Detroit 60 Thermostats??????

They should be temperature matched, and open/close basically together

 

There two thermostats, both are the same. Same coolant passages. They just use two for increased flow.

 

They should open at the same time, or pretty darn close anyway. They work together in the same port, there are two only for extra flow. T-stats don't usually cause erratic temp swings though, they most often fail open, so you have poor heat and it runs too cool.

Erratic temp swings could be a few things, slipping belts or fan clutch. Gauge, sending unit or wiring. I have seen some older S60's have the bolt that holds the impeller on the water pump come loose and it spins on the shaft. You can check that without pulling the pump by pulling the back cover of the pump. Before you do that, get a new back cover kit (cover, seal and snap ring). 99% chance that you damage all 3 getting it apart, so have the parts on hand. Then if you find that the bolt is loose or fell out, return the cover kit and get a water pump. Detroit makes the cover kit because those plastic covers sometime warp and leak.

If the temp swings really fast, I'd suspect the sending unit or a wiring issue. If it more gradual when you change RPM, I'd suspect something slipping, like the clutch, belts or water pump. The best way to know for sure is to hook it up to DDDL and compare engine temp to gauge temp, but that's not always a the cost effective option if you have to pay a shop to do it.

 

Do you a bottle test on your overflow system. Run you a piece of tubing from your overflow fitting to a clear bottle, hang it on your drivers side mirror and see what happens when boosted up under a pull. If your not familiar with the bottle test, use something like a windex bottle and fill it about 3/4 full of water. Put your hose down to bottom and watch for excessive bubbles when your temp goes to spiking. Be sure to clamp your tube to over flow spout and secure the tube where it won't come out of the bottle. Of course route the tube where it won't interefere with anything such as fan etc. and route it under the hood where it won't get pinched.

 

this is a wire connection problem, I.M.O. check the connection at the sensor in the thermostat housing.