Coolant temp conundrum Astra 1.7DTL - EnormousDerek

Sorry for the essay...

I've had this motor for 2 years. Always wondered why the cooling system works like it does, and whether something's knackered. Personally, I think it's just the way the jokers designed it. You really need to have the same exact car to be able to make comparative comments on this - Astra MkIII, 1.7TD (GM 'DTL' a.k.a 'Low Blow' engine, not the Isuzu engine.)

Haynes manual says that the thermostat opens at 92C, but is not fully open until something like 105C. Engine quickly warms up to gauge at 92C, and sits there with 'normal' (urban roads, not stuck in traffic) driving.

The fan on this thing is a 2-stage affair, with a low speed (provided by series resistor) kicking in when the gauge gets to 96C, when there's not enough airflow into the rad. This is also in agreeance with Haynes manual, so by now I believe the thermostats and gauge to be surprisingly accurate. The second fan stage comes on at about 102C, but only ever had that happen when the resistor was broken.

Motorway driving is something quite different. If the ambient temparature is more than about 20C, on a long motorway run at 70MPH or above, gauge temparature starts climbing higher and higher, eventually reaching the red (~103C), which is not really what I would expect. In winter, it doesn't even reach 95C for the same load/speed. Running at 100+C, the first stage fan is on (which is completely pointless, as there's a 70MPH wind blowing through the radiator!).

So, what's the problem? Here are my thoughts. Either:

a) Something's knackered.
b) That's just the way they designed it, which breaks down into:
1) The radiator's too small for this weedy, ageing little diesel engine design that's had a turbocharger and intercooler strapped to it to get it up to a paltry 67BHP.
2) The temparature gauge is a Vauxhall 'universal' one, which would be more at home in the petrol engined version. (The gauge on my Isuzu-engined Cav 1.7TD doesn't concentrate on such a small portion of the scale; it goes from 70 to 110, instead of 80 to 105, and the red's not until the very top.)
3) Perhaps they designed it to run with the water hot, so that the thermostat on the air-oil cooler comes on earlier. That doesn't really figure, since they could have just put a lower temp. thermostat on the oil cooler. The oil cooler is working BTW, as it gets too hot to touch (but only comes on at all after summer motorway driving.)

IMHO, this engine set-up is pants, compared to the old 1.7TD Isuzu that they used to put in the Astra and Cavalier: You get another 15BHP, which makes all the difference, you get another 10% MPG, and it doesn't try to blow itself up!

Anyone experienced similar? Lots of these motors still about...
Coolant temp conundrum Astra 1.7DTL - Malcolm_L
Reading the above - have you tried use a decent flushing agent on the coolant system?

It sounds like the flow rate is marginal which is showing up
in these temperatures - might be an idea to have the rad off and back-flush it.

Also worth checking that the thermostat is opening fully.

Best of luck
Coolant temp conundrum Astra 1.7DTL - EnormousDerek
I can try that. Thanks for the advice. If I could measure how hot each end of the radiator were getting whilst the car is running under these conditions, then I would have a better idea of what was happening.

I did change the thermostat when I first got the car, but that made little difference. All it did was make the gauge sit at 92C nominally, instead of only 90. I presumed the old thermostat was just a bit tired, and was opening a little earlier than it should.

I have noticed that this thermostat design has (in addition to the standard spring and wax-operated valve arrangement) a stainless steel disc on another spring, which sits proud of the narrow end of thermostat. This appears to sit in the bottom of the thermostat housing, in which is a small opening that comes from the cylinder head, I think. This arrangement would appear to be a non-return valve. I originally bought a thermostat from Halfords, but this didn't have the extra valve thing, so I took it back and got the proper one from a Vauxhall dealer. Anyone know what this extra valve thing is? Seen one before?

Anyone got one of these engines? Does yours get hot too?


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