My lessons learned while trying to keep my 1999 Jeep Grand Cherokee with a 4.0L engine cool.
Below are a checklist of possible causes of engine overheating.
ELECTRIC FAN: The electric powered fan should come on when the engine reaches xxx degrees F. Reach down (with the engine off) and try spinning the electric fan. Does it run freely? Does it continue to spin and take a few seconds to slow down and stop? If the fan doesn’t spin freely, there might be something blocking the blades or the motor is seized up and not moving.
The electric fan should automatically start to spin with the engine on and the air conditioner set to it’s maximum settings. You might need to wait for the engine to get warm before the fan starts to spin.
RADIATOR CAP: Check to see that your radiator cap is working correctly. The spring should be stiff. Only at extreme temperatures should the spring open up and release the extremely hot water into the plastic container near the passenger’s side of the engine.
FUSES: Check fuses with an ohmmeter or visually look to see if they are broken or burnt inside.
RELAYS: Swap relays in order to troubleshoot if there is a faulty relay. There are several relays used by the Jeep that have the same part number. Swap the relay for horn with the relay for the fan. If after swapping the fan works but now the horn doesn't work, then you have a faulty relay.
WATER PUMP: Feel the pressure on the upper radiator hose while the engine has been running and is warmed up. Use hand gloves if the engine is hot so you don’t burn your hands. If you can easily squeeze the hose and collapse it while the motor is running, then your water pump doesn’t have enough power to move the water through the engine. If the hose is stiff and you can feel the motion of the water running through the hose, then you have good circulation and water pressure coming from the pump.
Common problems with water pumps are the propeller spins on the shaft and doesn’t force the water into the engine to provide cooling.
THERMOSTAT: A blocked thermostat could cause a lack of pressure in the top radiator hose and also create overheating.
LEAKS: If you are losing radiator fluid, then you have a leak. Locate the area of the leak. Is water coming from under the water pump? Water pumps will break at the center bearing and leak water before the bearing breaks off completely exposing the propeller inside. You will lose all the water at that time and overheat.
COOLANT TEMP SENSOR: Check your coolant temp sensor mounted on the front of the engine on the top of the water pump. A wrong reading might keep the electrical fan from coming on. Generally a code on the OBD-II will show if the sensor has failed.
OIL FILL CAP: Open the oil fill cap to see if there is any yellowish foam showing. This could mean you have coolant water leaking into your crankcase and mixing with the engine oil. This could show that you have a cracked head gasket or engine block. Seek a service professional's help if you find foam in the oil.
CURRENT CARRYING WIRES: Check the metal pins of the fan connections. The fan draws a lot of current while running and any connection that isn’t clean will eventually build up a carbon deposit and fail. Replace any connector that doesn’t have a shiny metal look to it. If it’s blackened and the plastic is burnt around the metal, the connector should be replaced.
Remove the fan harness and apply 12v and see if the fan turns. This is not a true test because even though the fan turns while connected directly to the battery, the motor might need a lot of current to turn the fan and that current might not be available from the fan relay module. I had a fan that would turn with 12v but not while connected to the relay. I replaced the motor and it started working great. The fan relay is located below the passenger-side headlight. You will need to cut an opening in the plastic to get access to it.
SERPENTINE BELT: A worn out or slipping serpentine belt can lose its friction on the drum of the water pump and not turn the water pump properly. Check the belt to see if it’s cracked or needing to be replaced.
FAN MOTOR: You don’t need to replace the whole fan assembly and shroud. It’s possible to replace the motor only.
RADIATOR: Once I had a busted radiator and it needed to be replaced. Just by changing the radiator, I got a noticeable improvement on the temperature of the engine. The new radiator kept the engine much cooler. Over time the inner cooling pipes of the radiator build up a crusty corrosion and isolate the hot water from the cool metal pipes and it reduces the cooling. You should replace the radiator fluid on a regular time schedule or try to flush the corrosion out using a pressured water hose or service shop. If you ever need to replace the radiator, I recommend a metal radiator has more water-to-metal cooling surfaces and provides more cooling for the engine. However the metal body radiators are more expensive.
Radiators on the Jeep should be the hottest at the upper hose area. Feel along the bottom of the radiator and if the bottom fins of the radiator are cooler on the passenger’s side, you could have blockage in the radiator. The tiny tubes of the radiator travel from the passenger side reservoir to the driver side. The cooling happens in the tubes with the fins which should have air flowing through the fins to cool the tubes. The temperature should be warmest on the passenger side and coolest on the driver’s side.
RADIATOR FINS: Keep the radiator fins clean and free of bugs and debris that ends up clogging up the fins. Air should be able to flow freely through the fins and small opening of the radiator. Use a high pressure water hose to blast off the bugs, leaves and dirt that gets trapped in the openings. If water can’t penetrate through all the bugs and debris, then air will have an even tougher time. Air traveling freely through the openings is what keeps the water in the radiator and the engine cool.
HEATER COIL: Do you have water on the passenger’s floorboard? Do you have a fog on the inside of your windshields? Can you smell radiator fluid on the inside of the vehicle? The heater coil is located near the passenger’s feet inside the dashboard to the left of the fan motor. There isn’t an easy way to view or replace this coil. It requires the dashboard to be removed in order to replace the heater coil.
OVERHEATING WHILE DRIVING: If you are driving and the engine starts to overheat, turn on the heater with the fan and temperature on the maximum settings. This will blow air through the heater coil located inside the dashboard. The heater coil is connected to the radiator and will help to cool the engine while running. If the temperature continues to rise and overheat, pull over before you crack the engine block , split a head gasket or worse.
FAN CLUTCH: Add a clutch fan with a “reverse rotation” to the water pump shaft. The reverse rotation fan has blades bent in the correct direction so air flows from the radiator to the engine. Most jeeps already have the room and a shroud made for a mechanic fan located on the front of the engine and mounted to the bell of the water pump.
The clutch has an oil filled cavity that allows the fan to spin freely without being mechanically connected to the shaft and putting a load on the engine. The fan will run at the same time as the engine and never turn off while the engine is running like an electric fan will.
This fan is installed as an optional “towing package” on new Jeeps but there isn’t an easy “replacement fan” listed on most auto parts websites. Most websites don’t list a mechanical fan for a Jeep Grand Cherokee or they list the wrong fan and clutch.
Get the Hayden Automotive 2736 Premium Fan Clutch and the Hayden Automotive 3620 Fan Clutch Fan . I also had to buy four 5/16 inch -18x3/4 inch screws and lock washer to mount the fan to the clutch.
I needed to add a spacer (I used an old nut and washer as a spacer) inside of the clutch shaft so the fan blades won’t rub against the other components of the engine.
I dropped one loose nut and one loose washer into the shaft of the fan as a spacer to keep the fan blades from rubbing on the idler pulley.
I dropped the washer and nut into here before screwing onto the shaft of the water pump.
With the washer and nut inside the fan clutch threads, I screwed it onto the water pump tightly with a wrench on the nut and a wrench holding a nut on the bell of the water pump from turning.
Make sure the fan blades don't rub or hit anything else in the engine. There should be at least an inch of clearance around the fan in all directions. I got close to an inch from the idler pulley. I should probably add more spacer material inside the shaft to increase the spacing between the fan blades and the idler pulley.
After installing the fan and clutch and making sure there are no parts hitting or rubbing, you will immediately see an improvement on the engine temperature during long idles in summer traffic. The engine temperature stays a constant temperature even when traffic is not moving.