It is possible that the fan problem is related to the by-passed heater core. I would hold off on the fan problem until the core is fixed.

Of course, if it is inaccessible, that would change my strategy.

You should also inquire about the predicted life of your evaporator. I had to replace the evaporator on my '96 855R when it was 6 years old - now at 17 years it needs another. Eleven years like your car?

The evaporator is probably a dash out job also. Doing both at once might be a good move, as the fat part of the labor is in the dash work.

Post a new question like - How long do S 40 evaporators last? I'll bet you will some good responses. Hopefully Klaus may have something to say.

--
'96 855R,'64 PV544 driver, '67 P1800 basket case, '72 Yamaha Rd400, '68 Honda 350-4, '12 XC70, the first 5 are mine, heh, heh, 525,000 miles put on 10 bricks James A Sousa

Like Al said, replace the heater core and do not use stop leak products. The prices are from less than $100 to over $400. I would go with the least expensive core, plus a couple of good O rings.

Google 'volvo S40 heater core replacement' and there are a couple of things to read and cores for sale.

EDIT: I just read the Google stuff, and you are not going to like it. The dash must be removed! No wonder the PO just clamped the lines! Depending on the leakage, you just might want to try a stop leak!!!

--
My name is Klaus and I am a V♂lv♂holic

I don't like additives.
My opinion? If it's leaking, replace the core with a good one, don't band-aid it.
No idea how to change the core on an S40. Haven't had to...yet.

Heater fan speeding up and down? Not sure if that would be the resistor. I thought a typical symptom for a bad resistor is that you'd get high speed only. Maybe it's whatever controls the climate system.

I guess this hasn't been much help.
--
"Differences of opinions should be tolerated, but not if they're too different' - Sharon Craig