Engine overheats when idling. Top of radiator exploded! Now I must replace radiator. Did bad thermostat cause this, or was the culprit a bad radiator fan relay?

HI run engine until heat gauge reaches half or 90degrees .then feel top and bottom radiator hose for equal temperature ,IF not thermostat faulty. once you have replaced thermostat and coolant with required water dilution run engine by 90/100degrees engine cooling fan should switch on/off automatically .IF this happens system is fine.

the water coming out of the front would be from the water pump, it also would be an indication that the pump is bad, which may have been the reason of why the car overheated in the first place.

overheating has many symptoms, not just green on the ground
do tell?


temp gauge lies its buns off... use a real tool called a thermometer to see real temps. we use a IR pyro gun to see it and not GET burnt.

gauges is telling truth, so......................


a bad rad cap, or bad thermostat are common fails.
test one: (dead cold engine) never hot.
radiator tank filler cap off , you look down there, is the level really low?
(forget the silly side expansion tank for now)
if found low , then fill it and burp it. no air allowed there
running 50% antifreeze now? no?, that its WRONG.
the tool is $2, test it. (Prestone sells it)
the coolant is all rusty color, (ill not comment ) try service.

1. Green on the ground,? for passengers feet.
   
2. Water exploding from overfill side tank? ( oil in RAD or Milky engine oil) (same milk in transmission pan?)Exploding gases, on cold start is bad head gasket or warped head or cracked head/block, or just simple overheating, The Rad leak down test and the exhaust test (green/blue ) tests solve this riddle easy.
3. Steam rising from hood or Radcap, or hose to Cap and recovery tank. Over heating or leaks.
4. Dash gauge pegged. (or WILD GAUGE swings "air pockets below thermostat") or gauge bad. sender bad.
   
5. Motor pinging and loss of power. (do not allow pinging, it will wreck the engine)
6. Pistons landing on ground? (just joking....)
7. A/C dies?, does it?, due to A/C engine overheat switch ,tripping off. a 235 ?F (spec) see FSM. A/C means Air conditioner clutch dead.
8. Does the engine over heat , if left idling a long time (30min-1hr). or only at stop signs or red lights? (clutch fan not activating , check it, this way.)
9. Does it only over heat while moving ? ( a bad pump, or Rad)
10. Overheats, only on hills? (bad RAD)
11. Only overheats pulling heavy loads ? (inside, full of bricks, or pulling a 1000 trailer ? ONLY?
12. Never overheats moving fast on flat ground. (so the RAD fan is dead, no mater if stock or hacked in electric fan, it's DEAD)
13. Do you have carbon or oil floating around in the Rad, filler CAP neck top tank ? yes?, that is bad head gasket or worse cracked block/head. Do the Exhaust test.
14. Overheats under all conditions, Catastrophic overheat. ( can be bad pump (rotor gone) or RAD or system is packed up)
15. Seems to overheat , even at first start , in 1min or less I have gases shooting into the overfill tank, that is a bad head gasket (mostly a warped head, at least)
16. Antifreeze (and water) in the pans, Engine or Transmission.
17. The RAD fan is DEAD ( just because a mechanical RAD fan coasts, don't mean it WORKS RIGHT.) See the clutch tests below. if electric fan , there is a test in the FSM covers this 100% the fan must come on over heated,.
18. my electric fan , starts to run idling engine long time at 225F. It kicks on, some cars have 2 fan and staged, come on trip points.
19. RTM read the manual the FSM>
20. there is more im sure, cant think of them all now.

That pyro gun allows me to test any RAD easy , ask.
ok , i check all rad tube top and bottom and bingo find any tube clogged fast. if all are equally clogged.(partial) i cant find that.
that is caused by zero service, so...and there are signs for that too.

Remove the thermostat and run the engine at idle until it starts to overheat. If it does then the radiator may have clogged veins and needs replacing.

replace thermostat.then bleed coolant system, this is how you do it.fill the cooling coolant system with 50 / 50 dexcool and water to raise boiling point,straight water will evaporate boil away cause engine to overheat catch fire thats why you need 50 / 50 mixture antifreeze and water.to bleed coolant system first fill coolant system until coolant stay at the full cold mark on the the coolant overflow jug,then you start engine allow it to idle,place the heater and ac control in any ac setting except max and the temperature control to the highest setting,run engine until lower hose to water pump is hot. with the engine hot run the engine speed up to 3000 rpm and allow it return to idle, repeat this 5 times. slowly open the bleed valve on the thermostat housing or look for bleed valve at the water pump heater hose.open bleed valve for 15 seconds to expel any trapped air.then top off the coolant as necessary,and when top radiator hose hot thermostat open up.keep adding cooling to overflow until coolant level stop at full cold mark dont over fill coolant.if engine seem like overheating while looking at cooling gauge turn off engine wait a while until cool off add more coolant until coolant level stay stable and dont drop any.when open radiator overflow cap use large rag over the radiator cap to keep from getting scald open radiator cap a little at a time but dont open radiator cap until engine cool down first.replace the radiator overflow cap, thermostat,and engine coolant temperature sensor and check the cooling fans fuses and relays see what happens.

Bad thermostat

CAR OVERHEATING FIRST CHECK FOR LOW COOLANT IN THE RADIATOR COOLANT OVERFLOW JUG.IF COOLANT LEVEL IN OVERFLOW JUG LOW REFILL ADD COOLANT BACK TO THE FULL COLD MARK.PUT RADAITOR CAP ON COOLANT OVERFLOW JUG START ENGINE LET IT IDLE UNTIL IT OPERATING TEMPERATURE.MAKE SURE TOP RADIATOR HOSE GETTING HOT WHILE ENGINE WARM UP.IF TOP RADIATOR HOSE DONT GET HOT WHILE ENGINE IDLING YOUR THERMOSTAT COULD BE STUCK CLOSE CAUSING CAR TO OVERHEAT.IF TOP HOSE IS GETTING HOT.CHECK FOR COOLANT LEAKS CHECK TOP RADIATOR HOSE AND BOTTOM RADIATOR HOSE FOR LEAKS.CHECK FOR LEAKS AT THE RADIATOR CORES AND PLASTIC SIDE CONTAINERS.IF ALL IS GOOD ENGINE OVERHEATING WHILE IN A LONG TRAFFIC LINE, COOLANT FANS NOT COMING ON.COULD HAVE FAULTY ENGINE COOLANT TEMPERATURE SENSOR OR BLOWN COOLING FAN FUSE OR RELAY.COULD HAVE PCM FAULT.MAKE COOLING FAN ELECTRICAL CONNECTORS AND WIRING NOT DAMAGE OR BROKEN.

I can't answer that question for you without seeing the vehicle, but I can give you another possible cause. It could be that the radiator is plugged.
The thermostat is easy to replace and if you haven't changed it before, it would be a good place to start. If that doesn't solve the problem, have a qualified radiator shop test the radiator for you.
If it doesn't overheat when idling, it's probably not related to the radiator cooling fan, but it's possible.

ENGINE NOT GOING TO START IF GET TOO HOT FIRST THING I WOULD REPLACE THERMOSTAT AND RADIATOR PRESSURE CAP THEN ADD COOLANT UNTIL COOLANT LEVEL CORRECT THATS WHEN THERMOSTAT OPEN UP AND TOP HOSE HOT.CHECK FOR LEAKING RADIATOR HOSES CHECK TOP RADIATOR HOSE CHECK BOTTOM RADIATOR HOSE.CHECK FOR LEAKS AROUND THE RADIATOR LOOK DOWN BELOW COOLANTS FANS CHECK FOR LEAKS AT RADIATOR CORES OR LEAKS AT THE RADIATOR PLASTIC SIDE CONTAINERS. CHECK FOR LEAKS AT WATER PUMP WEEP HOLE AND CHECK FOR LEAKS AT THE HEATER HOSES AROUND ENGINE BLOCK TO THE HEATER CORE HOSES. THERE IS A REASON CAR OVERHEAT.IF CAR OVERHEAT WHILE DRIVING YOU HAVE FAULTY THERMOSTAT OR LEAKING OUT COOLANT OR BLOWN HEAD GASKET.IF ENGINE OVER HEAT WHILE SITTING DURING A LONG IDLING PERIOD.YOU COULD HAVE FAULTY COOLANT TEMPERATURE SENSOR OR FAULTY COOLING FAN FUSE OR RELAY.HOT WIRE COOLANT TO SEE IF IT RUNS IF NO REPLACE FAN MOTOR. IF YES FAN MOTOR COULD HAVE FAULTY WIRE OR PCM FAILURE. CHECK ENGINE CRANKCASE IF OIL LOOKS LIKE MILK SHAKE ENGINE HEAD GASKET LEAKING.

if you mean "thermostat" valve .. then that can fail either open or closed . if it fails open then the engine will not reach normal operating temperatures .. if it fails closed then yes the engine will overheat .. another common failure is the radiator fan .. it only runs when the car is not moving .. as long as the car is moving then cooling air is available so the fan is not needed ... so if your car does not overheat when driven but overheats when stopped or idling then suspect the radiator fan or thermal switch (mounted on the radiator) .. that switch turns on the fan when the radiator gets so hot ... you can just listen for the fan when hot .. you should hear it .. .. if your car gets too hot while driving then suspect a bad thermostat or clogged radiator .. sometimes a bad water pump (broken implellers) will cause the trouble ..