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Coolant flow: top to bottom or bottom to top?


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17 replies to this topic

#1 RemcoW

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Posted 23 November 2006 - 08:45 PM

I have a 94 Impreza and it is overheating.
The thermostat looks fine. I've replaced the pump last year when I replaced the timing belt. The fans do not come on. I've can fake the system out and turn the fans on by putting a variable resistor in the place of the thermosensor. I've replaced the thermosensor but no joy.

So now I am wondering whether the fans don't come on because they may need coolant flowing by them so feeling around I noticed that the bottom radiator hose appears to not get very hot at all.

I always thought that the the coolant flow on a Subary is reverse to most cars, coming into the radiator from the bottom and comes out the top.
My service manual mentions that this is a "down flow" type radiator.
Someone knows for sure?

On a down flow, one would imagine a blocked radiator could cause this. But if this design is reverse, it is probably less likely, right?

TIA.

#2 porcupine73

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Posted 23 November 2006 - 10:45 PM

Hi, on the suby's I have the exits the block and enters the radiator at the top, then the lower radiator hose goes to the water pump suction. OK, if you can get the fans to go on by using a variable resistor in place of the ECT sensor, then either the sensor is faulty or the engine is not reaching sufficient temp to make the fans go on. They usually won't come on until like 202 deg. F which if it's cold out and/or heat is on engine might never reach at idle.

#3 Setright

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Posted 24 November 2006 - 02:02 AM

As far as I can say, it's a down flow.

The hoses have arrows on them indicating the flow direction - when new,
they may have worn off.

The pump circulates coolant around the block and past the thermostat. Once the temp is up the thermostat opens and lets a little bit of coolant in from the rad, then it may well shut again as the flow across the thermostat cools and closes it. Then it opens again a little later.

I reckon the thinking is that this gives a more consistent engine block temperature.

#4 RemcoW

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Posted 24 November 2006 - 09:30 AM

Thanks, guys! That clarifies a lot.
So the coolant enters the radiator at the top and leaves at the bottom.

Overheating is maybe the wrong term: I see coolant trickling out the radiator cap. When it first happened, I didn't want to take any kind of risk so have not driven the car until fixed. For diagnostics purposes, I might run it for 10-15 minutes but not long.

It just occurred to me that maybe it is the radiator cap? I did not notice the overflow reservoir filling..

#5 porcupine73

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Posted 24 November 2006 - 11:25 AM

Yes the radiator caps can and do go bad, that little rubber seal on there could get gunky, spring weak, etc., make sure the radiator neck is clean too. Just replace it and you should be good to go then. :banana:

#6 Legacy777

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Posted 24 November 2006 - 09:22 PM

No,

The flow is from the bottom of the engine to the top of the engine.

I can prove this. These are pics of the front & back of the water pump:
http://www.main.expe...10/DCP_4741.JPG
http://www.main.expe...10/DCP_4743.JPG

Water enters the front and is spun (pumped) outwards by the impeller. This flows into the lower portion of the block.

Here's a pic of the engine:
http://www.main.expe...14/DCP_4873.JPG

You can see the lower coolant crossover passage in this pic:
http://www.main.expe...02/DCP_4290.JPG

Coolant flows into that and flows upward around the cylinders and then into the top coolant crossover pipe....and then into the radiator.

Josh

#7 porcupine73

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Posted 24 November 2006 - 09:54 PM

OK it goes from bottom to top in the engine and from top to bottom in the radiator. The diagram I see in the service manual shows coolant exiting the engine through the upper radiator hose with the lower radiator hose going to the water pump suction after the thermostat.

#8 Setright

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Posted 25 November 2006 - 05:44 AM

Ermmm...Josh, there's nothing to prove. We all agree - as does the factory service manual! - that the coolant enters the RAD at the top and is pumped out at the bottom :D

#9 porcupine73

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Posted 25 November 2006 - 07:49 AM

It all seems correct, Josh is talking about the flow in the engine (bottom to top) and RemcoW is talking about the flow in the radiator (top to bottom). I think the manual said the rad is a crossflow? So maybe it's top to bottom and right to left.

I would have thought it was the same way in all vehicles because that flow just makes sense, I mean isn't that how boilers and water tanks are designed, the warmer water having a propensity to rise and the cooler water having a propensity to sink?

#10 nipper

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Posted 25 November 2006 - 06:37 PM

FLow is always from the thermostat out.

nipper

#11 RemcoW

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Posted 25 November 2006 - 10:39 PM

Thanks guys -- it certainly seems like the coolant comes out of the top of the engine and goes in to the bottom. The top hose is warm while the bottom is lukewarm.
The last comment about it usually going out of of where the thermostat is what had I always thought as well -- that's actually what caused me to ask this question.

If someone has orginal hoses with arrows (one post mentions they had them) on them, could you please clarify the flow issue?

I replaced the radiator cap. My cap have been bad because it now does seem to build up pressure where it certainly did not before.
In fact, found a tiny weep hole in the radiator that was hiding before.

I'm replacing the radiator. Thanks, all.

#12 ron917

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Posted 25 November 2006 - 11:01 PM

I just finished an engine install today, and the genuine Subaru radiator hoses have arrows that show the flow as you described - out from the top of the engine into the top of the radiator, out of the bottom of the radiator and into the bottom of the engine at the thermostat/water pump. This is on a '99 Outback 2.5L DOHC.

#13 nipper

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Posted 25 November 2006 - 11:52 PM

Subaru Thermostat Assembly Description

The Subaru thermostat can be located on the top of the engine, in the outlet going to the radiator and some may be mounted towards the bottom of the engine near the radiator hose inlet to the engine. Cooling system is rated in degrees that reflect the typical cooling system temperature maintained by the thermostat. You can purchase Subaru thermostats in varying degrees based on the environment or the type of driving you are doing. The Subaru thermostat is the engine cooling systems temperature control. When the Subaru engine is cold, the thermostat stays closed and keeps coolant from flowing thru the radiator. This enables the Subaru engine to warm very quickly. As the Subaru coolant increases in temperature the paraffin element causes the valve in the thermostat to open slightly, allowing some of the coolant to flow thru the Subaru radiator. As the Subaru engine coolant continues to increase, the thermostat opens more in relation to the heat of the coolant. Under normal operating conditions with a fully warmed engine, the thermostat will be completely open. The Subaru thermostat is a very critical part of the cooling system that can cause many symptoms and problems. An open thermostat can result in low heater output, overheating and poor fuel economy. A thermostat that does not open or open all the way can cause knocking or pinging when accelerating and overheating. A faulty thermostat can even cause your car to fail an emissions test. Drivewire carries all the OE; genuine top brands of thermostats for your car at discount prices.


Dear Tom and Ray:

I need some help. My roommate and I have a serious bet going on. He thinks he knows everything about cars, and I DO know everything about cars (just kidding). He says that the cooling system runs water from the engine to the lower radiator hose, then up through the radiator and out the top radiator hose. I say it's the other way around; that the water runs into the radiator at the top, and out the bottom. We're talking about good old American V8s here, not a little Subaru. What's the answer? If he's wrong, he has to wash my truck once a week for a year. -- Tim
Ray: We're enclosing a good set of rubber gloves for your roommate, Tim, because he's going to have some serious dishpan hands.
Tom: For traditional, old, American cars, you're right, Tim. For ages and ages, the hot coolant came out of the top of the engine, through the open thermostat, down through the radiator, and then out the bottom of the radiator where the cooled water went back into the bottom of the engine.
Ray: But now, on a lot of cars, the flow HAS been reversed. The hot coolant comes in the bottom of the radiator, gets pumped to the top, and then goes back into the top of the engine. And for a very good reason.
Tom: Engineers realized that the top of the engine really needs the most cooling. That's where the cylinder head is, and, more important, where the head gasket is. So they turned the flow around, so that the coolest water would come out the top of the radiator and flow right into the cylinder head where it's most needed. And since they've done this, the number of blown head gaskets has decreased dramatically. Ray: So if you want to cut your roommate a little slack, you might mention that if you were talking about modern cars, you'd both be right. But why do I suspect you're going to wait about 11 months before bringing this up, Tim?


interesting

nipper

#14 Setright

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Posted 26 November 2006 - 04:16 AM

Nipper, I can say 100% sure that coolant flow in an EJ series engine is from bottom of the block to top.

It enters the radiator via the TOP hose. The thermostat will oscilliate quite a bit, but that means a more stable block temperature.
It also means air is less likely to get trapped inside the block. This is a boxer engine after all.

This direction has always been my "reasoning" and I've looked it up in a Subaru Service Manual at my local authorised dealer. :D

#15 Snowman

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Posted 26 November 2006 - 06:26 PM

Nipper, I can say 100% sure that coolant flow in an EJ series engine is from bottom of the block to top.

It enters the radiator via the TOP hose. The thermostat will oscilliate quite a bit, but that means a more stable block temperature.
It also means air is less likely to get trapped inside the block. This is a boxer engine after all.

This direction has always been my "reasoning" and I've looked it up in a Subaru Service Manual at my local authorised dealer. :D


Ditto. I've seen tons of videos on cooling systems in class, and there are lost of "reverse flow" systems out there. There are even cars with a thermostat on each end of the cooling system. However, Subaru has not done that yet. The thermostat controls the coolant flow into the engine from the bottom of the radiator, rather than controlling the flow out from the top. I've also been told that this provides more consistant temperatures.

#16 porcupine73

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Posted 26 November 2006 - 07:43 PM

Cool info nipper, thanks. Do you guys think maybe the newer Suby's like '06 might be the reverse flow setup? Just wondering because I see the list on the thermostats for these is like $10 more than the rest.

#17 ferret

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Posted 26 November 2006 - 07:50 PM

Attached .pdf for EJ engines

#18 ryry46d9

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Posted 26 November 2006 - 09:11 PM

WOW i think i just learnned too much for one night zzz

ryry :drunk:




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