(1998 civic, 1.6L single cam, no vetec) So this all started about 6 months ago, sometimes when driving the car would get a little hotter then normal and if i was driving a really long distance it would eventually overheat. as time went on sometimes the car would overheat like 5 minutes into driving but the temp would randomly drop back down and then rise again, this would continue until the car eventually just stayed hot. more recently it got so bad that every time i drive the car it just gets hot and wont stay down, the gauge usually doesn't go out of the zone though, it will stay just under the H white line. Let me say now that i always have the coolant level topped off to the right spot. So after this problem got bad i said okay let me try and replace some stuff. Yesterday i replaced the thermostat and radiator. I then bled the cooling system the best i could without one of those nice coolant funnel devices. I drove the car around a bit and floored it a bit and no overheating! Yay! i then went out to get food and on my way home the car climbed up to the H again and quickly dropped back down, it did this twice on my way home. Im at a complete loss, i don't think its the water pump because there is no coolant leak and the car is cycling coolant well by the looks of it. The radiator is brand new and so is the coolant so it should be pretty gunk free (although i didn't do a full flush). and i changed the thermostat and know that's working because the car doesn't overheat immediately. All i can say is that i noticed i put too much coolant in and the reservoir was overflowing when I drove to get food. Other then that I have no clue. So my questions are, 1 did too much coolant cause it to overheat a little, 2 did i not bleed the system of all the air, 3 is the thermostat sensor going wack and randomly closing when its not supposed to (causing brief overheating), or 4 is that water pump actually bad and just slowly failing on me. I really have no clue and have been trying to diagnose this problem for months, i have never had a car overheat like this where its almost random, hopefully one of you honda experts has seen something like this and can give me some help, it would be much appreciated. P.S. i forgot to mention that yes hot air always blows through the vents.

 

What car and engine?

 

98 civic 1.6L single cam no Vtec, sorry should have said that lol

 

Remove the radiator cap.
Top off with coolant if necessary.
Start engine and let it idle until warm.
Watch coolant in radiator fill hole.
Do you see bubbles in the coolant when the engine is idling or slightly revved?

 

Whe you bled the coolant did you have the heater control on full hot and fan on low?

 

Ide remove the thermostat and boil it to the specified temp, see if it pops. Could also drive the car w/o it if you wanted and see if that changes anything. Clean the ground on the thermostat housing while your there.

A full flush wouldn't be a bad idea after you figure out the issue. You can drain it all just using the petcock on the rad and the rest with the plug on the back of the block. Also only fill the overflow to max when its dead cold.

When i bleed coolant i like to raise the front of the car on ramps. Heat turned to high, fan on low. Keep it going until you hit operating temp and the rad fan kicks on a few times. Seems to do the trick.