The V8 Owners Forum -

Sounds like the thermostat is sitting in cold water and not activating, might be caused by the changes you’ve made to the system ie the heater bypass not being connected. It’s eventually opening with heat conduction because of the high temps. Then shutting again quickly once cooled from the rad side, keeping the temps high. Whip the thermostat out as you suggested. if all is good, take a close look at the plumbing.

Hope this makes sense, not sure how to best explain it. Came across it before, the signs and symptoms never seemed to make sense. I was eventually enlightened by someone else.

Does your thermostat have a bleed hole in it to allow some bypass flow till the thermostat opens?

Perhaps time to drill a couple of 6 mm holes in the surround to let some flow

Ian

Thanks guys and apologies for lengthy post but last weekend I managed to do a good test run and this is the result …

Went on my usual afternoon run of some 35 miles on a meandering A road, very small town and then another pretty straight A road with a terrific ‘blast all the way up’ hill up onto the South Downs and then a long fast open stretch to the outskirts of Goodwood. Convenient car park to turn around and back home. First part 45 to 55mph and second part 60+ if no traffic – and there was none!

I also set up my laptop to record the trip taking temp readings from the Megasquirt ECU for the whole journey.

First part as oil warmed up to indicated 90 degrees, water temp kept around 90 degrees. Once oil and water got to running temp, both fans came on indicating first stage at 92 and second stage at 95. (Off set at 87 and 85 but that never happened as per usual).

At mid-way point, I kept the engine running whilst I used a temp gun to check radiator temperatures. This was at least five minutes and probably nearer ten so simulated heavy traffic.

The CAI water temp gauge had been cleaned and matched to its sender by CAI a few years ago. However compared to calibrated ECU sender, it reads 10 degrees low.

Water temp gauge showed (adjusted) generally 105 except at mid-way point when it went to 110+ and basically stayed at 110 all the way home. However the laptop trace shows an different story. Running temp is generally 95 (94.7 actually) and at mid-way point reached 104.8 but then cooled back to 95 soon after starting to come back home. Hmm.

First conclusion is that once the water temp gauge gets high, it fails to return back when temperature cools so that’s why I have been assuming normal temperature is 100 to 110. It’s not. I have noticed occasionally it jumps back by 10 degrees or so but not very often

When I used the temp gun on my radiator at the halfway point, I got these results:
- Temperature at inlet around 90 degrees.
- Temperature at outlet around 40 degrees
- Temperature on right and left of rad and not covered by fan around 90 degrees
- Temperature at bottom of area covered by fan 26 degrees.

(Rad fan covers 100% top to bottom, but about 75% side to side. Rad fan is rated at a whopping 3.126 m3/hr)

Second conclusion is that whilst my engine is generally 95 and sitting traffic is 105 degrees, if I cowl the inside of the radiator to force the fan to cover as much as possible, I should get both these temperatures down. Having looked at the radiator, you can see that some 25% of the rad is not covered by the fan. Whilst things are tight, I should be able to slant the fan giving up to 40mm gap at the top and cowling the rest.


Third conclusion. Send back to CAI the temp gauge and sender to get things fixed.

So I now have another winter’s project!
Photo of my Temp gauge and ECU sender locations.


Photo of my flared side panels (with just a glimpse of my TR6 I’ve just started to totally strip to fully restore).

That looks like a really really small fan....you need more core coverage for sure.

Although if water temp at the rad outlet, ie into the engine is 40degC, as to why there are cooling issues seems odd. That's quite a substantial drop in temperature, unless water is not flowing properly and hence a false reading.

I cannot see how it is possible to get a temperature drop of any huge number between engine and radiator.

I would hazard a guess that it is not running that hot.

I would also hazard a guess that as soon as the fan kicks in the overall voltage from the battery drops enough to be affecting the readings your gauges and ecu take off. Some of these fans can draw 40 amps.

Lastly I would possibly run another earth strap between engine and chassis, you could be getting a duff contact

Ian

All ecu related sensors are regulated, voltage issues elsewhere will not affect the readings at the ecu.

The external guage could be different.

Some cars that use Motronic injection have their dash gauge fed from that ECU. And it doesn't read true engine temp. It appears to move normally during warm up, but when fully warm will sit on a 'normal' mark over a very wide range of true coolant temp (at the point where it is measured). In other words, the range of temperate Bosch have decided is normal. Only if the temperature goes higher than this maximum does the gauge start to read high.

As our US cousins say, go figure.

Absolutely, never trust an OEM gauge for an accurate reading, especially on modern stuff.

I think it's also expecting too much to have the coolant temperature stay exactly constant under all driving conditions.

DaveEFI wrote:I think it's also expecting too much to have the coolant temperature stay exactly constant under all driving conditions.

If it's all working well, it should remain within a fairly small range. But definitely shouldnt be getting too hot, and staying too hot.

It is not overheating would be my guess.

Water temperature at inlet to radiator is 90 this is also the temperature at outlet from motor so motor is around 95 .

Water coming out of radiator is 40 degrees proves radiator is working extremely well.

The fan switch should be based on outlet temp from radiator. If the outflow is at 40degrees the fan should be off, as it should be when driving at over approx 20 mph when more air will move through it than the fan can move.

Most temperature gauges are volt meters measuring a voltage drop across the sender. If one side is positive from battery and this varies from 15v when charging to perhaps 12 v when lights, fan, ecu, fuel pump etc are on this is a 25% voltage difference which will make a difference on the meter.

That said I still believe there is too much flow through the heater circuit and that water is missing the radiator. Mole grip it to stop flow for a trial. The temperature swings will be a lot slower and less alarming.

Ian

Problem is...I'd question the accuracy of that 40deg reading, and whether that stat is actually open and water flowing when it was taken.

As previously mentioned, there’s an issue with the plumbing and it’s affecting the thermostat function.

Ditch the thermostat & pump and go electric with a Davies Craig and electronic controller. I thrash my 3.9 and it struggles to get over 90c. £300 well spent imo.

Thanks guys and all posts seem difficult to summarise. However, in no particular order …

Having done the rounds visiting several specialist rad manufacturers, the rad was specified to give maximum cooling in the available space so has a fine 70mm core. It is maximum size for the opening in the front nose cone. Perhaps with the thick and fine core, non-fan assisted air is difficult to pass through but that’s how it now is. I have the nose cone fully cowled too with little chance of air going around the rad.





My fan fits 100% top to bottom so I cannot get a larger fan to fit (as rad is a rectangle). However, I will now make a cowling on the inside so the fan will cover much more of the rad. The fan has the highest throughput possible with a two stage (two motor) arrangement. It draws 35 amps on initial startup and then runs at a constant 18 amps – I’ve measured this on a bench. BTW, Kenlowe no longer make fans. Shame. My alternator has a 100amp rating and there is just a visible .5 volt drop (on volt gauge) when fans are on from 14.5 to 14 volts.

The ECU temp reading for the trip was a very constant. Here is the compressed trace of the trip out of about 30 minutes (until the USB cable fell out blasting up a steep hill!) showing temp vs RPM.



Here is the compressed trace showing the reconnected laptop at the mid-point car park where it idled for about 5 – 10 minutes. The temp here shows it at 103.7 and then shows it cooling down on the drive back until the laptop battery ran out. It had got to 96.



I am pretty sure that the ECU temp readings are quite accurate as these were calibrated by me and the figures entered into the ECU. Yes, they could be out but not by much, I suggest.

I have noticed occasionally the temp gauge suddenly jump back 10 degrees or so. So my theory is that as the gauge reads higher temps, it does not reduce when the temps decrease. It seems the gauge gets stuck and this was born out by manual readings taken throughout the test run. When the ECU temp reduced to mid 90s, the gauge stayed showing 100+.

The gauge is from a TVR and was made by CAI who supposedly checked it for accuracy when they cleaned it. Needs a second visit back to CAI in Wales, I guess.

Good point about measuring fan switch from output temp of rad. That’s my view but many others disagree. Anyway here I have two fan switches – the ECU for fan stage 1 and the otter switch (in picture on earlier post) in inlet pipe. I thus can’t really logically do this and I do not think they make otter switches for outlet temps – mine is a 95/87 one. I can easily adjust the ECU on/off to anything I want but seems logical to keep it at 92/85. Theory is both fans on when very hot and one fan stage when needed as a top up.

The rad temps were taken with engine running in car park using my temp gun. Bonnet was off, obviously.

At the moment I do not have a thermostat fitted as I wanted to ensure maximum flow. Also I have temporarily removed the heater circuit bypass so the water always goes through the heater. It is supposedly a 2.5kw heater but I did not have the fan going so no air flow at all. However previously I turned my heater control off and it did not make any difference but I have to agree I was not checking the ECU temps, only the gauge. Perhaps another trip out to test this is worth it.

I have often wondered about an electric pump. May be an option but I’ll try cowling the rad fan first, I think.

So, my current conclusion is still the same being to cowl the rad fan and send the temp gauge off to CAI to be looked at.

Whilst having an engine run at generally circa 95 on the open road and circa 105 stuck in traffic probably acceptable, I would prefer 5 degrees off those figures. If I was ever really stuck in traffic for a long time on a really hot day, I would like a little more margin than what I have got.

It is likely to be next Spring when I come back with the results.

Just a thought
Have you tried cracking open the two temperature sensors to burp or release any air bubbles trapped in the vacinity?

Should not normally be necessary but it may be a cause!

Ian