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Andy Field

Over Pressurised Cooling System

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Hi,

 

After about a mile of driving my 1962 TR4, the coolant hose from the engine block to the radiator was blowing off and subsequently all the coolant was dumped. I did notice that the pressure cap was rated at 7lbs. I changed it for one rated for 4lb, as per the cars specification, and the hose no longer blew off. However the 7lb rated cap has been on the car since I purchased it in 2008 and I have never had this problem before. Also the 4lb cap is lifting and dumping coolant via the overflow pipe. I have replaced the thermostat and carried out a compression test - all four cylinders have identical readings. The car does not overheat and there does not appear to be signs of oil in the coolant. After the latest long run I removed the spark plugs and noticed what appears to be rust deposits covering the plug ends which you could wipe off with your finger. There was previously, the classic 'mayonnaise' on the inside of the rocker cover but I put that down to the short runs I had while the hose was blowing off. There were no signs of the 'mayonnaise'

after the latest long run.

 

Any pointers to where the problem may lay would be very much appreciated.

 

Thanks

 

Andy

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Hi,

 

4 lbs is for TR3 and does not fit a TR4. It will leak coolant all the time. What you need is a 7 to 10 lbs cap (varies according to manuals). Start with 7 lbs.

 

Badfrog

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Hi Badfrog,

 

Thanks for the response. Unfortunately with the 7lb pressure cap fitted, the coolant hose blows off. I decided to fit a 4lb pressure cap which is the rating specified in my workshop manual (which seems to contradict what you have stated). This change stopped the coolant hose blowing off. But as I have run the car for 5 years with a 7lb pressure cap with no problems there must be an underlying reason why the cooling system is being over pressurised.

 

Regards

 

Andy

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Perhaps you need to replace or tighten the hose clip as the hose should not 'blow off' when subjected to 7 psi.

Ian Cornish

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Hi Andy,

could the old 7lb cap have seized up!! However if it did what is causing the pressure to build up in the first place.

I think your head gasket has gone.

Have you run the engine with the rad cap off to see if any bubbles appear.

 

Roger

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Andy,

 

I know where you got this 4 lbs figure. The Triumph workshop manual states 4 lbs indeed, but it's a mistake: they refer to the very early TR4. The change for 7lbs was done during production (don't ask me when exactly, could be for late TR4 andTR4A).Conversely, the values of 7 lbs for TR4 and 4 lbs for TR3 can be found on most other manuals (Bentley, Haynes, ..).

 

Today's consensus:

http://www.moss-europe.co.uk/Shop/ViewProducts.aspx?PlateIndexID=2008

 

The Rimmer catalogue even proposes a 10 lbs cap that I have tried in the past. Works very fine in lowering water temp (temp vs pressure equation). I use a 7 lbs cap now, found in Malvern among other goodies. Ah, Malvern ....

 

Badfrog

Edited by Badfrog

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Andy,

 

I know where you got this 4 lbs figure. The Triumph workshop manual states 4 lbs indeed, but it's a mistake: they refer to the very early TR4. The change for 7lbs was done during production (don't ask me when exactly, could be for late TR4 andTR4A).Conversely, the values of 7 lbs for TR4 and 4 lbs for TR3 can be found on most other manuals (Bentley, Haynes, ..).

 

Today's consensus:

http://www.moss-europe.co.uk/Shop/ViewProducts.aspx?PlateIndexID=2008

 

Works very fine in lowering water temp (temp vs pressure equation).

Badfrog

???????????

 

Raises temp at which the water boils perhaps.( Autoclave addict.)

Is JF referring to that story about putting the amphibians into cold water on the stove so they get cooked feeling happy all the while.

(We don't do that with prawns.)

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Hi all,

 

Badfrog - Thanks for clearing up the 4lb/7lb pressure cap contradiction. At least I now know which is the correct one to fit.

 

Roger - I checked over the 7Ib pressure cap in so much I could press the spring. Perhaps I should purchase another one just to be sure. I have run the engine with the cap removed and could not see any bubbles. There does not appear to be any trace of oil in the coolant either. I have also done a compression test and all four readings were close on identical - not sure whether that confirms that the head gasket is OK.

 

Ian - Prior to this problem, I had renewed the hoses and swopped out my hose clips for T-bar ones. I first thought that they were not gripping enough when the hose first blew off at the point the engine inlet hose joins the vertical rigid pipe from the radiator. I reverted back to the original style wire clips, doubled up to be sure, then the hose blew off where the hose joins the radiator. The clip there is as tight as it can be.

 

As far as replacing the head gasket, there must be many makes on the market. Presume I am OK going with the normal suppliers like Moss & Revington. Also I have read that the wet liners can lift once the head is removed. Is there a recommended procedure to avoid this?

 

Regards

 

Andy

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I assumed the change from 4lb (with overflow pipe)

to 7lb (sealed system) came with the deletion of the

header tank.

Am I wrong?

 

AlanR

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Alan,

 

Logic indeed, but I don't know.

 

LittleJim,

 

I wouldn't know. The mere idea of eating frogs brings me on the verge of porcelaine tango. You know, that's when you hug the bowl close to you head and do bizarre body moves?

Talking prawns, I eat them fresh. They have no eyes, you know, so they can't look at me.

 

Badfrog

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Whilst talking caps and pressure for anyone who does not know it is worth mentioning that 7lbs caps come in different depths, dimension from washer on underside of cap to sealing face on washer at rad neck end. I think most TRs use the 3/4 cap but Moss Cat shows which is for which car/ rad.

My local shop has caps with various depths get one too shallow and it won't give the pressure stamped on the cap!

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I think the main issue with the radiator caps is their length. The 4lb cap for the 3 has a 1inch depth, the 7lb cap for the 4 has a 3/4inch depth so they can't be inter changed. The 7lb won't fit in a 3 radiator but a 4lb 3 cap will fit in a 4 radiator but it will not seal. If your hoses are blowing off you may have put something slippery on them or your hose clamps are no good.

Graham

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And don't fit higher than 7 lb if you have a bellows type thermostat or it won't open properly.

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Before clamping on the Hose rub the inside with some 120 grade Emery Paper do the same to the Radiator Neck make sure both are Clean and Dry before Clamping Up tight with New Clamps.

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Another item to check. Pressure can build up in the radiator if the thermostat is not operating properly. You can check this by removing the thermostat and heating it in a pan of water, and using a thermometer to check the temperature when opens.

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Andy already replaced the thermostat in an attempt at a cure. I suppose he could have two dud ones but it seems unlikely.

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