At 04:12 PM 10/5/99 EDT, Kim Tonry wrote:
>My '79 1500 Midget has had problems lately with sudden overheating ....
>
>.... Radiator was recored 2 years ago. Water pump and fan clutch replaced
last year. Running with a gutted thermostat in place. .... It runs hotter
at higher running speeds, will usually drop back down at lower speeds.
>
>.... The first time it happened .... after the car was shut off for a few
minutes at a gas stop. .... then after the brief shutdown to gas up, it
immediately climbed into the red as I reentered the Interstate.
That part is not terribly unusul. Often after a hard run and a brief
shutdown the heat soak from inside the engine will raise the cylinder head
coolant temperature considerably. However, it shoold settle back to
"normal" running temperature soon after getting under way again. "Normal"
is of course relative, meaning back to whatever it was doing just before
you stopped.
>.... the heater fan was on when the temp started to climb, but the air
from the heater outlet was blowing cool air, ....
That sounds much like lack of circulation of the coolant. Heater cores
generally do not get clogged quickly, let alone start to flow again later.
Also a stuck thermostat does not impeed the flow of coolant through the
heater. First be sure the coolong system is nearly full of coolant. Then
check to be sure the fan belt is tight and the water pump is turning, and
hopfully also the fan. If all is well there, then the next suspect is a
broken or deteriorated impeller on the water pump.
>.... I pulled off as soon as I could. The only sign of heating was a small
amount of oil bubbling under one side of the valve cover and there was
about 1 pint of antifreeze blown into the overflow tank. After about 45
minutes, the temp gauge was down considerably and we went the rest of the
way home on surface streets with no problem.
Yup, sounds like no circulation of the coolant. I once had a generator fan
get fouled on a bolt of the tensioning bracket. The generator wouldn't
turn, so the fan belt stopped, and the water pump and radiator fan also
stopped. The engine was overheating to the point of loosing power and
pinging, but because of the lack of and coolant circulation the temperature
probe at the front of the head was reading nearly normal coolant
temperatures. The dead give away was that the heater was on full but was
blowing cold air, even though there was plenty of coolant in the system.
>The second time it happened I had not stopped or shut down the car, it
just started climbing while we were about 30 miles into a 40 mile trip on a
50 degree evening. Since then I have refilled the system with the front
raised thinking it was an airlock but still had it happen again last weekend.
In mild weather and light load conditions the coolant will circulate some
from the thermosiphon effect, hot water rising in the engine block and
cooler water falling in the radiator. This can suffice to keep the
temperature within tollerable limits, until you increase the heat load on
the engine by pulling higher road speed. Then the thermosiphon circulation
can't keep up with the heat load, which is exactly why our cars have water
pumps.
>.... when it did it for the 3rd time. It was only 50 degrees out, the car
was running fine when I stopped .... I shut down the car for 30 seconds to
jump out and fasten the top. When I restarted and took off again, the temp
gauge immediately climbed to the red. ....
Same effect. Heat soak from inside of the hot engine raises the coolant
temperature as soon as the engine is shut off.
>.... I am about to pull the engine for a 100K rebuild but the spare engine
is still not ready. I'd like to drive the car to the NAMGBR AGM in St.Paul
in two weeks but not if I don't figure out what is going on here.
Pull the water pump and check the condition of the impeller. Newer water
pumps with plastic impellers are particularly subject to this type of failure.
Good luck,
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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