Inline Thermostat Mod

[db*****]

Hey all, 

Recently I've been discovering posts about changing the location of the thermostat from the bottom of the engine, in front of the water pump, to the top radiator hose right after the coolant exits the engine. 

This is supposed to reduce cavitation, and I've been having problems with an ej25d overheating. Ive ordered new hoses, radiator, and thermostat, and I'm thinking about doing this inline thermostat mod. Am I wasting my time thinking about this mod? I've always had a hard time burping the ej25d system so maybe this would help? This would also be easier to replace, and in theory reduces cavitation at the water pump.  Has anyone done it? What are some experiences with it?

Edited November 14, 2018 by db*****

[carfreak85]

I've never heard of this mod on a Subaru engine, which don't really have issues with cooling (blown HGs not withstanding).

To burp the system properly, I usually raise the front end of the car as high as I can, turn the HVAC temperature to hot and run the engine while occasionally squeezing both radiator hoses.  You need to wait until the thermostat opens and the radiator stops bubbling before the burping is complete.

[db*****]

Yes I do the same to burp. Subarus do have issues with cooling. I have found a good amount of information, mostly based on eg33s and ej25s, suggesting that head gaskets aren't the issue, but air in the system is the issue. Head gaskets going bad could just be a symptom of air in the system. 

https://www.mrtperformance.com.au/resources/technical-documents/keeping-your-engine-cool-when-all-others-are-boiling-theirs

Here's a tech document written by an SVX owner. (I also own an SVX, which is why I stumbled on this. )

[idosubaru]

Absolutely not - properly burp, diagnose, and repair the overheating before wasting time and money. 
It's not going to mitigate headgasket issues - you can drill a hole in the t-stat  or get a vent release radiator cap if you're trying to limp this thing to a slow death. 

As hard as burping might be - it's done thousands and thousands and thousands of times without issues, it's not magical or mysterious.  If you can't burp it then that probably means you have a blown HG.

Once you don't have overheating, then decide if relocating it is helpful for various reasons.

[1 Lucky Texan]

yeah, never read of it across multiple forums/newsgroups for the last 15 years or so. Not saying it hasn't been done, but VERY uncommon I'd say.

there is a water circuit involving the heater core that is important, dunno how that would work with T'stat relocation either.

 

[db*****]

Not trying to limp it to a slow death, just trying to keep my engine efficient. Yes I know burping is not magical and mysterious, jeez. 

Head gaskets were replaced around 2 years ago. Should still be good, unless the mechanic messed something up. 

I guess I was looking for open minded discussion on this. Did anyone read that tech document? Cavitation causes air in the system at an increasing rate after 3,000 rpm. Anyone have thoughts on that guys experiences?

 

[1 Lucky Texan]

I can't argue that someone may have created a specific improvement for a specific racing application, just that it isn't common for our daily drivers as far as I have read.

I just scanned his article It's more like a personal anecdote than a 'tech document'. - I'm not convinced he understands that a rad cap can maintain over 15 additional psi (over 1 bar) on most of our cars. Also not sure he's using the term 'cavitation' appropriately.

plus, in general, you don't want an engine run too cool as it reduces fuel efficiency.

an oil cooler could be a better mod but, I dunno.

Subaru engineers seem to know there is something odd about the cooling system though - compare an OEM t'stst to a 'typical' aftermarket one. The wax capsule is quite a bit larger - probably needs more hysteresis.

 

 

 

Edited November 14, 2018 by 1 Lucky Texan

[db*****]

Yeah I know it was kind of a personal experiment of his. Only reason this stuff caught my eye, other than owning an SVX, was because Subaru is one of the only manufacturers left to put the thermostat on the bottom of the engine. I know they have their reasons for doing things in certain ways, but the thermostat is literally right in front of the water pump. This is sure to cause some cavitation isn't it? May try to experiment with it if I ever have to rebuild my motor. Obviously, if this actually does cause cavitation, it's not enough to hurt the motor immediately, but over a long time.

[db*****]

I have noticed before how much larger Subie thermostats are. Another point on the upper thermostat thing, wouldn't you want your thermostat to be getting coolant immediately after it leaves the engine instead of coolant going through the radiator, then to the thermostat? I'm trying to figure out why the t'stat is in front of the water pump, before coolant gets heated by the engine? Do you want the coolant to pass through the radiator before hitting the t'stat? 

[idosubaru]

14 minutes ago, db***** said:

Did anyone read that tech document? Cavitation causes air in the system at an increasing rate after 3,000 rpm. Anyone have thoughts on that guys experiences?

 

The key thing you said is "you have current overheating issues" - that's a problem. Fix that and then start looking at additional options. 

I guess you're asking if this coolant mod can prevent headgasket issues - is that your question?

i gave it a 20 second cursory glance -  
Racing is not daily driving.  If racing/high performance is an end goal - the first question is even more importantly proper diagnosis and repair before deciding which upgrades are best.  It's not worth reading because it's not addressing any pragmatic/practical issues for EJ25D daily drivers.

Facts:  EJ25D's have been perfectly maintainable and repairable with high success rates for 20 years - billions of miles, hundreds of thousands of vehicles, tens of thousands of headgasket jobs.  EG33 engines are highly touted as reliable as well. 

I guess your question is whether some coolant modification would prevent headgasket issues?  I highly doubt it and that would be hard to prove.  GD might have the best data driven experience on that, he could talk circles around that guy.  I'm pretty sure he'd say that oil supply issues will be the death of a race engine before any tstat issue.  He says high performane subaru engines pushed hard only have a 40k lifespan and it's not due to cooling. 

 

A significant issue with EJ25D's is that they're let go and allowed to have multiple overheating events.  diagnose and repair properly and they're good to go. 

The issue is that they are highly prone to multiple overheats and do not respond at all to the typical methods of "turn the heat on high to dump more heat" or "let it cool down, top off, drive again, wash-rinse-repeat" that you can do with most cars overheating/blown headgaskets. those don't work at all with factory installed initital headgasket leaks. People try to use them, think it'll help, then run them hotter on average than other vehicles that do respond to that.  

1.  They're hard to diagnose.  They can overheat randomly and have weeks and months between overheating episodes, so people tend to limp them along longer than normal.  They were confusing shops frequently in the 90's and early 2000's who couldn't get them to overheat or would see them not mixing/oil coolant, not blowing coolant out the exhaust and passing compression tests and passing the cheap liquid "block tester" kits.  I'm not even a trained professional and shops and mechanics would ask me to come look at a EJ25D overheating .  They would replace radiators, hoses and scratch their heads, then the car is let go to the customer - where it will overheat again and comes back to them with coolant blown everywhere and theyr'e confused.  

2. Turning the heat on high or letting it cool/topping off/driving - are common methods to drive a car with overheating issues.  This doesnt' work on EJ25D's - further promoting additional miles and excessive overheating. 

Extrapolated out to thousands of samples - this means EJ25D's are frequently overheated more often and more severely than other Subaru engines.  The main issue then is oil/bearing compromising.  Anyone around a lot of subaru's for a long time is not surprised to hear this story "i replaced the headgaskets on my EJ25D and then the rod went through the block" - yep seen that plenty of times.  They have new radiators, hoses, headgaskets - and a neat Exxon Valdez replication on top the block.

[scar13]

If you have overheating problems with a ej25d, the hot gasses escaping past the head gaskets are displacing the coolant, the resulting pressure in the cooling system then over powers the spring on the rad cap and you get the boiling over in the overflow reservoir.

In the article you linked the author is working with extreme conditions. You have a stock unit which should be adequate for daily driving.

On your model, the radiator, thermostat and head gaskets are suspect/questionable.

His mods will not help you.  

[GeneralDisorder]

If you have overheating on a 25D after doing all that work and burping the system, then you need HG's. Changing the location of the thermostat isn't going to change that. 

Subaru uses a cold side control on the EJ's. It works just fine. Cavitation is not a problem unless you have no flow through the heater bypass, or you run straight water that has too low of a boiling point. Cavitation is the formation of vapor pockets due to the pressure behind the impeller blade being too low (insufficient flow through the pump). I have only seen cavitation damage a few times in many years and it's always due to running straight water with insufficient bypass flow. 

GD

[idosubaru]

26 minutes ago, db***** said:

Y Obviously, if this actually does cause cavitation, it's not enough to hurt the motor immediately, but over a long time.

if cavitation was an issue wouldn't we see 250k and 300k water pumps, housings, and blocks, or something with damaged vanes?  i would guess this topic would be well known in the forced induction world if it were common. 

[GeneralDisorder]

Actually the cavitation will damage the block. I've seen it (and had to repair it) a couple times. Always on modified turbo cars from running water due to track requirements, etc. Not properly warming up the engine, insufficient bypass..... blown HG's also can contribute. If the system is properly assembled and proper antifreeze is used it isn't a problem. We build 600+ HP engines and run STOCK thermostats and water pumps without any issues whatsoever. So no you don't need to relocate the thermostat - that's not going to do anything useful on a 165 HP 25D. 

GD

[idosubaru]

11 minutes ago, db***** said:

I have noticed before how much larger Subie thermostats are. Another point on the upper thermostat thing, wouldn't you want your thermostat to be getting coolant immediately after it leaves the engine instead of coolant going through the radiator, then to the thermostat? I'm trying to figure out why the t'stat is in front of the water pump, before coolant gets heated by the engine? Do you want the coolant to pass through the radiator before hitting the t'stat? 

placement is an interesting thought exercise but ultimately it's a system wide design, not a single choice.

while I can poke holes in this comment and I don't subscribe to it precisely, one could also say the tstat at the water pump guarantees every engine "sees" the same incoming coolant temps.  one could view that as a plus...and so it is with stuff like this - you could pick apart hoses, temp senders, fans, tstat, diameters, pressure, flow volume....etc in the same way. 

[db*****]

15 minutes ago, idosubaru said:

The key thing you said is "you have current overheating issues" - that's a problem. Fix that and then start looking at additional options. 

That's why I bought all of the cooling parts that need replaced on my car.  I plan on replacing hoses, radiator, and t'stat this week. If its still overheating, Head gaskets are probably bad, I'm not doubting that at all, in fact, I bet my hg's are bad.  I'm curious as to why they've gone bad so quickly. Probably air trapped somewhere in the system. I know how 25d's are usually treated, and I don't treat mine that way, although I'm not sure how it was treated before me. I know that when a car overheats, you don't just turn the heat on to fix it. 

[whynot]

I do not know who did the head gaskets, or what they used for pI did mine in april. used fel-pro gaskets, and made a few mistakes on surface prep, that were fine for the dodge and ford I have replaced head gaskets on before. I just finished doing them again, with Subaru gaskets and better prep. The subaru gaskets are much different. So maybe look into what was used, and what prep work they did? 

I just got mine back on the road, so we will see if that resolved the issues

[GeneralDisorder]

The HG's are bad because the heads and block were not properly prepped and surfaced and/or the head bolt threads were not dressed and torqued correctly. 

GD

[db*****]

I got most of my information on this mod on the SVX forum where they have researched, experimented, and followed through with doing this. 

[idosubaru]

1 hour ago, db***** said:

 I bet my hg's are bad.  I'm curious as to why they've gone bad so quickly. Probably air trapped somewhere in the system. 

Inferior gaskets, surface prep, bolt torque - happens all the time.   

Who installed the headgaskets?

Cheap gaskets typically fail by one year.
The heads always have high and low spots and issues around the fire ring mating area and aren't at an ideal finish for MLS gaskets.
Were the heads resurfaced properly (a wire wheels or disc doesn't count)?
Was the block face cleaned and prepped?
Bolt torque - was it done right? 

The days of easily graphite gasket slapping a head are gone, modern MLS gaskets need precise prep and install.

Trapped air did not cause the headgaskets to fail unless they were driven for extended periods of time with additional cooling system issues (leaks, cap, burp, etc) which then caused the HG failures...but that's operator error, not air. 

[carfreak85]

50 minutes ago, db***** said:

I got most of my information on this mod on the SVX forum where they have researched, experimented, and followed through with doing this. 

I'll say it again, for those in the back:  Generally speaking, there are no Subaru engines that have an inherent weakness or Achilles heel regarding the cooling system.  The radiators are large enough (for any non-racecar engine), the water pumps move enough fluid (even for 800+ hp race engines), the blocks flow well enough to keep things from melting.

The only reasons you hear about Subarus overheating is 100% related to either: head gasket failures, using non-OEM thermostats, old cracked/plugged radiators or electrolysis due to lack of maintenance.

I will say this, however.  It has recently come to my attention that factory turbocharged EJ engines can benefit from installing a coolant tee between the driver's side head and the HVAC heater (GDT Cylinder 4 Chamber Cooling Kit).  This modification doesn't have anything to do with overheating engines, but everything to do with balancing thermal loads between the cylinders.

[1 Lucky Texan]

carfreak - thanx for posting that!

[idosubaru]

20 minutes ago, carfreak85 said:

I'll say it again, for those in the back:  Generally speaking, there are no Subaru engines that have an inherent weakness or Achilles heel regarding the cooling system. 

are you listening?  you're ignoring that one guy on a website who races, with zero credentials, no EJ25D experience, but has solved it's headgasket issues!

man some people.....

LOL

[db*****]

I'm sorry is this post about me?

13 minutes ago, idosubaru said:

are you listening?  you're ignoring that one guy on a website who races, with zero credentials, no EJ25D experience, but has solved it's headgasket issues!

man some people.....

LOL

 

[db*****]

Seems as if there is some hostility in this thread. Lol. Sorry for trying to explore my options. 

Thanks for all the help!