Weird circumstances:

On the way home for work, the coolant light comes on.

So I watch the temperature gauge as I drive it to the nearest gas station, about two miles away, the temperature NEVER GOES UP from 90-C, stays steady (it's about 84-F and 70% humidity in Florida).

I finally get to the gas station, and feel the upper-radiator hose and there is NO pressure.

There is also no visual leaks or puddles of coolant anywhere.

I carefully remove the coolant-cap, and add some water, and drive to my destination another 3-miles away.

I park, and notice no leaks during my 30-minute pizza diversion.

I start the car, temperature is fine, and no coolant light, so I drive home another 3-miles.

When I get home, I pop the hood, and again there are no visible leaks, and the upper-radiator hose has NO pressure, hence I carefully remove the coolant-cap.

Then all of sudden medium-pressurized hot-coolant comes bubbling out, but not under the normal pressure.

I felt the thermostat bypass hose and it is hot, so it seems like the thermostat is working.

I know I need to pressure test it cool, but any ideas ?

Thanks,
:-) neil
1993 500E
(new hoses, radiator, thermostat about 3-years ago)

After about a 30-minute cool-down, I noticed the following:

1) the lower radiator hose is cool to the touch, but the upper-radiator, thermostat by-pass, and block-to-thermostat housing hose were HOT to the touch.

2) the lower portion of the radiator is cool to the touch, but the upper half of the radiator is HOT.

Are these the symptoms of a failed waterpump (110K miles, original) (hence no pressure) or a clogged radiator ?

I highly doubt the radiator at 3-years old is clogged . . .

I put a stethoscope on the water-pump at idle, but I don't know what I should hear ?

TIA,
:-) neil

A couple things. First, the water pump does not create the pressure in the cooling system. Heat does it, and the pressure is held by the radiator cap. Ideally, the cap should vent air when the pressure rises above whatever the cap is rated at. When the coolant cools off, the cap should then allow air to enter the cooling system and thus avoid a vacuum. A failed pressure cap could account for the small loss of coolant.

Second, if the water pump bearings were bad, you'd hear it through the stethoscope. Listen to other bearings under the hood and compare what you hear.

Pressure test the cap when you pressure test the rest of the system.

__________________
1989 300 SEL that mostly works, but needs TLC

likely either the cap or the reservoir. This will eventually cause overheating.

__________________
89 300te 222,222
92 300e 190,000

Are we certain that the "coolant" light that came on is the RED one with the thermometer indicator?

I had a heater hose blow once with coolant splaying everywhere, temps over 120C and smoke out of the hood, and that light never came on!

To me, I would guess that the AMBER "coolant level" light is the one that lit up. The symbol appears as a cross-section of an engine with a water wave-like icon.

I ask this since temps seemed to be normal nonetheless...and when the coolant pump goes, there WILL be a pool of coolant all over your garage floor as the engine cools down.

The coolant level sensor does corrode over time and give false readings. A FastLane replacement may be in order. Inexpensive and simple to replace.

__________________
2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

False positives are the normal failure mode of the coolant level sensor. I had to replace the one in WMBO's 1993 300E within the last year. About $20 for the part, IIRC.

Keep tabs on the coolant level. If it doesn't drop, then it's time to convict the level sensor. BTW, if you have the reservoir more than half full, you might lose some coolant due to normal heating and expansion. In that case it will, over time, drop to the normal level and remain there.

No pressure in the hoses is not right - they should be quite firm with the engine at 90C. Pressure cap on the plastic tank is the first item to replace. The plastic tanks are known to not last forever either.

Only 84 degrees? Must be nice.

- JimY