94 850 Turbo - Coolant Temp Sensor

[bockreiter]

Hi all,
I need a second opinion. Yesterday on the commute home (after I passed a particular rough section of pavement) the temperature gauge suddenly dropped all the way down to nill. The engine then proceeded to surge. A few miles later and a few more bumps from the rough pavement, the needle came back up to the mid range and the engine ran smothly. For the rest of the drive the needle kept droping all the way down and then jumping back to mid range. I used my wife's car today and this evening I started the volvo up from basically a cold engine. Ambient temperatur was in the 70 degree range. The engine started right up but idles rough and fast (almost 1200 rpm). My hunch is the coolant temp sensor is bad or has a bad connection. I based this assumption on the resuming of normal smooth operation when the temerature gauge actually displayed a reading and rough running / surging engine when the gauge fails to display any temperature. However, my car is equipped with the OBD II connector. The only code I could retrieve is P0133 slow sensor. (I believe that's a slow response from the front O2 sensor). Since the car just turned the big 2K, the O2 sensors seem to be due. No code for the coolant temp sensor however. Now my question is this: Before I pull the coolant temp sensor, do I have to drain the coolant or can this be done without significant loss of ccolant? Any hints will be appreciated!

[MadeInJapan]

Could be the sensor but if you're replacing it also replace the thermostat...but before you do this, pull the connector loose from the sensor and spray it with contact cleaner to see if it makes a difference. You can do the swap without much loss of coolant...what you wan to do is drain the over-flow tank and then clamp the large hose to the radiator...put a large old towel down under the thermostat housing when you open it up...you'll get some coolant loss but not much. Also, make sure you have a long torx driver as the fuel line gets in the way and you bugger up the screw...you might want to buy a couple of hardened screw as to put back in from the dealer before you start and spray the ones that are coming out with PB Blaster the night before.
Good luck and let us know what you do and the outcome.

[bockreiter]

The contact cleaner did no help. But it was worth a try! (Thanks MadeInJapan) So I ordered new parts. I got a replacement sensor and thermostat from FCP Groton yesterday. (I was too cheap to pay more than UPS Ground). This morning I pulled the old thermostat and sensor and replaced both. The enginge purrs like a kitten!
The swap was straight forward: To get more room I completely pulled the hose between the thermostat housing and the radiator. This allows easier access to to the temp sensor harness, but is not required. Then I removed the small hose between the overflow tank and the thermostat housing. Now I had a clear shot at the thermostat. One of the torxhead bolts is difficult to get to because of the fuel rail. but I have Torx bits to fit int my socket wrench and with a 3" extension (1/4" drive), access was no problem.
My wife won't let me use her camera with dirty hands... So I don't have the procedure documented in pictures.
After the exchange, I took my trusty Ohm meter and checked the old sensor. I read that the resistance is supposed to go down as the sensor heats up, but I could not get any reading thru the cables (pole to pole). Like there is a dis-connect in the wire.

[MadeInJapan]

Glad you got it sorted!
Another satisfied cutomer!