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