RV Engine Overheating: What to Check, and in What Order

First: what to do when the gauge climbs

An engine that is running hot is telling you to stop, and every maker says the same thing about the order: stop, shut it off, and find the cause before restarting. Ford's manual is direct about the last part: "Do not restart the engine until the cause of overheating has been resolved."

The gauge itself has a defined meaning rather than a vague one. Ford: "If the needle enters the red section, the engine is overheating. Stop the engine, switch the ignition off and determine the cause once the engine has cooled down."

Once it is cool, the first thing to look at is the coolant, and the cap is where people get hurt. Ford's warning: "Do not remove the coolant reservoir cap when the engine is on or the cooling system is hot. Wait 10 minutes for the cooling system to cool down. Cover the coolant reservoir cap with a thick cloth to prevent the possibility of scalding and slowly remove the cap." Spartan's diesel manual carries the same warning with one addition that matters if you are topping up at a truck stop: "Do not add cold coolant to a hot engine as serious damage to the engine could occur."

The two numbers that decide it

Most overheating questions come down to which of two things is happening, and the makers publish a number for each.

Is it losing coolant? Ford gives the threshold: "If you have to add more than 1.1 qt (1 L) of engine coolant per month, have your vehicle checked as soon as possible. Operating an engine with a low level of coolant can result in engine overheating and possible engine damage." A system that holds its level is a system whose coolant is not the fault, which moves the search to the air path.

Is the belt doing its job? Spartan publishes a deflection test that needs no gauge: "Check for adequate belt tension and unusual belt wear or shiny spots. In the absence of a gauge to check belt tension, apply a firm force to the belt (approximately 30 LB) halfway between the 2 pulleys. The belt should press in no more than 1/2"." A belt that moves further than that, or one with shiny spots where it has been slipping, is a belt to replace rather than adjust.

The coolant, and what goes in it

The level is checked cold. Spartan: "Coolant level in the reservoir should be at or above the sight glass when the engine is cold." What goes in is decided by the label on the component: "Use the fluid specified on the component label. Never mix different types of coolant."

Ford's manual adds the rules that decide whether a top-up helps or hurts:

• Use prediluted coolant to the right specification. "It is very important to use prediluted coolant approved to the correct specification in order to avoid plugging the small passageways in the engine cooling system."
• Do not mix colors or types. "Do not mix different colors or types of coolant in your vehicle. Mixing of engine coolants or using an incorrect coolant may harm the engine or cooling system components and may not be covered by the vehicle Warranty."
• Do not add sealers. "Do not use stop leak pellets, cooling system sealants, or non-specified additives as they can cause damage to the engine cooling or heating systems."
• A color change is not a fault. "During normal vehicle operation, the coolant may change color from orange to pink or light red. As long as the coolant is clear and uncontaminated, this color change does not indicate the coolant has degraded nor does it require the coolant to be drained, the system to be flushed, or the coolant to be replaced."

Concentration is the other half. Ford's default is a 50/50 mix, and the manual gives the limits at both ends: "A coolant concentration of 60% provides improved freeze point protection. Coolant concentrations above 60% decrease the overheat protection characteristics of the coolant and may cause engine damage." In extreme heat the manual allows going the other way: "You can decrease the coolant concentration to 40%", with the warning that below 40 percent the freeze and corrosion protection drop.

Water alone is an emergency measure, not a fix. Ford: "In case of emergency, you can add a large amount of water without engine coolant in order to reach a vehicle service location", after which the system has to be drained, chemically cleaned and refilled, because "Water alone, without engine coolant, can cause engine damage from corrosion, overheating or freezing."

The air path

If the coolant is holding its level and the engine still runs hot, the next question is whether air is actually crossing the radiator. Spartan's inspection list is short and worth doing in this order, because each item is a known cause of overheating:

• Radiator and charge air cooler. "Inspect for wear points, loose connections, tubing damage, or punctures that could cause leaks."
• Coolant hoses, pipes, tubes and clamps. "Inspect for wear points, loose connections, leaks, damage, or punctures."
• Engine cooling fan. "Check for nicks, cracks, loose or bent blades, loose rivets, or any other type of damage." Spartan is blunt about what to do with a damaged one: "Do not attempt to repair or continue use of a damaged fan blade. This could cause component failure, personal injury and/or property damage."

On a diesel pusher the radiator sits at the back of the coach, and that placement is the reason a coach that runs cool on the flat can climb on a grade. The rear radiator pulls air through everything the road throws up behind the coach, and a core that is coated with dust and oil passes less air than it did when it was clean. Cleaning it is a service job rather than a roadside one. Ask for it by name at a service interval.

The fan is driven through a fan drive, and the maker's description of the failure mode inverts the usual expectation. Horton's description of these drives: "Electronically controlled viscous fan drives have a zero-voltage 'Fail Safe' design. When there is any type of electrical issue (zero-voltage), an electronically-controlled viscous fan drive will default to full 'on' mode. This keeps the engine from overheating until the vehicle can be repaired." So an electrical fault makes the fan run all the time, not stop. A fan that never seems to engage is a different fault from a fan that roars constantly, and Horton publishes an owner-level test for the first: "You can also check engagement by simply disconnecting the fan drive from the engine harness. The fan drive should engage within 3-5 minutes. Also check for any fluid leakage in and around the fan drive; if leakage is present, the fan drive will also need to be replaced."

The fan drive is also a maintenance item rather than a lifetime part. BorgWarner's Kysor drive manual: "Fan clutch maintenance should be performed at every 'A' PM schedule, at every oil drain or every 25,000 miles (40,225 km), whichever comes first." The clutch lining gets its first check at 100,000 miles and every 50,000 after that.

The belt and the hoses

The accessory belt turns the alternator, the water pump and the fan, which is why a belt that is slipping shows up as overheating rather than as a charging fault. Ford's maintenance schedule puts the interval in writing: at 100,000 miles, "Inspect the accessory drive belt(s)", and "Replace the accessory drive belt(s) if not replaced within the last 100,000 mi (160,000 km)."

Spartan's deflection test, above, is the check that goes with it. Two things to look for while the belt is in front of you: glazing, which shows as shiny spots where the belt has been slipping on a pulley, and fraying at the edges. Either one means replacement rather than adjustment.

The hoses are the other half of the same inspection, and they fail in a way that is easy to miss: a hose that is soft, swollen or hard where it clamps is on its way out even if it is not leaking yet. Ford's schedule inspects the coolant hoses at every service interval, which for a motorhome is every 10,000 miles, 12 months or 450 engine hours, whichever comes first.

What the engine does on purpose when it is hot

Two behaviors look like new faults and are the engine protecting itself.

Fail-safe cooling. Ford: "If the engine reaches a preset over-temperature condition, the engine automatically switches to alternating cylinder operation. Each disabled cylinder acts as an air pump and cools the engine." The manual lists what the driver notices: "Engine power is limited." and "The air conditioning system turns off." An engine that has gone flat and stopped cooling the cab is not necessarily failing; it may be shedding load on purpose to get you off the road.

The chime stages. Ford's warning chime escalates in three steps, and the manual names them: "Stage 1 is a single chime when the engine temperature begins to overheat. Stage 2 is multiple chimes and engine power becomes limited in order to help cool the engine. Stage 3 is multiple chimes and the engine will shut down." A single chime is the point to start looking for a place to stop, not the point to keep climbing.

What is not owner work

Three jobs sit past the line this page draws.

• The cooling fan itself. Spartan's instruction is not to repair a damaged blade at all, and a fan drive replacement means removing the fan and its drive from the engine. The owner-level test above tells you whether the drive is engaging; it does not make the replacement an owner job.
• Anything that opens the cooling system under pressure. The cap warning is the maker's, and it is not a formality: a hot system releases scalding coolant the moment the cap comes loose.
• A radiator that needs cleaning or replacing on a rear-radiator coach. The core is stacked with the charge air cooler and often the transmission cooler, and getting between them is a job for a shop with the coach on a lift.

What is owner work is everything before those: the coolant level and its strength, the cap, the belt deflection, the fan engagement test, the hoses, and the decision to stop driving.

What it costs

The cost of an overheating fault is decided by which branch it lands on, and the cheap branches are the common ones.

At the bottom is the diagnosis and what it usually finds: a coolant top-up, a belt, a hose, a clamp, or a radiator that needed cleaning. None of that is a major part, and all of it is found by the checks above.

In the middle is the cooling system's hardware: a thermostat, a water pump, a fan drive. These are modest parts on a gas chassis and expensive ones on a diesel, and the checks decide which of them is actually needed.

At the top is the radiator itself on a rear-radiator coach, and an engine that has been run hot for long enough to damage it. That is the outcome to arrive at last rather than first, and it is the reason the maker's order starts with the coolant level rather than with the parts.

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