Overheating after head gasket, radiator flush, and new thermostat

I recently replaced the head gasket, flushed the coolant and checked for blocked cores, and replaced the thermostat. Unfortunately, my car (2003 Mitsu. Eclipse 4 cyl) still overheats, though not as much.

Last night i took the thermostat out and drilled a TINY hole in the housing to allow for the passage of more coolant. It still overheats. Would enlarging the hole help, or should I budge for the new water pump?

Also, could belt tension have any effect on the performance of these parts? I’d never replaced belts, and was unsure of the whole process…

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Enlarging the hole will not help. Under what driving conditions does it overheat? i.e., idling, high speed, etc.

Make sure you electric fan is working properly. Temp sensor in the radiator could also be the problem. Even if the radiator is flowing properly, if the sensor has debris around it, it cannot read the correct water temp.

it overheats after it has been running. the harder it’s driven, the quicker it overheats, though it generally doesn’t reach higher than about 3/4. idling doesn’t affect it much. Running the heater brings it back down to normal operating temperature.

The fans have been checked. I cleared the sensor and checked the fluid last night while working on it. Also made sure the system was bled.

Also, if it does turn out to be the water pump, as long as I am not overheating (assuming i find a work around) there will be no damage to my vehicle, correct?



What is the idea behind the hole in the thermostat and why is it not standard on all new thermostats? It certainly can’t allow that much more coolant to flow, I can see it being used as a way to prevent a blockage due to air in the system (which should be able to be bled normally).

I have heard of the DIYer doing the “hole in the thermostat” thing for 30+ years but no professional mechanics (at least that I knew) ever used it.

OP, myself I think you have a headgasket sealing problem that allows exhaust gases to contact the coolant, even in the smallest amount, or a cracked or not flat head.

Water pump flow is easy enough to check. Usually, what you need is a new radiator…

I just replaced the head gasket, and made sure of it’s sound placement.The hole is to allow more coolant to pass through the system, though it was mostly me grasping at any possible solution.


How exactly does a radiator stop functioning correctly? The cores aren’t plugged and the fans work. What else is there?

How do you know the radiator is not partially clogged? If you peer into the radiator the upper holes you see in the crossover passages may be clear but the clogged ones are often down low; out of sight where you can’t see them.

The hole in the thermostat is unnecessary but I wouldn’t worry about it at this point. For years Subaru used a T-stat with a 1/8" hole in it from the factory. This hole was outfitted with a small loose-fitting brad in it. It was referred to as a “Jiggle Pin” and was designed to bypass a tiny amount of coolant into the radiator at all times.
Pointless in my opinion but I didn’t design or build the cars.

A small hole in the outer ring of a thermostat prevents the drastic drop in temperature at hot spots in the heads in engines without recirculation passages engineered into the system. In all engines with leaking heads/gaskets, it prevents the accumulation of exhaust gas under the closed thermostat from insulating the thermostat and slowing its opening which causes the “dry” upper cylinder head temperature to go ballistic, which is indicated by temperature gauges climbing well beyond what is normally seen, then suddenly dropping. I drill all thermostats to make filling and bleeding cooling systems faster and simpler. How many have filled radiators and started the engine and W-A-I-T-E-D for the thermostat to open to see the radiator tank level drop to finish filling it? I don’t wait.

The radiator was backflushed and had a flow check done on it. As for the 1/8" hole, mine already had one in it with a brad through it.

Any chance of a gauge error due to a faulty sending unit? I don’t know the specs on the sender but it should be very easy to test with an ohmmeter.

Comming from GM over to BMW I was used to the GM practice of running the vehicle to get the thermostat to open and then adding coolant. I was doing this one day and the shop froeman came over and told me this is not how it goes with BMW since they do have a bleed port and a "jiggle valve"on the thermostst like 4450 speaks of there is no need to run the engine to either fill completely or bleed completely.

You simply fiil with the bleed port open and engine off.

If that was the case, then running the heat would have no effect, correct? As it stands now, running the heat drops the temperature back to the normal operating temperature, always.

I never have really figured out how a heater core ( no matter the fin densitity)being the size they are, can pull a car that is in the “red” back down to “normal”. It just doesn’t seem like the core could dump that much heat.

The car is not running in the red, though it gets about 2/3 to 3/4 of the way there. I too have no clue what is going on that it would be able to dump that much heat…