Engines these days are built with what would be considered for terms of this discussion as an internal bypass for the water pump so that the pump always has the ability to flow free, regardless if the thermostat is open or not. The old Buick engine we've got in these Rovers does not have this internal free flowing bypass in the engine block or water pump as it was designed for an external bypass. Take a look at water pumps from an old SB 350 or an newer LS compared to the LR engine water pumps, for anyone with any mechanical knowledge it's easy to see (and I'm ashamed of myself for having trusted this forum and it's recommendations, rather than thinking for myself for so long, but that's on me, and it'll happen NO more). So instead, the LR engineers used an external bypass to compensate for the early engine technology, and it worked for the most part. Unfortunately when they mistakenly mounted the thermostat low in the cold flow area, rather than up top where the heat is concentrated, and then made it with plastics that don't handle the temperature fluctuations long term, they caused us all lots of problems down the line. Regardless, the external bypass was integral for the proper operation of the system, otherwise, you basically dead head the flow coming from the water pump, and then direct that flow and pressure to the overflow tank and heater cores, neither of which was ever meant to do that job, and will inevitably cause failures in both. When using the "Inline thermostat", you basically take what should have been flowing thru 1.25" hoses, and restrict it all down to 3/4" hose (that's a minimum 50% restriction there, and a big increase in pressure as a result), and then force it thru components that were never meant to deal with all that added pressure, all while stopping the flow of coolant out the hottest part of the engine, the top. Then, the "Inline" also has a tiny hole in the thermostat that allows for a bit of coolant to flow (has too or it would literally suck itself into a vacuum because of the blocked flow), which in cold climates means you will struggle to keep your engine at operating temperature (lowering your fuel mileage), and will regularly set a CEL with the P0126 code if you have an 03-04 model. These are well known issues, reported many times on this forum and others, but still, nobody thought hmm, somethings not right here? So, now that we've taken the time to consider the needs of the Discovery systems, and hopefully now understand why it needs a bypass, we should be able to understand how the "inline" thermostat basically cripples the water pump flow, puts coolant in areas that confuses sensors and sets codes, causes stress to other components in the system, and traps the hottest coolant at the top of the engine, all while putting added parasitic drag on the engine while it tries to turn that restricted water pump. (Can you imagine the pressures the system is seeing at hwy speeds when that "inline" thermostat is closed due to low ambient temps???)

I feel like I also need to give a bit of an explanation / apology for things I've said in other posts. I am an old mechanic, been working on all makes and models of vehicles for over 40 years, 30 years professionally (and have professionally designed 4x4's, race cars, and motorcycles from scratch). Which means I fix things with tools and facts, not emotions and conjecture. If we as mechanics make a mistake, we call each other out on it, as it has nothing to do with how someone "feels". The truth is the truth, it doesn't care if you like it or believe it or not. And in my world, we care about "WHAT" is correct, not "WHO". If someone in the Automotive world were to design a mod for a system that they didn't take the time to fully understand, while purporting themselves to be knowledgeable and qualified, just to find out they didn't even take the time to research or problem shoot before taking peoples money for those products, all while having never even worked in the industry.... we'd ridicule them into oblivion and make sure folks know exactly what kind of yahoo they're dealing with. And if that person kept selling those products, knowing full well the issues they cause, they'd likely get sued out of business, as well they should. So if this forum wants to keep legitimizing this silliness by making it a "sticky" so all the ignorant newby Disco owners waste their money on a product that is basically damaging and crippling their vehicles... well, guess I'd have to hope not having a conscience is at least fun for ya, cuz I'd like to at least think the karma bill would not be a fun one to pay. (but I'm prolly wrong...lol)

One more thought, I've been on this forum for a few years now, and from my perspective, the issue of blatantly bad information being given by folks who have not taken the time to fully understand the systems they are modifying, seemingly solely to take peoples money for useless and badly designed products, while being legitimized by others who also lack any real automotive systems comprehension is a common issue in this forum, one I've not experienced in any other forum before this. One person said this "Inline" thermostat has been a sticky and the go to product for like 14yrs here? Seriously? Not one person thought better of that and spoke up? There are multiple sticky threads up there that from an actual experienced certified automotive mechanic, should not be, as they are full of the types of misinformation and products that would get an automotive repair shop shut down and sued.

For those looking for an alternative for both the OE and "Inline" thermostats, there is some info out there pertaining to a high side metal body external bypass thermostat housing mod that should work much better, and hopefully not cause all the silly issues of these others. I'll be experimenting with this mod myself soon, and will make a thread about it when complete and tested. But till then, if you'd like to get started yourself, here's a few links...
https://discoweb.org/index.php?threa...mod-3-0.95160/
https://landroverforums.com/forum/di...36/#post920585

And, if on the off chance that these products were actually being sold out of ignorance, rather than predation, and the seller would like to do the right thing having learned how things actually work, then by all means, stop selling the inline version, and replace it with the bypass versions so you can have a clear conscience. If you REALLY want folks to know you weren't purposefully predating them, then alert all past buyers of your product about it's issues, and sell them the bypass version at a deep discount, like any auto manufacture doing a recall would do (and in the future do actual research BEFORE selling products). And if you really, really want to do the right thing, send a percentage of the new units sold to the guy that educated ya and saved your bacon... but yea, pipe dream I'm sure, lol (this will be an easy way to see the character of these folks tho, won't it )

And as always from me, Good Luck, and Happy Modding!

 

if nothing else I'm willing to bet that jag housing seals better with far less effort to stop it from leaking than the inline kit. Neither of mine regardless of seal type seal nicely. Defeats the purpose if you have to permatex them shut for easy t-stat replacement to wait 24 hrs before you can run the new one.

 

For those of us non mechanics but DIY using the inline for summer is this still an issue or an issue waiting to still happen. I just spent the fall slowly rebuilding my 04 yeah HG finally went after 12 yrs on my original rebuild and pinning my cyl sleeves mod. I was noticing the long warm up this winter in the 3 times I took it out in the snow. I can see in that thread you posted the concerns raised in cold climate. i dont have the hot bypass posted there in my brain yet but if I consider driving my Laney anymore this winter I will restore to factory hose and thermostat until I get visual clarity on the hot side bypass therm mod. Always learning vut k owing who to learn from isnt always 100%. Thx for your insightful post.

 

running the inline in the summer doesn’t solve the lack of bypass for the water pump that he highlights. The long warm up times in the winter isn’t the only concern he raises

 

On all the inline T-stats I've installed with the high performance gasket I haven't had any leaks. The key I think is to make darn sure the thermostat is sitting dead center. As the inline kit really doesn't have any small indention for the thermostat to center itself in. I've done that on probably 8 of them that I've installed for friends and then I tighten it evenly back n forth and after a heat cycle I go back and check the bolts for being tight. Zero issues. I have 1 D2 with the inline 180F and I have 1 D2 with the OEM LR Grey 180F thermostat. I get better heater performance out of the OEM LR Grey setup as it doesn't take as long to open as it flows differently. Overall temps during the summer = Inline starts off cooler, but eventually they even out at least here in TX. Once you get out on the road the actual operating temp is determined by engine load, ambient temp, airflow, and so so many other factors. At one point I had 4 D2's with different setups, and my overall temp range were within 1-3F from each other.

 

The 87-99 RRC/D1 coolant setup is pretty much identical to what the inline thermostat achieves. They used a regular GM thermostat (190F was stock if I'm not mistaken). BMW introduced the heart valve looking thermostat to LR's plain n simple. The P0126 fault you get with any D2 with SAI is due to a second temp sensor located in the radiator. The BMW style thermostat allows the engine to warm up quicker as the flow bypasses the thermostat (190/195F OEM temp) so that secondary sensor within a certain time will of coarse read a higher temp quicker (BMW did all this trying to squeeze better emissions from a 1960's GM Engine). When you put the inline on taking the cooling system back to the 70-99 config the thermostat reaches 180F quicker vs 190/195F so that second temp sensor doesn't read as warm of a temp so sets the P0126 fault. If you were to sit and idle after startup for a few extra minutes vs instantly take off and drive P0126 wouldn't trip. Whenever I got that on an SAI equipped D2 that just told me it was running cooler than stock!!!

I created the PCV Mod for the D2, and it certainly helps these engines from the poorly designed baffle which clogs and causes excess crankcase pressure and for the engine to leak from every gasket. It's completely NOT stock, but it fixes 2 well known issues. Mods are the only thing keeping these 4.0/4.6L engines alive. BMW didn't give a darn and by 04 Ford was already working on the LR3 with a Jag engine.

PS Also the OEM Thermostat has a hole with a jiggle pin in it just like the thermostats I've installed into the Inline thermostat kit. The hole is to allow coolant to flow even when the thermostat is closed allowing some coolant to flow (IT DOES NOT RESTRICT COOLANT FLOW).

 

During the winter simply run a warmer thermostat like a 190-195F or do like truckers do and block part of the radiator to stop some airflow going thru the radiator. That's why you see 18 wheeler's in the winter with a radiator cover [X] that allows their trucks to run warmer. All depends on your location and the winter conditions.

 

Mntnceguy, at this point I don’t want to weigh in on whether you or Extinct is right. None of us has all the answers, even when we’re very sure we’re right, we might be wrong. Over the last couple of years, you’ve made a lot of very good comments here and have become a respected member. Extinct has been doing this for longer. He is a very well respected and helpful member here. I’ve also dealt with him in his business. He great to deal with, very upfront about everything. You have every right to disagree with him and post your disagreement. But please don’t call his integrity into question. These forums are for us to discuss our perspectives and ask questions from others. Perspectives are perspectives. This is not blatantly bad information. You certainly have the right to disagree and say so, but please do this respectfully.

 

Thank you for saying that. No one should be questioning Extinct's morals or "calling him out" like this. If you disagree, fine. But always be respectful.

 

So what will be curious to see with this set up as Mntnceguy gets it installed and hopefully shares his findings, is how will the temp react during the winter (mainly bc it is right now). One talked about benefit touted for the inline is a more consistent temperature, I've found this to be true in the warmer months but very much not so when colder. For instance this morning taking kiddo to school never going over 35mph in stop and go traffic it was maybe 30 degrees ambient and I only hit 180 degrees when at a stop light. When moving the temp would drop all the way down to 160 and when in the mountains on down hill sections I've had the dummy gauge read cold the coolant temp dropped so much (which when I checked my app it was reading like 135-140 degrees)! While temp swings on the lower side of things is less of a concern compared to fluctuating 20-40 degrees above 180-190 degrees that rapid heating and cooling or change in coolant flow is still concerning. Warmer months the stat just stays open trying to keep it at 180 or close to 100 degrees above ambient.