I worked for the number one Oldsmobile dealer in the U.S. for 15 years. During that time, I witnessed one of the greatest mistakes in the long history of great mistakes: the conversion of a perfectly good Oldsmobile 350 cubic inch naturally aspirated (carbureted) engine to a diesel engine. Since diesels and cold weather do not play well together, every one of those cars had a block heater.
On days when temperatures were below freezing, there would be dozens of extension cords running out of the shop. They were all plugged into block heaters, trying to warm up the cars because the fuel or motor oil had congealed and wouldn’t start.
Read along as we discuss block heaters, their benefits, functions and problems.
What Is a Block Heater?
A block heater is an electrical device with a built-in heating element that warms an engine’s oil or coolant before first starting in cold weather. The block heater warms up either the coolant, motor oil or the engine’s metal block. This lets the engine reach operating temperature faster.
Fluids that are already warm flow faster and more easily, helping reduce engine wear and tear, improve cold-start performance (especially in diesel-powered cars), reduce tailpipe emissions, and heat the passenger compartment more quickly.
How Block Heaters Work
Frigid temperatures weaken batteries, but they also affect other automotive systems that can keep an engine from starting. Plugging a block heater into an ordinary 120-volt outlet completes a resistive-heating-element circuit.
“The block heater heating element either warms the metal around it, which then warms the fluids, or the heating element is in direct contact with engine coolant or motor oil, warming up the fluids,” ASE and General Motors Master Technician Alvin Bud Mangin explains. “In freezing temperatures, oil thickens at the same time batteries lose cranking power. Warm coolant and engine oil circulate better, and the engine will crank more easily.”
Bud compared it to starting your boat motor after it sat all winter versus during the middle of summer on a warm sunny day.
Types of Block Heaters in freezing temperatures
There are numerous types of engine block heaters. Most can be installed by a DIYer, but leave it to the pros if you’re installing freeze plug (core plug) block heaters.
- Freeze plug block heaters are the most effective type and are installed in place of a freeze plug, heating coolant from inside the block.
- Pro Tip: One of the great automotive myths is that as water or coolant in an engine freezes and expands, the freeze plugs pop out to keep the block from cracking. Here’s the truth: the holes in the engine block are there to remove sand after an engine block has been cast. Freeze or core plugs simply seal the hole.
- Oil pan heaters attach to metal oil pans with magnets or to composite (plastic) oil pans with self-stick adhesive pads. In addition to improved lubrication at startup, warmer oil reduces idling time, thereby increasing fuel efficiency.
- In-line coolant heaters are usually inserted into a heater hose. They heat up quickly and may use a small pump to circulate warm coolant through the engine.
- Dipstick heaters are the least effective block heaters, but are easy to install. They replace an engine’s oil dipstick, warming the engine oil.
- Battery warmers, while not exactly a block heater, can be used with or without a block heater to keep your battery warm, delivering greater cranking power during severe cold weather.
Benefits of Using a Block Heater
Mangin also noted that warmer fluids on cold wintery days provide several advantages, including:
- Less strain on the starter, battery, and internal engine components.
- Reduced engine wear by oil immediately lubricating and protecting bearings and cylinder walls.
- The engine reaches peak operating efficiency faster, reducing fuel consumption and lowering emissions.
- Heater and windshield defroster deliver warm air much sooner.
When Are Block Heaters Necessary?
Block heaters were primarily designed for climates where winter temperatures regularly drop below 0 degrees F. Manufacturers such as Ford, General Motors, Toyota and Subaru recommend plugging in a block heater around 5 degrees F.
Although newer diesels are less problematic in cold weather than diesel engines of a decade ago, a block heater is definitely warranted. “Consider using a block heater if your battery or car is a little older and you’re trying to increase battery life and reduce engine strain,” says ASE and Toyota Master Technician Joe Simes… or if you just want to get into a toasty car on a frigid morning.
FAQ
How long should I leave a block heater plugged in?
“Most engines [block heaters] only need to be plugged in two to four hours,” Simes explains. “After a few hours, temperatures across the entire engine gradually warm and stabilize.”
Simes recommends using an outdoor-rated timer to automatically turn the block heater off to help save energy.
Is it safe to leave a block heater plugged in overnight?
Yes, but it’s not necessary. “Block heaters are designed to run safely for extended periods,” says Simes, adding that “it’s important to apply common sense when working with anything electrical.” This includes:
- Use the correct wire size/gauge, an outdoor-rated extension cord and a rated circuit/outlet for your specific block heater.
- Keep all plugs and connections off the ground and away from water or puddles.
- Never use damaged, frayed or repaired cords.
- Make sure the outlet and extension cord are protected from the weather. Remember, even in freezing temperatures, the sun’s UV (ultraviolet) rays can damage electrical cord coverings.
“Block heaters are excellent for preheating the engine and its fluids before starting on cold mornings,” says Simes. “As a firefighter, I have seen car fires from lack of proper [block heater] maintenance.”
Experts
Bud Mangin has over 50 years of experience in the automotive industry as a master technician, shop foreman, service manager and service director. During our interview, he recalled how we had to replace block heaters in dozens of Oldsmobile diesel engines.
Joe Simes has over 25 years of experience in the automotive industry as an ASE and Toyota Master Technician, a Pennsylvania Department of Education certified automotive technology instructor, as well as a PA certified emissions and safety inspection instructor and inspector. He is currently an automotive technology instructor at North Montco Technical Career Center in Lansdale, PA. Joe was also the former deputy fire chief and training officer for Montgomery County, PA (retired).
Resources
- AutoZone: “What Are Engine Block Heaters?”
- Canada Drives: “Does My Car Need a Block Heater During Winter?”
- Engineering Fix: “What Is a Block Heater in a Car and How Does It Work?”
- U.S. Department of Transportation, NHTSA: “Engine Block Heater”
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