Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Temperature readings: Diagnostic help please!

Bit of a long one this chaps, but hope you can stick with it and offer any words of advice?

A few days ago, I popped out in the Morgan for a short but vigorous drive, only a few miles (less than 4 to be specific). When I got home I noticed that the engine fan had kicked in, something that heretofore I had observed only very rarely. The water temp gauge on the dash hadn't moved at all.

So I let the engine go cold overnight, and the following day started it up with the computer plugged in to see what was going on. Remember my car is thoroughly non-standard. The engine fan is controlled by the ECU, the water temperature gauge in the cockpit is fed from a temp sensor (taken from the original CVH engine, where it sits in the block) which is sited in the water return feed pipe from the top of the radiator, immediately before the pipe enters the thermostat block on the engine.

Prior to starting the engine I removed this sensor, it's an old-fashioned copper stubby thing that just protrudes into the water flow and changes resistance as it warms up. I stuck it in a pan of water on the stove with an ohm-meter attached, and observed that, as expected, the resistance dropped as it got hotter. So I assumed it was working ok and put it back.

The engine performed entirely normally, the water circulated round the block until the ECU temp passed 100C, then the thermostat opened, water started circulating through the radiator, the dash gauge started measuring the temperature of the now-circulating water as it heated up, the engine fan slowed down as the head temperature stayed steady on 100C.

So I assumed that everything was fine and that what I'd observed on my short drive earlier was the engine not getting hot enough to open the thermostat, and worried no more about it - until today.

Today, I went for a 30-mile jaunt. The engine performed entirely normally - if anything, it was going better than usual. But the temperature gauge rose relatively quickly to indicate around 100 to 110C and stayed there. When I got home I got the infra-red thermometer out and did a bit of measuring.

Temperature of the head itself was as expected, around 110C. Temperature of the water passing the sensor was around 80C. Everything checked out fine - apart from the flipping temperature gauge. The car's been in this configuration since 2008, I've never had any issues with it at all, and typically the temp gauge reads much lower than this, I wouldn't have expected it to show more than 90C. Nothing has been done to the car over winter apart from fitting new rear brake cylinders and changing the brake fluid.

So, what do we reckon? Faulty gauge? Faulty earth on the gauge? Faulty voltage regulator (but the other electric gauges work fine), or faulty temp sensor? Obvious thing is to start with getting a new temp sensor as it's the original from the 1986 CVH, but I'd appreciate any and all ideas.

Hamwich, look in "car builder solutions" on line and replace the whole in-line sensor housing and sensor as a starting point. The thermostat opened at 100'c you said.... mmmm, that is probably a little high! You will need to check but I have a feeling that it should be a thermostat that opens at either 85' or 90' c. that could explain you raised head temp on measuring. Is your engine still a CVH Ford and is it injection or carb fed. I'm sure there is a difference in thermostat opening temp however if you cant find the info out whip the stat out and it should have a temp setting on it, as it was working fine before a replacement should be ok. When the stat is out check it in a pan of water with a thermometer and see the true opening temp.

That's a quick answer but i'll read your tale more slowly later and try and get some better logic into it!

It's a 2007 Vauxhall Ecotec Z18XER 1.8 VVTi engine, Sewin, as fitted to the Astra and Zamora SRi at that time. The ECU is a MBE unit with custom map by John Ravenscroft of TVR. The operating temp range of the head is 120C, the ECU is set to switch the fan on at 100C but you're right, I would expect the thermostat to open a bit earlier. Maybe it's that that's knackered.

EDIT:Just checked and the thermostat on these engines is set to open at 105C, which I found surprising.

The feed to the dash gauge is from a traditional sender plumbed into the radiator return, so I could keep the original Smith's dash gauge. I've already ordered a replacement as they are easily obtainable, I'll see if I can find a replacement thermostat from somewhere.

The temp sensor that's been installed in the return pipe is as you quite rightly say of the type normally found in the block or lower third of the radiator and sounds like the coil may be breaking down. Better here to change the whole housing and sensor out for a specifically designed in line (pipe mounted ) sensor. Where does the ECU get its control signal from? This would be my first point of investigation, you have checked flow on the stat opening and presumably it closes again as the temperature drops. The fan came on when you didn't think it should have and indeed hasn't previously. If you want to get better control of the temperature the fan switches in at then perhaps an electronically controlled in line sensor switch might be a good idea, The whole kit including T piece, sensor and control unit are less than £70. Once you have found the problem then this might be a way for you to fine tune when the fan kicks in, in other words, fine tune your unorthodox car for best function. I think you have to view your car as you would a kit car, Clearly the variables here are from a non designed set up, It may have worked alright when all the parameters of everything in the system were managing to overlap and the simplest remedy would be to find the failing area/part and replace it or perhaps look at the cooling system as a whole.

You say the resistance measured across the sensor DROPPED as the water heated?
That's the reverse of what should happen. As temperature increases so should the resistance. I remember doing a practical in A-level physics measuring this. A coil was immersed in a bath and resistance measured over a range of temperatures. It was a standard practical in a long list covering the whole range of physics. Mechanics, electrical, heat, radioactivity etc.
My guess is that your sensor is worth changing. Also try immersing it in boiling water whilst connected to your car temp gauge remembering a small temp drop from boiling. Clean all contacts in the circuit as dirty ones can have higher resistance affecting the current/voltage your gauge receives thus misreading.
Here's a link that describes this
https://www.bbc.co.uk/bitesize/guides/znh7382/revision/8

Sorry Sospan, The temp sensor probe drops when heated.

Ah! The circuit compensates then. We had thermocouples measuring the temp of molten steel in the furnaces at 1600degC and I think they had compensation . Throw away Pt/Rhodium thermocouple in the probe.

The ECU gets its data from the standard Vauxhall sensor, which on these engines is part of the thermostat housing. Watching the temperature rise on the computer (I have an app called EasyMap which monitors all the variables in real time) you can see that the fan is triggered exactly on schedule at 100C. So I don't think there's an issue with the data that the ECU is getting, more that the coolant may not be circulating as effectively as it should.

Having done a bit of digging around on t'internet, it seems that it might be the thermostat itself which has failed, apparently there's a seal in there which can break up. If it's not opening correctly that would explain the higher than normal operating temperature.

So I've ordered a replacement thermostat along with a new switch sensor for the gauge. Not tremendously surprising I suppose, the Smiths sensor is 40 years old and the Thermostat nearly 20. We shall see.

What material is the thermostat housing on the Vauxhall engine?

On recent Ford engines it is plastic and uses an elongated O-Ring as a seal rather than a gasket. Worth checking as you will need a new seal if similar to Ford.

Yep, it's the same arrangement. Supposedly the new thermostat comes with the seal but I have the part number if it doesn't. The thermostat itself is integrated with the top section of the housing along with the temp sensor and is alloy, but the main body that mounts to the engine is plastic for some reason, probably cheaper as it would be quite an intricate casting.