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250 saloon engine overheating

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Re: 250 saloon engine overheating

Post by classiclife » Tue Feb 19, 2019 12:47 pm


Something that is often overlooked is the rad cap.

A small element in its own right, but critical to correct operation of the cooling system.

Do you have the correct cap pressure, is it in serviceable condition and is it the correct length.

This may not be the whole answer, but if incorrect or non-serviceable it will certainly not assist the situation.

Good luck and I'm sure your final sentance is more frustration than anything. I know, I have been there with many a car, but you cannot help but love them !!


1968 Daimler V8-250 Saloon
DLOC East Sussex Area Representative.

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Re: 250 saloon engine overheating

Post by bakergh » Tue Feb 19, 2019 3:44 pm

From the symptoms you describe it sounds like insufficient flow of coolant through the radiator. I know you said the radiator has been rebuilt but has it been tested? A simple test is to disconnect top and bottom hoses, plug the inlet and outlet pipes on the radiator ensuring the bottom plug can be removed instantly. Fill the radiator, remove the bottom plug and see how long the radiator takes to empty. It should empty very quickly.
The reason I am suspicious of the radiator is that you said that the engine ran for two hours on a test rig which I am assuming did not use your current radiator.


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Re: 250 saloon engine overheating

Post by Paulkennedy » Wed Feb 20, 2019 8:41 am

If the water that is expelled into the overflow bottle is not sufficient to replace the water back into the radiator on cooling does that not indicate that there is a lot of air within the engine yet to be got rid of iie an air lock present. I found that by being fairly accurate with measuring what quantity of water that was required that I put into the engine I would know if there was air present preventing a proper fill up of the system.

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Re: 250 saloon engine overheating

Post by Phillmore » Wed Feb 20, 2019 8:57 am

Personally I would drain the system and start again. Open heater and fill VERY slowly. Start the engine and bring up to temperature with the rad cap off squeezing the hoses to help expel any air. Put cap on before level overflows and then allow to cool. Remove cap, top up again and repeat the warm up process a few times until the level when cool has stabilised.

1954 Conquest Mk1, 1956 Conquest Mk2, 1957 Conquest Century Mk2, 1955 Austin A90 Westminster

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Re: 250 saloon engine overheating

Post by lioneleast » Wed Feb 20, 2019 10:53 pm

I suggest you put the standard fan back in. I pointed out the restriction to air flow caused by the grill in the thread about heaters. That is why these cars have a shaped cowl and a close fitting fan. If you can't put the fan back then maybe the electric fan is not up to the job and it is probably only blowing/sucking air through part of the radiator. When the M1 was first opened Mk 2s gained notoriety for being stuck on the hard shoulder with their engines boiling. They had only a simple fan typical of the time.



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Re: 250 saloon engine overheating

Post by Chris_R » Thu Feb 21, 2019 10:29 am

Dick, A continuously running electric fan is a waste of time. Over 30mph the fan contributes nothing. As you have just put the fan on have you made sure it is turning the right way and not trying to blow the air back out through the radiator?

Ian Slade
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Re: 250 saloon engine overheating

Post by Ian Slade » Thu Feb 21, 2019 6:33 pm

Should you want the water to return to the header tank/radiator the cap on the header tank cap should be plain and the overflow tank should have the pressure cap. When testing the engine how was it cooled? I am assuming the radiator from the car wasn't fitted, can you replicate the testing cooling system with the engine in the car or jury rig another radiator of similar dimensions
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Re: 250 saloon engine overheating

Post by simonp » Mon Feb 25, 2019 5:39 pm

I have had several problems with water pumps causing overheating. If you have a spare one I suggest you change it. Mine was leaking air and it happened twice.

If the rad has been dealt with - maybe some crud has moved from the block to the rad - the block itself may have blocked water ways still. They are well known for having a huge amount of silt/casting sand.

Daimler SP 250 - "To feel its eager response as you open up is to know a new motoring adventure"(Sales brochure) The adventure continues!

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Re: 250 saloon engine overheating

Post by tjt77 » Thu Feb 28, 2019 12:07 am

Id agree with others who state that removing the thermostat will have negative consequences. If it has been determined that it is actually overheating (if it is losing coolant via the radiator cap.. and the cap is sealing properly..than its an indication of overheating) here are my suggestions for finding cause :- (1) do a 'leak down' test of the cooling system. This is carried out with a kit that consists of a special radiator cap with and atatchment for compressed air and pressure gauge.. most radiator repair shops have the equipment and can perform the test. this test will tell you if you have issues with head gaskets or 'other' issues allowing coolant leaks . (2) check cylinder block for crud build up by removing the drain plugs which are located just above sump flange to rear of block both sides.. if plugs are removed ( 1/2" socket fits) and no coolant comes have sediment in the cylinder block, which will decrease calling system efficiency.
for symptoms as described and the fact that heads have head work more then once.. Id suggest you issue is most likely cylinder head porosity or gasket problem.. either way..a leak down test will confirm if the system has leakage issues.

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Re: 250 saloon engine overheating

Post by DickSpeedy » Sun Mar 03, 2019 12:45 pm

Hi Guys, I have finally got to the bottom of the over heating problem. After months of having a vibration at the back of the gearbox I discovered it was the the original heavy metal fan that was out of balance.

So I discarded the original fan & replaced it with a constant running electric fan. This is what proved to be ineffective and not pulling enough air through the radiator when stationery.

I had an engineering firm machine me up the original fan housing to fit a large plastic fan that I bought off the shelf. I removed the electric fan & fitted the new plastic fan. That solved the balance and air flow problems. The car runs smoothly and does not over heat. I will refit the thermostat next week.

Thanks for all the help over the past few weeks, the solutions would have taken a lot longer with out it.

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