D24T - Volvo 740 - slow to warm up?
 

Hi Folks,

I'm new to the forum, having recently bought a 1990 Volvo 740TD. I'm sure I'll have plenty of questions in time, as I do most maintenance on my cars myself...

My first question is... how long should a D24T take to get up to temperature?

In the UK at the moment it's about 0-10 degrees C (32-50F) and my Volvo takes ages to warm up. On the 120 mile run home from collecting it, I didn't exceed 60mph and the temp gauge never got above 1/4 mark.

I've found that if I sit at 70mph (GPS, 75mph speedometer) it does get up to 1/2 on the gauge, but it takes a very long time at this speed to get warm. If I slow to 50mph for roadworks, the temp goes down to 1/3 on the gauge...

This evening I went for a drive, 8 miles including 1 mile at 60mph and then 4 miles towing another car, and it didn't get above 1/4 on the gauge...

Does this sound right to you?

I've already changed the thermostat and the cold-start wax stat, so these should hopefully be OK... my next thought is to change the temp sender, but tucked under the alternator bracket it looks a real pain to get at! :)

Any thoughts much appreciated - thanks :)

Mine warms up to operating temp. within ~10 minutes in similar ambient temperatures to yours. When it had a broken thermostat it exhibited behavior similar to yours.

Failed Thermostat

Did you test the new thermostat before installation? I had one that was defective right out of the box. Is it the correct thermostat for the engine? It needs the tophat disc to close off the relief (bypass) port when warmed up.

Does the cabin heater put out insufficient heat also? If the heater core produces good heat then the problem may be the temp gauge sender. Located just forward of the radiator hose outlet in the head.

Hi, and thanks for the reply :)

No, I can't say I tested the new thermostat before fitting. So far as I know, it was the right thermostat - what do you mean by the top hat bit? :)

Have you a photo of the correct orientation for the thermostat fitment? :)

The new stat seemed to be slightly larger in outer diameter than the one that came out. I might pull it out next weekend and test it...

The heater does seem to put out reasonable heat, but I'm used to 1960s cars which never had great heating...

Tofufi? Is that you James?
My 88 TD takes about about 2/3 miles of careful driving to get up to temp.

Note the coinlike disc attached to the tstat in this photo, that is what blocks off the coolant bypass at operating temp:
Thermostat 240/260 diesel 85-93 (87°C, 189°F)
Thinking more about that, lack of that blocking disk would worsen an overheat situation more than underheat.

RE: correct orientation, that disc should be facing into the head, with the peaked part of the tstat facing outwards. Then the rubber seal (hope you did install that?) is sandwiched between the tstat and the bolt-on flange, as shown in this dwg.

Tis I. I didn't realise your 740 was a TD :D

Thank you - I am pretty sure I've installed it correctly. I wouldn't be certain that I fitted the O ring on the correct side - I will pull the 'stat out again, test it, and re-fit ensuring everything is correct.

Thanks :)

My gauge sits around 1/3 on flat ground unless running over 90MH for too long, then it heats up.

Thanks for the reply, did you mean 90MPH or KM/H? :)

I'd just like to establish what is normal behaviour :)

I tested the 'stat I removed and it seemed to operate fine. So I'm still at a loss as to the slow warm-up I'm experiencing... I think I'll change the temp sender on the weekend :)

Both of mine sit straight up halfway after 5-10 minutes driving from cold. Moving maybe a needle width higher when under high load.

thats MPH

With an 87C stat at normal speeds it sits below the half mark on flat ground, but will rise on hills