Red Tornado

my low coolant light starting coming on, intermittently.

no known leaks, no overheating.

i noticed the white plastic overflow tank is well past the "full" mark, its almost 100% completely full. it's usually about 1/2 full.

am i right in "diagnosing" this as a bad expansion tank cap? that it's failing and not holding pressure in the system correctly, thereby passing the coolant directly to the overflow tank?

c4cruiser

You could have a bad pressure cap on the expansion tank or a hose that is collapsing causing excess pressure in the system and expansion tank forcing coolant out to the overflow tank.

With the engine cold, remove the pressure cap to see what the coolant level is. It should be to the bottom of the filler neck. Start the car and let the engine run while watching the level. Once the thermostat opens, you should see coolant moving and the level should drop a little. If the coolant starts to flow out of the expansion tank, you have a blockage somewhere, possibly a thermostat that is sticking.

Suck the fluid out of the plastic tank so that coolant is at the COLD mark. With the pressure cap still off, run the engine at around 1200-1500 rpm and fill the expansion tank to the bottom of the neck. Replace the cap and watch coolant temp and check for coolant going into the overflow tank.

Sometimes it helps to have the front of the car raised as you fill the cooling system. That helps to get air bubbles out of the system.

CFI-EFI

Possibly. It is easy enough to test the cap. Then I'd be looking at a compression test and/or a CO in the coolant test. A leaking (blown) head gasket could be causing the pressure to blow the coolant out, into the reservoir, and breaking the vacuum to prevent it from being drawn back in. Capital letters at the beginning of a sentence are out friends.

RACE ON!!!

Z51L9889

My 89 showed the exact same symptoms you described a few days before the head gasket let go. The low coolant light would flicker on and off and yet the radiator would be full when I checked the level. It turned out that combustion gasses were being forced into the cooling system and the bubbles would contact the coolant sensor causing the light to come on. I would suggest starting the car with the radiator cap off and watching for bubbles in the coolant.

Red Tornado

the rad cap was starting to go bad which allowed for a slow leak. additionally, the coolant sensor failed. that's 2 strikes, i was almost out

i fixed both and all has been a-ok over the last week. the coolant level remains steady in the expansion tank, no bubbles, and no excess to the overflow tank. back to hammering it, banging thru the gears and ripping it up on the roadway.

thanks guys, for your thoughts