Hi All,

I'm currently considering that my thermostat may not be functioning as I want.

I have read that turbo engines take a 87C/189F thermostat, while NA models take a 92C/198F thermostat.

While my car has the turbo engine, I am pretty sure the last time I replaced the thermostat I bought a 92C/198F because (I reasoned to myself, without reading up on it) that I wanted better heater output in the winter. It seemed reasonable that hotter engine = hotter heater output = warmer me. It doesn't /really/ get that hot in the summer here (max is about 38C/100F for a couple of days every summer but more often we won't see many days above 32C/90F) so I wasn't concerned about that.


Can anyone explain to me the reason why the turbo engines use a colder thermostat? Conversely, why do the NA use a hotter thermostat?

I think I understand the problems with a thermostat that's too cold, but can anyone give testimony as to the repercussions of using a thermostat that's too HOT?

All thoughts are welcomed. I"m looking for a theory lesson here.
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1998 V70 AWD->FWD->AWD Turbo 220k+

There could be a number of reasons for the lower figure.

Both engines share a lot, including - space available - exhaust system - radiator - etc.

So -

More power equals more heat

the engine compartment has more stuff in it

The turbo restricts the exhaust and probably keeps exhaust heat in the engine longer

The intercooler blocks some of the flow through the radiator.

The turbo engine probably has more "hot spots" in it, requiring a better flow of water and heat.

Etc.


The thermostat may not just open when the engine warms up - it may cycle under some conditions - for instance, if the thermostat fails open, the temperature will drop on the highway on a cold day. This indicates to me that full flow may be too much for some conditions

BMW has used a by-pass system with two thermostats that by-passes the radiator under high speed conditions in really cold situations.

Your temperature gauge is not a good way to judge what is going on as far as temperature goes because it has circuitry to compensate the reading so that it stays at normal. If the gauge moves around, the temperature is changing quite a bit.

I do agree with other posters that say that a cold engine probably uses more fuel - and that the small difference in thermostat values does not make much difference in comfort.

I am for using Volvo's recommendation.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 485,000 miles put on 9 bricks

I just had a brainwave about this last evening and I thought I'd run it past you guys to see if I'm on the right track here:

- there are two sides of the thermostat: the hot engine side and the cold radiator side.

- a thermostat rated for a specific temperature is supposed to start opening at that temperature, not snap to the fully opened position.

- the ETC (temp sensor for fuel computer) is on the 'cold' side.

So with this in mind, I think my fuel mileage might go up a bit with the lower temp thermostat (87C for turbo cars, as per Volvo).

My short commute is killing my mileage right now. The fuel computer is dumping fuel in the engine while it's cold, trying to keep it running, and warm it up, and feed the catalytic converter, and lubricate, etc.

If I were to use the lower temp thermostat, the thermostat would open up sooner in my commute, the ETC would 'see' hot coolant sooner as well. The computer will see that the engine is starting to warm and lean the mix out a bit.

Right now the thermostat doesn't start to open up until 90C or 92C (I forget which one I have) which of course will be later in my commute than 87C.

The temp inside the engine should be relatively similar to what it is now, as the thermostat will slowly open after 87C, reaching fully open somewhere around 100C (I guess).

How does this sound to you folks?
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1998 V70 AWD->FWD->AWD Turbo 220k+

Last time I looked, the ETC sensor is below the thermostat, in the hot stuff.

If your heater is running full blast to warm up the cabin, that can be like a small radiator. Watch your temp gauge - the blower for the ECC should not come on until the needle has moved a little bit. My 1998 usually opened the thermostat in about 2 miles, no warm up idling, but from a garage at 40F. I found that the needle would drop a miniscule amount when the Tstat opened.

Are you sure that the thermostat is operating and not just stuck open?

Klaus
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Did you do your Random Act of Kindness today?

Yeah I think you're right about the location of the thermostat. My brainwave was off a bit I guess:)

I'm not sure the thermostat is stuck open or not. It's pretty new FWIW (~2 years old) and OEM as I recall.

I'm trying to get some better gas mileage out of this old girl and I'm trying to hunt down everything I can that could be contributing to the problem.

I'm pretty sure I opted either for the 92C t-stat because I wanted a hot cabin (as explained earlier) or a 90C t-stat because it was OEM. I can't remember.

Either way I'm pretty convinced I should switch to the 87C t-stat, I'm just trying to figure out if my mileage is going to improve from this move or not.

FYI in-town mileage right now is about 14.5-16mpg. The car is an AWD but the prop shaft is removed (for summer - will soon be re-installed).
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1998 V70 AWD->FWD->AWD Turbo 220k+

14-16 is dismal and it's not even cold yet. I think 18 is relatively poor around town. Mine needed a new O2 sensor to get the mpg back to 20-21/28 highway.

Unplug the ECT sensor and prob the sensor for resistance when cold and hot. It should be:

0C = 7300 Ohms
20C = 2800
40C = 1200
80C = 300
100C = 150

Almost a straight curve. Any resistance much higher than those numbers indicates a colder engine = equals more gasoline used.

Klaus
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Did you do your Random Act of Kindness today?

haha yeah it's getting pretty bad.

It has a pretty new O2 sensor in it - I put a new Bosch on about 2 years/20k miles ago hoping to fix my fuel economy problems back then....

I can see the live output on my scanner and it really looks good. The scanner has a 'test o2' mode and it always passes, and my O2 system monitor always passes as well. I haven't set an o2 code either. I /think/ the O2 is fine right now, but I guess there's always the chance that something could be off...

I will definitely test the sensor out, but I can see the output from my scanner and it looks to be pretty accurate to me. I think the best way to check would be to test the temperature of the thermostat housing and compare it to the output as seen on my scanner. If they're too far off the ETC should be replaced?

Strangely enough I got an EVAP leak code last night P0442 (small leak) and I did a quick test with pressurized air (~4psi, which is the official test described in VADIS BTW) on the intake system and found no leaks at all. I was surprised! So maybe I've got a small leak in the EVAP system, perhaps in the purge valve like this guy:
http://www.matthewsvolvosite.com/forums/viewtopic.php?f=1&t=31691

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1998 V70 AWD->FWD->AWD Turbo 220k+

Tested the ETC - seemed to be pretty much right on. Without something to accurately test the actual temperature of the coolant independently, it's pretty tough to know exactly whether it's right on or not, but it seemed to be in line with the numbers provided.

BTW what is the proper fully warmed up operating temperature for these cars? I think last time I checked with my scanner it was reading ~96C/205F.


Tested the MAF as well while I was in there - again, it seemed fine. I think the voltage on pin 2 to pin 4 was .178 volts which is about what they say to look for right (0.1-0.2)?


As to the EVAP leak... I replaced the only easily accessible soft line back there. I have a feeling I'm headed for an EVAP system smoke test real soon.

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1998 V70 AWD->FWD->AWD Turbo 220k+

My p0442 came from the lines by the fuel filter. Smoke testing the manifold will not diagnose the p0442, it is in the EVAP system. Go ahead and test the purge valve. If you want to tear it apart, find one in a junk yard to play with.

Klaus
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Did you do your Random Act of Kindness today?

Lots of reasons. The 87C will heat your cabin just as quickly as the 92C. The higher temps these days are purely for emissions control, a hotter engine will produce fewer NOx.

The turbo model also cools the turbo, and we want that to be as cold as possible. (my guess)

Klaus
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Did you do your Random Act of Kindness today?