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620 Cooling - Radiator Bypass Loop


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I'm creating this post as an alternative to the 620 Cooling thread which now focuses on remedial work being used to correct the overcooling issue affecting the 620.

I would treat this solution and thread cautiously,  I'm not sure what I'm doing.  I don't have the experience or technical knowledge shown by the amazing team in the other thread.  Though I do actively invite input, and critique to help optimize the concept I'm using.

To be clear I'm doing this as I don't have the confidence to remove the intake manifold with all the extra ancillaries it has on the 620 due to the supercharger.  This confidence may change over time, and I may fit the solution that the team have come up with.  I'll probably purchase the parts and leave them on my workbench attempting to find the bottle *irked*.  For the time being, I'm going to try an easier approach and compare the results.

The approach I'm using is to install a coolant bypass in front of the hot side of the radiator, diverting coolant sub 89c from the radiator back to the thermostat housing.  Coolant over 89 will be sent to the radiator and then back as normal.

To do this, I'm cutting into the hot side top coolant hose between the engine and radiator,  into which I will install a bypass thermostat housing.  I will also cut into the bottom radiator return hose and install a T-piece.  The T-piece will receive the bypass coolant from the bypass thermostat housing.  Coolant under 90 should flow through this loop back to the OEM thermostat housing.

The team in the other thread have confirmed and also acknowledged by Caterham Cars part of the overcooling issue is due to coolant pressure forcing the thermostat in the OEM housing open.  Nullifying its presence, ie it appears stuck open.  I'm therefore going to leave the OEM thermostat in place.  The thermostat used in the OEM housing receives the pressure at the rear of its surface, hence reacting the way it does.  The bypass housing is a T junction, my hope is that the pressure will behave differently in the housing and won't force the thermostat into a set position. This might be a failure point in the design.  Only time will tell.

My parts list are as follows, all purchased from carbuildersolutions.

By-Pass Thermostat 87 Degree SKU: BYST87 £32.40 1 £32.40
Gates Green Stripe Flexible Hose 32mm 5ft Long SKU: GRE32 £69.60 2 £139.20
Stainless Steel Hose Clip 30-40mm SKU: H30SS £2.16 7 £15.12
Hose & Pipe Cutter SKU: HOSEC £7.80 1 £7.80
32mm Welded Aluminium Tee SKU: TEE3232 £24.00  1 £24.00 

Link to the bypass https://www.carbuilder.com/uk/by-pass-thermostat-87-degree

You might notice I've opted for flexible coolant hoses.  I'm not sure what these are like.  I'm using them as I expect to connect the middle port of the bypass thermostat to the hot rail, and then attach the housing to the cruciform.  As both output ports from the bypass housing are at 90 degree I'll need to ensure I have the flexibility to route the coolant hose around the intercooler.  This might be overthinking things, but I'm trying to predict unforeseen issues.  I think I'll also source some standard silicon hoses in case they fit as I'd prefer them.  Connecting the bypass loop to the bottom T should be simple, but I will need to cut and reuse the factory hose.

I think that covers it, I'd love to know your thoughts.


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I believe the bypass thermostat will close the bypass loop when it gets hot.  However I've never seen or used one before, so I might be wrong.  I'll drop it into a bowl of boiling water first before installation to see what it does.  Even if it doesn't completely close, it might still be enough to keep the temps from falling too much, of course that scenario might be an issue during extended track use.   But that's an unlikely scenario for me.

There's a handy picture, from the suppliers website, it's this logic I'm working on.0028561_by-pass-thermostat-87-degree.jpe


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Hi Mark 

One comment on your initial post , you say that you think the OEM thermostat is being forced open by the water pump pressure causing the cold water to flow , Based on what we've found running our modified design in the other thread  i don't think that's the case , the cold water flows in the OEM set up because of the big machined cut out in the housing, its akin to trying to fill a bath when you've left the plug out , the water keeps escaping  .

Have a look at this picture and you will see what i mean .


Good luck with your alternative solution , Just have a think about what the effect of having the original thermostat left in position might have on the way the system responds , each thermostat is going to be responding to different sources , personally i think you need to remove the OEM stat if you are  fitting a remote one .


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Ive also just noticed that the thermostat in that bypass unit is 87 degrees , that means that it will circulate water back to the engine until it reaches quite a high temperature . The standard 620 thermostat is 82 degrees , just check you wont have a miss match with the fan switch on setting on the ecu . If you do the fan may run continuously .

The remote stat idea should work but you just need to think through all the affected issues .

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Lots to cover guys,

During my email comms with CC, they replied with the following statements, who knows what to think.

Lee Bristow (Dealer Support Engineer) "However even with the thermostat fitted, it has been proved that the force of the cooling system can force the thermostat open and begin to bring the temperatures down prematurely. In an attempt to prevent this, we now remove a small section at the top of the thermostat, to act as a pressure bypass and allow a small amount of coolant to pass though, which I believe is what some people wrongly determine as being the thermostat failing and sticking open."

Lee Bristow (Dealer Support Engineer) "Duratec engines are renowned for forcing their thermostats open. We think that they even do this at high revs in the standard Ford application, but the duty cycle never reveals this; plus the wide-band ‘normal’ of the temperature gauge means it would be hard to spot. We have looked into solutions in the past and one thing that was tried with some success was pre-loading of the thermostat spring, to enable it to better resist the high water pressure. Without significant testing (which is always a challenge for us), it is not really suitable for rolling out into production. However, I think that we could probably provide you with a special order thermostat that would more closely match how you use the car."

I told CC I wasn't prepared to do their testing for them, they didn't have another offer so the conversation dried up.  Also surely if you preloaded the stat, and there is low rpm, will the extra tension in the stat keep it closed, causing overheating?  Oakmere wouldn't help either unless I complied with the modified thermostat from CC.  

Just on the topic of two thermostats, surely the additional one is only controlling flow around the radiator, rather than what is entering into the engine.  It is possible I'm completely missing something?  I might be misunderstanding what is happening.  I'm still not comfortable gaining access to that area either.  If I could get that far I'd be waiting for your solution at this point,

With regards the higher temp stat, yes it would be better if it was 82, however the stat in the 420 is higher, so I'm working on the assumption the engine will be happy with it.  If I could find an 82, I would use it.

Warranty, hmmm.....  Yep and this is the part I have a significant beef over.  60K on a car and I have to kiss good bye to my warranty after 700 miles.  I'm writing off CC and Oakmere, their handling of the situation has been a disgrace.  Oakmere won't see me through their doors again.  My 620 will be serviced by an independent only.  I think it's scandalous.  During the earlier days of the other thread someone mentioned a comparison with Porsche GT products and problems they had with engines denotating.  The difference, owners didn't need to intentionally denote their warranty fixing the problems.  Any how, I've accepted who I can trust,  CC and Oakmere aren't on that list.  Seven ownership will be independent only from this point forward.  But let's not get into this topic.

Guys, if you think my plan is flawed or a bad idea please let me know,  it is possible I've misunderstood the entire circuit.   I'm not going to be offended. I'd rather know.




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Igors fitted a larger water pump pulley which reduces the water pump speed , this might help with any issues of the thermostat being forced open at higher revs . Ive left mine as standard at present to see if there's any difference in performance when all the other parts are similar .

Don,t forget in the standard CC system on the 620 there's no volume bypass ( only the small diameter  bleed to the expansion bottle ) so water has nowhere to flow when the thermostat is closed, This might cause it to be forced open.With a proper, larger diameter  bypass from the rear water outlet on the block back to the pump the water can circulate through the engine without having to force a route elsewhere . Thats the main reason we ended up going for the solution we did .

I will have a look at the standard CC map settings for you tomorrow and tell you what the fan switch on setting is .

The standard OEM thermostat is operating against water temp on the engine side of the stat , its expecting the rad feed temp to be lower .

I repeat what i said earlier , i think the remote stat solution could work but i have reservations about leaving the original stat in place .

https://www.eliseparts.com/shop/engine-br-gearbox/cooling/pressure-relief-remote-thermostat-kit/ This appears to be 82 degrees 


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Starting to see the point...the differing temps in the stats may cause the fan to run against the rad when the rad isn't getting flow etc.  I'm learning here!  I'm not too concerned about the pressure, at the end of the day I'm not changing that behaviour.

If I'm going to try this, I will need an 82 stat.  I need to find that Rover stat on its own.

Love this from the link you sent  

Designed to help to prevent thermal shock suffered by the engine and reduce head gasket failure by keeping the engine temperature change at a slow and even rate.

Should send that to CC.

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I've been looking at how the PRT works, I've pinched someone's diagram and modified it to represent how I believe it would be connected.   Now my concern here is that the shape of the PRT will result in the full force of the coolant flow hitting the back of the stat causing it to stay open just like the factory stat.  It won't be able to close the bypass.

I know the housing I picked is only subtly different, but I was hopeful it would alleviate the pressure issue.

I'm starting to think this isn't going to work, it's going to need your modification, or remove the factory stat?



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Whilst you are thinking about thermostats think about the standard Duratec one , its double ended , when it heats up it  opens to let cold water flow from the rad whilst at the same time it closes the bypass loop . 

Not trying to be obtuse here just want you to think and understand how the system on your 420 works . 

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doing my best here.... 

Having posted what I mentioned in #19 about pressure pinning the stat closed, I'm wondering if the fact there's 32mm exits either way there won't be the force on the plate, so it can move, unlike the OEM housing with only the 16mm bypass hose.

Does the force/pressure only occur at the factory housing, and drop beyond the expansion bottle, 

I'm getting confused  *nuts*

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I can’t see why that remote stat wouldn’t work OK, in terms of being or not being forced open. 

Presumably, looking at the pic of the original stat, the flow on that one is in the opposite direction to what it will be in your remote, if indeed there is a problem with water flow forcing it open?

Why not try what you're proposing and if it’s not quite right, consider if removing the original stat would help with any remaining issues? At the end of the day it’s all easily reversible.

I don’t see why having 2 stats would be an issue, plenty of cars do. Nor do I see a problem in them being different temperatures, again not unusual, though I think 82 is more appropriate for this application in terms of performance and overall cooling. 



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Thanks for that Mark, I think I'll follow your recommendation and switch to the Rover PRT 82c unit, rather than the carbuilder one.  There is some cross over, but I guess having the rad work properly from 82 will work better than just 87 to 92 etc.

To be honest I am still a little unsure about what is happening when the temps are below 82c, assuming the thermostat is closed and not opening under pressure.  I'd appreciate your (everyones) help to make sure I'm approaching things correctly.   Again, sorry about this, I know everyone here is so much more competent than me. Some day I'll get there.

I'll try and describe what I think is happening.....

  • Sub 82c coolant exists the rear of the engine (nearest the driver) and flows to the funny Y shaped joint.
    • Some coolant will flow from the Y joint over the exhaust headers to the radiator, around the radiator to the thermostat housing.
      • At the thermostat housing the thermostat should be closed (<82), therefore coolant will be forced into the 16mm bypass and back towards the heater....  I'm not sure if this correct.
    • At the same time, some coolant, from the Y joint will flow from the lower Y joint back towards the heater, not flowing to the radiator.  That coolant exits the heater and flows to the thermostat housing, but flows down the intake side of the car.
    • The Y joint also has small port going to the expansion tank, the bottom of which flows to a joint on the heater circuit, and then to the rear of the thermostat.

My understanding about the Y joint <82c.



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Mark , thats not quite right , 

Water exits the rear of the block , and returns to the rad as you have indicated . The temperature isnt sub 82 degrees , its whatever the engine has heated it to , it flows through the rad and is returned to the block via the thermostat housing .

The expansion pipe bleeds air from the system and some ( minimal ) water flows this way and is returned to the thermostat housing on the block side of the thermostat .It is the temperature of this flow which is actuating your thermostat .

The heater circuit mirrors the route of the last para .

The arrows on your diagram are correct for the direction of flow but your words "therefore coolant will be forced into the 16mm bypass and back towards the heater" ... are not .

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