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Cold Weather Starting for Diesel Engines


One of the most popular questions by owners of diesel engines is how to operate the various "preheat" functions on their diesel engine during cold weather. There isn't a whole lot of information in the owner's manual on this and some of the terms can be misleading. Hopefully this article will help clear some of this up.


Diesel engines are different than gasoline powered engines and have both operating and starting techniques that also are different. Gasoline engines burn gasoline, which is fairly volatile and flashes easily. All it takes is a spark from a spark plug at the right time and the cylinder fires. Diesel fuel is much heavier than gasoline and doesn't burn as readily. But, when it does burn, it produces more BTU per gallon than gasoline, which results in greater power and efficiency. A diesel engine fires by compressing the air-fuel mixture to a very high compression ratio, then it fires by itself, without any ignition system or spark required. All it takes is for the fuel to be injected into the cylinder at just the right time and amount. If a diesel engine is real cold, that cold will affect it in two ways. First of all, the drag on a cold, stiff engine will cause it to crank over much slower than a warm engine. All that mass and heavy oil really tends to slow it down and inhibits it's firing process. Secondly, the diesel fuel (just like gasoline) flashes off and fires once it reaches it's ignition temperature. On a gasoline powered engine a spark plug does this but a diesel is relying on the heat generated by compressing this mixture very tightly. If the air-fuel is cold or the cylinder is cold, it may not fire.

Types of Heaters:

There are two ways to help a cold diesel engine get started. One is by warming up the air-fuel mixture entering the engine. The other is by warming up the engine itself so that the cylinder isn't as cold. Each has it's place. Under moderately cool conditions preheating the intake air works fine to get the engine started. It may also allow you to start an engine in really cold weather but your engine will take some time to crank and it'll probably run rough and smoke badly for a while. By preheating the engine itself, you'll not only ensure that your engine will start but you'll minimize the wear and tear on the engine. Remember that the majority of wear on a diesel engine occurs in the first minute after starting. Having a warm engine will minimize this and provide you with better life for your engine.

Glow Plugs:

Originally diesel engines were fitted with glow plugs. These small spark plug like devices were placed in the cylinder head. 12 volts was applied to them and they would glow, producing heat just like a miniature toaster. These were generally tied into the ignition switch and a thermostat and timer was integrated into the system. As soon as the ignition key was turned on a "Wait to Start" light would illuminate on the dash and the glow plugs would begin their preheat process. When the thermostat deemed that they had been on long enough, the warning light would go out and you would start your engine. Glow plugs were not trouble free and required occasional replacements so some engine manufacturers moved them to the intake manifold, where they weren't subject to the harsh conditions inside the cylinder itself. They were less effective this way but held up longer. Glow plugs are typically only used in smaller diesel engines and you won't find them on your class A diesel pusher.

Intake Air Manifold Preheaters:

Today's RV diesels use an intake manifold preheater instead of glow plugs. The system functions similar to glow plugs in that a "Wait to Start" lamp illuminates on the dash when the key is first turned on. After the designated time delay (the colder it is, the longer the delay) the light goes out and you're ready to crank over the engine. The intake manifold preheater is a large grid (think "big" toaster) inside the intake manifold. It warms up the intake air and manifold so that the cylinder receives warmer air for easier starting. These preheaters are much more powerful and reliable than their smaller glow plug cousins and you'll find them on virtually all diesel pushers made today.

Engine Block Heaters:

Engine block heaters have been around forever. You can buy aftermarket units for your car or truck that simply attach to your engine with heater hose. Then you plug these units into 120 volts and the heater warms up your coolant, which in turn warms up the engine. Technically, these are not "block" heaters, they're "tank type" heaters. A true block heater is an electric element that is inserted directly into the engine block. The engine block has an access plate that is removed and the block heater's matching plate is installed. There is a heater element that can be between 750 and 1,500 watts that extends into the engine's cooling jacket. The benefit to this system is that you don't have any heater hoses to worry about replacing or leaking and the heat is put directly into the engine, where it's more efficient. Because these are 120 VAC powered units you need to plug them in someplace to make them work. Recent Tiffin motorhomes have these plugged into a dedicated outlet in a basement bay. This outlet is controlled by a low voltage relay that is actuated by a 12 volt rocker switch on the diver's side console. Many of these coaches have another plug that crosses over to the driver's side and plugs into another outlet. It's best to secure these plugs to their receptacles with nylon cable ties to prevent them from falling out.

Hydro Hot/Aqua Hot Preheat System:

Another type of engine coolant heater is the engine preheat feature available on the Hydro Hot or Aqua Hot hydronic heating systems. Your hydronic heating boiler is filled with boiler type antifreeze. Inside that boiler is a coil that is connected to a pair of heater hoses from the engine. When you drive down the road the engine's hot coolant helps to warm up the Hydro Hot boiler, in effect giving you free heat. However, if your Hydro Hot came with the optional Engine Preheat function then you can also use the heat in the Hydro Hot boiler to help warm up your cold engine when parked. At first this feature was not included on Tiffin's coaches but it now is. If you have a third switch on the Hydro Hot control panel labeled "Engine Preheat", then you have this feature. If you don't have that switch, then you don't have the feature. Although, it can be added later if you want it. It's basically a pump, some hose and wire connections, and a switch. When you flip the switch on, the circulating pump will circulate your engine's coolant through the hot Hydro Hot boiler and pick up that heat, warming the engine. In this way it functions very much like a tank type engine heater exept it uses the heat in the HydroHot boiler rather than a dedicated 120 volt electric element.

How to Use These Heaters:

Part of the confusion is the terminology or naming of these items. Technically they are all "preheaters" because each one will warm up something before starting your engine. Whenever you turn your ignition key switch on, the "Wait to Start" light illuminates. This is when your intake manifold heater is active. As soon as the light goes out, the intake manifold preheater goes off and you are clear to crank your engine. This preheater only stays on for a few seconds and is designed for intermittent duty so it won't stay on for any extended period of time. The engine block heater can stay on pretty much forever. There's no problem with it staying on 24/7 for a week or more. The only drawback is that it will consume electricity, which costs money if you are paying for it. It takes time to do it's job. When it's only "cool" out, it may warm up your engine in one hour. When it's really "cold" outside, you may need to keep it on a minimum of 8 hours in order for it to effectively put enough heat into the engine. The block heater is controlled by a rocker switch on the side console labeled "Engine Preheater". This can get confusing so remember that the Intake Manifold Preheater is automatically controlled by the ignition key switch and only operates for a brief time while the engine block heater is controlled by the rocker switch on the side console and requires quite a bit of time for it to do it's job. If you have the Hydro Hot engine preheater, that too operates in a similar manner as the engine block heater with the exception that you do need to also have the diesel boiler turned on in order to heat up the boiler. You'll find that the Hydro Hot's engine preheater is more powerful than the electric engine block heater and will cut the time required substantially. I've found that what takes 2 hours for my block heater to do can be done in 30 minutes with the Hydro Hot preheater. Note that you can run both the block heater and Hydro Hot preheater at the same time if you want for even faster warm-up.

While your engine may even start at -10 F with just the intake manifold preheater, you aren't doing it any favors by doing this. Your stiff engine will be spinning over with thick oil and you won't have very much lubrication going on. This is the time when scoring and scuffing occurs and you'll take a bit of lifetime out of your engine every time you do that. Once it fires it'll shudder and shake and run unevenly until all of the cylinders come up to operating temperature, much like running a gasoline engine with the choke on. This can foul injectors and build up carbon on the piston heads and in the combustion chamber. The excessive unburnt fuel will run down the cylinder walls, wiping away the lubricating oil from the piston rings and thinning out your crankcase oil. You are always better off using your block heater to minimize this. If you are at a campground for a week or so that is supplying your shore power, just turn the block heater on and leave it on. It won't hurt anything and it's one less thing to remember to do the day before you plan on leaving.


Submitted by Mark Quasius - 12/10/07

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