Thermostat stuck open???

[cfiiman]

Hey all,

Took the Viper out today since it was 20* cooler and low humidity for once and maybe that is why I noticed this. On my 94 my temp always stays at 190* unless stuck in traffic or going slow when it is hot and humid out. Anyway it usually reaches operating temp quickly, before I get out of the neighborhood. So today I start it up and go and didn't really notice till I was a few miles out of the neighborhood that the temp gauge was still down around 160'ish I would say, I was like that has never happened. So I kept watching it and was cruising and it just never really came up much further, maybe 170* or so when I finally had to stop at a light. Then as I got into traffic it slowly crept up like normal so I decided to do a test and jumped on the freeway for about a 5 mile stretch in 6th gear. Sure enough it slowly kept going down in temp till it got to around 175* or so and I never remember it doing that before, it would sometimes get around 185* but then the thermostat would close and put it back up to 190*, so what is going on? Is this just coincidence that the lower temps/humidity caused for much more efficient cooling today or is my thermo stuck open? I have never seen the car not warm up quickly to 190 before today, thanks for any help.

 

[DrumrBoy]

I agree with your diagnosis....thermo stuck open...or at least partly open.

 

[jdeft1]

Sorry to hear it. It's a PIA to swap a t-stat on a '94. It's tight back there but it can be done without removing anything but the cowl and wiper blades...

A good little project for this weekend IMO.

 

[viper067]

fix sooner rather than later, a stuck thermostat will not properly cool the engine and could lead to serious damage

 

[JonB]

If you have air trapped in your motor, the TEMP SENSOR is not immersed in coolant. Therefore, you get a FALSE reading. Its just reading the hot air in the head. Sometimes the coolant sloshes and you get a spike....then a drop.

Until you rule out trapped air, I do NOT suspect a 'stuck T-stat"

 

[Mopar Steve]

If you have air trapped in your motor, the TEMP SENSOR is not immersed in coolant. Therefore, you get a FALSE reading. Its just reading the hot air in the head. Sometimes the coolant sloshes and you get a spike....then a drop.

Until you rule out trapped air, I do NOT suspect a 'stuck T-stat"

X2. Even more on a GEN 1 car.

 

[cfiiman]

Thanks guys, I'll keep everyone posted, I am vigilant about burping the car before every trip so I don't think it is trapped air but I'll keep an eye on it. As far as damaging, how could there be damage if the car was cooling a little too much, I mean some guys put in 170* thermos, so I don't get the comment about engine damage if mine ran a little cooler on the highway at high speed, seems almost like a good thing in a fire breathing beast like this car

 

[JonB]

Thanks guys, I'll keep everyone posted, I am vigilant about burping the car before every trip so I don't think it is trapped air but I'll keep an eye on it. As far as damaging, how could there be damage if the car was cooling a little too much, I mean some guys put in 170* thermos, so I don't get the comment about engine damage if mine ran a little cooler on the highway at high speed, seems almost like a good thing in a fire breathing beast like this car

How? Because the Temp reading will be FALSE if the air has replaced the coolant at the Temp Sensor.

How many miles on your 94? Have the head gaskets been replaced? The OE 92-95 gaskets will likely fail. If you have been vigilant, but with OE gaskets, maybe NOW they have failed and you are losing coolant. You do know where your true overflow bottle is? Inside the front fascia, in front of RF tire? "Step One" of burping dilligence.

In a few hours I head to PIR for Kids Hospital Weekend... Closed Til MONDAY....

 

[rpm9000]

I went for 2 years with a stuck open thermostat on my 95. It just takes it longer to warm up. It won't hurt the engine. Of course it doesn't get very cold here in the winter time.

 

[viper067]

how could there be damage if the car was cooling a little too much, I mean some guys put in 170* thermos.

If the thermostat is stuck open, then coolant/water is constantly flowing through the engine .... meaning the engine doesn't heat up properly and doesn't cool down properly. Rings, pistons, and other things that would normally expand when heated, won't on an overcooled engine...on the flip side, as the engine gets hotter, so does the coolant and without the thermostat closing to hold coolant in the radiator it isn't going to effectively cool and you are in an overheating situation

as for the 170 and 180 thermostats, they still close and allow the coolant to cool in the radiator .. they just open at a lower temp so they are more likely cycling more. When moving on the hwy at low RPMs you'll get a cooling effect like you noticed as the air passing the radiator is able to better cool than in traffic...I'd imagine that doing the hwy in 3rd would yield different results

Jon is certainly much more the expert than I, but I would expect that trapped air would be hotter than the coolant else there really wouldn't be a need for coolant.

been there, not on my Viper ... with unhappy consequences, the stupidity of youth

 

[AAKVIPER]

agree a stuck thermostat is not allowing the motor achive it's thermal efficacy.

 

[bluesrt]

i would not complain about it running cool!! if it has a excellent heater at idle,its not a thermostat

 

[cfiiman]

Well took her out tonight to test it all again, and I think my thermo is def. on the fritz. It is back to closing where it should, not like yesterday when it never heated up until I was stuck in traffic. What I notice now (albeit I'm being a freak OCD about it) is that it heats up a tad faster then normal now I think, still only goes up to 220 or so like it is suppose to then starts to come down once the fan is on. BUT, it takes longer to come down now, much longer then normal so I'm thinking it was partially stuck open yesterday which makes sense, and tonight it is unstuck to close but not opening all the way, anyway that is my guess. Since it is a gen 1 with original head gaskets I do a hydrocarbon block test frequently to make sure the head gaskets are ok (car has about 10K miles) and I'll be doing one tomorrow again. If that goes ok I'll be ordering new thermo/gaskets from Chuck, I think I have a small leak on the crossover anyway. Does anyone else think my hypothesis could be correct? Thanks guys.

 

[speedracervr4]

I don't know how mechanically inclined you are, but I would start gathering parts/researching head gasket replacement. Not saying that's the issue you're have right now (I agree with you about Thermostat sticking), but it is a well known issue on Gen 1 cars. I would want to replace the HG so it wouldn't be something to worry about down the road. Maybe through some ported heads/Cam on there at the same time. It's easy for me to spend your money though....I would at least change the thermostat.

 

[cfiiman]

Yes I an investigating all that...my system held presuure this morning on the way to a show and on the way home had none. I'm thinking I could also have a bad pressure cap even though this one is fairly new I'm picking up Anotherone today.

 

[jdeft1]

Welcome to 'old car love'. I made the mistake of NOT changing the t-stat when I did my gaskets two years ago. My T-stat failed last winter...same as yours. stuck open....... Just do it.