NCdrift

ok well when i drive the engine/coolant temp stays fine but if i get stuck in traffic or held up at a light for a while the gauge will go up fast. but will go down when moving once again. it wont get to the hot part but the little line right before it. so i stop at the mall and i pop the hood ( which the way it opens is a great feature on FB33's ) and the overflow cap is open and its boiling like all hell has broke loose. i let it cool down and such put it back on and drove home. got home and it did it again. any idea at all what could be causing this????

Jeezus

iTrader: (4)

Check for leaks if you are not running hot. The water/coolant shouldnt boil when under pressure and I am thinking somewhere there is a leak. My guess would be the Rad cap.

Rx-7Doctor

iTrader: (13)

Need to inspect the operation of the fan clutch. When the engine is at operating temp there should be resistance when you spin the fan.

Also might want to do as a precaution, Replace the radiator cap, t-stat, and put new coolant in.

ChaosAura

Plugged rad.

I work at a radiator shop, and thats the most common problem when people overheat when idling in traffic, but not when moving. The rad flows enough that when moving and air flows over it cooling it off, it will keep the engine cool, but it doesn't cool the coolant enough when you're sitting in traffic.

Rx-7Doctor

iTrader: (13)

You might wYou might want to rethink your theory.

If the fan clutch is not engaging, there is no Air at idle, which makes the temp go up.

After you get going the temp goes down because ram air takes over and starts cooling. Not because of a clogged radiator. A clogged radiator would have the symptons at higher speeds because of the demand for coolant to circulate thru the block to carry away the heat.

Jeezus

iTrader: (4)

Wouldn't the water pump pump out more or harder as RPMs rise? Maybe he is getting a small clog, like calm before the storm haha.

ChaosAura

It may be the fan, yes. But a clogged rad will still flow the coolant through it quick enough to keep the engine cool, as long as the coolant that is in the rad (smaller amount all the time) gets cooled off enough by air passing through, along with the air passing over the engine itself cooling that off.

Rx-7Doctor

iTrader: (13)

It does not matter how much the water pump puts out when a radiator is clogged or partially clogged. Only so much can flow thru the radiator at that point. Under load or higher rpm's the engine needs more coolant circulating thru the block to cool it down. If the engine cools down as he is driving , the radiator is not the issue.

Jeezus

iTrader: (4)

Ah cool (pardon the pun). I love learning things

steve84GS TII

Being centrifugal,the waterpump is limted in its ability to move water,although RPMs will affect its output somewhat.Its not like a positive displacement pump that moves "X" amount more fluid for every RPM gained.

Although a plugged radiator is certainly common and will affect cooling ability in all conditions,I agree with Doc.
When sitting idle,the cooling system is having to remove only a minimum of heat from the engine.However,its totally dependant on the cooling fan for airflow through the radiator to transfer the heat.If the clutch fan is bad,youll overheat despite the lack of load on the engine and cooling system.
When at speed,the cooling system is working harder,since the engine is under a load and consuming more fuel.But,the cooling system is able to cope since the RPMs of the water pump are increased a bit and the ram effect of forward motion is feedng plenty of air into the radiator.If you can cool off when moving,then you likely dont have a plugged radiator or bad pump/belt.Failed fan clutches are plenty common and will be wet,greasy and typically free-wheeling when they go bad.

Rogue_Wulff

I'm with Doc and Steve on this one. Fan clutchs are prone to failure after more than 20 years of service.
A plugged raditor will usually cause hotter temps when moving, than at idle. Of course, there are exceptions to almost any rule.

ChaosAura

I'm not trying to start bad blood or anything here, but I've seen it a few times since I started working at a radiator shop. Car is fine when moving, but over heats while sitting in traffic. Every time it has been the radiator.

I just started working here, and I know that there is much I have to learn, but by the information provided here I would give an educated guess as to it being the radiator.

Rogue_Wulff

Newer cars, with electric fans, sure, it's very likely the radiator if the fan is actually working.
Older cars, with a clutch fan, the clutch is usually the more likely suspect. Of course, these older cars also had radiators with a much larger coolant capacity than the newer ones have. This gives more area for crud to build up in.

Rx-7Doctor

iTrader: (13)

We don't have a problem with new ideas or input. When you use the term "Educated guess", only that comes with time.

Something that you may not realize is that you are on a forum that deals with a "Specific" type of Automobile and especially a very specific "Type" of engine.
Alot of us have been around these engines and cars for anywhere from 10-30 years. We generally know the characterisitics of these engines.

NCdrift

it is nice to learn some new stuff. thanks guys :]

Rx-7Doctor

iTrader: (13)

Anytime.

Only thing we ask is that when you get it figured out that you post up what fixed her.

Remember to test the fan clutch first.

NCdrift

i will do :]

NCdrift

Ok..
Well i replaced the radiator cap which the seal on it was dry rotted. flushed the coolant and got a new cap and new coolant. took it around town and down I-85 a good way. and let it run in the drive way a while when i got home. And it didnt heat up alot and the overflow bottle didnt pop open and boil. so i think the cap may of been sucking in air and building pressure in the overflow, and when it would come open i would loose coolant fast. Its been running fine all day with no issues. thanks for your help

Rogue_Wulff

A bad cap will not hold pressure, thereby allowing the coolant level to drop. Also, pressure is needed to prevent the coolant from boiling. Higher pressure = higher boiling point.