Overheating

[Paul Southworth]

My 1.9 petrol dg engine overheated and blew off a pipe. She has since had a new dalek cap, a temperature sender, a thermo switch at the radiator and a new thermostat. We have bled the system until water appears at the rad bleed screw and thermostat housing. The problem is the temperature gauge reads very hot yet the front rad is warm. We have had the rad fans come in when the temperature gauge is virtually at max. I drove her ten miles without any further overheating yet the temp gauge was virtually at max, the red light never flashed either. The temp gauge should be half way. What's the problem?

[kevtherev]

You could change the temp gauge sensor (Brickwerks) to see if it was cooked during the overheating.
What year is the van?

[Paul Southworth]

She is a 1987 transporter, we've changed the one behind the thermostat but there is another to the side. Should we change both? Ideally where should the gauge be? Midway? Three quarters?

[kevtherev]

Mid way
what colour was the sensor you changed?

[Paul Southworth]

Brass with a circular coupling and one spade conector, the other has a blue surround with two wires connected.

[kevtherev]

do you have a metal thermostat housing?

[Paul Southworth]

Plastic upper and alloy lower.

[kevtherev]

Ah a syncro housing.
Did you get the new sensor from Brickwerks? as they are usually OK
If you did I can only suggest that the sensor could be a duffer.

The thermostat opens at 85 deg C and is fully open at 105 deg C.
So the engine will run nicely at around 85 deg

Was the thermostat changed?

[Paul Southworth]

New sensor from gsf. We have replaced the thermostat. Would it be a good idea to flush the entire system?

[kevtherev]

and the thermostat was GSF too suppose.
Now personally I have had some duff sensors and thermostats from GSF. I'm not saying you have , but it is possible

OK I would now try to find out if the cooling system is overheating or the sensor is not reading correctly.

Regarding the symptoms, assuming the sensor is correctly reading (resistance based)
overheating is caused by..

blocked or partially blocked radiator core
poor or ineffectual water pump
Thermostat faulty not opening fully or at the correct temp, or at all.
cooling system not filled correctly

you can check a lot of this
putting the stat in a kettle of boiling water should see a 10mm gap appear at boiling
and a just visible gap at 90 deg.
For the sensor
the electrical connector unplugged, and the engine off, proceed to check the coolant temperature sensor with a standard ohmmeter by setting it on ohm x 1000, connect the positive to the connector and negative probes to an earth point

- The normal resistance on an engine with 50 to 80 degrees (Cold) F = 2,200 to 2,700 ohms. As the sensor temperature INCREASES. , the resistance value will DECREASE

If you are going to check the sensor with the engine running, make sure that the testing equipment is away from all moving parts.

[Paul Southworth]

Many thanks for all your great advice. My mechanic seems to think that it would be a good idea to install a stand alone temperature gauge, he reckons the sensor is reading hot but the actual engine temperature isn't as hot as the temp gauge says. The engine itself is fine because it is a wege block which has done only two thousand miles. The previous owner melted the last engine whilst in France ( must have been a nightmare) he said the temp gauge was reading low when in actual fact the engine was hot.
Too tell the truth everyone who has looked at it has a different opinion, I'm beginning to realise that these engines are a bit like voodoo I.e.quite mysterious even too experienced mechanics.
I agree that gsf aren't great and always try to buy original parts. I think I will give brickwerks a call because everyone seems to recommend them. Hopefully we can resolve the problem this week and I will post the outcome asap.
Regards,
Paul

[kevtherev]

It's important where you take the temperature of the coolant from.
assuming the thermostat is working correctly, and you have a late cooling system (post '86) then the temp can be taken from the metal pipe just after the thermostat housing.
This pipe carries coolant to the pump when the thermostat is not open.
When the thermostat just opens this pipe carries cooled coolant from the radiator mixed with the returned coolant from the engine
When the thermostat fully opens it carries coolant from the radiator only.

there are temperature guns that measure temperatures by infa red, a useful hired tool that can instantly check cylinder head temps.
If you drop an earthed temp sender in boiling water with a thermometer in it you can calibrate you gauge an see if it's reading correctly
It will be half way at 85 to 90 deg C

[Paul Southworth]

Today and after lots of work and head scratching I have managed to sort out the problem.
The coolant temperature gauge has been replaced with a genuine VW part and hey presto everything is hunky dory.
I'd like to remind everyone that fitting cheap parts from euro parts or gsf and the like can be false economy.
What a relief, I shall be demanding my money back from the so called specialist who fitted this part and I will of course buy oem parts whenever possible in the future.

[kevtherev]

Like I said I have had enough duff to use brickwerks for as much as possible
I thought it might be the sensor. It happened to me too

[AdrianC]

I'd like to remind everyone that fitting cheap parts from euro parts or gsf and the like can be false economy.
What a relief, I shall be demanding my money back from the so called specialist who fitted this part and I will of course buy oem parts whenever possible in the future.

To be fair to ECP, GSF etc - a lot of their "non-genuine" parts are the exact same bit that you'd get in an OEM box, except for the branding. Sure, some are cheap junk - but not all.