(OT) Car stuff, but not Ford or Toyota

I have a '93 Mazda MPV that sprung a coolant leak from the housing that the Lower Hose connects to. This housing has a small-diameter hose coming off of if that goes to the housing where the Upper Hose is connected.

The Lower Hose Housing also has a passage, obviously, that lets coolant into the engine.

The housing also has a theromostat inside of it. I've done my share of engine work, but I'm not familiar with a thermostat in the housing where the Lower Hose is connected. I know all about the housing where the Upper Hose is, and the thermostat that goes there.

It looks like this housing diverts coolant during cold-temp operation (initial warm up) to the intake, or other such system, to warm it faster than might happen if the lower hose simply was connected to the block -- which is the method I'm familiar with.

What is the default state of the t-stat in the lower hose housing? What does this t-stat do? Since I'm scratching my head over this part, what are the symptoms associated with its failure? Does this t-state replace the one I expect to find in the upper hose housing, or is this one in addition to that one?

FOR THE RECORD It looks like my only problem here is that the mounting bolts were a bit on the loose side, and the gasket gave out.

Reply to
Jeff Strickland
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Subaru does this too. It's a standard thermostat. I wonder why they do that, because that means changing the thermostat means loosing a LOT of coolant rather than...er, NONE!

I guess the logic is that if the thermostat is shot, so is the coolant...

Reply to
Hachiroku

Probably a reverse flow cooling system - as in, the reverse of the "traditional" direction, where it entered the engine through the lower hose and returned to the radiator through the upper hose, said layout being determined by the fact that hot (whatever) rises, dating back to the days before water pumps were used and coolant circulation was solely done by the thermosyphon effect. Makes for more even temperature distribution between the head(s) and block.

nate

Reply to
N8N

So, in this configuration, the t-stat is moved away from the top hose to the lower hose?

I am not familiar with this arrangement. I think I can understand how it works, but I was thinking there was something with the smog equipment that gave me two t-stats for some reason.

Reply to
Jeff Strickland

It DIDN'T work! I have a blown headgasket on that car!

Reply to
Hachiroku

1 t-stat, moved from the top hose to the lower hose; reason as Nate described.

When you say "default" state of the thermostat, there is no "default" state of a t-stat. The "state" or position of the t-stat depends on temperature of the medium it resides in. Below its opening temperature, it will be closed, and above its opening temperature, it will be open.

Reply to
Ray O

I don't see much difference between a Ford and a Mazda so to me it's on topic.

I don't know anything about your lower thermostat but my guess would also be some kind of emissions control to heat up the engine more quickly but I don't see how it would help. If it opened at a lower temp the engine would run cooler and if it opened at a higher temp then it would be pointless.

Reply to
Ulysses

I shoulda said "cold state"

I can see what I have cold. Of course, this does not tell me what happens when it gets hot.

I'm not sure I understand what Nate said about the reverse flow.

My top hose is the return from the engine. Coolant gets picked up at the bottom by the pump, circulates through the motor and comes out the top. All cooling systems work that way as far as I know.

Reply to
Jeff Strickland

"Jeff Strickland" wrote in news:gr0vih$87c$ snipped-for-privacy@news.motzarella.org:

Indeed they do. Heat rises. Coolant is pumped in the direction the heat wants to go anyway, so the upper house is always the outlet.

As for the thermostat being in the lower hose, Honda has done this since about 1990 (on all injected engines). Toyota may have gone to this about that same time.

In this arrangement, the water pump is at the opposite end of the block from the thermostat, with (often) a large tube running behind the block from t-stat housing to water pump. The pump sucks fluid through the t-stat housing, through the tube, then pushes it into the water jacket in the block. From there it rises through the head gasket and out the upper rad hose. It's a "straight-through" system.

There are a couple of small bypass hoses (or channels in the various housings) that recirculate coolant between block and t-stat housing bulb. When the thermostat is closed, flow through the bypass hoses allows the thermostat to "see" the heat being generated in the block. As the heat builds up and the thermostat opens, the bypass is slowly closed off, diverting /some/ flow through the water pump, until eventually /all/ the flow is through the water pump.

My personal belief is that this arrangement was adopted in order to ensure better thermal control of the block and head, and the lower emissions that would be made possible through this control.

An older setup, for contrast: My '82 Corolla had the thermostat in the traditional location at the upper rad hose. Both thermostat and water pump were located at the front of the engine. Coolant traveled a longer, U-shaped path, with some of the coolant passing through the head gasket before the rear of the engine. I imagine the rear cylinders ran hotter than the others.

Reply to
Tegger

My motor is a V6, and the water pump is adjacent, perhaps even adjoining, to the water pump. Clearly it is not at the opposite end.

I get the bypass hoses you speak of, but it is right in front, and connects the lower and upper housing. I was thinking it helped heat the intake quicker, but now that I've spent the day messing around with all of this stuff, I think it just feeds the heater core. I know from past experience that the heater is itself a bypass system from the main waterflow that passes through the block. The idea is, you would want hot water at the heater core even if the t-stat was closed. It appears the heater circuit works by the effect of hot water rising and this creates a water flow all by itself.

That was the theory I was working with as well.

Reply to
Jeff Strickland

Reply to
Ray O

You're getting warmer (sorry, I couldn't resist)

The heater core gets coolant circulation when the heater control valve is open because the water pump is circulating coolant whenever the engine is running.

Reply to
Ray O

The thermostat is located in the lower hose in order to allow faster factory filling of the cooling system. The default state of a thermostat is - closed - it is held closed with a very strong thermally sensitive spring - since it is installed in a cold (new) engine that's never been run. When a closed thermostat is installed in the upper hose, it creates an air pocket and the engine must be started and brought up to operating temperature to allow the thermostat to open. Then, water or a coolant mixture may be added to top off the coolant system. I've seen lots of new cars come off the transport truck low on coolant. That's why there is a "make ready" department. One of the items on the make ready list is to top off the coolant - in the older cars by removing the radiator cap (open systems) and in the newer ones by verifying the overflow reservoir is filled to the proper level (closed systems).

With a thermostat installed in the lower hose, the air pocket does not form and therefore the cooling system does not need topping off which saves time (and money) on the production line. That is the main reason manufacturers design engines with the thermostat at the lower (intake) hose of the cooling system.

The smaller hose from the thermostat housing going to the upper hose area is a by-pass only hose that allows water to circulate and provide some cooling of the engine if the thermostat should stick either partially or fully closed. Some water from the block would be forced through the by-pass hose, into the upper hose and into the radiator. It isn't very efficient, but may make the difference between a blown head gasket or worse while the operator receives enough feedback that something is wrong and shuts the engine down.

Now, to the original problem of the bolts being loose to cause the leak, my thought is that over the 15 years of service, electrolysis has taken a toll on the gasket and surrounding metals which has degraded the gasket and metals to the point of causing a leak.

Those little flat ground wires that are standard on every vehicle between the engine block and body are there to keep a static electrical charge from forming in the metals with different numbered electrons - block - head - exhaust and intake manifolds - brass or steel "freeze" plugs which are really to plug holes left in the block and head from the casting sand used to produce the correct interior shapes in the casting.. A lot of "mechanics and others" don't think the ground straps are needed and don't replace them when a repair is made where they must be loosened or removed. A case of the mechanic knowing better than the engineer who had the ground strap installed in the first place.

I've heard more than one mechanic state "I'm smarter than the engineer that built the engine. Know why? Because I know which bolts I can leave out and the engine will run just fine." Well, they leave off the ground straps too!

I've spent 35 years issuing parts to "know it all" technicians of all sorts and I've petty much seen it all.

Reply to
Steve

Well, of course the heater control has to be on!

But the water that flows through the core is desired regardless of whether the t-stat is open or closed.

Reply to
Jeff Strickland

True, but that wasn't my point. You said "It appears the heater circuit works by the effect of hot water rising and this creates a water flow all by itself." My point was that the coolant circulates in the heater because the water pump operates when the engine is running regardless of whether the thermostat is open or closed, not because of the effect of hot water rising (convection). In other words, the water pump doesn't stop pumping when the thermostat is closed; instead, the coolant circulates within the block and also the heater core if the heater valve is open.

Reply to
Ray O

I spent over 20 years as a service tech at GM dealerships and I've seen a lot of parts guys that thought they were mechanics, too. I really hope you're not such a dick in real life as you come across on the internet.

Oh, and Jeff, I'm with the others. Better thermal control for emissions purposes is the primary reason for the change in location of the t/stat. Even with these vehicles, the coolant gets checked before the car goes on the lot.

Reply to
Old Crow

Well ,in the case of a Scoobaru, thermosyphon cooling doesn't work particularly well anyway - the heads are even with the crankshaft! My comments were more general in nature and I was envisioning a traditional vee or inline engine. Having no experience with them I have no idea if the coolant enters the heads first or the cooling jackets for the cylinders.

nate

Reply to
N8N

"Jeff Strickland" wrote in news:gr14gg$fpa$ snipped-for-privacy@news.motzarella.org:

If you check very carefully, I think you may find (I could be wrong for your Mazda) that the water pump discharges into a big tube in the valley of the vee. This tube goes to a sort of splitter at the rear of the engine that feeds coolant flow to the water jacket in both banks.

In other words, even though both pump and thermostat are at the front, you still have a straight-through system. My original description was specifically for 4-cylinder engines, but the concept is the same for V6's.

There will be several bypass lines which are not necessarily external hoses.

The thermostat bypass is one of them. There are others which are used to heat the throttle body and idle air control valve in order to prevent icing. There may be more on your particular vehicle.

As Ray O has stated, water pump operation causes the coolant to circulate even when the thermostat is closed. This is absolutely necessary to prevent severe damage from localized overheating when the thermostat is closed. It has the added benefit of providing early heater operation. Thermosyphon would be utterly useless if relied on for effective heater operation.

Reply to
Tegger

N8N wrote in news: snipped-for-privacy@n20g2000vba.googlegroups.com:

All engines of any sort of modernness (would include Studebakers!) push the coolant through the block first and the head after.

Reply to
Tegger

My 1994 Corvette uses reverse flow, but I was told that the main reason was to circulate the coolest water through the aluminum heads first in order to improve power.

Reply to
Mark Jones

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