Malcolm Stebbins wrote:

> John Rodgers :  we seem to agree, except for what
> happens under NORMAL operating conditions - IMHO the
> expansion (overflow) tank is inactive under NORMAL
> operating conditions and is only used when the system
> begins to over-heat and the cap "lets go" above its
> specified setting.
>
> I have read today's posts on this topic.  What I meant to
> say was that  AFAIK, the Coolant Reservoir cap pressure
> relief valve is set higher! than the "normal" operating temp
> of the engine - when warmed up.  My experience is that I
> can drive around all day (even with external temps at 44
> deg. C) and the coolant reservoir cap will NOT allow the
> coolant to flow into the Overflow tank - as long as the van
> keeps moving!  And (just for the record) when the engine
> cools down it is the pressure that drops and no coolant is
> sucked from my van's overflow tank - under NORMAL
> operations.
>
> When going up a looooong hill (in 3rd gear), or stuck in
> traffic, on a HOT day then YES, the fan comes on and if
> that does not cool things down, then the coolant reservoir
> cap "lets go" and some of the pressure is relived from the
> system and, upon cool down, then, yes, coolant is
> sucked back into the Coolant Reservoir tank from the
> overflow tank via the coolant res. cap.  This "sucking
> back" may happen while I drive - as the engine temp
> returns to "normal".
>
> So I hope that we agree.  Malcolm

For some of us being stuck in traffic on a hot day *is* "normal
conditions".

Larry A.
Chicago