'94 850 GLT Boiling coolant

[fle]

hi again,
so I received the new cap, and this did not solve the problem.

Today, while further debugging this issue, I noticed the following :
the car was iddling since15+ mins, and the coolant overflew. The fan kicked in, and I could see the coolant level drop rapidly (10 sec) to a normal level. The fan would stop, and within 2 mins, the coolant would overflow. Fan again, coolant level goes back to normal, and so on.
I stopped the car, but no overflow, as it did once.
The car did not overhead at any time (I mean, the temp needle was always at 3 o'clock).

I wonder if the problem could simply be that the fan is starting at a temperature too high ?
like the temperature gauge would be wrong ? is this possible ?
thank you for your help.

[rspi]

Connect an OBD2 Tool to check live data. Fan should kick on at 216° and back off at 206°.

[timmybdaddyof3]

another sure fire way to confirm whether you are looking a leak or a bad component (because you can have one without the other), go rent/borrow a radiator pressure tester. wait until the system is completely cold and run up the pressure to with 1~2psi of the cap spec and let it sit for 20-30 mins....If you got an actual leak you will get pressure bleed off (and it will be easier to find because everything will be cold; if it holds pressure you are looking at system components.

[phils94850]

Also.. your fan is a 2 speed fan, Hi and Low considering on engine temp, etc. Are you sure its working on both speeds ? If you follow the wiring back from your fan motor it should go to a relay, there you will find a couple of large gauge wires, you can ground the wires one at a time and it should give you two different speeds the fan motor runs

[fle]

hi,

at last I replaced the expansion tank with a new one. We'll see if it solves the problem.
I 'll keep you informed.

just for the record, it is possible to test both fan speed using the diagnostic mode A6. I put the plug in the A6 socket, push 3 times (?) the button, and the inboard computer switch the fan, on both the speeds.