so we fixed the symptom, but the cause still remains.

temp stays cool only with AC on.
when shut off, fan comes on, but coolant boiling over in overfill tank.

new radiator
radiator cap
thermostat
water pump and timing belt were "fine', not replaced

my mechanic is replacing the new thermostat with a cooler one (170degrees)
i havent heard back from him yet.

what do the experts think?

 

Does it overheat when driving?

If it's only when stopped, check the fan switch (2-wire switch) in thermostat housing?

 

do both fans come on at the same time with a/c on or when engine reaches normal temperature,, make sure that both fans come on .
if fans do NOT come on,, chk the switch at t-stat housing like Jim suggested,, unplug the connector and jump the 2 wires together at the connector, if fans come on most likely you got a bad switch

 

yes. it overheats when driving unless i have the AC on.

at operating temp, one fan should be on all the time?
then with the air on, both fans should be on?

 

sorry if i appear dense, but could you explain the fan operations:
driving without AC = 1 fan?
driving with AC = both fans?

jump fan switch = both fans should come on?
and if they do the switch is faulty?

 

If your 91 is like my 2002, the fans won't "always" be on - except with the A/C. The fans should cycle on and off when the A/C is off as the engine warms up.

driving without AC = fans will not always be on - they should cycle (both on, both off)
driving with AC = fans will both be on when the compressor engages

jump fan switch - both fans will be on, if they don't then you have a faulty switch.

 

I think it SHOULD work like this...

Driving with AC turned on:
Both fans run when AC compressor engages (which is not continuously). When parked & idling, you can see the compressor engage by watching the center hub of the compressor pulley.

Driving with AC turned off:
Fans generally don't run because driving at any speed at all pushes plenty of air thru the radiator.

Stopped in traffic with AC turned off:
When the temperature climbs (maybe a bit higher than 1/2-scale) then both fans run until the temperature goes back down.

Turn off the engine:
One fan will run if the coolant temperature in the upper radiator hose is too hot. In summer that seems to be quite often. Fan timer will prevent it from continuing to run beyond 15 minutes or something like that.

The ability for the fan to run after the engine is turned off was eliminated from the control circuitry beginning in 1998.

Now, if the fan doesn't turn on when the temperature gets real hot, you can unplug that fan switch and jumper the wire. Key on, that should make both fans run. If it DOES, then the whole circuit (EXCEPT that switch) is OK. That should leave a faulty switch by process of elimination.

 

even with a/c on both fan will still cycle on and off ,,

 

Thank you for correcting what I had written. I will edit my original post as well, as as not to spread incorrect information.

 

thank you both for the excellent information.
after over $1000 invested in this car i am hoping we can finally get it going!

one (hopefully) final point to clear up:
when i jump the fan switch:
both fans on = switch is good or bad?
(again i apologize for my obtuseness)