V8 4.0 coolant loss

My car has started to loose coolant, a lot of it. There are no signs of leaks anywhere that can be seen underneath, although I have seen some water in the area of the expansion tank, and there is a smell which may be burnt anti-freeze. The car starts perfectly first time (no initial misfiring) and continues to run smoothly. Oil is clear with no contamination. It has been pressure tested & had a couple of new hoses just in case, also a new thermostat. The LR dealer said he had used a gas analyser & it did not detect exhaust in the coolant (not sure I believe him on that). Still losing fluid I fitted a new expansion tank cap yesterday & this morning there was much less pressure when I removed the cap to check the level, so I was hopeful there was some improvement. But no, I had to add about 1 1/2 litres to top up - only did abut 50 miles yesterday since last check.
So the symptoms are a bit mystifying. It seems the system must be pressurising and blowing out of the overflow, but where from? I would have thought a head gasket blowing would clearly be identified by fumes coming from the header tank, but could it be a slipped liner, and wouldn't this give the same ?
All ideas gratefully received.

1-1/2 litres in 50 miles is a lot of leakage, favourite place is at each end of the head gasket where the water passages are.

I'd rig up a catch bottle over the overflow pipe and see if it is in fact from there or not.

Water pump seal? Look up under the pump and see if there is any leakage.

Throttle body heater gasket and the plastic pipes.

Peter

Thanks Peter I'll have a look. Latest update, the new header tank cap needed an extra twist to ensure it is tight enough - it is now. No water leaking from this area at all now, and also not holding pressure inside, which the old cap was, so the new cap is doing it's job properly. Interestingly the smell of burnt coolant has gone as well, I think this was caused by excess pressure and was coming from the overflow & onto the exhaust manifold, so this is now fixed & again I hoped for the best - but still losing water at about the same rate !!

Can't explain this but now losing less, more like 1/2 litre in 50-60 miles. Maybe the reduced pressure due to new cap takes some time to work it's way through the system ? Doesn't sound likely though. I'll monitor it for a week and see what happens.

Yes !! it is now fixed, no loss at all and level staying where it should. It seems that after replacing the pressure cap it takes some days for this to work through ! Doesn't sound possible but it must be the case I have made no other changes. Also the back-up evidence is the hissing in the morning as pressure is released when I do a daily check. This has diminished each day to the point where there is now no noticeable hissing at all. Before I changed the cap it would take about 30 second gentle releasing of pressure to be able to take the cap off.
No I have to old gurgle from the heater matrix back again - all back to normal. I expect this to work its way out and I will have an extra top up but that should it it. Phew

AArgh. No it's started using water again at an alarming rate. Engine running sweetly, no water in oil either, but blowing water out of the overflow again. Just ordered a new pump just in case but I cant see how that could pressurise the system. I'm guessing head gasket - but does the intermitted nature of the problem suggest a failing water pump?
Wont get a chance to look until the weekend now :-(

Oh dear, the suspect will be a head-gasket blown between a cylinder and a water gallery, the pump does nothing other than circulate the coolant around the engine and rad.
The only issues with the pump would be a failed bearing, shaft seal or impeller, it will not pressurise the cooling system.

Sounds like a head gasket to me. Based on personal experience, the 'sniff' test carried out by the dealer is not infallible & there should only be a slight pressure release upon removing the cap after an overnight stand. The way to sus a slipped liner is to start the engine from cold & listen for a tapping noise, occurring at piston speed, as it starts to warm up. This is totally different from valve-gear noise that is at half piston speed & normally when the engine is cold.

Phew, no tapping noises, the engine sounds sweet so fingers crossed it's not a liner. Have ordered a gasket set (with head bolts), so looks like i'll be busy soon.

Good Luck. We look forward to the pics on what u found.

Are you going to get your heads checked / skimmed???

That depends on what I find really. I suppose the "purist" view might be that you should do this as a matter of course, and if time (& money) were no problem I probably would. But I need to complete the work as quickly as possible as I currently don't have a garage so will be borrowing space to work in. Also it seems that many people find a gasket with a visible fault and simply replace with new gaskets and the problem is solved. If there is no fault seen with the gasket it will be a different matter of course.
However as I have a new pump I will fit that first just to see what happens - I'm still not sure that the problem isn't one of poor circulation. The car seems to also have developed strange noises a bit like the belt slipping (but it isn't). Once I eliminate poor circulation (I will check the rad as well), then I will try some other type of test to try to see where the pressurisation is coming from.

i feel your pain im having similar issue with my 4.6 losing coolant over night i know where its comming from just not why gonna sniff test it soon as my kit arrives.

try running it for at least 20mins with the coolant bottle cap off to make sure all the air is out if its not it will expand in the system poss causing your problem. im proberly gonna back flush mine with the hose worked for me last time i had an air lock

If you do remove them, check heads and block with a straight edge as a minimum. Better to be sure than do it twice

These symptoms sound suspiciously like ones I had a year ago. Turned out to need the liners replacing due to a corroded water gallery. It wasn't liner slip, it leaked under pressure between the liner and the block into the cylinder, then out the exhaust. All the other symptoms were there also, pushing water out the overflow and 'waterfall' sounds from behind the dash.

You can go through a load of problems that MAYBE this or THAT, but taking the heads off and having a look will draw to a conclusion. You could do a compression test first as this will tell you where to look when they are off.

Compression test shows all cylinders exactly (well nearly) the same, no issues. Gas analyser on the header shows no problem. Additive in the water test (to show colour change if CO2 present) no problem found. Oil is clean.
Here is a good one... on Saturday I did about 25 miles and then checked the header tank - no loss at all !! Did another 20 miles and checked again - needed 2 litres to top up to the same level. Next 20 miles - needed just 1 litre to top up.
The water loss seems to coincide with faster driving (faster revs) but I am pretty sure it is not pressure that is the issue. So I am tending to think the pump is the only moving part that will be faster... could it be the pump after all. I have a new pump to fit tomorrow, will post my update here (maybe after a few days as it seems so intermittent).

Grrrr. The old pump squeaked and felt loose so I was hopeful. New pump fitted and belt at the same time. Now all nice & quiet but... still losing water somewhere. Have to get the heads off I suppose - won't be able to do it this weekend, just have to keep topping up for now and no long trips until sorted.

I remember going through something similar with a classic. Turned out to be a pin hole in one if the hoses to the pump. Have you tried revving for a while and observing when up to temp?

I think it is fixed - or rather will be soon. A while back when this first started I changed the expansion tank filler cap it was holding massive pressure in the system - the top hose was still rock hard the next day when cold, and it took about 30 secs to remove the cap and let the pressure out. Since changing that has all gone away, BUT I have found that the thread is damaged. Even though the cap was tight it was still able to leak, and by tightening with two hands and a tight as possible without ripping the tank to bits it has been possible to stop coolant loss. I have ordered a new tank so will fit this weekend & confirm the findings.