Cooling system not pressurising

[MJK 24]

Mercedes 190 2.5D

Started to run slightly cool (just below 80 degrees no matter how hard it’s driven).

Using no oil or water.

Warms up quickly - heater like a furnace in no time.

Thermostat replaced last year - genuine Merc item.

Header tank cap replaced last year - Eurocarparts item.

Backstory: same problem approx 12 months ago. Was running very cool then. New thermostat solved it. Then it was using quite a bit of water. Pressure tested to 25psi (hoses rock hard) and held that for 5 minutes with no loss of coolant. We concluded the cap had a weak spring and it was replaced.

I’m thinking my crappy header tank cap isn’t up to the job and not sealing correctly after just 12 months? I’ve just done 5 miles down a NSL duel carriagway and stopped in a lay-by. Top hose very warm (wouldn’t say hot) and squashy. Bottom hose stone cold. Is a faulty cap not letting it pressurise and making it run a bit cool? Or is the ambient temperature stopping it warming up? It’s a big lump of an engine and possibly over cooled with 7 litres of oil and 8 litres of water in a relatively small, light car.

All advice welcome!

[stevieturbo]

Pressure test it...simple.

If cap leaks, it's the cap, if it doesnt...it isnt lol.

And the cap should have blown open before 25psi on the previous test.

[GreenV8S]

The symptoms are contradictory. The quick warmup suggests the stat is closing OK but the lower running temperature suggests it's overcooling. Having the bottom hose cold is not unusual in the winter - it would only need a dribble of cold water under cruise conditions so the stat will be nearly closed. If the only definite symptom is the gauge reading, perhaps the gauge is mis-reading. These gauges typically use a thermister with a resistance that goes down as the temperature rises. Any extra resistance anywhere in the circuit would tend to make it read low.

[227bhp]

So what's wrong with it? It sounds like it's fine to me.

[MJK 24]

stevieturbo said:

Pressure test it...simple.

If cap leaks, it's the cap, if it doesnt...it isnt lol.

And the cap should have blown open before 25psi on the previous test.

It was pressure tested by removing the cap, and fitted an adaptor to the header tank which was then pumped up to 25psi

So the only part of the system not tested was the cap itself.

[MJK 24]

Last year before replacing thermostat:



After changing thermostat for new Merc item:



I’m now back to the to it running as per the top photo.

The new thermostat has a ball bearing fitted which rattles when you squeeze the top hose. What this signifies, I don’t know!

[GreenV8S]

MJK 24 said:

The new thermostat has a ball bearing fitted which rattles when you squeeze the top hose. What this signifies, I don’t know!

That would be a jiggle pin - commonly used as a valve to let air bleed through the stat while restricting the flow of coolant.

[stevieturbo]

MJK 24 said:

It was pressure tested by removing the cap, and fitted an adaptor to the header tank which was then pumped up to 25psi

So the only part of the system not tested was the cap itself.

Then test it properly with the cap in place.

[MJK 24]

stevieturbo said:

MJK 24 said:

It was pressure tested by removing the cap, and fitted an adaptor to the header tank which was then pumped up to 25psi

So the only part of the system not tested was the cap itself.

Then test it properly with the cap in place.

How would I go about that?

[anonymous-user]

MJK 24 said:

How would I go about that?

Try a new cap?

[PositronicRay]

V6 Pushfit said:

Try a new cap?

This is the correct answer, they're only a couple of £.

However if you attach a take-off pipe somewhere in the system it'll find a leak. From memory I disconnected the overflow/balance pipe from the top of the rad to the pressure canister, blocked off one end and pressurised it with a partially deflated spare tyre

[stevieturbo]

No, the correct answer is apply pressure to somewhere in the system to test other than the cap hole ! It's not like there arent pipes everywhere to add a fitting into.

It really wasn't that difficult.

[thetrickcyclist]

If its "not pressurising" surely it'll be running hotter?

[Megaflow]

If I am reading this correctly, then engine warms up quickly, then stays constant, doesn’t use water, but is running slightly lower temperature than it used to?

Are you sure the gauge is reading correctly?

Apart from the fact the temperature is lower than it used to be, there doesn’t seem to be anything wrong with it.

The temperature drop, could be due to the ambient temperature dropping.

[anonymous-user]

I would still try a new cap. It’s s faff and a watery mess putting unions on and pressurising without trying that first, and maybe increasing the poundage too.

[Tony1963]

Megaflow said:

The temperature drop, could be due to the ambient temperature dropping.

If ambient temperature drops, thermostat closes slightly to bring temps up again.

OP: if you go for a drive, get it as warm as it's going to get, then 'go for it', does the temp rise at all?
If you get it up to as warm as it's going to get, then stop and let the engine idle, does the temp rise at all?

[227bhp]

My experience of older Merc diesels is that they do run very cool hence my earlier comment - there's probably nothing wrong with it, the heater working properly points to that. When it's really cold the stat will close, but there is still enough water in that circuit for it not to heat enough and it to open again in certain conditions.
I remember sitting in a newish one idling away in zub zero temps waiting for someone as the temp gauge slowly dropped. In the 10yrs I owned it I heard the rad fan cut in just once and that was pulling a trailer with a car on it through heavy traffic on a very hot Summers day. Modern diesels do run very cool, that older one I owned had a diesel powered aux heater as standard to heat up the water, modern ones have what look like glow plugs in the water ways to heat them up.