool the engine. It also lubricates the water pump, reduces cavitation issues, reduces oxidation, and lowers the fluid's freeze point. Aside from the other mentioned issues, water expands when it freezes, causing cracked blocks, heads, gasket issues and more. If you're running pure water in your system, swap it out for a 50/50 mix or coolant and water ASAP. 

Symptoms: Coolant loss, oil in coolant, external leaks, or visible cracks in the block or heads.

Why it happens:

Water expands when it freezes causing cracks in blocks, heads and more.

Fixes:

• Use the correct ELC (Extended Life Coolant) or Motorcraft Gold coolant with tested freeze protection. C
• Never top off with plain water — always use a 50/50 mix for freeze protection down to at least -30°F. Do not use a mix with more coolant than 50/50 as this can actually raise the freezing point. More is not better here.
• Flush contaminated systems to prevent electrolysis or liner pitting.