thermostat question

[Hooked_on_HP]

I have a 98 Chevy 3500 dually with a 7.4 in it. I have had this truck for 7 yrs. When I first got the truck the temp would come up to around 195 and stay there. After owning it for about 2 yrs I changed the antifreeze replaced the thermostat and some hoses. Ever since then the temp gauge comes up to 195 the thermostat opens, the temp goes down to around 160, the thermostat closes,then slowly back up to 195 then down to 160. The temp never stays steady. I have probably put 4 different brand thermostats in trying to correct this.Should a thermostat open slowly or all at once. Maybe it is coincidence but at the same time my fuel mileage went to crap. I have had several different shop owners that I know tell me that this could be affecting my fuel mileage. The computer is constantly changing the air fuel ratio because of the temp changing. Has any on had issues like this and how did you solve it.
Bill

 

[WTA]

Have you ever checked or replaced the radiator cap? If it's not holding pressure at the right level then it will do that.

 

[Diamondpilot]

Put a good Stant Cap on it and a new T-stat and give it another try. The other thing I would check is the fan. I know there were problems on trucks of that era. I would loose the factory fan and clutch and get a $100 electric fan with electric on and off t-stat for it.

Chris

 

[Hooked_on_HP]

I will try a cap. That is the only thing I haven't tried. I replaced the thermostat 2 weeks ago so it should be good. The fan works properly. You can realy hear it when it engages.Thanks for all the help so far.
Bill

 

[crashz]

I have heard, but not experienced this problem and what I remember is that it had to do with aftermarket thermostats. When tested, the aftermarket ones had a different rate of opening than the stock units, causing erratic behavior. Once coolant is detected below a certain temp range, the ecm will go to closed loop cold operation and your fuel mileage will tank.

Like I said, I have not personally seen it.

 

[WTA]

That can be true. The computer can never get the best mileage when it's cold. Aftermarket thermostats do suck too. About everything aftermarket these days is a crap shoot. I really hate the dealer prices though. Stant used to be a good brand but quality has gone down hill lately for sure.

As for the fan clutches, you can't do better than a properly working fan clutch and original fan. Keep it on there and you will be happy.

 

[EE_Bota]

It is my understanding that the thermostat should open gradually. I believe that it is little more than the expansion of paraffin wax opposed by spring pressure.

 

[jimainiac]

I hope the new pressure cap solves your problem. It's hard to believe you could have gotten that many bad thermostats in a row. Don't take offense, but are you sure the thermostats were installed correctly, with the proper side up? As for the declutching fan, they definitely make a different noise when they're engaged or not, so if it sounds normal I guess it's okay... Could a worn water pump cause erratic temps?

Wish I could help more, We also have a '98 Chevy K3500 with the 7.4 I've had to fix a lot of things on it over the years, but not this particular one.

 

[Diamondpilot]

Having the t-stat upside down is a good point. I chased a problem like this on a 350 in a boat and it ended up being the t-stat in wrong. GM's are also notorious for needing burped of air after the system is opened but I have only had this issue on 4 and 6 cylinders not the old push rod v8's. Check the simple stuff like the sending unit to the gauge, belts, hoses, ect.

Chris

 

[thatguy]

I have heard, but not experienced this problem and what I remember is that it had to do with aftermarket thermostats. When tested, the aftermarket ones had a different rate of opening than the stock units, causing erratic behavior. Once coolant is detected below a certain temp range, the ecm will go to closed loop cold operation and your fuel mileage will tank.

Like I said, I have not personally seen it.

I was just researching this for my Dodge/Cummins and found quite a few most recommending to stay away from the aftermarket stuff.- the OEM thermostats are +$75 so ppl were using the NAPA/Advance/Autozone $25 thermostats and a lot had the same problems..

Brian

 

[jimainiac]

I still say, what are the odds of getting four bad thermostats in a row? That said, I have gotten two bad replacement windshield wiper motor units in a row from NAPA, for the afor mentioned '98 Chevy truck. It can get very frustrating.

There could be something to the air pocket thing, but it seems like it would eventually work itself out, I would think.

 

[redharley]

I work in a fleet garage with 90% GM and aftermarket t-states will somethimes do that.We do not seem to have any problems as our trucks are on the road 6-10 Hr's a day.We use stant and will notice a wider temp swing than the Gm stats, another issue could be if you replaced the radiator in the past,if a thicker one was installed for towing reasons this will also cause a wider temp swing.If you have or know someone with a $50 point and shoot temp gauge and direct at your t-state housing you can see what temps you are really running.Keep us posted

 

[Hooked_on_HP]

The thermostat is not in upside down. I am thinking about getting one strait from GM. I have even went as far as changing where the heater hose returns. The one heater hose returned coolant to the radiator and I changed it so it returns back into the engine. The shape of this thermostat is different than any other Chevy thermostat I have ever changed. I have had big block Chevys forever and never seen this style thermostat in one. I am not sure what the lower spring is suppose to do.
Bill

 

[jimainiac]

Yep, that's right side up, alright. Sorry, I just had to ask. I went looking for the old one I took out of the truck last year while changing coolant, but I guess I chucked it. I did find the thermostat I just took out of our backup generator, 4 cylinder Mitsbishi engine, and it looked the same as your picture. It has that lower spring, as well.

I think NAPA has pretty decent quality parts as a rule. Nobody gets it perfect all the time, and you can't help notice that some of the stuff isn't as well made as it used to be. Seems to be the way of the world.

 

[Treemonkey1000]

I am a bit mechanical but I tossed your question at a friend of mine that is a bit of a whiz kid. Here is his reply for what it is worth :

"Interesting problem which I've seen before and was caused by a bad thermostat. To resolve the issue I switched from Stat brand to a performance, high flow thermostat from Mr. Gasket. Temperature is now rock solid at the rating of the thermostat, 180 degrees. Go figure..."

 

[Hooked_on_HP]

Treemonkey, thanks for the idea.For some reason I never thought of buying a thermostat from the companies I buy my race car parts from.. I am sure some of them make some quality thermostats.
Bill

 

[Stimw]

As for fuel mileage, I bought a used 1989 1500 Chevy, 4.3. The heater barely put out. I changed thermostat (195*) (had original 195* in it). I got 2 more miles per gallon!! And the heater worked great.
The computer adds more fuel to a colder engine. I see/hear too often people changing a computer vehicle T-stat to a 180* or 160*. Don't do it! If you have an overheating problem fix that. The wrong thermostat is not a fix.

 

[timswi]

Treemonkey, thanks for the idea.For some reason I never thought of buying a thermostat from the companies I buy my race car parts from.. I am sure some of them make some quality thermostats.
Bill

I'd do that too...I get my stuff from Summit Racing for the trucks and the Chevelle. Some of this over the counter stuff is pure garbage. I have run into this t-stat problem as well. They work, but the temp swings are too wide.

 

[Hooked_on_HP]

So far No good. Aparently no one in the hi perf world makes a 195 thermostat. I checked Summit and Jegs and neither has one. I did a google search and could only come up with Stant.
Bill

 

[Diamondpilot]

My father is VP and plant manager of Stant. He has worked there for 42 years starting in the maintenance department, then on the line, then up though management. Something like this would never happen today but anyway they make a quality thermostat. I am not just saying this because he works there but because they are a good home grown Indiana company that makes aftermarket and oem parts. Give them a try.

If you need any inside info or technical info give me a PM and I will pass it on to the guys I know in the R&D dept and the engenring guys. Many who work there are ex motor heads, race car drivers, crew members, ect who know this stuff.

Chris