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4AFE Corolla Cooling System Design & Leak Diagnosis

I’ve never really understood how it works on my Corolla. It’s a bit of an unusual set-up, at least compared to my truck. First off it uses something called a “bypass thermostat”. Second, there’s a weird metal tube that runs back behind, between the engine and the firewall. You can see it in the diagram below. On the diagram it looks like it goes through the engine, but it really is entirely on the outside . Labeled “water inlet pipe”.

Is that external water inlet pipe necessary b/c of the bypass thermostat design?
What exactly does a bypass thermostat do? Advantages and disadvantages to a non-bypass type?

Also, I’m getting some indications of a leak – blue-ish stains – at the point the water inlet pipe connects to the water pump. Anybody seen that too? Is it most likely the gasket? Or could it be the water pump is starting to spring a leak?

Click to enlarge the diagram, the 4afe is the top one.

My Saturn had a similar pipe. It puts the coolant into one end of the engine and takes it out at the other end. As for the leak, could be either.

Honda also uses this system. It controls the coolant temperature going into the block rather than coming out of the head.

I agree . . . there is absolutely nothing unusual about George’s cooling system

Very common design

I’m not trying to be argumentative, I’m just pointing out a fact

I’ve seen this design plenty of times, on many different brands

I also agree that this is not an unusual design on modern cars.

Yup, “bluish” stains are an indication of a leak. It’s most likely the seal on the water pump shaft, not a gasket. You may even see coolant drops directly under the pump. Time to start shopping for a new pump.

Good comments, thanks. )One not yet addressed: Why does that design require a “bypass thermostat”?

This t-stat has two valve discs which move together. The “conventional” disc opens to allow circulation through the radiator (like in the old days). The other disc (at the other end) simultaneously closes the “bypass” tube to stop flow from bypassing the radiator.

Interesting. Is the “bypass tube” shown in that diagram above?

On your engine, the “bypass” all takes place in the t-stat block on the right in the diagram. On my Insight, the “bypass tube” is a rubber hose connecting the exit from the head to a separate t-stat holder. Yours doesn’t have a “bypass tube” per se. Functionally, they’re the same. Sorry for the confusion.

George, here’s your system when cold:

Oh, ok, that makes more sense. I see what you mean now. As the engine heats and the thermostat opens, it does two things at the same time

  • allows coolant from the bottom of the radiator into the water pump inlet, which then is pumped through the engine
  • forces coolant heated by the engine into the top of the radiator, rather back to the water pump inlet uncooled

I wish whoever drew the same diagram for Toyota’s 4AFE FSM made it as clear to understand as your drawing.


I remain a little unclear however on the advantage of that method. There must be a reason to do it that way, b/c it is more complicated than just a single action thermostat. One thing I notice about my Corolla vs my truck, the heater blows warm air considerably faster after a cold start on the Corolla than the truck (which uses a non-bypass type). Is it about making the heater work faster?

It maintains full flow through the block and head at all times which keeps temps uniform, especially during warm-up (vs old-style). I assume this has emissions and mpg advantages.

It seems like – to my pinhead way of thinking – that when the stat is closed both systems work the same way. All the coolant recirculates through the engine and head, none through the radiator. But the difference is when the thermostat opens. With the old style single action t-stat, some coolant flow continues to bypass the radiator. While with the double action thermostat, all the coolant flows through the radiator. Or am I not following your explanation totally?

You’ve got it right, but there’s one other subtle difference.
In the old system the bypass path needed to be more restrictive, otherwise too much coolant would continue to flow through the bypass when the thermostat is open, reducing the effectiveness of the radiator.
With the bypass thermostat the bypass loop can be “wide open”, increasing flow when the thermostat is closed.
Also, 100% of the coolant goes to the radiator when the thermostat is open, which maybe allows the radiator to be a little smaller and lighter.

My '88 Accord had the old version; the bypass went through a small 1/2" hose, which got soft and needed to be replaced before the bigger radiator hoses.
I think it deteriorated faster due to the strong flow during warm up.

Ah, good explanation, and I think that also explains why the heater works faster w/a bypass thermostat. A less restricted bypass allows the pump to push a big volume of coolant through the engine when the t-stat is closed, so the coolant heats up faster.