'94 850T-Temp gauge dropping slightly. Seems random.

IG-88A · Post by **IG-88A** » 26 Feb 2016, 07:42

To start with, I replaced both the thermostat and ECT a little over a year ago. New radiator and hoses shortly thereafter. Lately, I've noticed that even after the car is up to temp (temp gauge sitting at the 3 o'clock position) the temp gauge will drop slightly lower for a moment or two. I've noticed this maybe three or four times over the last month or so. I've always noticed it fairly close to home, and just shortly after the gauge showed the car was up to normal temp. So I always thought that the car wasn't truly up to temp just yet, and the gauge was giving a false positive shortly before the gauge dropped. However, I noticed this happen this morning after about 15-20 minutes of driving. The final ten minutes of my trek were on the interstate at a steady 70 mph. As I was approaching my exit, I had to accelerate to about 80 to get around some cars that were blocking me. After that is when i noticed the temp gauge dipping again. It'll maybe get to the 3:20 mark or so. Nothing really extreme. But this type of reading is what I was experiencing with the bad t-stat, even though it was constantly at around the 3:10 or so mark, instead of intermittently dropping. My heater is working good. Not sure what to make of this.

Post by **rspi** » 26 Feb 2016, 07:55

IG-88A · Post by **IG-88A** » 26 Feb 2016, 08:46

Thanks for the video, Robert. One question though. If the temp gauge was failing, wouldn't that mean that it would probably drop completely to the off position and then "turn back on" so to speak, rather than just slightly dipping and then slowly and steadily climbing to 3:00 again? The drop to back to normal time-lapse is usually about 30 seconds or slightly longer. I did think of one more thing. When I bought my new thermostat last year, The seal that came with it wouldn't fit. The groove was too shallow. So I had to reuse my old seal for the time being. Since it seemed to be fine, I didn't bother with ordering a replacement. Is there any chance a faulty t-stat seal would cause these types of symptoms? Although, I think I'd likely be leaking coolant from the housing if that was the case.

I'm kind of wondering if I just got a thermostat that is dying on me prematurely. Now that I'm thinking about it, I'm noticing my temp gauge dropping mostly after acceleration on the freeway or coasting down a long hill. Which would mean more airflow through the radiator. More airflow+open thermostat=slightly low temp reading.They're cheap enough. I ought to just toss a new one in and see if it fixes it. And hopefully get a seal in the box that actually fits this time. I just never thought that a thermostat's life span would be so short.

IG-88A · Post by **IG-88A** » 26 Feb 2016, 14:01

So I went for about a 20 minute drive to do a couple of deliveries for work. I decided to plug in the Scan Gauge to monitor coolant temps while driving. While underway (after gauge was at the 3:00 position) I was noticing temps ranging anywhere from 173 to about 202. At first, I seemed steady at 177, then at a red light I hit about 195. On a street with a 30 mph speed limit, and occasional light throttle on a downgrade, I was at around 172. At my final stop, I left the car running for a couple of minutes while I was inside. When I returned to the car, I was reading at around 202. Is it typical to see fluctuations like this? I would assume that temps should fluctuate by a few degrees at best.

Post by **polskamafia mjl** » 26 Feb 2016, 14:57

Those fluctuations don't seem unusual. The temp gauge is pretty vague so a wide range of temperatures will move the needle to the 3:00 position.

Post by **rguzz** » 26 Feb 2016, 15:45

Having just gone through this with my Audi (I am somewhat convinced my "new" Wahler thermostat is weak) I will replace it with OEM Audi which is Behr if I recall. I assume your new one was OEM? I don't know, just a thought. T stats are crap these days overall...weak springs? But I get dramatic drops in the gauge, more obvious that yours appear to be. For peace of mind, think about replacing yours, depending on who made it.

IG-88A · Post by **IG-88A** » 26 Feb 2016, 16:00

My new one was/is Wahler, as they were the only ones who offer the recommended 87 degree t-stat for the 850 turbos, and I'd heard they had a good reputation. I'd read that the turbos had an 87 degree unit for some reason, and the NAs had a 90 degree one. Apparently Volvo OE t-stats are only available in 90 degrees now days. I thought I was doing the right thing by going with the 87. Maybe it doesn't make that big of a difference. Maybe I should just try an OEM one. I ordered another Wahler today, just to test it out and see what happens. Now I'm wondering if I should return it and go with the OEM. Thanks for the reply.

Post by **rguzz** » 26 Feb 2016, 18:20

OEM. Might not make a difference, just a suggestion. Then at least the doubt is off the table (mostly).

Post by **rspi** » 26 Feb 2016, 21:24

Was the gauge moving when the temp dropped under 177? At what temp did the gauge dip? I noticed mine temp was dropping to about 172 last night, I even recorded it and mentioned it in a video. However, my temp gauge never moved. They are usually at 3 o'clock at 172 and stay there until 230.

IG-88A · Post by **IG-88A** » 27 Feb 2016, 07:33

I didn't see it move. But I noticed it had moved slightly. With the range of temps I recorded on the Scan Gauge, I shouldn't have had a lower reading on the actual temp gauge at 173, going by what you noticed with your needle staying put. Yet I never saw any readings from the Scan Gauge lower than that. But I will add that when my thermostat was going bad last year, I noticed a similar reading from the temp gauge. I never checked actual temps with the Scan Gauge then. I may not have even had it at that point. However, at that time, the gauge would never make it to 3:00. It would stay slightly lower at all times. So maybe the current one is working intermittently or working somewhat, rather than being all-out failed. I'll find out when I remove it.

I ordered a new thermostat, and we'll see if that fixes it. I just never would have thought it would go bad so quickly. I should just pick up several of them if they're going to fail so soon. How old is the one in your car that's going bad? And which kind do you like to use? I used Behr on the VWs as a preventative measure in the past, without incident. Maybe I should try theirs instead of the Wahlers. Another question though, does the temp gauge only get its readings from the one ECT sensor in the thermostat housing?