Hello all,

My Civic EX had a coolant leak that I could not find; as long as I kept on top of it, the temperature gauge consistently topped off in it's normal position (just about horizontal).

The leak got worse & car overheated & I bit the bullet & took it to local garage. They replaced the thermostat, upper & lower hoses, cap, 2 temp sensors (1580154 & 201-1530) & fan switch.

For a month of short to medium long trips, the temperature gauge was steady, and coolant level consistent (just above the radiator core, about an inch below the cap).

On a 8 hour highway drive yesterday, about 3 hours in, the gauge began creeping up & would vary as the load and speed varied - raised when going uphill or when car speed slowed, and dropped with light load & high speed. Upper limit of the gauge was a bit less high than in the photo; normally lower.

Beginning the drive, the reservoir was empty. When I stopped, the reservoir was full. When I let her cool down, the radiator level was normal.

I continued driving the next 5 hours and the pattern continued. The next day, the radiator level was normal, reservoir empty. I idled the car & when she got to temperature, the radiator fan kicked on for just under a minute - so the fan works (at least that much). I had the cap off & was looking for flow from the water pump - did not seem to be much.

HELP!!!

I'm baffled. Water pump, fan controller, thermostat?

Any thoughts on how to troubleshoot? I'd like to avoid another garage bill, or at least go in with a good idea of what the source of the problem is.

Thank you in advance.








Temp gauge

 

If your not seeing "flow" replace the thermostat, even if it was just replaced, [it may be defective].

The "reservoir"/overflow tank should never be empty, there will be a cold level and a hot level, make sure overflow tank is at the cold level and rad is full to the top when engine is cold also I would bleed the system, you may have air in it. 94

 

Normal level in the radiator is full to the very top. No air space. Anything less is a problem. Check the radiator only when completely cold.

 

when was the last time you replace the water pump? did they bleed the system after replacing all of that?

 

Thank you everybody!

Flow:

With the cap off, I can see some movement, but I'm not sure how much I should see?

The upper hose feels warm/hot, but not much pressure, & I can't feel flow through the hose - should I be able to?

If this indicates too little flow, any ideas on how to tell whether it's the thermostat vs. the water pump?


Air/bleeding

I will search on how to bleed. I have driven over 5 hours since the repair work with no overheat, & my mechanic says any air should of self-bleeded by now. No?

(Water pump has not been replaced for many years)



For what it's worth, I just drove about 20 minutes, light to medium load, and gauge remained in normal, "happy" position the whole time. Does that help to diagnose?

I'm also curious why the problem came on all of the sudden, after driving over 5 hours since the repairs with no issues...

Please keep the thoughts coming & I'll continue to troubleshoot & post.


PS: Noted that the level should remain topped off, and the reservoir full.

 

How to bleed the rad is listed in the FAQs also this video is very helpful:

How To Bleed A Cooling System - EricTheCarGuy- Stay Dirty!

 

If the engine begins to overheat specifically while at idle or low rpm, the likely culprit is low level of coolant. Also, thermostat typically doesn't open on these engines until they have reached operating temp and then have been driven or given higher rpm for a couple minutes. Both hoses should feel hot at that point if your thermostat is functional.

If you increase rpm and the temp drops the water pump is likely functioning. If there appears to be no leaks on the exterior but coolant level gradually decreases, I'd perform a combustion leak test and see if you have a bad head gasket perhaps.

 

Thank you.

Bled; not much air seemed to come out.

Does not overheat at idle.

Temp does not always drop with high rpm, seems to be correlated to the load & speed (air across radiator?).

Car runs well, clean exhaust, so pretty sure the head gasket is ok (fingers crossed).

Thermostat or water pump....or...???