Water temperature if there is no water...

[shielsy]

So I gave my work horse (1997 Shogun V6) a service this weekend; fluids, filters and brake pads. I noticed the water level was nigh on empty so filled it up. Took the car out for the afternoon and parked up at the shops, and on my return I noticed a rather large puddle under the car. No problem I thought, I've probably put too much in and it's just coming from the overflow.

It got to a point after a few trips out there was puddle after puddle under the car. On my way home I kept my eye on the temperature gauge... about 3 miles from home it rose quite quickly and then dropped, rose again and dropped, the third time it rose I had to pull over before it went into the red. I got out and noticed there was no coolant dripping out. I put two and two together and figured there was probably no coolant left. I opened up the cap and this was confirmed by there being no pressure in the system. I got some water and filled it up so I could get home.

I started looking for a leak and managed to find the culprit behind the timing belt cover; it was the water pump. Unfortunately it looks like it may have been leaking a while:



I plan on changing the pump and also the timing belt while I'm at it. I am just a little concerned about the radiator being low on coolant before I serviced the car. My question is... if there is no coolant, how accurate is the temp gauge? I drove 300 miles last week before the service and the temperature stayed bang on in the middle the whole time. I am hoping that the act of me filling it up has just put the nail in the coffin, rather than it running hot for a prolonged period.

[FD3Si]

If there's no coolant it's not reliable at all, as it's just measuring the temperature of air. It going up to the red, then dropping, then swinging up, will be it dipping in and out of the remaining coolant as it sloshes around.

[TurboHatchback]

Yep, when the needle suddenly drops that means it is measuring air not coolant temperature as all the coolant has escaped. This happened to a car we took on holiday once on an autobahn, within a mile or two the engine completely combusted .

[kambites]

As above not very, but it's highly unlikely that it would read a consistent, sensible figure if there was no fluid in the system. If the temperature gauge was reading "normal" for a 300 mile trip, I suspect you had enough coolant to cover the sensor.

[Marlin45]

....or it's just measuring the temperature of the passing steam!!

[shielsy]

Thanks for the replies, I'm just really hoping the heads are OK.

[TheRealFingers99]

Don't hope, compression test.

[kambites]

I'd probably change your cam belt tensioner (although that's probably part of changing the belt, it isn't always done). It should be waterproof, but it's probably not designed to get quite that wet and for the sake of a relatively cheap part, I'd do it anyway.

As above you could compression test it to make sure the head hasn't warped, also regularly check for coolant in the oil and vice versa. I suspect it'll be fine, though.

Edited by kambites on Monday 2nd June 11:52

[Mave]

Whether or not sensor accurately measured the steam / air temperature is kinda irrelevant. Cooling the engine with even ice cold air is far less effective than 85c liquid....

[Garybee]

kambites said:

As above not very, but it's highly unlikely that it would read a consistent, sensible figure if there was no fluid in the system. If the temperature gauge was reading "normal" for a 300 mile trip, I suspect you had enough coolant to cover the sensor.

Sadly this isn't the case in plenty of modern cars. Some drive the coolant temp gauge from the ECU and will point the needle at the centre of the gauge over an alarming temperature range.

[kambites]

Garybee said:

kambites said:

As above not very, but it's highly unlikely that it would read a consistent, sensible figure if there was no fluid in the system. If the temperature gauge was reading "normal" for a 300 mile trip, I suspect you had enough coolant to cover the sensor.

Sadly this isn't the case in plenty of modern cars. Some drive the coolant temp gauge from the ECU and will point the needle at the centre of the gauge over an alarming temperature range.

Hmm, fair enough I didn't realise that. My car actually gives you a temperature in Celsius and updates it every 5 seconds or something. It seems to correlate directly to the sensor reading through the diagnostics port.

[AJB]

Garybee said:

kambites said:

As above not very, but it's highly unlikely that it would read a consistent, sensible figure if there was no fluid in the system. If the temperature gauge was reading "normal" for a 300 mile trip, I suspect you had enough coolant to cover the sensor.

Sadly this isn't the case in plenty of modern cars. Some drive the coolant temp gauge from the ECU and will point the needle at the centre of the gauge over an alarming temperature range.

Yes modern cars do "lie" a lot when it comes to the temperature gauge. But I think they normally read centre if the measured temperature is, say, between 70 and 100 degrees. If the coolant was low and the sensor wasn't covered I think it'd be out of this range and would show the error fairly quickly, so I'd be inclined to agree with Kambites that "normal" reading meant the sensor was covered even allowing for the ECU's interference.