The 75 and ZT Owners Club Forums - How To Understand Pressure Caps and Header Tanks

HowTo Moderator - would you copy/paste this update over the faulty version in the HowTo Forum.
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TC

The images in this HowTo were hosted by Photobucket, but following their sudden policy change they became inaccessible. So, I've been slowly recovering them and am now putting my stuff on a more friendly site. It may take a while.

This is a work in progress essay, so may be revised at any time.

This is my revision dated 9 July 2017

The header tank and pressure cap are regularly discussed on the forums, yet their workings aren’t fully understood. I therefore decided to find out what goes on inside them. It may shed a little light, though I remain unsure about certain aspects of the pressure cap design. My interpretation of what I found is subject to revision, so if anyone has alternative explanations or useful information, I'll update it periodically.

Pressure caps are exceedingly tough, so in my initial attempt to open one up destroyed it completely. But eventually I dissected some more systematically to try and find out what makes them tick. As for the header, a rather tatty one was scrounged for nothing and a nearly new one came from a boot sale. Using these, I took photos and measurements and made some sectional drawings, which should help understand their construction and operation.

What's clear from all this is that the pressure cap has gone through several iterations over time and that the '100' type (below left, fitted to the early petrol engines) and the '140' type (originally on diesels, but now used for all engines) are structurally different. There's also a '200' type as fitted to BMWs that incorporate the same sort of header tank. The ‘200’ cap is also prescribed by a few 75/ZT owners.

https://www.mediafire.com/convkey/c9...g1hvl2dd6g.jpg

I haven't examined a ‘200’, though I expect it will work like the '140'. They’re all branded ‘Reutter’ and there seems to have been design changes within each pressure type. What I mean is they differ not only in their relief pressure ratings, but in their internal construction. These differences are generally subtle and don't affect the way they work. To keep things simple, this essay covers the '140' version in the main, but where I've seen or read of interesting differences they'll be mentioned.

The header tank comes in two types (earlier black and the later buff) and the way they handle pressure relief is different. This might complicate things, in theory anyway, because not all combinations of cap and tank may work as MGR intended. I haven’t examined a black tank but I've collected a few photos over time, so I've made my conclusions from those. Photos Google Images.

https://www.mediafire.com/convkey/df...bnpcmerh6g.jpg

The Pressure Cap

Simplistically, it seals the header tank so that the heated coolant will operate under pressure. It thereby allows higher coolant temperatures to be accommodated without loss by evaporation or boiling. A point to emphasise here, is that the pressure rating doesn’t control the engine’s normal running temperature in any way. The pressure rating description (100, 140, 200, etc) is an inheritance from BMW, who calibrated in the French SI system. So, pressure is measured in Pascals (Pa). A Pascal equals one Newton per square metre, a rather small pressure. It’s lower, for example, than that £5 note exerts on a table top. The numbers printed on the caps are kiloPascals (kPa =1000Pa). Helpfully, there's a convenient if approximate relationship between kiloPascals, atmospheres (atm) and pounds per square inch (psi).

100kPa = 1atm = 15psi (approx)
140kPa = 1.4atm = 21psi (approx)
200kPa = 2atm = 30psi (approx)

The cap is made from a precision moulded glass reinforced plastic (GRP). The polymer code is PA66-30G, which translates to Polyamide 66 with a 30% glass fibre fill by weight. Polyamide 66 is hexamethylene adipamide, more commonly known as nylon’ (specifically nylon 66, since there are theoretically hundreds of nylon polymers). The cap’s individual components are locked together in a way that prevents dismantling. So, to figure out their secrets you have to pull them apart using hand tools, a Stanley knife, Dremel disc, hammer & chisel or whatever works.

This is where we start (photo Google Images).

https://www.mediafire.com/convkey/03...n1ngeqze6g.jpg

If you're familiar with the pressure cap, you'll know that there are three rubber seals (two o-rings and a flat washer), all fitted on the central body. This housing or cartridge is fastened to the outer cap/hand wheel, but is quite loosely mounted so it turns independently. The end of the cartridge has a yellow plug/insert with the pressure rating (140) indicated on the face (the colour may vary). The insert has a gap around it that's open to tank pressure. If you blow into this gap, the air escapes higher up through a hole in the cartridge.

The next photo shows how the cartridge is fastened to the outer cap by four lugs (one is highlighted). These engage around a flange on the cartridge and are shaped so it will snap into position during manufacture. There's no way they'll separate in normal use, even though they're a slack fit.

https://www.mediafire.com/convkey/0a...q2o3vp6j6g.jpg

The lugs can be persuaded to release the cartridge if you apply a Stanley knife in the gap and whittle one of them down. You can then extricate the cartridge sideways using brute force.

Here's the outer cap/hand wheel with the cartridge removed. This one's from a grey '100' type, but the '140' black type is identical. You can see the retaining lugs, with one cut away.

https://www.mediafire.com/convkey/3c...eo7j80b86g.jpg

The next photo shows detail of the lugs. You'll see they're designed for one way assembly but are a slack fit on the cartridge.

https://www.mediafire.com/convkey/1d...kswcc7zr6g.jpg

The cartridge will snap into position during manufacture by applying high downward force.

The next photo shows the outer cap venting holes around the periphery, above the thread.

https://www.mediafire.com/convkey/82...xs33hnw86g.jpg

These four holes open into the finger grips, which are hollow and allow excess pressure or ejected coolant to be discharged downwards onto the header tank. One wonders if the earliest caps (used with the black header) didn't have these vents? With the black tanks, ejected coolant was directed to the discharge pipe in the neck.

Once you've got the cartridge out, this is what you get (rubber seals are removed). You can see the retaining flange at the top. Notice how the edge is abraded by the lugs when screwing the cap on and off.

https://www.mediafire.com/convkey/1f...yx1ls2gs6g.jpg

In the photo you can see that the lower half of the cartridge is tapered. This ensures a pressure tight fit when it's pressed by the cap into the tapered neck of the header tank. Efficient sealing of the rubber rings doesn’t need very much downward force. The taper action ensures it, but will also cause taper locking if too much pressure is applied. There are reports of this happening when a leaking cap has encouraged the brute force solution. It’s possible to lock the taper so tightly that outer cap is actually snapped-off. Extraction of the cartridge from the header tank may then require even more brute force.

In the photo, you can also see the primary valve spring. This controls the release pressure. The valve is located inside the cartridge and the whole assembly is held together with a cross-shaped insert on the flange. This appears to be bonded/welded in position. However it's held, this end piece is impossible to remove without destroying it. The next photo shows what it looks like before you attack it.

https://www.mediafire.com/convkey/79...fhm0an9f6g.jpg

Visible through the hole is a conical hairspring which holds the vacuum relief valve. This allows air to be sucked back into the header tank if any pressure is released when the system’s hot. It's very light and you can blow it open with your mouth. So, won't need more than -1psi or so (vacuum) to open. Without this relief, the radiator could collapse following a boil-over.

Before going into the valve housing, it's worth looking at some other features, especially the vent holes.

https://www.mediafire.com/convkey/03...2rdt9c9o6g.jpg

At the very top are the four large vents for the release of over-pressure. Any coolant ejected will come out through these and into the outer cap. Between the upper o-ring and the flat washer is a rectangular vent, highlighted above. This connects with the annular space mentioned earlier. When you blow past the end plug, the air escapes here.

On the opposite side, between the two o-rings, is a small hole/drilling. I've labelled it bleed hole.

https://www.mediafire.com/convkey/8f...lo0p7wtu6g.jpg

You'll see the hole is yellow on the inside. This is the inner wall of the '140' end plug.

The only way to get the plug out, is to cut the cartridge vertically so you can expand it and pull the plug out using pliers. It's compression-fitted into the cartridge and has two sets of teeth to lock it in place. When you manage to get it out, a thin rubber diaphragm comes with it.

https://www.mediafire.com/convkey/ee...bbx0dbd46g.jpg

The insert is hollow and is sealed by the diaphragm. The 'chamber' so created is connected to the outside via the inter-ring bleed hole.

The next three photos show the chamber, the bonding teeth and the bleed hole.

https://www.mediafire.com/convkey/f2...z9a5468a6g.jpg

https://www.mediafire.com/convkey/2b...middqj8z6g.jpg

https://www.mediafire.com/convkey/8b...okm41c1d6g.jpg

The teeth stand on two raised sections, so when fitted there's the annular space between the plug and the cartridge body. This allows you to blow past it and is the route high coolant pressure takes during a release. An interesting point here is that this bleed hole is absent in the earlier ('100') type cap. The end plug (black or grey in that case) is identical to the 140, except for this hole. It's a bit of a mystery as to why there’s a diaphragm in the ‘100’ cap. Maybe I'll add more about it in due course.

To get into the valve housing, I found the easiest way is to slice the top off using a Dremel disc. The four brackets forming the pressure vents and were cut through along the lines shown in red.

https://www.mediafire.com/convkey/0f...s729ft2h6g.jpg

This allows the main spring to relax, so you can remove it and the valve unit from the cartridge.

The next photos show the main spring from the 140 and a 100 type for comparison.

https://www.mediafire.com/convkey/38...24acmcn56g.jpg

They have different compression rates, so the longer 100 spring is weaker than the thicker 140.

With the spring removed, the main pressure valve comes out. It's quite a clever little gizmo with an integral vacuum relief valve.

https://www.mediafire.com/convkey/12...omidmabb6g.jpg

The white washer is the rubber seal and sits on an annular seat in the cartridge. The plastic body is hollow and contains the vacuum relief valve. This vents atmospheric air downwards, through the lower holes. The narrow domed end of the unit passes through a short guide collar and protrudes slightly with its end siting on the diaphragm.

Here's all the components set out in order, with the end piece cut off.

https://www.mediafire.com/convkey/df...5pc7f61v6g.jpg

I've combined all this into a sectional drawing which might help explain how it works.

https://www.mediafire.com/convkey/1f...vokcdbez6g.jpg

Here, the blue parts are made of rubber. The pressure and vacuum relief springs are green. The drilling between the o-rings into the yellow chamber is labelled ‘head pressure’ for reasons that will be clear when the cap/tank assembly is discussed.

Adding the outer 'hand wheel' to this drawing gives the cap assembly.

https://www.mediafire.com/convkey/1f...qtsifvoo6g.jpg

This drawing shows the cap relaxed so it's easier to understand. When it’s screwed down, the o-rings and flat washer are more compressed than shown here and in the later drawing. The underside of the hand wheel presses down on the top of the cartridge.

The Header Tank

The header provides a small reserve of coolant and permits thermal expansion and pressure control as the engine heats up. It’s located as the highest part of the cooling system, so any circulating air bubbles will be collected through its connection to the cooling circuit. This assists in bleeding air from the system during a refill. The principle is exactly the same as the header tank in a domestic central heating system.

The connection to the cooling circuit is by a single, small bore hose from the tank to a T-piece in the bottom radiator hose. It's therefore standing on the return leg to the pump where cooled coolant is cycling to the thermostat on the inlet side of the pump. Obviously, there’s no formal circulation through the header under normal running conditions.

The tank is a three part moulded GRP (again PA66-30GF) assembly consisting of the main tank body (lower part) the upper tank section and the filler neck. These are sealed together around the outer edges.

The filler neck is threaded externally to accept the pressure cap and incorporates a small bore hose connector. This receives the air bleed hose from the inlet manifold, the cylinder head/heads and the upper radiator. These individual bleed points are Tee'd together before going into the filler neck connector

https://www.mediafire.com/convkey/3f...w7d3ysf06g.jpg

The black tanks have two pipes connected to the filler neck. The second one discharges any ejected coolant downwards behind the radiator. This is shown in the next photo, which I collected from the forum. I can’t remember whose it is, so hope the owner doesn’t mind my using it.

The overflow connector opens into the annular space in the neck, so it's a simple gravity drain. In the event of a violent pressure release, this drain is unlikely to cope with a significant volume of coolant. Most of this will discharge via the cap finger grips as per the buff tank.

https://www.mediafire.com/convkey/cd...19gb593b6g.jpg

The internal fins/baffles in the header are well known to owners who regularly check the coolant. However, they may not realise how complex the internal structure actually is. It’s difficult to see the interior by looking into the filler neck and even a small endoscope is impossible to position well enough to see everything. This is the best I could manage. These fins run along the top of the tank.

https://www.mediafire.com/convkey/b8...ilwuuc1a6g.jpg

If you shine a light through the buff tank, it’s apparent that it’s divided into three sections by solid walls. To allow coolant to pass through they have a small ‘door’. More on this below.

The following graphic shows the general layout of the fins/baffles as perceived externally. Note the three sections.

https://www.mediafire.com/convkey/8f...2omr21kf6g.jpg

Their presumed function is to reduce sloshing of the contents. The isolated central section may be a vestige of the level sensing system that MGR intended to fit, but never did. This is evident on the earlier tanks, which have a sensor bracket at this point. Photo Google images.

https://www.mediafire.com/convkey/34...xq4tsllu6g.jpg

Because the fins are positioned along the full length of both the upper and lower tank, straddling the down pipe, they may also minimise the volume of any ejected coolant.

The next few photos show the layout of the fins. I split the top off this one using a hammer and chisel. It looks like the parts are actually ‘glued’ together with a resin bead that squashes around the seam before curing. Here’s some of it I loosened with a knife.

https://www.mediafire.com/convkey/9c...mvbo3yh26g.jpg

Note the blue staining and pinkish sediment, which is present on all the coolant contact surfaces. The pink crusting seems to be a characteristic of OAT antifreeze and may be caused by calcium hardness in tap water. If so, it certainly shouldn’t happen. I don’t know if this is something the current OAT formulas do, but I don’t think this deposit appears when deionized water is used.

https://www.mediafire.com/convkey/01...7jc3ik4u6g.jpg

Next is the top half, which shows the blue staining rather better. I suspect this is due to a change of antifreeze from red OAT to a basic blue type. Interesting that the sediment remained pink.

https://www.mediafire.com/convkey/73...61aa89576g.jpg

The walled off central section is shown in more detail below. The base here has a circle marked where the ill-fated sensor would have been. Note the two small ‘hatches’.

https://www.mediafire.com/convkey/17...k983fgfr6g.jpg

And the all-important Min/Max level indicators (and more pink sediment).

https://www.mediafire.com/convkey/d3...0m2dm5066g.jpg

The difference between the Max and Min inventory is about 100ml, so the smallest leak will quickly push the contents to the attention zone. No wonder MGR planned on fitting a sensor! It also explains why refilling the system is quite a slow process. The small doors shown above throttle the flow significantly. This was observed when filling the lower tank section under the tap.

Of some importance is the design of the filler neck and the bleed system. The neck has a concentric inner sleeve with is tapered to take the pressure control cartridge on the cap.

https://www.mediafire.com/convkey/b7...wukpzsgq6g.jpg

Note the thin raised ridge on the end of the sleeve and the o-ring recesses on the tapered wall. These are where the o-rings are positioned. It’s often assumed that these are wear grooves, but they’re actually moulded into the surface. I guess this may protect the seals from crushing if the cap is over-tightened. Over-tightening is a common error that accelerates the deterioration of all three seals. The taper system doesn’t need much compression to be effective.

The bleed hose connector on the outside has a 5mm bore and leads to a much smaller hole in the wall of the inner neck. This is seen in the next image. It’s around 2mm diameter and very prone to blockage.

https://www.mediafire.com/convkey/a5...d0mwv7tv6g.jpg

Note the hole is positioned between the o-ring recesses, so this area is sealed top and bottom when the cap’s in position. It's clearly impossible for coolant to pass through here and into the tank under normal running conditions. So, this small hole connects the head bleed to the pressure chamber in the valve cartridge.

Another facet is the position and direction of the hole. The above photo shows that it's drilled through the inner neck at an upward angle and so connects with the 5mm passage positioned higher up. This poses an interesting problem. How do you unblock the hole in the neck when it becomes choked with debris? Clearly, you can't use a length of wire or a long drill from the outside. Drilling it through would actually ruin the tank. It has to be cleared using a piece of wire bent at 45 degrees, so you can probe it upwards from within the neck and extract a blockage. Avoid pushing stuff ‘through’ the hole because it won’t go anywhere other than into the bleed pipe. So, if this hole gets blocked, the connected part of the pressure cap’s mechanism won’t work.

This begs the question, how can the head bleed operate when the pressure cap's screwed down or indeed if the little hole’s blocked? A closer look at the top half of the tank reveals the answer. Another drilling is present, about 3-4mm bore and hidden from view. This photo shows the underside of the filler neck.

https://www.mediafire.com/convkey/7d...h13xh3dp6g.jpg

https://www.mediafire.com/convkey/a9...urwb43vu6g.jpg

Here, seen from inside the tank, the hole labeled '1' is the small inter-ring drilling and '2' is the true bleed that drains directly into the tank. This hole joins with the 5mm channel at right angles. So what we have is this:

Cross Section of Filler Neck

https://www.mediafire.com/convkey/cd...debdinv76g.jpg

Adding the pressure cap gives the complete assembly.

https://www.mediafire.com/convkey/0d...fkcs88ft6g.jpg

The bleed system automatically vents air from the head, radiator, etc. when refilling with coolant. During this operation the pressure cap is off and the small vent hole is visible in the neck and the large vent out of sight. As the system fills up, displaced air is released via both bleeds. When all the air has been vented, the bleed hoses and the connector pipe remain full of coolant. If the engine is running with the cap off and the pump pressure is high enough, it will transfer coolant into the tank through both these bleed holes. This is illustrated in the following image taken from a video sequence. It’s a KV6 header with the engine at high revs.

https://www.mediafire.com/convkey/31...xb37zp706g.jpg

Note the two flows. The primary bleed flows downwards into the tank. The smaller bleed exits sideways.

In a normally running engine, I have some doubts that this flow is of any particular significance other than demonstrating that the smaller bleed hole is clear. However, I’ve been unable to replicate it in my 1.8T after refilling and bleeding it according to the MGR Manual. The manual makes no mention of the bleed holes or any flow through them. However, with the engine running from cold, I have observed flow through the lower bleed hole during. I probably didn't have it running fast enough to get enough pressure to see any upper vent hole flow as seen in the video.

There are Caps and there are Caps

The 140 cap is clearly a dual acting system, so the system pressure acts on the main valve in two ways. There’s the direct route through the annular space around the insert and into the void around the valve. This is a pneumatic pressure, air and water vapour. There’s also the indirect route via the drillings into the pressure chamber and diaphragm, which also acts on the valve. This is essentially the hydraulic pressure in the head, because the bleed route is flooded throughout. Both systems are in equilibrium under normal conditions.

It’s clear from the drawing that the o-rings have an equal pressure at top and bottom under normal conditions. In other words, they don’t have to seal the tank pressure, but simply isolate the pressure chamber. It’s the flat rubber washer at the top that alone maintains the running pressure. This washer needs minimal compression because the annular ridge on the inner neck enhances the sealing force (check out the surface of your washer).. This is where many caps are damaged by over-tightening. With a good flat washer, the system should pressurize normally without any o-rings in place. Anyone fancy trying it?

The older 100 cap has the same internal construction, but it doesn’t have the pressure chamber connecting hole. The chamber itself is there, as is the diaphragm. So, the second route is blocked and this means it’s a single acting system and doesn't actually need any o-rings. It’s as if the basic cap was produced for two different applications. This proposal is to some extent borne out by a note I found regarding MGR/BMW’s intentions at the time the 140 cap was introduced.

The basic 100 cap was fitted to all early petrol models until BMW come into the picture. In some of their own engine’s they had a problem with coolant over-pressure and discharge. This is due to heat soak.

Heat Soak

When an engine is worked hard, particularly in hot environments, a significant heat reservoir is built up in the head and block. This slowly dissipates after shut down. Without pump circulation, the head contents (a 50% glycol solution) can superheat and boil. This happens at around 110°C at 15psi (or 120°C at 22psi) and so creates enough back pressure in the entire cooling system to force coolant up into the header tank. Some will be ejected and if the pressure exceeds these figures the release would be hazardous..

BMW anticipated this would happen with the turbocharged 1.8 and the diesel. The latter has a high heat capacity cast iron block which is a bad thing in heat soak situations. The 140 type cap was therefore introduced. One wonders whether MGR’s ‘100’ cap before this point was somewhat different from the 100 used by BMW? I suspect the reason why the ‘current’ 100 and 140 caps are basically the same (except for one hole) is due to Reutter, the cap’s manufacturer. They apparently offered a price incentive if BMW fitted the same design to all the 75/ZT engines. We therefore see caps of different ratings of identical construction except for a single hole. But it raises the question as to why the 100 cap has the o-rings? It should work fine without them. Anyone ??

So How does it Work?

This is how I think it works, but if anyone has a deeper insight on this I’ll amend it periodically.

In normal running, the header tank is not within the coolant loop and is not affected by the pump other than receiving the tiny bleed. It stands on a stagnant leg located on the lowest section of the pump inlet hose. The tank contents may warm up from the bleed and by thermal conduction up the leg, but should never reach the temperature of the engine.

Pump speed should have no effect on the volume of the coolant in the tank. This applies whether the pressure cap is in place or not. Thermal expansion of the coolant is accommodated by the header tank. The surging of coolant level sometimes reported when an engine is revved is abnormal in my opinion. It suggests an increase in total coolant volume or a reduction in the capacity of the hardware. Located on the pump inlet side of the circuit it’s difficult to envision that physical changes in the hoses are the cause.

The pump generates around 1-2psi differential across the radiator outlet at 2000 rpm. This was determined experimentally on a PRT removed from a 1.8T system known to relieve at 2000 rpm. I assume all pumps will have a similar displacement/pressure. The effects of engine revving on KV6 header tanks under ‘normal’ and ‘abnormal’ conditions are illustrated in the milosnikolic videos mentioned earlier. A link to these can be provided for anyone interested. As noted earlier, I have doubts about what’s regarded as ‘normal’. Another discussion point.

In a heat soak situation, the hydraulic pressure in the head increases dramatically and much faster than the cooling system as a whole. The head bleed thus receives a hydraulic ‘shock’ that must be sufficient to pressurize the sealed chamber and lift the relief valve. This must occur even though the bleed circuit is open-ended to the tank. I guess this pressure initially exceeds the capacity of the bleed. With the main valve opened hydraulically the over-pressure in the head is momentarily relieved by the cap’s main vent. Little or no coolant will be expelled when this happens.

Conclusion

There’s no doubt that the header tank/pressure cap assembly is surprisingly complex. The drawings I’ve made are accurate translations of the parts I have to hand and there may be a variation not examined. The mechanisms proposed are for discussion and may require changes. If corrections are needed or additional information becomes available, I’ll amend this write up periodically.

Disclaimer:
You are responsible for any work or modifications carried out on your car and you undertake any such work at your own risk. Neither The 75 and ZT Owners Club nor the original author of these How-To's can be held liable for anything that may happen as a result of you following these How-To's.
Any modifications should be reported to your insurance company.