Overheating - please help!

[Colin Mill]

OK, here are the results of my initial round of tests.

 

First of all, I looked at what resistance in place of the sensor was needed to get the gauge to read various temperatures. As mentioned the gauge is somewhat sticky so some tapping was needed to get it to settle (amazingly, engine vibration alone wasn't enough 😬)

 

Gauge reading.......Effective sensor resistance

40C...................600 ohms

60C..................440 ohms

80C..................200 ohms

100C.................130 ohms

120C...................75 ohms

 

As for the sensor I used a contact thermometer on the club foot close to the gauge sensor and when the contact thermometer was registering 80C the gauge sensor was at 230 ohms suggesting that the gauge slightly under-reads at 80C. Since my engine does not naturally get above 80C I will need to get the sensor out and do my further testing in a water bath.

 

Interestingly, I tried using an IR thermometer on the club foot and, presumably because of convection from the exhaust the surface temperature seen by the IR thermometer was well above the contact thermometer readings.

 

I have tried to fit the gauge requirements to a standard NTC thermistor and the best fit at the important top end of the gauge is obtained by a thermistor with a B value of 3800k but this is not too good at the bottom of the gauge.

 

I'll be better able to sort this out once I have some better reading of the sensor itself.

[jon_travers]

Thanks Colin - very much appreciated for this - those figures seem to confirm my gauge's readout matching very similar to sensor resistances we found as yours below.

 

We tested by disconnecting the feed from the sender and connecting up our own wire to the temp gauge - this seemed to prove there no grounding out within the original feed from the sender - and the gauge is indeed showing a near true value. (although we're going to retest tomorrow to be sure).

 

I also took the thermostat out and just tested it in boiling water - seems to be working fine - although I did think it's moving a little on the slow side (but i'm no expert 😬) - i'm going to get another one tomorrow from halfords to rule out that out anyway - I'm also going to take off all hoses and check for any blockages.

 

I'm going to fix this, even if it kills me 😬

 

 

[Colin Mill]

Hi Jon

 

If you continue to have problems it could be worth checking that the ground voltage at the engine is the same as the chassis (a poor earth on the engine could cause there to be a difference). If the engine were, say, slightly negative with respect to the chassis it would cause the gauge to over read. Conversely, if it were positive WRT the chassis the gauge would under read.

 

Just to complete the info from the testing of the gauge I found that it had an open circuit output voltage of 5.1v and an output resistance of 120 ohms. I guess that, as someone with a passion for instrumentation I shouldn't have started looking at this as I was bound to be underwhelmed with what I saw.

 

However, given how important it is I think I want to improve this - it would be silly to cook the engine without knowing it. My thoughts at the moment are to use an LM35 IC temperature sensor (or similar) and a PIC to drive the gauge. Because this sensor has an accurate 10mV/degC output it would be possible for me to accurately calibrate the gauge at 40, 60, 80, 100, and 120C (with a set-up mode in the PIC to allow these adjustments) and a power-up check (gauge pausing briefly at each of these temperatures before going into read mode). An alarm drive output would be easy too (a switched ground for a lamp or buzzer at some preset temperature.

 

There would be enough drive coming out of the gauge to drive the circuit so I don't need to mess with running a supply wire to it.

[jon_travers]

Thanks for the tip, really good point with the engine ground voltage - what would you say is the best way for testing the two, just simply stick the voltage metre on the engine? (sorry newbie 😬).

 

That also make sense as all this started after taking the engine out/then back in and something so basic can easily be overlooked. Is there a best way to make sure the engine is grounded?

 

You've made some interesting findings - I wonder why caterham doesn't utilise better technology for something which is important - would all that gear be expensive?

[Colin Mill]

Hi Jon

 

You can check the engine is reasonably grounded by sticking the DVM on ohms or continuity buzzer and stick the probe on say the cylinder head and one of the ECU mounting screw on the scuttle. If that seems OK try measuring the volts between the same two points with the engine running and the headlights on. If the ground is poor you will see some voltage between them which will probably vary as the alternator output changes.

 

As for doing the job more accurately it is very cheap. The sensor I'm looking at using, which is good to better than 1 degree C is about 40p and the PIC will cost about 80p. A hand-full of passive components, a PCB and it could be made for a couple of quid - a reasonable profit at 8 quid retail. Given that the standard sensor is about 8 quid retail its seems worth doing the job right.

 

I'm up to my neck in R&D just at the moment (playing with accelerometers which is fun!) but when I get a moment I'll CAD a PCB for this and get a few made up by my tame circuit board outfit. I'm wondering if it's worth putting the sensor in the water flow or simply bolting it to the back of the head.

[jon_travers]

quick update, we've taken the hose coming off the coolant return pipe and it doesn't flow water freely until we shove a metal stick down the shaft a few times = then it stay open and water flows - is this normal? Any advice greatly appreciated!

[Colin Mill]

Is this the pipe that runs down from the expansion bottle to the 'submarine' section (the metal section between the rad and the pump intake)? If so, then it is certainly intended to be open.

 

 

[jon_travers]

thanks colin, nope - this is the return pipe at the top of the coolant tank - coming from the engine - it seems the value in the engine part could be sticking, although we'll need to investigate further (I think the value is there to stop any back flow into the engine)

[Colin Mill]

Hi Jon

 

Ah, yes, the dreaded joggle valve! From the descriptions from Rover this valve is a float valve that is supposed to open when there is air in the cylinder head and close, or at least seriously restrict the flow of the liquid out of the head once any air is expelled. On mine you can hear it operating and see small amounts of fluid coming through into the expansion tank when you squeeze and release the top hose violently.

 

If it stick shut, and there are reports of ones doing this, it could trap air in the head. I can't find a really convincing explanation as to why this valve is a good thing. On this thread about the only thing we all agree about is that this valve is better removed 😬 Unfortunately, AFAIK it's an intake manifold off job.

[henry21p]

what was the outcome of this one?

[brucehc]

My engine is overheating at the moment. I lost a load of coolant through a loose hose and have been refilling it and trying to bleed it for the last couple of days. I thought I had it cracked yesterday - hoses left and right of engine and to the heater were getting hot. When the fan did cut in the rad cooled quickly and fan switched off.

 

But a short run last night proved otherwise into the red and coolant everywher - I will get a nex exp bottle cap.

 

I have unclipped the top hose to the heater (the long one that runs from front right ('stat?) round the side to the heater valve), and bled that. I don't need to undo the two short right angle hoses that run from the heater valve into the heater itself do I?

 

I don't have a bleed T, yet, so have filled the system via the end of the long hose (once filling rad and exp bottle).

 

Do you have any more advice?

 

Very many thanks

 

Bruce

[Colin Mill]

I fitted a bleed T from the word go and have never had any trouble bleeding the system. You can make one up out of bits from the local plumbers merchant using a 15mm 'Yorkshire' T and a bleed valve (that simply solders into the leg of the T). I then slightly extended the arms of the T with stubs of 15mm copper tube. I think I also cut a 15mm Yorkshire straight joiner in half and soldered the halves over the stubs to increase the diameter.

 

When filling I have taken the screw completely out of the bleed valve and put a short length (about 9 inches) of 16mm pipe and a funnel onto the leg of the T to aid pouring coolant into the T.

 

You can see the bleed T on mine in this picture here

[brucehc]

Sorted.

Thanks for advice. Problem was loss of coolant through leaking expansion cap and top radiator hose.

Sorted Bleeding by parking car on steep slope and filling expansion bottle - T Peice on order for future.

Also changed heater hoses round - to corrcet plumbing!

Is the expansion cap from a Freelander?