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250 saloon engine overheating

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Joined: Sun Mar 20, 2016 2:07 pm

250 saloon engine overheating

Post by DickSpeedy » Mon Feb 18, 2019 8:48 am

I am having trouble trying to stop my newly overhauled V8 from overheating. I have replaced the fan with a continuously running electric fan. The radiator core has been replaced at great expense. I have fitted an over flow tank. The thermostat has been removed. A 7 lbs long neck radiator cap has been fitted.

The cylinder heads have been skimmed a number of times, giving a compression pressure of 130 psi.

Within 15 to 20 minutes of driving the temperature is right up to boiling point and the amount of over flow is less than that required to top up the header tank when the engine is allowed to cool down.

Does anyone have any suggestions?

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Re: 250 saloon engine overheating

Post by fredeuce » Mon Feb 18, 2019 8:58 am

Put your thermostat back in. Running without can be a trap water can circulate to quickly and not properly cool.

Before you insert the thermostat ensure you bench test it to establish that it actually works in accordance with the stated opening temperature.

Do all of that then review your engine temperature on road.
Last edited by fredeuce on Mon Feb 18, 2019 11:02 am, edited 1 time in total.

Christopher Storey
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Re: 250 saloon engine overheating

Post by Christopher Storey » Mon Feb 18, 2019 10:21 am

I could not agree more with Fred. Removing the thermostat completely upsets the dynamics of coolant flow within the engine and the radiator and is probably the cause of your problem . I should also try measuring the temperature at various points in the system using an infra red gauge

Ian Slade
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Re: 250 saloon engine overheating

Post by Ian Slade » Mon Feb 18, 2019 11:37 am

The thermostat is not the problem, it is there to regulate the engine temperature to a given range and will not cause overheating if missing. Despite the engine being rebuilt head gasket failure could still be a problem as could sludge in the block, there are various chemicals that can be introduced to the coolant to scour the block as will neat anti freeze clear the block over time. Have you tried turning the heater on and off to see if the temperature alters.
As the heads have been skimmed, has a modified gasket been fitted, depending on the amount of skimming the waterways may not line up with the block.
Owner since the 70's, Genghis is slightly to my left.

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Re: 250 saloon engine overheating

Post by DickSpeedy » Mon Feb 18, 2019 3:01 pm

Thanks guys for your input. Where would I get special gaskets from, especially as I am not aware of how much has been skimmed off the heads?

I did have a brand new thermostat in place but removed it after the motor overheated more quickly.

The motor was run on a test bed for two hours before I put it in the car and had no overheating problems with it.

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Vortex O'Plinth
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Re: 250 saloon engine overheating

Post by Vortex O'Plinth » Mon Feb 18, 2019 3:46 pm

As Ian says, running without a thermostat cannot in itself cause overheating - it will usually mean the engine runs too cool. Likeliest causes of overheating in an engine that has previously run without problem is either an airlock or a failed water pump. When refilling the cooling system did you ensure that the car heater control was set to maximum before adding fluid? If not this could result in air trapped in the system. Even removing or replacing a thermostat can introduce air.

Suggest you remove the radiator cap and check the fluid level. Leaving the cap off and setting the heater control to maximum, start the engine. Let engine idle until normal running temperature is reached. You may now see some bubbling of coolant from the radiator filler neck. Unless this is severe (in which case you may have a failed head gasket) continue running the engine at idle for a few minutes then switch off. Let it cool down and check the coolant level and if it's dropped, you've expelled some air. Refill and repeat the process until the fluid level is constant, Replace the pressure cap and check that the engine now runs at normal temp or, since you have no thermostat, runs cool.
Last edited by Vortex O'Plinth on Mon Feb 18, 2019 3:47 pm, edited 1 time in total.

Pas d'elle yeux Rhône que nous.

Warsash 2
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Re: 250 saloon engine overheating

Post by Warsash 2 » Mon Feb 18, 2019 3:46 pm

A bit off a diversion but I am having Margaret's engine rebuilt. At the moment we have a compression os between 160 & 170 psi. This seems a bit high and I am wondering if the thicker head gaskets would be advisable? There is a firm in Christchurch advertising them. Has anyone used them and what pressure should I expect?


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Location: Thundersley, Essex

Re: 250 saloon engine overheating

Post by johnwager » Mon Feb 18, 2019 6:14 pm

Is the engine actually overheating or is it just showing hot on the temp gauge?
Maybe a check with a hand held laser temp gauge will confirm either way.
Could just be a faulty gauge or sender.

Christopher Storey
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Location: Cheshire

Re: 250 saloon engine overheating

Post by Christopher Storey » Mon Feb 18, 2019 10:45 pm

I am sorry to disagree with both Vortex and Ian Slade, but removal of the thermostat on all Jaguar Group cars is a recipe for overheating, because all the systems on Jaguar/Daimler saloon cars ( the dart may be different ) use the thermostat as a restrictor for both the main and the bypass systems in the cooling system, and the net effect of removal is, as was originally said, that the "dwell " time of the already heated coolant in the radiator is far too short for proper heat transfer. This is why I suggested that you check the temps at various parts of the system, most notably the difference between the top hose inlet to the radiator, and the bottom hose return to the engine. If the inlet at top is at , say, 180F, the return to the block should be at most at about 140F and preferably rather less than that. Any smaller difference will inevitably lead to overheating

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Re: 250 saloon engine overheating

Post by DickSpeedy » Tue Feb 19, 2019 12:30 pm

I did not 'bleed' the heater unit, so I that could be a problem which I will look into.
The water is definitely at or very close to boiling point as you can hear it next to the radiator cap. Also, after the water expands into the overflow bottle and cools down it does not fill the radiator leaving an air gap similar to when operating without an overflow bottle.
The water gauge might not be accurate as the moment it goes over 70 degs C she boils, but I am aware of this.
I'm beginning think that fault might be in the cylinder heads, but as my compression pressure is at 130 psi it might be a gasket, although I have replaced them twice since I rebuilt the engine for varying reasons.

I'll keep you posted over the next week and let you know progress. She's a beautiful car, but sometimes I could dump her!

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