Over heating Rover V8

[peterc]

Since the winters lack of use the water temp of the Rover in my Cobra seems to like exploring the wrong side of 95 C mostly during the warming up process e.g. first 10 miles.
With the Edlebrock inlet manifold it had often liked to explore around 93C whilst waiting for the thermostat to open but then settles to around 87.
However things have taken a big step further just in the last week and for reasons totally unknown the temperature has been off the clock! ( one trip 105 and then 120C? )
I have this weekend changed the thermostat in the belief it was sticking. No improvement. I checked it later in a saucepan and it was opening OK at around 82-84C.
Checked the cars (capillary) temperature gauge which showed it is only 2 degrees out at 100C.
Checked the hoses for any kinking / reduced sections. The hoses were all replaced last year.
Checked the water pump can be rotated by hand. That was new not many years ago.
My next step will be putting the hose on the radiator to see if flushing would produce any result although I’m not sure why it would dramatically become blocked over the course of the last few days.
If that doesn’t result in any obvious improvement then I will reluctantly remove the water pump to check that impeller is still connected.

I have never suffered any airlocks in the past but am aware that air can send the temperature rocketing e.g. a bit difficult having 120 C water.
I have also removed both rocker covers to check for emulsified oil in case there was a head gasket issue but they were both really clean.

The car was meant to be having a new exhaust designed and fitted starting this week but clearly I can’t drive the 20 miles to get there.
Any suggestions as to what to check next?
Peter C

[clive]

Hi Peter,
You say the temp is high for the first ten miles. Does it stay there or does it fall after that? I had issues with my Ford many moons ago and the problem I found was whilst cruising in fifth gear the engine was only doing about 1,500 RPM and the water pump was not circulating the coolant fast enough. Dropping down a gear was sufficient to speed up flow and reduce the temp. I did plan to get a smaller pulley for the water pump, but needless to say, I never quite got round to doing it!

[peterc]

Hi Clive,
Thanks for the comment. Yes, I went through that learning curve many years ago and changed from 6” to a 5” pulley which mostly resolved the elevated temp whilst in slow moving traffic.
For many years the temp would always go a bit high at around 8 miles before the thermostat opens and by 10 miles it would settle into its norm at 87.
With the temp sender so far away from the thermostat I have got used to seeing the rise in temp due to acceleration etc before the thermostat reacts.
Down hill on a trailing throttle it could drop below 80.

No, today this situation is very different. At 10 miles temp only fluctuates between 100 and off the clock!
As mentioned you can’t have water at 120C so air must be getting into the system. I have never had issues previously with bleeding the air out after refilling with coolant. Two trips needing the over flow tank to be topped up and no over temps during those trips.
Over night thoughts are now to perform a compression test as a leaking head gasket could be putting hot gasses into the water without affecting the oil system. E.g. the clean rocker covers.
If it turns out to be a head gasket issue it would be rather ironical as the engine is booked into having top hat liners fitted in Oct as a precaution against this type of issue.
Apparently the narrow width of the liner wall can allow the gasket to fail.
I will look at the cost but new head gaskets and the relevant new head bolts might seem a bit expensive for just 3 months use.
Peter C

[peterc]

All but one cylinder pressures between 170-180 psi and that’s cold with a closed throttle. The odd one at 165 which I don’t think indicates an issue.
So plucking up the courage to rip out the water pump but that is only 4K miles old.
Run out of ideas to be honest.
Peter C

[clive]

Have you drained and refilled the cooling system recently? When I refill my 302, I remove the top heater hose and lift it up higher than the rest of the cooling system and slowly fill it through that with the heater set to hot.

[allan horsfall]

Morning all,
My first thought was to check the temperature sender. In the past I've experienced problems with the oil sender supplied by Speedy Cables, the most recent change was after 500 mls, Im now on my 4th one. Looking at them I suspect water and oil senders have the same origin?

Regards
Allan

[Migge]

74°C Thermostate?

[peterc]

The system has been perfectly OK since May 21 when I replaced all the hoses. I have only been partially draining down since the over heating started. Firstly to replace the stat and then to check the temp sender. As already said the sender appears to only have a 2C error at 100 assuming my wife’s jam making thermometer is accurate. I assume Alan you are talking about an electrical sender not capillary.
It turned out the original stat was opening as it should.
.
Bleeding the air out after any draining is a simple task. The main system is filled to the brim of Gerry’s header tank and the over flow bottle a 1/3 filled. Get the engine hot and then get cold over night. It takes in the water from the over flow bottle during cooling. Refill overflow to a 1/3 and repeat. After the two runs the system is totally full. During that process I have not noticed any severe over heating.

I have this afternoon made arrangements for a cooling system pressure test as my gut feeling says it might be a head gasket as I have not found anything else amiss.
Peter C

[clive]

Before removing the cylinder heads to check the gaskets you could try a coolant test. There are many kits on the market which test for exhaust gasses in the coolant. Just Google "Coolant Gas Analyzer"

[peterc]

Clive I think our posts crossed in the ether. Yep, coolant test booked for Tuesday before I bother to strip out the water pump to see if it’s all in one piece.
Peter C