'78 300D losing coolant
 

My nephew has been driving this car for the last couple of years and I went over to look at it yesterday - he said that it had leaked some coolant out the little overflow hose that is attached to the neck of the radiator. I added coolant - over a gallon - and started it up. After running for 10-15 minutes or so, the upper radiator hose was hot and the lower hose was cool - not even warm. Then coolant started running out the overflow hose again on to the ground - didn't see any other external leaking. Any ideas? Thermostat, maybe?

Thanks, ryan

The cap

The thermostat would be pretty high on the list of possible culprits. Considering that it is inexpensive and easy to replace, that would be a good place to start.

The coolant should circulate with no cap at all.

The fact that the top hose is warm and the bottom cold is just an indication that the radiator is working and the coolant circulating. I second the idea that the radiator cap could be bad, especially if the car is not running hot. If the car is running hot, then there are other possibilities.

A cold lower hose on a warmed-up engine would certainly mean that the radiator is doing a great job. It is also outside the realm of possibility, given normal function of the system.

Either the thermostat isn't opening, the water pump isn't pumping, or the radiator is hopelessly obstructed. Or some combination thereof.

One other item is that the heater is not working...doesn't blow warm air at all...

He did indicate that it got hot - approaching the 120 mark on the car before he shut it off and noticed the coolant leaking out the overflow. But, it hasn't been running hot before this...

10-15 minutes of idle might not open the thermostat

Agreed, but I shut it off because it was starting to leak out coolant...

I don't think 'leak' is the right word...coolant was running from the overflow tube on to the ground.

Sounds like air in the system, Did he have anything apart?

Nothing apart...but I also thought that might be a possibility, but only because it was so low on coolant. I had to add over a gallon before we even started it up!

The basic question is why did it get hot to begin with? I'd put the nose uphill, fill the block thru the upper radiator hose, put the heater on full blast and get the air bubble out. I'd then replace the radiator cap and do further diagnosis. It may have gotten hot just because the coolant was low and the radiator cap bad. It may have a coolant leak somewhere.
I'm not sure why Tango thinks there's restricted coolant flow? Has that been determined.? Simply looking into the radiator with the engine running after it's warmed up should answer that question.

I agree – air in the system, he must have run it low.

You must bleed air out of the system thoroughly. Localized boiling in air pockets in the cyl head will blow open a new radiator cap even if coolant in the lower radiator is cold. These engines are particularly quirky in that respect.

1) Get the car facing up a steep incline.

2) Open both heater valves wide.

3) Remove the bleed plug on the thermostat housing and fill the radiator with the engine idling.

4) Keep adding water until no more air comes out of the housing and replace plug.

Well, got most of it fixed. Put it up on ramps and bled the air (although I don't think that was the problem). Put a new cap and thermostat in and drove it around for a while - laser temps were all well within specs and no leaking!

Only problem, is no heat still - since it's summer time in Oklahoma we'll tackle this problem later. I replaced the dreaded servo with an aluminum one several years ago and have had no problems with it.

ryan