Coolant boils at shutdown, worry or not?
 

Hi all,

My 87 Civic's coolant boils as soon as I shut the engine off when it been operating at full temp (1.5 liter CVCC).

I do not drive it hard just prior to shutdown and the temp guage only shows 50% when this happens (it will boil more severely when the temp gauge is over 50%).

I noticed that it did this last fall when I purchased the car, but it only overflowed the stock coolant recovery bottle a few times so I was OK with it. I bought a combustion gas test kit for the antifreeze to be sure I didn't have a head gasket leak, but the result came up negative for cumbustion gasses in the coolant.

It only boils over at shutdown. I have installed an extra large capacity overflow tank (2.5 quarts) since it had been routinely overflowing the stock overflow tank now that the weather is warmer and it has been running a bit hotter. It will nearly fill this large overflow tank and it will always suck the coolant back in as the motor cools.

I have put a hotter thermostat in (195 degree), but I have the right antifreeze mix (50/50) and a new radiator cap. I also put on a new water pump. I have even retarded my timing a little, but that didn't seem to help.

The best I can figure, there must be a hot spot in the engine somewhere which is OK when coolant is circulating by it while the engine is running, but when the watwer pump/engine stops, its hot enough to boil the coolant for 20-60 seconds (I hear it bubbling as soon as I shut off the engine even if the temp gauge only reads 50% up the scale and I can watch it fill the overflow bottle).

Should I be worried?

Another issue I have noticed is the on the interstate at 65 mph, with my 90% grille block, and cooling fan power switched off, on a warm day my temp gauge reads 75-80% up the scale (100% would be the red "hot" zone). I assume I am OK letting it get this hot as long as I never get into the red zome or see steam, right?

I am getting some great mpg on the interstate when its nice and hot like this (42 mpg going 65-70 mph, and this car is not even fuel injected- I did an HF trans swap). I am hoping its OK to run at the top end of the temp gauge. What do you all think?

Do you now at what point or temp your fan is come on? You don't want the fan running all the time. If it running while in cruise I think You might need to open up the grill block so that it will cool a little more and not run the fan.

No, I don't know at what temp the fan is supposed to come on on this car, on my last Honda, it kicked on at about 60% up the gauge.

My fan thermostat was bad, so I rigged a switch up so the fan is either on or off (the previous owner had it wired so it ran all the time the ignition switch was on).

I leave the fan switched off, sometimes for a week or two at a time. I only switch it on when my temp gets up to about 90% on the gauge. That has only happened while sitting in city traffic. It doesn't get up quite that high on the interstate.

The boiling at shutdown will occur even when the temp is only halfway up the gauge. I can try to turn the fan on 1 minute before shutdown and see if this helps.

My main concern is for the wear and tear on my engine.

I'll let someone with a Honda chime in but I wouldn't let it get above 60%. With the cars I've had (mostly non-hondas)the temp gauge never moves between 165-225 and then it starts moving higher quickly. I wouldn't want the temp above 215 for long periods of time.

My Honda does this too. For some reason, as soon as I remove the keys from the ignition, the fan comes on. My friends accord does the same. I don't know if this is a coincidence or a Honda thing.

The problem is that Honda gauges aren't super-accurate. At 225*F, mine shows half-way to over-heat, but it'll only take another 30-degrees to boil-over. When that happens, the gauge moves past 1/2 pretty quickly. 70-80% is pretty high, I would believe.

I'm guessing your car is running hot enough that the pressure drop in the cooling system when the water pump stops turning is enough to cause the coolant to boil. I'd recommend changing back to the stock thermostat. You might consider one of the higher pressure radiator caps you can get on Ebay. Most Hondas will bring the temp guage up to around 1/3 from cold and will sit there. If yours is going higher (and it is accurate) I wouldn't recommend continuing to drive it like that. Might not be a bad idea to get a real water temp guage to see how hot it is getting.

In my civic, after the 195 degree thermostat install, the temp gauge is half way. I wouldn't run much more than half way. You really don't want to overheat your engine. The oil gets much thinner when you're running that hot and your pistons can expand till they fuse to your cylinder walls. I personally would install the factory automatic fan switch/sensor and call it a day. I can sell you one from a 92-95 d15b8 if you want (shipping mainly)...PM me. I would also install an aftermarket temperature gauge. I think they are like $15 from most auto parts stores.

Thanks guys, today I flipped my fan on about 2 minutes before I parked it and did not have any boiling at shutdown.

Good points about the factory gauge not being very accurate and increased wear at high temps. I'll cut a larger hole in my grill block so it runs no more than 50% up the gauge on the highway. Actually I bought another screw in rad fan temp sensor at the junkyard and I installed it in the bottom of my radiator a few months ago, but I liked the control of having the fan switch and letting it get a little hot to keep me from using any electricity (gasoline) running the fan. I'll hook up the sensor and see if it will operate as it was designed to.

I was about to buy a stronger radiator cap, but the stock is supposed to be 13 lbs and I am afraid that jumping up to 16 lbs might be enough to make me more prone to a head gasket failure- for all I know, my head gasket might be the original (20 years old).

Thanks for all the responses.

I don't think the different radiator cap would cause the HG to fail, it might blow the plastic top off the radiator, or a hose, but the HG is sealing such higher pressures, I don't think it would notice the extra 3 psi in the cooling system. Other then highly modified engines, I think it is thermocycles that eventually wear out head gaskets.