V8 Thermostat

[acaie]

I'm still working on the cooling system, started by the poor performance of the heater out in the cold weather.

 

I put in a new 82C thermostat - was a little surprised to find the old one was 74C. I found my way down the well trodden airlock path and out the other side, with the help of putting in a top hose bleed valve and drilling the thermostat.

 

I'm now finding the temperature gauge reading when the thermostat is well open (after 5 mins on the M4 at 70) to be at 3/4 and a bit. I checked the thermostat on the kitchen stove before installing - starts to open at 82C, fully open about 87C. I've changed the temperature transmitter, because its easy and cheap, and it makes no difference.

 

There is no evidence of a major boiling problem. The temperature gauge rises steadily, and is steady after the thermostat opens, going up and down about 1/16 of full scale depending on usage. Little coolant appears an ad hoc catch bottle on the expansion tank overflow, which would get a fair bit if there were a major problem.

 

So, you guys and gals who report a steady half way temperature gauge reading, what thermostat do you have?

 

 

 

 

 

 

[Alec Pringle]

74C, which gives a steady halfway reading, whereas 82C gave 3/4 on the gauge and led to blown head gaskets . . . . .

 

Cheers

 

Alec

[acaie]

What I don't understand is why 74c is necessary for 7v8 whereas 82c is preferred for Sd1v8, and later Sd1 use 88c.

What actual coolant temperatures do the 1/2 and 3/4 scale readings correspond to?

Thanks for your thoughts

Al

[Guest Wayne S]

82c is the standard factory spec for a carb TR8 too as far as I was aware and its certainly the thermostat I have in mine. There is an 88c one marketed as for use in winter / cold weather environments and Rimmers at least always used to sell the 74c one as a "summer upgrade."

 

My car has always run happily with an 82c thermostat, just over 1/4 on motorways then dead on 1/2 in traffic and city driving with my electric fan kicking in if it ever nudges over the 1/2 mark.

 

If I ever have to drain the system though it is a right game to refill without airlocks usually involving running the engine with the cap off and frantically pumping the top hose by hand to "burp" all the air out of the header tank that I try and elevate by jacking the car up. Its messy and looks like you're giving CPR to the thing but works in the end!

[stansoldman]

I am running a 82c with a efi unit. mine is exactly like waynes. 1/4 on the run but does rise to 1/2 in traffic. Once again the kenlowes do kick in once in a while.

[acaie]

This is really intriguing.

 

 

The mid point of the gauge on a 2 litre TR7 and a carb V8 SD1 is about 90C for both. Both run with an 82C thermostat, and in both the temperature is about mid point in everyday running in reasonably cool weather.

An 82C thermostat is normally fully open by about 87C. It’s partly open in everyday running, so probably about 85C, so a temperature of 90C at the measuring point measuring point is no more than about 5C hotter than the temperature at the thermostat.

So, as Alec reports and as I (and I’m sure lots of others) have found out, when we have the standard carb V8 engine and 82C thermostat in a TR7, it runs shows about 100C instead of 90C on the temperature gauge. Interpreting this another way, instead of the temperature gauge measuring point being no more than 5C hotter than the temperature at the thermostat it is about 15C hotter. So the temperature difference between the two positions is three or more times as much - a very large factor.

So what can be going on? Something has to be different. The insides of the engine, head and manifold and their cooling passages are the same; the water pump’s the same, the thermostat’s the same; the bypass return from the front of the manifold direct to the pump is the same.

So logic says it has to be something in the other cooling circuits outside the engine is markedly affecting the coolant flows inside the engine and head. There’s the circuit through the radiator itself; the circuit through the vent line from the top of the manifold to the radiator or expansion tank; and the bypass return from the rear of the manifold to the pump via the heater.

There’s an interesting post about an MGB V8 conversion,

http://www.v8register.net/FilesV8WN/V8NOTE374%20V8%20engine%20cooling%2029.1.08.pdf

which really brings out the importance of the bypass return through the heater.

A question for those who successfully run an 82C thermostat is whether have you done anything which might have affected the bypass return flow through the heater?

Or do I need to think about whether it is something to do with the bypass flow through the vent line from a carb manifold to the header tank/radiator?

Thanks for all your input

Al

[Guest Wayne S]

Al,

Are you absolutely sure you're radiator is functioning at 100%? The only time I can recall having problems similar to this were either airlocks OR a blocked radiator. I flushed it and did all sorts but the temp kept creeping to 3/4. Often it would cope well in traffic but get hot when bombing down the motorway! In the end a re-core was the only option and sure enough this solved the problem.

 

Something else that may be of interest, I use a modern VW group otter switch in the stainless joiner pipe (top hose) which is rated at 90c - I tested this before it was fitted. It switches the fan on as soon as the temp nudges over 1/2 suggesting the coolant in the top hose at that point is 90c.

 

Also, very occasionally you see V8 conversions in TR7V8's that are still running TR7 radiators which just don't have sufficient capacity or surface area.

 

Poor heater performance is a clue as it might suggest that there is a lack of flow or pressure round the system. You should find the heater in a TR7V8 car has 2 settings, "OFF" and "BLAST FURNACE" !!

 

Sorry I can't be of more help!

Edited March 17, 2013 by Wayne S

[acaie]

Hi Wayne

 

All this info is definitely a help.

 

I'd not been aware that the heater had blast furnace potential. Is bleeding the heater itself something I should try - maybe running the engine for a while with the front up on ramps so the air runs forward into the manifold and/or water pump, and then bleeding the system through the top hose - or is it better to undo one of the heater hoses?

 

I'm far from sure the radiator is OK. What I'm coming round to is next to give it a good dose of radflush. I's not done this since the receipts from the PO show it was filled with 4 (litres?pints?) of antifreeze in feb 2012, shortly before I bought the car. I'd assumed it would have had a good flushing at the time, but maybe not.

 

Best wishes

Al

[Guest Wayne S]

You can back flush the heater by removing both hoses from the copper pipes that enter the bulkhead, clipping on a decent tap pressure water hose on the opposite direction of flow and giving it a good blast through. I would be surprised if its the source of your over heating problems - more a symptom of poor flow and pressure I would guess. Removing hoses with the engine running I would suggest will only induce more airlocks into the system, best way is to elevate that header tank, run without the cap and burp the top hose until it stops belching at you if you pardon the expression. Refill, put the cap back on and test - repeat until running with no air locks.

 

Definitely worth checking all those other components to make sure you've not been chasing a red herring. Have a feel for cold spots in the radiator while its running with the thermostat open if some areas are colder to touch than others then it gives you a clue towards a blockage or poor flow. Also worth seeing if you can hear any rumbling rattling noises from the water pump as well while your there.

 

Just a brain dump of ideas for you really!

Edited March 18, 2013 by Wayne S

[acaie]

Many thanks, Wayne

Al

[TR8IAN]

Just changed the thermostat in my 8 today for an 88deg version - ran it for a while to get the air out and then went for 25mile blast up and down the A12.

 

Now my car has an engine that was rebuilt two or three years ago as well as the rad and the water pump - I run a standard US8 carb set up with water fed chokes, these vent back into the expansion tank and take a feed directly off the top of the thermostat housing so burping the air out is generally no problem.

 

Having done the run and left the car running on my driveway I had a chance to have a play with my new infrared thermometer. The thermostat still hadn't opened and I had a reading of about 45deg on the rad and about 55 on the themostat housing - the expansion tank which is mounted over the top of the alternator was showing 32deg. The temperature gauge only just got up to the first mark.

 

This change has made an improvement to the cabin cabin temperature but I think I'm going to have to take another look at the heater rad to make sure we have full flow through that.

 

Acaie - have you got all of the correct boards around the rad, so that the air flow is directed through it and not around it?

 

Cheers

Ian

[BlueTR3A-5EKT]

Filling The V8 Cooling System

 

The MGRV8 cooling system was filled on the build track at Rover from a coolant barrel 6 foot off the ground, via a tube fitted to a connector in the tapping at the highest point by the manifold.heater tubing. We used a similar system when we built a new MGRV8 at The Essen Classic Car show in the late 90's.

 

Peter W

[TR8IAN]

It shouldn't be too difficult to tap a hole in the top of the thermostat housing and add some sort of valve or the like to be able to do as Peter (above) says and run a remote water tank for filling and "winding" of the system.

 

Cheers

Ian

[acaie]

Thanks for all the info and suggestions. I'm hoping to get back on the case later this week.

Best wishes

Al

PS Howards day at Castle Combe was very good and very very very cold.

[acaie]

So I eventually remembered that I had changed the temperature sensor - to a new one from Rimmers for TR7 and carb rover v8. Put the old one back, and the gauge now makes sense, and I am no longer fearful about taking it on a run. When I roll along the m4 at 70 or so, the temperature gauge is rock steady just above 1/4, with the heater working fine. Very much the behaviour I would expect with a 74C thermostat.

 

I still think the cooling's not quite right, because when I potter gently round the Wiltshire villages the gauge rises to 1/2 or a touch above. Also the heater doesn't work so well. Unless the valuable register experience tells me this is normal, I intend to explore some more in the next week or so. Current suspect is a flow problem, so I will try a dose of some sort of rad flush next, before I start to thinking about taking off the water pump.

 

Thanks to all for your advice, experience and encouragement.

Best wishes

Al