Jump to content

Rad Cap Pressure


Guest mcramsay

Recommended Posts

Guest mcramsay

What factor decides what radiator cap pressure you should run? The stock m3 cooling system has a 30 psi cap fitted, but I'm not using the exact m3 set up.. I have a 20, 13 & 7 psi cap. With a 13 fitted the silicone hoses are really hard... wondered if dropping to a 7 psi cap might take some of the stress out of the cooling system... I know the coolant will boil at a lower temperature but I'm aiming to be running around 85 degrees

Link to comment
Share on other sites

Engines tend to get better efficiency the hotter they run.

So they need a higher coolant pressure the hotter thay are run, and MPG is a selling point.

So running a bit cooler due to lower temp thermostat and lower temp switch on for fan will allow a lower presure coolant system.

If fitting a lower pressure cap obviously frequent checks on coolant level required on the first few runs.

Link to comment
Share on other sites

The stat and cooling efficiency of the radiator are responsible for maintaining a stable engine temp. Aided by the rad fan when going slow or stopped.

The question I would ask is what are your desired max and min engine temperatures? You quote 85 so say 85 and 95.

How efficiently does your system keep the engine to its desired temperature range even when you thrash it?

If the answer to the second question is 'Well or very well' then you can go with 7psi and spare your engine hoses and their fixings a trial by pressure? 13 would give 119C. 20 will give 125C. But I would have thought a 15 to 20 margin was adequate.

If your cooling system doesn't maintain your desired temperature range then the correct way to sort it is to improve the cooling system, not rather fudge the issue and fit a 20psi cap.

 

Nigel

ps BMW must be very confident of their hose quality and efficiency of their hose fixings or worried about the efficiency of their cooling system possibly. 30psi seems odd unless they want the engine to run at about 105C! Might be a 'reducing emissions/mpg' issue.

Link to comment
Share on other sites

I would second Nigel's comments. Although in my case I went from a 13psi cap to a 21psi cap a couple of years ago when I was having cooling issues and the coolant was boiling over. On the way back from the End of Season Bash one of the flexible lower hoses blew off a stainless steel cooling pipe. Turned out that since initial build the hose had not been properly aligned and had caused a slow leak under pressure, which gave me all the problems. So in my case it turned out to be a good thing in the long run. Maybe fit a high pressure cap to highlight any problem areas then revert back to a more reasonable value ?

Link to comment
Share on other sites

Guest mcramsay

So after a bit of testing today with the new set up I think I have found the fault. I would like second opinions before I go down the wrong path. After all my mods i.e. Ducting, moving the rad, etc I took the car out this morning, initial results were 92 degrees going at slower speeds, dropping to 85 when driving faster. Great!

 

Parked the car up for 2 hours and then went back out, I now noticed the constant running temp at speed was up around 93-94, carried on another 10 miles and gave some very spirited driving, temps increased to about 96 but started to drop.

 

Then when town driving home temps crept up to 100 with the fan on full speed the temp ramped up to 110 within a few seconds, luckily I was just on my drive when it got that bad.

 

Felt the hoses and they were completely soft. So with some care and a towel I undone the 7 psi cap i fitted as a trial..... no pop or release of pressure.....

 

Suspecting a coolant leak letting pressure drop off I can't find anything.

 

Rigged up my electric pump to the car and put some pressure in till the cap lifted and reseat. I can now hear air escaping from the top seal of the cap, & overflow... just very slightly. Tried 3 caps with the same result.

 

My theory is my system is building pressure as the water expands, but over time the rap cap is leaking and the pressure in the system dropping, to the point the water starts to boil and cooling efficiency goes out the window.

 

I assume once the coolant has expanded and the pressure is released it won't repressurise until the coolant volume lowers when cooled and it expands in a closed circuit again?

 

I think the rap cap sealing plate on the eBay expansion tank is the issue.. thoughts please?

Link to comment
Share on other sites

Guest mcramsay

It's actually down to the aluminium slopes on the underside of the header tank that pull the tabs on the cap down, it looks like they are worn with the amount of times I've put the cap on and off! I put a little more of a bend in the 13 psi cap and went for a longish spin, temp ls fluctuate from 90ish to 96. With the fan on 92 and above! It depends what direction the car is pointing! Same road going out running temp was 94! On the way back facing into the wind I was sat at 90! And only got to 92 kicking the balls out the car!

 

I think the way it is now is marginal but I need to get more confidence in the car on longer trips, I know a larger rad would fit but I just can't find one, the other marlin m3 owner uses a 450mm wide 420 high x 50mm thick versus my 465 wide 310mm tall 50mm thick rad.

 

I'm going to enjoy driving for now and see how things go. If it still isn't solved then I need to think of alternatives... perhaps using the heater core pipework which has been blanked off and fitting a smaller aux radiator further back in the engine bay low to the floor

 

Or splash out on a Davies Craig electric water pump

 

Or give waterless coolant a go and see if that makes any difference. I just don't want to overheat and blow the head gasket as that would cost more than all the above added together!

Link to comment
Share on other sites

I see what you're saying having lost pressure it can't re-pressurise till it's cooled, sucked air or water back in and heated up again(except with steam). What you're really saying is you can't pressurise a leaky system! But you're still overheating if it got to 110C regardless of the leak.(Heat soak on slowing suggesting poor circulation. ?design?pump.)

I imagine you were balancing pressure/volume of steam generated at 110 against the rate of loss of pressure/volume of steam leaking. So were probably running at 110 and 7psi but the 7psi was steam pressure. With a slow leak it would not look like much and by the time you pulled the cap it could have dropped to 100C.

It would be interesting to see a diagram of your cooling system. I think the basic problem is still lack of cooling capacity either because the rad is too small(unlikely) or the system is somehow not working efficiently. Could be anything from an airlock, a tired pump, water bypassing the rad or something else.

You could check for air locks(system must self bleed air), make sure you have a 3mm bleed hole at the top of the stat, have a look at the pump to make sure the vanes are not worn(electric pump could well be an option).

Your rad is only 76% the size of the other M3 Marlin. That could be significant. Not a problem to ignore and hope it will go away. Your cooling is marginal today with a lowish air temp. You will suffer in the summer.

But see if you can do a diagram for us.

 

Nigel

Incidentally my rad is 149400mm2 to your 144150mm2 on a 196bhp ford ST engine but copes with summer, winter and trackdays with capacity to spare.

Edited by Longboarder
Link to comment
Share on other sites

Guest mcramsay

So here is a diagram,

 

59AD1AC9-478F-4E69-8921-2AEFBDE0BF79_zps

 

Pipes 13&5 to the heater core have been blocked off with ventable plugs. And the expansion tank is an aluminium ebay job not the one shown in the pic.

 

all pretty much as BMW designed

Link to comment
Share on other sites

My immediate thought is to join pipes 13 and 5 together, without any restriction to aid flow through the head. Blocking them changes the flow pattern through the head and quite possibly the ability of air to exit the head. Certainly can't do much harm to connect them and try a test drive. You might feel happier if you put a restrictor in the join reducing its aperture to half (15mm restricted to 8mm?) to simulate the heater matrix.

Presumably the stat is in the top of the pump and the pump pushes water into the head and scavenges from the block when the stat is closed.

 

I'm sure I saw your previous thread with an expansion tank with two hoses plus an overflow. If so what two hoses have you used? (or just the one shown 10/14 to the bottom connector and blocked the other?)

 

Nigel

Edited by Longboarder
Link to comment
Share on other sites

Guest mcramsay

Most of the BMW blokes block two pipes off, with the idea being it forces the coolant through the head. And would also replicate having the heater valve closed. I also fitted plugs with bleed nipples which I have opened when the engine is running to get any trapped air out.

 

Definatley worth trying though you never know! I did think of buying a small heater matrix and mounting it very low in the engine bay or half way down the transmission tunnel and linked into pipes 13 & 5 to try and add a bit more radiator surface area... the difference this would make i don't know seen as the pipework is only 18mm rather than the 40mm on the main rad.

 

My expansion tank is Plummed with pipe 14 going to the bottom connection, then I have an 8mm from the top of the rad going into the upper connection on the expansion tank (under the rad cap pressure seal) which I get a constant flow of coolant when running the engine with the cap off.

Link to comment
Share on other sites

End up with something like this. The 8mm rad bleed, dark blue, might need a restrictor to limit flow as might the simulated heater circuit, pale blue.

And you do eventually need to sort the pressure leak.

 

Nigel

post-21-0-63323900-1490491252_thumb.jpg

Edited by Longboarder
Link to comment
Share on other sites

Fitted a heater to Florin for two reasons -- primarily to warm both foot-well & de-mist & also to act as an extra means of engine cooling -- I believe the heater matrix will dissapate 4kw of energy using the standard heater fan set-up.

 

Still can't understand why BMW fitted a 2 bar cap if they intended the system to operate 10/15 degrees below atmospheric pressure boiling point.

Edited by florin metal works
Link to comment
Share on other sites

Guest mcramsay

On the BMW forums it's a well known issue, as the plastic expansion tank seems to fail rather than the cap lifting! My 2014 Mini Cooper s runs at 105 degrees I expect for emissions.

 

 

I think I'm pissing in the wind. The core volume of my rad is 340ci the m3 had 700ci, I need to fit a bigger radiator some how!

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...