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Hi All,

I have a 2001 P38 DHSE with c. 118,000 miles on the clock.

It has had its Head Gasket done in the last 2 years. However I have noticed that is has been using coolant. The was checking the levels yesterday and noticed no coolant in the Expansion tank! It required c. 800ml to fit the system up. The temp gauge hasn't moved from the middle.

I would like to eliminate a leaking pipe, where are the common places that the M51 engine could leak?

I have checked the oil cap and can't see any white looking mayo.

Many thanks Tom

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Not normal.
There are several hoses that are hidden that might be weaping.
Back of engine near bulkhead under the manifold.
Front of engine above the fuel injection pump.
The two heater hoses going into bulkhead and finally orings into heater matrix which shows as a wet carpet on right side footwell.
Also check water pump.

If its got proper coolant in it, there will be telltale signs of leaks on the hoses.

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When we checked it when it was here, it was pressurising when hot, as you would expect, but the pressure went away when it cooled down suggesting it was purely, normal, thermal expansion. That would suggest a leak somewhere.

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possibly trapped air... each time you fill the coolant up to the line... engine on and the hot air expands wherever it's trapped, and blows the coolant through the pressure valve on the expansion tank.... an untraceable "leak", unless your overflow pipe runs straight out under the expansion tank in which case you've probably got dried coolant under there

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I probably shouldn’t say this, but mine never moves from the fill line on the coolant tank, month after month ..... unless I play with the cooling system, in which case out take a day or so to stabilise and then never changes.

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That's how it should be, but not how mine is!

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The car should not use any coolant at all. You must have a leak somewhere.
The coolant level on my 2000 DSE stays the same all year. It settles just below the seam on the expansion tank.
I must have had every kind of leak over the years though, You need decent hoses and clips all round. I have replaced a lot of mine with silicone hoses and I have fitted constant tension clips using the special tool. I am not a big lover of jubilee clips.
At the back of the engine there is a U shaped hose that connects the head to the block. It is out of sight and as Tanis8472 says, it can leak. It is a favourite place. Also take the plastic cover off the injectors and have a look underneath.
You can get leaks into the cabin but it would be smelly and get noticed. I have also had the bearings in the water pump go causing a leak. The fan also caught slightly on the radiator damaging one of the tubes. I had to practice my aluminium brazing skils.
The expansion tank is a source of leaks. You need to fit a new cap. Only get an OEM BMW one. The threads on the expansion tank can also get worn. My level was dropping a fraction of an inch over several months until I managed to come across a new tank. I have fitted a BMW M3/E34 tank with a level probe, but it is very similar to the p38 tank.
I also invested many years ago in a cooling system pressure tester. The one I have is a US made by Stant. It is just like a big bike pump with a pressure gauge. It wasn't cheap and it also needed a BMW adapter to screw the pump onto the tank.
It has proved it's worth. Just pump it up to 12 psi or so and leave it for a few hours. It does not test the cap however.

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Thanks for the replies everyone.

My turbo hose burst last night so I am looking for a hose kit and I understand that the silicone hoses are the better ones to go for. I understand that being a late 2001 2.5 diesel, I will need the EGR version?

Is there a silicone coolant hose kit available anywhere?

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Yes, yours is the EGR version, but you would be better off removing or disconnecting the EGR valve. It will run better.
I have not seen a silicone turbo hose kit as such. When I did mine, I just measured up and fitted a few bends with suitable aluminium joiners.
Try: Ash silicone hoses They are pretty good.
You will also need a couple of decent turbo hose clips. Jubilee clips are not up to it.

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Something that your £5k inspection didn't notice along with the bits that don't need doing.....

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Quite!

I would never go and buy a vehicle and use an independent inspection again. I think they don't have the knowledge of these vehicles.

It a shame I hadn't spent more time on the here before buying the vehicle as it was from your area.

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Had we known I could have gone and had a look at it. But I doubt I'd have spotted an iffy turbo hose as my experience with diesels is very limited so wouldn't know the difference between a good one and one that is about to burst.

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To be fair a split in the hose can be difficult to detect.
Years ago my car suddenly went down on power and put out a lot of smoke from the exhaust when I put my foot down.
There was also a sort of whistling sound in the background (most diesel owners are already obsessed with engine sounds!).
I was convinced it was a hose, or hoped it was the hose and not the turbo. I couldn't see anything amiss but I eventually found the split in the hose that goes from the turbo to the intercooler. It was on the engine side. The only way to check that area is to take the RH wheel and plastic inner liner off. I still couldn't see it but I could feel it blowing with the flat of my hand with the engine running.
Splits are usually a tear in the fabric used in the construction and will close up when there is no pressure. The turbo runs at 1 bar pressure but doesn't really reach that until 3000 rpm. At tickover it is more or less atmospheric pressure. Starts to build from 2000 rpm onwards and max'es out at 3000 rpm when I measured it with my Faultmate.