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Tdi coolant q

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I converted my 2.5NAD to 300Tdi three years ago, and it's always been the same - I fill the coolant up and it chucks a load out when it gets warm, then the level recedes to this condition:


Thing is, since the level in the header tank is below the pickup now I can't tell if the block level falls (until it siezes!!). It's been ok for 3 years but it'd be nice to know if I'm worrying needlessly.

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Why not fit a 300 header tank?

I can't see why it should do that, it would suggest there is something wrong. My 300 doesn't lose any coolant, unlike the V8 which steadfastly refuses to be topped up to the mark, it will drink a bit to get it down to about 1/2" below the mark and then it is happy, never moves below that :D:blink::angry:

could be something to do with a lower pressure cap on the older header tanks...? no idea, just a thought

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I fitted a 200 TDI to my B reg 2.5NA a year or two ago. It looks like I did the same as your self and retainined the 2.5 NA expansion tank. I suffer the same problems - expansion tank often nearly empty and engine running hot when pushed. I have done all sorts trying to identify the issue from mess with the temp sender to changing the head gasket -all to no avail.

I have come to the conclusion that it's either:

1/. the radiator cap is set at a lower pressure on the 2.5 NA - however unlikely in my opinon, they are all set to lift at a temperature of 115 -120 C (assuming that the radiator system starts at atmospheric pressure basic laws of physics mean that this is a common pressure)

2/. a TDI has more coolant in it than a 2.5 NA - I haven't looked this up it's an assumption - some one may have the data to confirm this. This would mean that the 200 TDI system would need a greater volume to allow for expansion of the water as it warms up. If you retain the smaller expansion tank off the 2.5 NA, it does not have enough room for this - hence the system over pressures and dumps some coolant out of the cap. When you stop the engine and allow it to cool, the system contains less water than it did originally. Therefore when it cools down to atmospheric temp it is actually under a vaccum as there is less water/air in the same volume. Next time you run the vehicle the coolant system then warms up, but because it started under a vaccum instead of at atmospheric pressure, the system pressure does not reach the required pressure to stop the water boiling. Hence the water pump cavitates (water boiling in suction of pump, pumps struggle to pump steam). End result you end up with poor coolant flow around the engine and eventully your parked at the side of the road with an overheating vehicle.

Not tested the theory yet - the 200 TDI expansion tank is sat waiting to be fitted after I fit the new back axle.


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Got black header tank on my 200Tdi, it's been fitted your years without any problems, not sure where they are supposedly splitting probably at the small hose fitting on the underside where the expansion hose goes to the radiator :D

naa, they go along the seam. I really can't see why the opaqe/white ones wouldn't do the same, as it appears to be (at least to me) the same tank/material without the colour added during the moulding (to make them cheaper) My black one started to open up along the seam and so I replaced it, although it never leaked.

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The white ones are a different part number and are supposed to be a better material to stop the splitting. I must admit my first thought was that it looked like the same thing but with no black stuff poured in the plastic vat, but I have seen a number of black ones that are split, and never seen a white one split so maybe the seam design is better or they used stronger Superglue or something.

As already said, the seam that runs around the waistline of the tank splits on the black ones, lots of coolant disappears, blooey new engine required pretty quick if the driver isn't watching the temp gauge :blink:

I have seen one such case where the driver wasn't watching the gauge and cooked it good and proper, the vehicle turned up on a tow rope and when started sounded like a stonecrusher trying to digest a Sherman tank. Bonnet up, filled it up with coolant, somebody started it up again with the cap off the header tank "to see if it was gassing". It is the only time I have seen anything pump all the coolant out of the filler in about 2 or 3 seconds, quite impressive and caused a bit of a scattering match among the assembled spectators (some of who got a bit wet IIRC).

But I also know of a lot of black tanks still running around, out of all the 300Tdi Defenders on our company fleet I don't think any of them have needed new header tanks and only 1 will be new enough to have a white one.

The other thing that is a weakness on Tdi engines is the stupid little 3 way "Y piece" (I think it is called an "ejector pump" though I have absolutely no idea why) in the small coolant pipe on the RH side of the engine, those can split with similar results. Worth changing if in any doubt as it costs nothing and can cost you dear if it goes.

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