Why are there two coolant drain plugs and should I use them?

[satmark]

From video research it seems like there are two coolant drains on an SL - a plastic petcock and a drain bolt. What is the purpose of the two plugs?

It seems like some people have had issues getting one or more of these plugs open or getting them to seal again once they are opened. On my 99 SL would you recommend I play it safe and drain instead via the lower radiator hose, especially if I'm replacing the tstat anyway? Thank you.

[OldNuc]

The bolt allows draining of the block. If you are changing a leaking thermostat you can leave the bolt along and just give it an hour or so to drain back through the leaking thermostat. Or you can just unbolt th ehousing and use a big pan to catch the coolant.

In the S-Series cooling system there are 4 separate cooling flow loops, 3 external and 1 internal loop. These loops are in parallel with each other and all terminate at the water pump suction and the water pump provides the driving force to the coolant flow in all loops.

1) The deaeration line flow is from the water pump discharge into the block up into the head to the high point of the head(SOHC) or intake manifold(DOHC) to the reservoir then from the reservoir to the pump suction. Flow in this loop is continuous.

2) Heater loop flow is from the water pump discharge into the block up into the head to manifold to heater core to the pump suction and the reservoir also sits on this heater loop line. Flow in this loop is continuous.

3) Internal loop flow is from the water pump discharge into the block up into the head and back down from the head area through an internal passage to the pump suction. Flow in this loop is control ed by the thermostat.

4) The radiator loop flow is from the water pump discharge into the block up into the head and out the head outlet into the radiator and back to the pump suction. Flow in this loop is controlled by the thermostat.


The radiator loop(4) and internal loop(3) flow are both controlled simultaneously by the thermostat. As the radiator loop flow increases the internal loop flow decreases. The disk on the back end of the thermostat is what moves to close the internal loop as the radiator loop flow opens. Both thermostat disks move in the same direction, one is opening increasing loop flow as the other is closing reducing loop flow.

The total cooling system flow passes the ECTS so you should always know what the bulk coolant temperature is. The thermostat control element is sensing the coolant temperature in the water pump suction chamber.

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[Signmaster]

Originally Posted by satmark

From video research it seems like there are two coolant drains on an SL - a plastic petcock and a drain bolt. What is the purpose of the two plugs?

It seems like some people have had issues getting one or more of these plugs open or getting them to seal again once they are opened. On my 99 SL would you recommend I play it safe and drain instead via the lower radiator hose, especially if I'm replacing the tstat anyway? Thank you.

If you drain by pulling the lower hose the engine will get drained just fine. If you look at the location of the lower hose on the radiator you will see how it's not at the bottom of the radiator, so it will leave coolant in the lower section. This is easy to overcome by simply also pulling the upper hose and flushing it well with a hose.

Any method works, and there is really no right or wrong way.

[brian74]

I did mine recently by pulling the plastic petcock at the bottom of the radiator, using a 1/4" drive extension. The part is fragile so I had to be gentle.

Once the petcock came out, I found that the o-ring was split. After visiting 2 separate auto parts stores didn't have the right size o-ring, I was actually able to match up a proper size replacement from the Home Depot plumbing section. I spread a thin layer of silicone lubricant over the o-ring to facilitate re-installation. No issues since.

Pulling the hose will make a mess, but the lesser evil if you have a big enough catch pan. If the lower hose looks worn out, razor slit it slightly at the bottom first to minimize the rush of the coolant, and then just replace the hose.

[satmark]

Thanks for that feedback! In general I do like to tread the path of lesser evil. I'll be looking for a nice big catch basin and trying to avoid that plastic valve.