Hello all, first time posting in a good while.

Car
1995 Civic EX 5MT D16z6, NO AIR CONDITIONING
175,000 miles

About a month and a half ago I had an episode where after sitting at a few lights in hot Illinois summer heat I got up to about 40 and noticed my temp guage on the rise. It nearly hit full HOT so I steered off into a nearby parking lot and cut the engine and went to take a look. A few years back this same thing happened to me a block from home, and a heater hose had ruptured, spewing coolant everywhere and nearly overheating the engine. To my surprise, there wasn't a drop of coolant anywhere in the bay, save for the overflow tank having overflowed completely all over the side of the radiator. I checked all my hoses. Everything was intact and there wasn't a sign of a leak anywhere. All of my hoses were very hot to the touch, top and bottom radiator hoses included. I got back in my car, maxed out the heater and opened the windows and the temp returned to normal quickly and didn't act up again.

See, last year I had a dozen or so oil gaskets, timing belt, and water pump done, so I knew my pump was good (Gates brand). I figured a trash thermostat, even though the one I had was only a couple years old. I replaced the thermostat on a weekend about a week after this incident, and replaced the hose between the IACV and block, which was corroded. I refilled the coolant, bled the system, and detailed the engine bay to remove all the residue, leaking oil, and coolant. All seemed well until a week later the same thing happened again. I didn't pull over this time, just maxed the heater again, and after a few seconds the temp dropped back to normal. I figured I left air in the system so I took the car out on my driveway, which is about a 10 degree incline, threw the front of the car up on stands to raise it even higher, and idled the car with the cap off for like 20 or 30 minutes, and I did notice the water in the radiator make a "gulp" every now and then when it was hot. I topped it off each time it did that. Eventually the fan came on, so I confirmed that was working too. Put everything back together, went another week, and it happened again. It's something that seems to happen in stop and go traffic but on the interstate at 4k rpm for lengths of time the engine will sit at just below 1/2 temp the whole way. I'm confused.

The water pump is a year old and is Gates brand.
The thermostat is a month or two old.
Vacuum guage is steady as all hell at -25 in/HG, no twitches, vibrations, swaying (rules out head gasket)
No oil in coolant or coolant in oil
No smoke out tail pipe unless the engine RPM gets to 5,000 or higher.
Electric fan works.
Engine leaks a bit of oil out the back of the head, and uses a quart a month, but I always make sure its full or near full

What do I look at next?

 

The fan may not always work. The switch or relay could be intermittent. It seems to only happen when sitting still, and you can remove the heat using the heater, suggests the fan is not on when it overheats. If it does it again, check the fan. If you have $30 burning a hole in your pocket, replace the switch, also swap the relay.

 

buy another civic. or ride your harley.

 

trade Civic for another POS Camaro.

 

thanks everybody. Will order a new fan relay on rockauto and start looking for an 80s camaro to cruise the trailer parks in

 

Look at next: Rad cap. Fan thermoswitch.

Gates brand water pump is sadly shyte. No metal fins. But if it's near new then unlikely that's the prob. But just FYI.

 

OK i installed the relay and sensor that goes into the thermostat housing. No change in behaviour. Could the head gasket be causing this??????? Like I said before, my vacuum guage doesn't support a head gasket leak, I don't have oil in the coolant (I loose too much oil between changes to see if coolant is in the oil), and I don't have steam out the exhaust pipe or water drops on the tip of the exhaust..... and 98% of the time the car is performing at optimal levels. I really am trying to replicate the situation so I can see if a certain driving behaviour is contributing to the problem.

You're saying a radiator cap could cause these symptoms? How so? and what is a fan thermoswitch so I can look it up on rockauto

 

The fan switch is on the thermostat housing. It's about $45 from Honda, or a little cheaper from aftermarket. 2 part number options 37760-P00-004 and 37760-P00-003

Unfortunately there could be a LOT of different small issues that cause this. 1st I would definitely check for proper fan operation before you start throwing parts at the system. There are several things that control the operation of the cooling fan, and the thermo switch is only one of them. Could be something as stupid as a fuse or relay. But you don't want to start replacing anything until you've DETERMINED the fan is not operating. You don't have AC so you can't use the "simple" AC button test. This means you will have to do the more complicated "flow-chart" test found in the shop manual.

Other small issues could be a problems with the overflow system. The overflow system is not pressurized so it could have a leak that only draws air in, but doesn't spit coolant out. Check that the elbow on the top of the reservoir cap isn't cracked. A crack in the plastic elbow could cause air to be drawn back into the cooling system every time the radiator fan comes on or every time you shut down. Also be sure the two hoses (one from radiator to elbow, and one inside the reservoir) are not cracked, and still make good seal with their connectors. Anything that draws air into the system when the coolant in the engine is shrinking is bad. This also goes for the radiator cap. OEM Denso radiator cap is only $10 so you might consider buying one anyways. 19045-PAA-A01

 

Oh and yes the Gates pump is crap. I had one on my Honda Pilot and it began to sieze up after about 40k miles. Started making noise my wife noticed. When finally got to it, the pulley was almost impossible to turn by hand, then when I finally got it to spin, a bunch of coolant spewed out of the weep-hole. So I guess also check for coolant leaking on the back of the water pump. The weep hole points toward the firewall on the D-series.

Both Aisin and Yamada make the Honda OEM water pumps. The Aisin brand can be bought in a Aisin box for WAY less. The Yamada pump can only be bought in the in Honda box.

 

Ok thank you buddy but listen up: as stated above I already replaced the relay in the fuse block, and I replaced that switch on the thermostat housing. IT wasn't 45 bucks either. Easiest job ever. Grab big socket, crack it loose, and screw teh new one in and reconnect plastic plug.


HOWEVER....

you brought up something important. I didn't know your reservoir had to be completely sealed to the radiator. A couple years back I cracked the screwcap on the overflow bottle trying to tighten it too much (cheap aftermarket plastic). I always left it somewhat loose, so it didn't fly off and the hose was submerged in fluid, but loose enough you could give it a 1/8 twist and it would come off. What's curious is why it took so long to act up. Basically replacing the overflow tank would alleviate the problem is what youre saying

and by the way I have confirmed fan operation within the last 1.5 months when bleeding radiator system.

 

I was confused because you asked what a "thermo-switch" was. Now I see you already replaced it. Yes of course aftermarket is cheaper than OEM.

It's not the cap so much as the separate plastic elbow #5 inside the cap. If THAT is cracked, you could have problems. Same deal with the outer seal on the radiator cap. They might not blow coolant out, but they could suck air in.

 

3 years ago I redid all the hoses and the radiator. The radiator was an ebay brand with its own overflow bottle. Honestly, its served me well for years and outside of the crack in the cheap plastic lid on the overflow bottle, it's been flawless. It's an aluminum half radiator. The likelyhood of being able to source just that lid/elbow combo is minimal without purchasing a new overflow tank entirely. My best recourse would then be to purchase an OEM radiator cap off rockauto. I believe I saved the original cap and tossed the ebay cap that came with the radiator when I installed it. The cap I have now has all the honda info, japanese mumbo jumbo on it, and is probably 20 years old. Either that or it came off my 94 integra when I had that...... POINT IS, I've had a surplus of random honda parts laying around through the years so there's no way to be sure where that cap came from or how old the rubber seal is. I will place an order for a HONDA OEM radiator cap, and maybe do some kind of tape around the crack on the cap of the bottle. I don't believe the elbow is damaged.

 

OK update. I ordered a new radiator cap via rockauto, and sure enough it didn't fit because the radiator is aftermarket. I tossed the cap and kept driving. That was a couple weeks ago. The car drifted into the hot zone maybe a handful of times since then.sp

Today the temp gauge started rising to 3/4 again and hung there so naturally i cranked the heater again and made sure to drive through clean air to get cooling on the radiator. As I pulled in my driveway I realized the air coming out the vents was luke warm, and the engine was approaching full "H" on the temp gauge. Is this indicating a clogged heater core? I've replaced the thermostat (twice now), sensors/relays, all hoses are good, radiator is good, overrflow bottle is good, water pump isn't even 10k miles. No coolant leaks inside the engine bay even when the car is overheating, but the overflow tank will sometimes overfill and spill out on the ground. I can't think of anything else I can do other than bypass the heater core and see if the problem goes away. Is this what my next step should be?

 

No heat from the heater means that there is a bubble of air or steam in the engine, and you need to pull over and shut it down. If you keep driving overheated you will have major problems.

It is likely there is a small leak in the head gasket. The only symptom of such a leak is that the coolant gets pushed out of the engine into the plastic tank, leading to eventual overheating.

 

Do you suppose that the bubble is from poorly bled cooling system? The last time I bled the cooling system I tried really hard to do it right. Perhaps I should do it again?
And please keep in mind that the car runs in normal temp range 99% of the time, its just on excessively hot days and after lots of road time the temp will start to rise randomly at a stoplight or in 10mph traffic. Seems it will heat up real hot and the next day run 30-40 miles without a single issue.

 

Before a trip anytime the engine is completely cold, check that the radiator is completely full. Don't take the radiator cap off if the car is facing downhill though.

You should try to find a new cap that fits your radiator. Inspect the rubber seals in the cap, especially the top one.

 

I'll check the spark plugs too. If the head gasket is bad wouldn't that give it away?
Remember, I don't see oil in the coolant, and although the oil is dark as dirt when I change it (old engine), I don't see water in the oil. And I don't have smoke in the tailpipe or smell coolant burning.

And if I check the coolant before startup and the radiator isn't full what does that mean

 

The only symptom of a small leak is displacement of coolant out of the engine.

If the radiator isn't staying full and the coolant stays in the plastic tank as the engine cools off, either there is a buildup of air in the engine, or the cap is not letting the coolant return.

 

^Not just the radiator cap, but the overflow tube, straw & elbow. There are 4 components to check.

 

Alright guys I'm back. This car still overheats randomly, I notice a pattern now though. If I really crack the throttle and let the revs climb, say up to 5,000rpm or so, the car will start to "overheat" shortly after. Well today, after a full 25 minutes of normal temp I got the temp gauge to start climbing as it does, right before I pulled in the driveway. I hopped out with the gauge between 1/2 and 3/4 temp and opened the hood. No coolant overflow. no coolant on anything, no oil on anything, the engine sounded normal under the hood...... except the radiator fan wasn't on. If the dash gauge is saying the engine temp is rapidly rising but the fan won't come on does that mean the ECU doesn't see the temp on the rise? I just replaced the temp sensor on the thermostat housing, as well as the fan relay. I KNOW the fan functions. Something else must be obstructing the command for Fan-On from reaching the fan/ecu/something

 

bump

 

Did you change the radiator cap and check the connection to the plastic tank? Have you been checking that the radiator stays full?

Jump the fan switch on the thermostat housing to make the fan run all the time the key is on. Test drive like that. If it still overheats it is not the fan. The fan is switched by the temperature of coolant leaving the radiator. If there is no flow through the radiator, the engine can overheat but the fan stays off.

 

The predicament here is that the radiator cap isnt something i can buy alone, the vendor doesn't have one. I bought the radiator off ebay years and years ago when i was living paycheck to paycheck, and i messaged the vendor recently to see if he would sell me just a cap but he said in broken english it's not possible. OEM honda cap doesn't fit on the throat either. The only way I could replace the cap is to put an OEM radiator back in. I'll jump the fan though and see if the problem recurs before going to that step. OEM radiator can be had for like 30 bucks on rockauto so I'm not too concerned, but I don't want to replace it if its not the problem.

 

Please post a picture of the inside of your rads fill neck and current cap! I also run an ebay aluminum half rad. Having a similar problem I ended up using the cap off of some trim of Hyundai Elantra, 1.1 bar and fits the rad perfectly. If your fill neck is the same as mine i can find the exact model number of the cap / vehicle it belongs too and post it up.

 

ok ill get you that tomorrow, thank you