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2000 Civic, started having overheating problems last summer that magically went away until just again recently. An out of town shop tried telling me I had a blown head gasket, but my local mechanic says I don't.

The car runs hot on long trips or when the A/C is on. When I pull over and pop the hood, I notice that the radiator fan isn't running. I tested the fan against the battery and I tested the relay using a paperclip in the switch connector. Both worked. If I start the car cold and let it idle, the fan kicks on in short cycles like it should. The driver side fan works fine when the A/C is running.

I drained all the coolant, ran water through it, changed the thermostat and the switch. I put in new coolant and bled the system out of air completely using a funnel kit and filled the overflow. No bubbles coming through the radiator, no white exhaust.

I'm somehow losing coolant from the overflow and have to keep filling it and topping off the radiator, but I see no obvious leaks. There is also a black, dirt-like grime that keeps forming on the underside of the radiator cap.

The only thing that keeps the car from overheating is if I leave the paperclip in place, disconnecting the switch and leaving the radiator fan running full-time. I'm not sure how long I can run on that solution; I'm supposed to be on a twelve-hour road trip as of yesterday.

Any ideas are appreciated.

 
Rep Power: 367

Magical words for headgasket breach.
first post have a video showing how to diagnose the gasket, but spilling coolant from overflow is more than enough evidence.

https://www.civicforums.com/forums/3...reference.html

 
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Sorry, I don't mean to say that the coolant is coming out of the overflow directly, just that I notice that the overflow keeps emptying. I'm not sure where the system is losing coolant. I don't have any telltale blown head gasket signs (no bubbles from the radiator, coolant and oil look normal, no white exhaust, heater blows hot).

I'm convinced that I do have a leak in the system somewhere now, though, and the leak is causing problems for the fan switch (air getting into the system and stuck in the thermostat housing, maybe?), preventing it from running appropriately/long enough/etc. Before, I thought I was losing coolant due to the overheating but now I'm thinking it's the other way around. Again, I can keep the car from overheating by bypassing the fan switch and leaving it running full-time.

It's in the shop now getting checked out.

 
Rep Power: 367

again, do the test that is in the video - not all shops know this way to test them and could tell differently. D-series engine gasket fails in different ways than other engines, period.
if it does not pass the test, it's 100% the headgasket. It's not difficult to do either in a shop
if pass, move to the others

 
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Are you referring to the funnel bubble test? If so, thanks, but as I've said twice now I have no bubbles coming up from the radiator while the engine is running, once the system has been bled (hoses squeezed, etc). I've tested it using a funnel kit.

Shop just got back to me and said the pressure test came up normal and there was no air in the system.

Something is preventing the fan from operating normally, which is what allows the overheating - but I can't imagine at this point what that would be.

 
Rep Power: 222

Fan switch on the thermostat housing might be bad, aftermarket ones don't work, get an oem one.

 
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Yeah, the shop is trying to source me an OEM switch and thermostat after ruling out leaks and head gasket. Hopefully I can get a refund on those parts from AutoZone if it turns out the OEM ones work where the Duralast ones are apparently failing.