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Temperature drops when revved - Sigma Supersport 140


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Hi folks, looking for some help with possible over cooling issues

My Sigma Supersport 140 was running cool, between 60-70deg on the temp gauge and would drop to 60deg when revved.  

I assumed sticking thermostat and replaced with a genuine 82deg item from Burton Performance, part no. QTH576K.

The car now runs dead centre 80deg when cruising, but still drops to between 60-70deg when revved. 

I've bled and re-bled, raising the front end as high as I can when bleeding. 

I've checked the thermostat in a pan of water using an infrared thermometer and it opens / closes as it should. 

I'm now thinking possible temperature sender issue. I believe they have 2, one for the ecu and one for the temp gauge. I've looked at the build manual and my car doesn't have the temperature sender just above the bell housing end of the engine as indicated in the manual. I have searched around the water piping, but can't find. Some thermostat housing appear to have a sender, mine doesn't.

Any thoughts on either my issue or the location of senders please?

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A friends 150 Sigma is the one Jonathan was referring too. What year is your car?

I have a 2019 built Sigma and it has two temperature senders, his is a 2007 car and just has one located on the back of the cylinder block. Mark changed the sensor but is still seeing the low temprature issue.

My car used to run a steady 90 but now runs at 80 or below following a radiator change to a Radtec unit.

Mark also has a new Radtec radiator.  

It might be worth trying and OBD reader to see what the ECU thinks the engine temperature is.


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Thanks Jonathan and Derek, mine too is a 2007. I have tried a couple of OBD readers, but they don't connect to the ECU for some reason, different operating system? 

I'll have a look for the sender. 

Thanks again folks

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  • 2 weeks later...

So, update and hopefully I've got to the bottom of this.

New temp sender, no change.

Went for a drive today and noticed the heater loses lots of heat when the gauge drops, so I guessed it was reading correctly.

When removing the old sender I collected any coolant in a little bowl. I noticed it glistened and upon closer inspection with my reading glasses, appears to have microscopic particles of metal within. All very uniform and I've assessed there is some form of coolant system sealing product within. Not that I've ever put this in. 

A bit of further research suggest these type of coolant sealing products can cause thermostats to stick. 

I'm now ordering another thermostat, new G30 coolant and will flush the whole system a number of times before refitting a new thermostat and topping up with fresh coolant.

Hopefully this will rectify my overcooling. I'll just sit back and await for my coolant leak which might currently be held at bay by the sealant! 

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Very interesting. Thanks for the update.

Have you found two senders?

One source of metal particles in coolant is the water pump. But the symptoms don't sound like that.

NB you can flush with water rather than anything expensive as long as there's enough wash-out with the real stuff at the end.


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Thanks Geoff, sadly been in for 2 years at least (my ownership). It will be gone in the next week or so with many flushes before putting in my new thermostat and coolant. I plan to put a hose in the radiator and heat exchanger alone as well as the whole system, including back flushing these two items. 

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Blimey! Another update, but sadly with no satisfactory outcome.

Flushed the full system 3 times, new temp sender, new thermostat, new coolant (5 litres) bled with front raised and still overcools..... 

After completing the above, temp now reaches 100 rather than the 90 it used to indicate before the radiator fan kicks in. Will happily cycle 90-100 when stationary. From older posts, reaching 100 (mark between 80-120 on gauge) seems to be the correct temp for the fan to activate.

As soon as I drive however, temp drops to 70ish and will not reach 80 (mid mark on temp gauge) until I'm stationary again.

From what I feel from the heater, temp definitely is dropping considerably and when stationary again, it returns to burning my left leg. I therefore assess the gauge is okay and reading correctly.

Thermostat fitted with spring side facing towards engine and small bleed hole at 12 o'clock. 

Thermostat housing has 2 engine outputs, one for the thermostat and another, slightly smaller and squarer, which leads to the t-piece and heater / header tank. Whilst these pipes are a smaller diameter than the thermostat outlet, I'm guessing as there's nothing to regulate flow from this outlet, does this not just simply cause hot water to bypass the thermostat unregulated?

Again, from previous forum topics, some folk never bottomed out and just ignore. I'm starting to think that way. Albeit, somewhat concerned of potential wear and cat damage if over fuelling constantly. 

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I know it's a different engine but my Duratec, with an 82 stat behaves somewhat similar. 

It fairly quickly warms up to 82ish (you can see on the gauge when the stat first opens) and will run around there  in traffic. It doesn't get much above 82 because the fan soon cuts in. I've never seen it over 85 or so.

As soon as you give it some beans the temperature falls to maybe 75. It will do this a few times but after maybe 20 miles it doesn't drop much below 80 though there's always a few deg drop after first opening it up after steady spell.  

My points are that the drop on opening the throttle is normal but not as much as you're seeing. 
I'm puzzled why your fan doesn't come on till 100 with an 82 stat. It should come on much earlier. 

The cowl on the back of my rad has several holes cover by rubber flaps that only open at speed but still impede airflow I would guess. Do you have the same and are they all in place? 

I know some Duratecs are 82 stat and some 90 or so. 
Are you sure your sigma is meant to have an 82 in it?

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I have a Sigma 150 and recently had cause to replace the radiator for a Radtec and larger fan. I haven't really noticed the symptoms you are getting with the temperature variance on your 140, but with the new larger rad the gauge temp dropped from 80 to 70. 

However when I look at the temps via the ECU (Easimap) I see that the actual running temp is 80 (when gauge says 70) and the fan kicks in at 101 (when gauge is 90). No mods done to the ECU or senders so everything standard.

Might be worth using Easimap to calm everything down!


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Hi Scott, I've never seen it run so hot, and although the new thermostat has the correct part number on the box, it did look slightly different and in wondering if I've got the wrong part. I know this won't affect the over cooling, but it did always run to 85 or so before the fans cut in. 

im going to change this, again

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I use the lead purchased from SBD and the free Easimap software


The lead is VERY expensive now but there will probably someone local to you who has one you can borrow.

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My Sigma 150 runs pretty cool most of the time 70-80°C on the road with radtec radiator. Only gets hot on track / in traffic. Fan also comes on around the 100°C mark. 

My gauge matches up to Easimap.

You could try a 89°C stat or similar if you use it mostly on the road and see if that's better. 

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A low cost ODB reader will give you engine temperature on a smart phone App

ODB.thumb.jpeg.6ee4180baf4e693cb91b996ec268a3f9.jpeg AutoDoc.thumb.jpeg.14436fcf81c60e377e88b166cf9d5ba9.jpeg

This is under £12 on Amazon using FOC Auto Doc app



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