I have a 78 Z that seems to run hot when running on the highway.I have found it runs at 195* all day in town but as soon as it gets to highway speeds it starts heating up to 215*.It is a big block with th400 trans.I have a HD 3 core radiator and 7 blade cluch fan w/shroud and overflow bottle.I put a new thermostat in today and same thing.Also has a trans cooler out front.Any suggestions?


Thanks,Mark

perhaps your driving your water pump too fast. there comes a point where you are moving the water so fast that it doesnt dissapate enough heat for the short time its in the radiator. so when you are on the highway running higher rpms for longer periods of time its not cooling as good as around town where the pump is running slower. unless this problem just popped up on a car that behaved correctly before, try overdriving the water pump pulley.

i can almost promise its the bottom radiator hose collapsing in on itself at higher RPM...

i can almost promise you your waterpump is fine...

how old are your hoses??

if your bottom hose does NOT have a spring in it... you can make one from a metal clothes hanger and put it in the hose.. it could fix the problem

[This message has been edited by 1981coupe400ci (edited October 09, 2005).]

Sometimes you can rev it up to cruise RPM in park in the driveway and see the hose colapse

It has a spring in it.It also seems to puressure the upper hose almost instantly upon start up before thermostat opens.This was the reason for replacing the thermostat I thought it was sticking open.


Mark

what condition is the fan clutch in? Thermal or non-thermal? Could be that something gets out of whack at highway speed, but pulls air fine at idle? Perhaps the clutch does not disengage when it's supposed ot, and at high engine rpm, the fan speed acts as a wall behind the radiator, preventing air from passing through.
My thermal clutch fan used to do that above 50 mph, and my temp would go up to about 190*. Below 50mph, temp would be 180*. Obviously my situation is not as bad as yours is, being that my fan clutch was in working order. Now that I have an electric fan, I don't see any temp variations at all at whatever speed.

[This message has been edited by engine (edited October 09, 2005).]

sounds to me your cap is bad my small block did the same thing finely changed the cap and fixed the prob it was not releasing enuff pressure or the blades in the water pump are half way ware off cooling only at slow speeds and not flowing enuff at hi speeds had that happen too i would look at the cap first tho make shure you get the right cap (psi) for the motor and rad

What temp. thermostat do you have....i had a 195* in my 406 and it ran about the same as yours....put in a 180* thermo and it barely breaks 190* now.

When was the last time you flushed the rad/block and replaced the antifreeze/water and mixed it to a minimum of 50/50 solution?

------------------
Mark D
1970 Z28 M22 4:10
http://http://ca.geocities.com/mlvd@rogers.com/ (http://ca.geocities.com/mlvd@rogers.com/)
WTB: 70 SS396 Chevelle 4-spd, flapper hood car, no rag tops, 1-2 owner car.

The cluch seems a little on the loose side.It has a new cap also.The coolant is new as this motor has about 200 miles on it.When I changed the thermostat I noticed that there was no coolant in the intake manifold and someone said it may need to be burped.If so how do I accomplish this?There is also a bypass on the manifold directly to the water pump that I was told could be blocked off and it will run cooler is this so?
http://i9.photobucket.com/albums/a92/marksLT/100_9652.jpg
Thanks,Mark

[This message has been edited by marks LT (edited October 10, 2005).]

[This message has been edited by marks LT (edited October 10, 2005).]

I just noticed that the upper rad hose is colapsing when I rev the motor.What will cause this?


Thanks,Mark

<font face="Arial,Verdana" size="2">Originally posted by marks LT:
I just noticed that the upper rad hose is colapsing when I rev the motor.What will cause this?


Thanks,Mark</font>

how old is the upper rad hose? a new one shouldnt collapse.. its a cheap thing to replace.. you can replace it and see if it help