Interesting Temperature Symptom

[Tony Hunt]

Gentlemen,
Since fitting a long fifth gear to my Renault 21T gearbox during the winter, I have been investigating my car's performance.
Generally, top gear acceleration is still pretty good, motorway driving is more comfortable because rpm has dropped by about 500 at 70mph, but overrun, because of reduced engine braking, is greater and caught me out slightly until I got used to it.
But that's not what I want to air.
At 'low' speeds in top gear (<80mph) the water temperature is rock solid at 85C. The car doesn't overheat when stationary if I switch the fans on, so I'm pretty satisfied with the cooling system.
However, at higher speeds (>100mph) the water temperature cycles continuously, working slowly up to about 95C then dropping quickly to about 70C, before slowly increasing again. Drop the speed and the temperature immediately settles down again.
Engine rpm is typically 3500 when this happens, which is less than it was before, obviously.
Any ideas, or have I just spotted something that has been going on for the last 2 years without my noticing?

 

[40 Bud]

It sure sounds like cavitation to me. Does your pump have a plate on the backside of the impeller or is it an open impeller?

 

[Stew Stau]

The other possibility is a sticky thermostat. I had the same problem in the Crossflow in my Striker (Lotus 7). Was fine at lower power settings but when I started pushing it it started to give trouble. Don't know why, but when I replaced it the problem went away.

As I had a air bleed above the housing, I can be pretty sure it wasn't air in the system.

Stew

 

[Paul Brameld]

Hi Tony,
I have had similar issues with cooling and had come to the conclusion that it may well be the thermostat.Mine like yours has been in 11 years and without constant use could well be stuck.
I am having mine replaced in the next few weeks and will advise result.
Cheers
Paul /ubbthreads/images/graemlins/driving.gif

 

[Howard Jones]

I had cooling issues also, I believe that the thermostat housing on GTD,s is part of the problem. I think that the way the pipe is attached to the mounting plate of the thremoatat housing causes air bubbles to be formed when there is high flow conditions because of the very sharp 90 degree pipe with no radiused turns. This is made even worse with the water going through the thermostat right where the turbulence is worse.

I took my thermostat out and put in a aluminum disk with a 5/8" hole in it to replace the thermostat. The car does take a little longer to heat up but once it reaches about 170F is seams to remain there instead of going up and down with engine load. The old thermoststat was a 160F and it never ran that cold. Most of the time it was above 185-195F. Much more stable now.

Give this one a try. It's cheap and easy.

I also added a bleed line to the radiator and to the top of the intake manifold where the water temp sensor goes. If you haven't added a self bleed system you should.

 

[Ron Earp]

Whoa, can you resize that Avatar? Hmm, need to look into that and see if this can automatically be handled. I know the limit is 80x80 now, but if they were not that earlier I am not sure what the system will do.

R

 

[Howard Jones]

Ron, I was wondering about that. The orginal forum software did this (sizing) automaticly. I'll see if I can figure out how to make it smaller when I get home tonight. I didn't like that picture anyway. I'll go with none for now.

 

[Tony Hunt]

In response to a query from Ian Anderson, here is my rationalisation of what is going on:

I haven't done anything to the car and have concluded that the Docking radiator is more efficient than the old GTD one, so at high speed, it is trying to over-cool the water.
As the temperature drops, the thermostat closes, so the temperature rises, the thermostat opens and the, by now, much cooler radiator water causes the temperature to drop suddenly.
The cycle repeats itself continuously. At lower speeds, the cooling airflow is less, so the cycling isn't so pronounced. It's still there, though.
I am running an MGB thermostat, available from Halfords as Quinton Hazell QTH100K (82C).
Don't go for the high temperature ones that Real Steel sell - they make the engine overheat!
Tony

 

[Iain]

Hi Tony,

Page nine on my build site has the single line diogram of our cooling system. Now we may well have gone over the top but the system sure works well. My previous race car had a detuned version of the same system which also worked seamlessly.

I think the single most important factor is to re plumb the system with a remote thermostat on the "wrong side" and then provide a trigger feed to allow circulation when the thermostat is closed, the same trigger feed also continuosly douses the thermostat with engine temp water which smooths out the open / shut cycle, dumping cold water into the two front cylanders can play havoc with the feul mixtures with all the associated problems.

(How is sunny High Wycombe ? I come from Great Missenden )

Iain