Three times now my dash Coolant Warning Lamp has come on while I'm driving. The first two times it was after about 20min of mostly Highway driving on my way to work. The light came on but the Temperature gauge was normal and rock steady at about 85 deg.. The light went out again after a few minutes. The 1st time this happened, I pulled over and checked under the hood, but everything appeared normal. No visible leaks, Top Rad hose was normally hot and pliable (no abnormal pressure), The coolant tank level was normal. I also checked again after it had sat for a few hours and all levels were normal.

Second time I was only minutes from the office so I kept a close eye on the gauges (normal) and kept going. Again light went out after a few minutes. The third time the light just came on very briefly when I was forced into some hard braking by the 'doughnut' in front of me , but went out straight away. Again all temperatures and levels appeared normal.

Any ideas anybody?

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown

Mine acted identical to what you describe as my cracked head was getting worse. The level would go down under higher rpms, then return close to normal when idling. It drove me crazy since the light would be on when on the highway, then go off when I slowed down. Finally had wifey bring the rpms up while I watched the coolant tank and sure enough you could see it slowly dropping. I chalked it up to air (combustion gases, actually) getting trapped in the system from the crack in the head.

Hope yours is something as simple as a bad coolant level sensor (definitely a possibility), but I would be checking for cold cooling system pressure. If you come out in the morning and your top radiator hose is stiff, start budgeting for a new head. Sorry to be the bearer of potentially bad news...

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Keep everything as simple as possible-but no simpler--Albert Einstein

I hope you are wrong too, but I will check the top hose tomorrow morning.
Did your running temperature change at all because of the cracked head?

Where does the coolant go when the engine is revved? If combustion gases were getting into the system from a cracked head, you would think the level would rise and be forced out the overflow.

Is it possible I have air trapped in the cooling system somewhere that is compressed at highway speeds allowing the coolant to drain out of the tank?

How do you check for a bad level sensor?

__________________
1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown

My 86 SDL had a similar symptom.

I would get the light at speeds over 50, but it went out when I stopped and idled for a moment. I was curious, so I opend the hood, revved and watched the coolant level drop. Sure enough, at idle the level went up.

It must have been an air bubble...

I topped off the reservoir, and all was well for a while, but the light came on the next week. Topped off again, and it came back, but sooner.

Ultimate problem was a radiator leak along the lower seam which allowed coolant to leak away slowly.

Top the coolant up, right to the rim. Some may overflow, but you'll know it's FULL. Wait and see if the problem returns.

If it does, you are losing coolant, and must find out from where.

Best Regards,
Jim

Thanks for all the input.

Checked this morning and the top rad hodse is still nice and pliable, so it doesn't seem that compression gases are getting into the coolant system.

The coolant level (cold) was absolutely normal, about half an inch from top of coolant tank, so I am not losing any coolant. Rad was replaced two years ago BTW.

I will try revving the engine when warm to see if the coolant tank level drops significantly. It does drop about an inch when the engine is running and fully warm, but returns to normal a few minutes after switching off.

I'm hoping this is just a bad sensor, Does anybody know how to check these?

If the warning light is on before you start the car, and goes off after you start the car, and then comes back on after some driving time, the sensor is most likely OK, performing as is should, and responding to reservoir level.

If so, the remaining question is why is the level dropping. Probably air in the system, but how much, and where is it coming from? Ah, that'll be the mystery.

Come, Watson, the game's afoot!

Best Regards,
Jim