To misquote Oscar Wilde, “To have one coolant leak may be regarded as a misfortune; to have two looks like carelessness.”


A few weeks back I had a coolant leak which took me a while to find although it was, in a sense, very easy i.e. I hadn’t tightened the top coolant hose clip as tightly as it needed, and coolant was escaping and leaking down the front of the engine. Having worked this out, it was easy to fix.

Yesterday I took the car out for a 10-mile ride. After I parked the car back in the garage, I noticed later half of cup of coolant under the car. This morning I started the car again:

1. Within a minute of so, there was some new drips of coolant on the floor (see photo 1)
2. I think it then stopped dripping (I am guessing maybe the thermostat had opened, temporarily releasing some pressure?). I am guessing that the leak would start up again, based on the coolant seen yesterday

I had a look under the car, whilst the engine was running (which is not fun given it is on front axle stands). Whilst I can see where the coolant is finally dripping from, it is coming from a leak somewhere else. Looking as best as I could under the car and from the top, I have been unable to find the source.

The leak manifests itself on the driver’s side of this UK car.

I have had a look at the oil on the dip stick and that looks fine – no sign of water in it.



At the back of my mind, I have a fear the leak is coming from the water pump I replaced, but

a) I was really careful putting this on with cleaning, gaskets, bolts, torques etc

b) the water pump is on the other side of the engine. Whilst I am sure it is possible for the leak to be on one side of the engine but it to run under the timing belt covers to the other side, I am thinking/hoping there might be some other cause.



Is my next step to remove the timing belt cover(s) as best as I can to try and find the leak, or is there anything else I can do?

 

No need to remove the timing belt covers. At least not yet.
Do a coolant pressure test and you'll find where it's leaking. Probably another loose hose clip. Or more than one.
Or go back to each coolant hose clip and make sure that they are all tight.

 

System static pressure has nothing to do with the thermostat opening or closing- static pressure is caused by expansion of the coolant compressing the air space in the expansion vessel thus its name.

In round terms the system contains about 14 litres of coolant- as this heats up from ambient to circa 100C the volume increases by approximately 0.5 litres. The expansion tank has a volume of approx 2 litres and when filled correctly it has about 1 litre of coolant in it at ambient conditions. When the system heats up the air space in the expansion tank compresses to about 0.5 litres and thus thanks to Mr Boyle who noticed that PV= a constant we can conclude [in round terms] that the pressure doubles from 15psia to 30 psia. Overfill the expansion vessel and chances are the safety valve in the cap will lift and blow off excess pressure quite possibly taking some coolant with it.

When the engine is running the dynamics change some and the water pump adds head [pressure] to drive the coolant around the closed loop so the presure will vary in the system depending on whether one is close to the pump discharge or the pump suction and that differential pressure will vary considerably as a function of engine speed. Centrifugal pumps do not fare well over a large turndown range and when such is greater than 3 to 1 the system becomes very inefficient - in our engine the turndown ratio is 10 to 1 so at max rpms the pump is consuming energy the system does not need hence why electrically controlled water pumps that can operate independent of engine speed are such a good idea such as the one Mike in upside down land has engineered into his astonishing build that he may just get working one of these fine days!

I solved a similar problem to yours earlier this year- apologies - I mentioned it earlier in some thread - I had a mystery leak that appeaared to be coming from the water pump area- in my case a 8 year old Laso pump. After removing the timing belt cover and pressuring the system to 10 psig whilst still filled with coolant I could not spot the leak. After draining the coolant and pressurising the drained system to 10 psig again I was then able to do a soapy water leak test and to my astonishment the leak came from the base of the tensioner pivot post that came installed on the pump and thus was never touched or checked when I installed the pump. This thing had no obvious sealing arrangement and how it had sealed for 8 years was beyond me. I applied a copious amount of PTFE tape around the base of the post and problem solved.

When the coolant was leaking it hit the deck under the front of the engine and a bit to the passenger side of the motor [LHD model].

 

based on your leak ,
I suggest you remove the aircleaner base,
then inspect the heater control valve and hose and clamps,
if the short hose and HCV are older than 5 years replace both of them.
Also look for the outlet flange bolts to be loose where the short hose attaches,
an / or, the collar might have pulled free of the flanged outlet.
Inspect the belly of V you could also have a leak at the water bridge gaskets,
or one of the hoses coming off the back of the WB

 

It WAS the heater control valve hose. I had replaced it with one from Roger when i replaced the faulty diaphragms and heater control valve. I hadn't tightened up the front jubilee sufficiently. The photo is the "before", and even without turning the engine on, you can the clip has moved and a hint of coolant. When i started the engine, there was a significant weep, which i guess would get stronger as the engine warmed up.
many thanks for suggestion

 

I did remove the bolt that holds the timing belt cover on the drivers side cover, which i had read went through to the water jacket. I had put grease on it previously to act as a sealant, but this time, i cleaned the grease off and put some gasket sealant on instead and popped it back on.
However, there was no sign of coolant on the end of the bolt which i would have expected - so i wonder if either it is a different bolt or they changed it on the S4?

 

that bolt doesnt go into a water jacket.

it does pass through one of the water pump attachment points.