Coolant boils at shutdown, worry or not?

[GasSavers_Erik]

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?