Rep Power: 0

2002 civic lx about 220xxx miles.
backstory of temp gauge issue:
While driving after 10 to 20 minutes or so - highway or not, temp gauge would go from normal and drop down to dead cold. Car would drive fine though. After maybe a minute or so, the gauge would return to normal. I have a obd2 monitor hooked up and the app on my phone. When the temp gauge drops, the temp showing on the app on my phone would also drop and pick back up when the gauge itself did also. If you need more info, please let me know.

I have no idea what this could be. Bad ect switch? How do I test this to make sure it is working properly? Could it be a short in the wire somewhere?

backstory of overheating:
After driving on highway and getting off, car would randomly overheat, temp gauge would shoot up quickly and car would shut itself off. After looking at threads on this site I diagnosed it to be a bad thermostat so I replaced it myself, bleed the cooling system best I could, made sure cooling fan kicked on and everything. Everything was good for a while then it started to overheat and shut itself off again however now I was getting a CEL for a tdc sensor. Had that replaced and my mechanic also said he bled the cooling system some more for me as well as he felt there was excessive air and pressure in the system. Everything has been fine for about three weeks to a month now in regards to overheating but the whole temp gauge issue as mentioned above (going from normal to cold) has still been happening. Yesterday while I was coming home from work (all backroads, 15 to 20 minute drive) the temp gauge shot up while I was pulling onto my street and into my driveway. I shut the car off, pop the hood and check the radiator hose - top hose is hot like it should be. Lower hose is cold to the touch as if no coolant running thru it. I let the car cool down, checked the radiator and fluid is low but the overflow tank looks filled almost to the top, I top off the radiator, start the car and once the car warmed up the gauge went slightly above normal range but no CEL or anything, the lower radiator hose was still cold and my fan did not kick on, I only had the car running for maybe 10 minutes in the driveway, so I just shut the car off and it is now just sitting in my driveway. I am going to run the car today when I get home from work and see if the fans kick on or not. I know while driving and at idle, they do sound like they kick in but it feels kind of rough when they initially kick in - you hear and feel this kind of thudish noise from the engine bay.

what could cause the overheating again? Did I possibly get a defective thermostat? Is my first issue causing the overheating to happen possibly?

IF ANY MORE INFO OR PICTUREA ARE NEEDED PLEASE LMK.
TIA!!

 
Rep Power: 211

Coolant level in radiator dropping? Reservoir over filling?
Most likely a head gasket breach. Try burping air from coolant per service manual, new (Honda Genuine OEM) radiator cap and clean out the reservoir and hose from radiator to reservoir. If symptoms persist then do a block test for exhaust gases in coolant (head gasket breach). If it passes block test do a leakdown test and look for bubbles in coolant system as well as for compression loss. Can rent (free) a block tester at O' Reilly or AutoZone and test fluid is $10.

 
Rep Power: 244

 
Rep Power: 519

You replaced the thermostat, so does it still do this as you describe? Temp drops, by how much? And how rapidly does it change?

You've described random temp DROP, and random temp RISE too?

(reads more)
Is the water pump aftermarket....could the impeller be slipping causing random loss of water flow?

Do check for head gasket breach using method shown in the 10 minute video in the thread linked earlier.

 
Rep Power: 0

after replacing the thermo it still randomly drops from the normal range to dead cold. It'll stay dead cold for up to 2 minutes and then take 30 seconds to go to back to the normal range. This is a more repetitive and consistent issue then the overheating.

the random rise of temp causing the overheat is after getting off highway after driving maybe 15 to 20 minutes. But again, it's random, doesn't happen every time I drive on the highway and get off and it doesn't seem to be related to the outside temperature. that's what confuses me, it's not consistent and it'll even overheat on a cool rainy day.

the waterpump, to my knowledge, is not aftermarket. And truthfully idk if or when it's been replaced. The previous owner was a stick by the manufacturer rules type of guy so if it was replaced it would have been by a mechanic or himself with OEM parts. Is there any way to check the weep hole on the pump without having to go thru the whole process as if I was replacing it?

 
Rep Power: 0

thanks for that! I am beginning to fear it is the HG.

 
Rep Power: 0

I don't see any coolant loss and the reservoir is not overflowing but is filled up to like an inch below the cap. I'm gonna look more into the block test, thanks for the suggestion.

 
Rep Power: 128

buy the millage usually thats when they develop slow headgasket leak..

mine i was able to drive it with occasionally topping it off every 2 weeks for 2 years till i got it fixed.

I do have a left over headgasket kit felpro .. un oppened box if you want it cheap...
and some extra parts lying around i can throw in.

 
Rep Power: 519

How cold is 'dead cold' ?

Be specific------- because -40* leads me to one sort of answer, a different number leads me to a completely different answer.
Once you can nail down the true cause, it won't sound random at all.

So...you have no idea if it's OE or aftermarket.
It's visible without disassembly, but you can''t tell anything about the impeller from there. The weep hole is only for leakage.
Not yet, but you said it's overheated. Stand by.

Is that where the level started? Does it always return to the same level?

Fork a colored liquid block test. You won't find a small problem until it's a huge problem. Read what has been posted in the overheat common causes thread.

 
Rep Power: 0

what I mean by dead cold is the needle would drop down to where it would be when the car is off. I've never really noticed with the overflow until I started having these issues but since I've first looked at it, it's always been right around that much in there. I would say it has went up just a little within the past maybe two weeks.

I'm gonna see if I can get in contact with the previous owner to find out about the waterpump.

new info: I checked the radiator late last night, it looked low on fluid so i topped it off to where it should be. I check this morning and it is low again. I don't see any visible leakage anywhere. Should I do a pressure test on the radiator? Would that help eliminate or confirm anything related to the HG?

 
Rep Power: 0

so I just topped off the radiator, let the car get to normal operating temp and then let it run at idle for about 15 minutes, this is what happened during the time. The temp gauge was sitting at normal operating temp for about 5 minutes then dropped to about a quarter and sat there for the next 10 minutes before I turned the car off. During these 10 minutes the fans never kicked on, idk if it is cause of the temp gauge or not, but the car started to smoke a little from the radiator and reservoir area, not much or thick but enough to see and i couldn't pin point if it was the cap or reservoir. So I turned car off but just to be sure the fan motor was working I put the ignition to the on position and turned the ac on. Both fans came on so I can eliminate a blown fan motor. Oh and one other thing, the lower radiator hose was starting to warm up just as I turned the car off, which is contradicting what happened with it the other day when it overheated while I was driving.

 
Rep Power: 211

I mentioned a block test because although it cannot detect all types of breaches it's cheap and if it does detect exhaust gases then it's confirmed a headgasket replacement is needed. Unless one does a DIY leakdown they can be somewhat costly performed by a mechanic..although worth it to rule out headgasket issues and isolate compression loss, if any.

 
Rep Power: 519

The gauge doesn't start moving until about 130*F, plus it's movement is heavily buffered, so it's kinda meaningless.

Remember when you wrote this: I have a obd2 monitor hooked up and the app on my phone. When the temp gauge drops, the temp showing on the app on my phone would also drop and pick back up when the gauge itself did also. If you need more info, please let me know.
I wanted very specific information here. I wanted the actual coolant temperature data from the PCM whenever the gauge drops.

Anyway, you've probably either got a bad temp sensor (for the computer) or poor connections. I'd probably screw in a new temp sensor.

The system is designed to keep the radiator completely full under all conditions. It doesn't take much loss to compromise cooling efficiency, which leads to overheating (or lousy heater output in the winter)
Once it's completely full and all trapped air is burped out, I will top off the reservoir to the full line with a cold engine. (I overfill it hot so hopefully it ends up real close to the full line by the time it cools off)

When the engine gets warmed up the reservoir level will rise a bit, then if all is well it should return to the original level when the engine is cooled off again.
When set at the full line cold there is still plenty of room for normal expansion of the liquid.

but If it overflows the reservoir that usually means the head gasket is breached.

It was probably just steaming off the liquid that was spilled when you opened the cap or filled the system
Engine warming up, thermostat opened, hot liquid entered the radiator and made it get hot, now it steams off any spilled liquid.

It takes a couple thermostat cycles to get the bottom hose warm. The temp sensor that controls the fans senses temperature from that bottom radiator hose so it can take a really long time before the fans run on their own.
If there's any air flowing through the radiator, the fans might not need to run at all.

 
Rep Power: 0

I'll run the car and the app later today and let you know the temps. I remember when the temp gauge actually drops down to the bottom and bottoms out as if it was sitting all night, the app says it has no connection to the sensor. I was thinking of replacing either the ECT sensor or the fan switch but if either sensor was bad, shouldn't I get a CEL? should I replace the ECT sensor or the fan switch that is connected to the thermal housing?

 
Rep Power: 519

No connection to the sensor, or no connection to the entire engine computer (PCM)? (no live data received at all)
If it lost connection to the entire PCM, open the hood RIGHT NOW and put wrenches on the alternator mounting bolts make sure they are all tight. Tighten them before the PCM gets ruined completely.

 
Rep Power: 0

I believe it just says the sensor lost signal/connection I have other graphs and whatnot to monitor other things and they continue to work as normal when the coolant sensor on the app and temp gauge itself drops and loses signal. I'm not home ATM but when I get a chance I'll warm the car up and grab some screenshots of what the app shows when the gauge drops.

 
Rep Power: 519

You're reading this through the OBD2 connector like any other scanner would? Weird.
I expected just about any scanner to display a temperature value, even when the coolant temperature sensor is disconnected the live data on my scanners shows -40*

Not all faults set codes, and a fault code is not always a diagnosis.
It also helps to know the set criteria and time allowance for each individual fault, but that info is hard to come by.

You can have a sensor glitch or skew that drastically affects things, but as long as the value is plausible the PCM won't flag it as a fault.
This is a good example of why one should not rely solely on fault codes.

The radiator fan temperature switch is not monitored by the computer at all, it cannot set fault codes.
You can overheat the engine until it quits running on its own and it won't code until the coolant temp sensor is so hot the computer thinks it's a short circuit. (I believe that's around 300*F)

 
Rep Power: 0

alright so I ran the car for a total of 30 minutes. Took 11 minutes to get to normal op temp - 179 180. After 5 minutes the temp gauge dropped to dead cold. The app did show a connection the whole time with the sensor but the temp shown 64.4 on the app - screenshot below, remained there for about 2 minutes and the app suddenly shown 195 and the temp gauge itself climbed back up to normal range. After 15 minutes or so from when the car first hit normal op temp, the fan finally kicked on, both rad hoses were hot to the touch as they should be.

the fan only remained on for no more then 10 seconds, idk if this is normal or not. After it turned off it took another 5 minutes or so and the fan kicked on again and stayed on again for no more than 10 seconds.

I let it run another 5 minutes and then shut it off, got called in by the girlfriend.

 
Rep Power: 519

ECT sensor is bad. Replace it ASAP.

Then top up the radiator and burp it of air, and all that neat stuff.
Then drive it daily and watch for the common sign of head gasket leakage. .

Fan cycle and run time is normal. It only runs when needed, and only long enough to cool what's in the radiator a few degrees.
You could replace that temp switch if you want for peace of mind, but it will act the same as you described here.

 
Rep Power: 0

thanks for all the help. I'm gonna try to replace that tomorrow and watch how it goes from there. Would a duralast one from autozone be good enough? Will let you know how it goes when I'm all done.

 
Rep Power: 519

You can't find anything cheaper, of even lower quality? LOL

If it works right at all, it's better than the bad one.

 
Rep Power: 0

could you give a link to one or a brand you would suggest?

 
Rep Power: 519

Not really. Install whatever you want. If it fails too quick you can decide if you want a factory part or not.

Dangerzone shows a Delphi brand sensor (link) for 2 bux less than their duralast.

FWIW the Majestic Honda parts catalog (link) shows MATSUSHITA as the original brand. For only 40 and some change plus shipping. Google search that brand?

 
Rep Power: 211

$18 Honda genuine ECT at HondaFartsNow: https://www.hondapartsnow.com/genuine/honda~sensor~assy~37870-plc-004.html?Make=Honda&Model=Civic&Year=2002&Submodel =&Filter=(bt=1%2c6;et=2%2c5)&Location=pd:3064693,1 7 Next thing you know the OP will want us to wipe his *** for him after a dook..LOL.

 
Rep Power: 519

Dude. It's 40 bux.
Where's turd ferguson when ya need him?

 
Rep Power: 211

What the *****? When I pulled that link up last night it stated $18. I must be pufnstuff