Overheating Issue

[zfifty]

My 98 Expy has a problem where it will be running at normal operating temperature, suddenly peg the temp gauge to hot and the service engine light comes on and there is a loss of power. If you turn the ignition off and wait a minute, the temp gauge will return to normal and it operates normally. This happens when I am stopping and going. Never happens when cruising the highway. Any thoughts at all here? (There are no signs of the vehicle actually being overheated.. ie noticeable boiling sounds, sounds of pressure trying to be relieved from cooling system) Bad sending unit? Any advice appreciated.

 

[Don Hall]

Check the radiator level before start-up. Fill, if necessary.
Check the coolant reservoir before start-up. Note the level.
Check the coolant reservoir when hot. The level should be higher.
Check the coolant reservoir after cool-down. The level should be lower.

You may have insufficient coolant at start-up. Fans not activating, or, a faulty gage/sensor.

If the coolant level doesn't reduce after cool-down, the siphon has been negated
due to a break in the closed system. This would cause a low level of coolant in
the radiator.

In stop-go traffic, the fans may not be activating. At speed, the problem is not evident
due to sufficient air flow.

 

[zfifty]

Thanks Don, I will do the cool and hot level checks and post results.

 

[Jim Brown]

If your engine is overheating in traffic but not on the highway, consider the fan clutch or loose belt. However, I recommend checking a few things first, starting with the coolant level.

Like Don said, first make sure the cooling system is properly filled and bled. Fill the degas bottle to between the coolant fill level marks. Select maximum heater temperature and blower settings. Put the discharge control to blow at the instrument panel (not defrost, not floor). Start the engine and allow to idle until normal operating temp, while it's idling feel for hot air at the A/C vents. The temp gauge on your dash should read in the middle of the normal range and the upper radiator hose should feel hot to the touch. Let the engine cool and top off the degas bottle to between the marks.

The coolant should be in good condition; consider having the system flushed if there are contaminants.

If you have to keep adding coolant then check for leaks. External: Look for damaged hoses, loose or damaged hose clamps, leak around the thermostat neck and water pump gasket and seal, leak at the intake manifold, radiator, degas bottle, degas bottle cap, and heater core. Internal: look for signs of engine or transmission oil in the degas bottle. You can get a loaner pressure tester tool from auto parts store but you have to leave a deposit that you get back after you return it.

Once you know the system is properly filled, bled, and no leaks, then you can check components:

a. Air flow obstruction - check the A/C condenser and radiator for dirt, leaves, mouse nests, or anything that could stop air flow.

b. Heater core - Start the engine and select max heater and blower settings. After about 5 min both heater hoses should be about the same temperature. If they're not, you may need a new heater core.

c. Thermostat - let the engine run for ten minutes. Feel the upper radiator hose and both heater hoses. If they're still cool then replace the thermostat.

d. Fan - with the engine off, spin the fan, it should have slight resistance. If there's no resistance or heavy resistance the fan clutch may need to be replaced. The fan and water pump are driven by the serpentine belt, the belt and tensioner should be in good condition or it may be loose and not drive the fan at proper speed. Checking fan max and min speeds involves using a strobe light to compare pulley and engine rpm. There's a test procedure I can share if you want it. Beware cheap fan clutches don't last long, most only come with 1 yr warranties. Motorcraft and other high quality but more expensive fan clutches have 3 yr warranties and should last longer.

e. Overheating may be a symptom of a failing water pump.

f. If all the mechanical components seem to be working, consider the temperature sensor or wiring harness.

This isn't an exhaustive list; there's more but it's a start and you may find your solution.

Good luck, Jim

 

[Yupster Dog]

From your explanation it sounds like a intermittent grounding.
Start vehicle and carefully wiggle the wire that connects to the coolant temp sensor to see if you get that loss of power and have someone watching the gauge to see if it pegs.

 

[ExplorerTom]

^agreed. If it “overheats” and goes back to normal after cycling the ignition, it’s electrical, not mechanical.

 

[Jim Brown]

If the mechanical cooling system is okay, put a scanner/ code reader on it and look at live data. You can see what the engine computer is reporting real time for engine coolant temperature as you drive around. Don't rely on the gauge on your dash; on my 2001 expedition with 5.4l engine the gauge always stays at the same position, regardless if the temp is 180 or 215. You can get an Autel 319 for about $40. You may also be able to read a code while the check engine light is on.