Bad radiator cap causing overheat when idling?
 

I had a guy tell me that a bad radiator cap can cause overheating when idling. I was thinking running too lean or thermostat (even though mine is only a month old??). It's an 85 GSL and just curious if anyone has heard or experienced this. Thanks.

the cap wont cause overheating, but it can do something that looks at lot like it. the cooling system is designed to run under pressure, which raises the boiling point, if the cap can't hold pressure, the water/antifreeze will boil....

so this looks like over heating, even though its just a bad cap.

also make sure the cap, and the sealing surface on the radiator are clean, i've seen specs of crud on the cap cause "overheating" too.

It stays cool when driving but overheats quickly when idling. The top hose swelled too. Never let it get hot enough to hear boiling.

looked into the thermostat?

Check your rad. fan
 

Driving (forward motion) forces air through the rad. If the car is at rest the rad. fan pulles air through the rad to cool it. If the car stays cool driving but over heats stopped, the fan is not pulling enougf air through the rad. to cool it. CHECK the fan.

As pointed out, its the fan not pulling enough air. If its the stock clutch fan then its
probably not engaging when hot. Go electric or replace the clutch fan.

Ding! Ding!! Winner. May be the fan CLUTCH, not the fan itself. If it spins freely with a push of the hand, it's bad.

Will check it tonight. Thanks! Why would the top hose swell then?

age or exposure to oils (inside or out)

It could be a weak hose but I think it is because the engine is hot and creating over pressure in the system quite possibly both :)

Fan clutch was fine. Might be coolant seals? Gonna flush the radiator and see what happens. Thanks for the replies!!!

The cooling system should never over-pressurize to the point that hoses swell up due to pressure. The only time you will see this happen is if the rad cap is bad and not opening at the pressure it should, or the hoses have been contaminated with oil. The maximum pressure the cooling system should see is about 15psi if the cap is working right, never more.

What condition is the radiator in? I would guess it's okay since the car runs cool when driving. The problem only occurs at idle, so its either the fan or a loose belt on the pump. Do you have the fan shroud on?

Also, the stock clutch fan should spin freely when the engine is cold, but "lock up" when hot. :)



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If everything checks out ok then im afraid you have blown a coolant seal. This same exact thing happend to me then one day i got green fluid comming out the muffler. If you take the radcap off and have some one try to start the car and antifreezes starts backing out the radiator you have blown a seal.

Bad coolant seals are easy:

https://www.rx7club.com/1st-gen-archive-71/engine-coolant-seal-repair-maybe-750774/ :)




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Thanks folks! Took the car to Longturbo to get a rebuild. The motor has 218000 on it and I decided to go ahead and bite the bullet. I work hard this time of year on farm and don't have a lot of time. I plan on keeping this car for a long time!!!