New failsafe thermostat stuck open???
 

First a little background info, I have a 96 4x4 blazer 4 door, about 3-4 months ago I had a leaky intake gasket so I replace that and the thermostat with just a normal thermostat, last week my waterpump started to leak so I replaced that, started it up and let it run and it was still overheating. So I decided to replace the thermostat even though it was just a few months old, I got a failsafe thermostat so if it fails it stays open. Now when I drive it takes longer for it to heat up (about 10 mins or so on the highway) and eventually (around an hour of driving) it gets hotter than it used to (over 210). It used to hit about the 210 mark and then the thermostat would open and the temp would drop down and stay a couple ticks below 210, now the temp never really seems to drop it just slowly increases. So my question is, I've never used a failsafe thermostat before, does this sound like it is stuck open all the time? Or is there any way I can verify if it is working correctly or not? It's not bad driving it right now but I'm afraid if I get stuck in some bad traffic in the afternoon I could have overheating problems again. Any help would be appreciated.

almost sounds like it is open. my old work truck did that in the winter. would take like 30 minutes to start getting heat but would never really get warm

Maybe try a lower temp thermostat.

To verify it is working correctly is a fairly simple procedure. Except you have to remove it.

I learned this years ago, this is old school mechanic 101.

Before installing a thermostat, put it in a pot of water on the stove. Turn the stove on and with a meat thermometer in the water, keep an eye on the temperature and when the thermostat opens up. This will tell you exactly what you ned to know. If it indeed works and opens fully and precisely what temp it does so.

Just don't let your mom/wife/significant other catch you boiling car parts on the stove in a good pot.
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Thanks for the suggestion on testing it, I don't have time during the week to pull it out and check it but I will this weekend. I've noticed the last few days of driving it that after 5-10 mins of driving the temp goes up to the small tick past the big tick between 100 and 210 (if that makes any sense) and stays there for most of my drive, it's not until after an hour or so that it starts to creep up. The day that it went over 210 it was 90 degrees and very humid out and I was running the AC so that probably didn't help. I'll let you know what I find out this weekend.

Borrowing this thread to ask a Q. My 1996 Blazer with 4.2litre motor never gets hotter that 160 degrees. I am guessing it should be around 210 or so.??

Any ideas why mine never gets hotter ?

Stock thermostat is 195, you may have a cooler thermostat in or else it is sticking open, I had to replace mine because of that a while back, it was sticking open and never went above 160 or so.. if it is running that cold your vehicle will run in open loop all the time getting worse gas mileage

Well I'm not sure what to do next, I replaced the fail safe thermostat (which was not stuck open when I took it out) with a new one (195 degree), this time a non-fail safe one. It drove fine for the last week but it hasn't been as hot out as it was before. With this new thermostat the temp now goes up to the 210 mark then the thermostat opens and the temp drops down and runs around 180, just like it used to. Now yesterday it was warmer out and sure enough after about 45-60 mins of driving my temp slowly started to go up, it got up to a bit over 210 by the time I got home, it's not as quick as before but it hasn't hit the 90's outside again. I don't understand why it will run fine around 180 degrees for 45-60 mins before it then starts to go up. Any suggestions would be greatly appreciated. Thanks.

Well my problem appears to be fixed, I went and got a Prestone flush kit and flushed out all the coolant (it was a bit mucky but not horrible), ran some Prestone cleaner through, flushed that out and then filled it with the Prestone green All Colors coolant. Then I put another new fail safe thermostat in, now the temp still stays around 180 and after an hour or so of driving it goes up a little but after driving in a very humid 90+ degree temps yesterday with the AC blasting and in rush hour traffic the temp didn't go over 210, so I am happy. Thanks for your help.

Glad to hear that you got it fixed!! thanks for letting us know as this may help others with similar problems

Hi, my Blazer, (a '96 with over 250K miles on it, first car, bought it for 1100) recently lost heat (AKA, heater stopped working).

I noticed this a week ago and yesterday it began to over heat.

I'm pretty broke so I'm hoping it isn't anything major but am prepared for the worst (putting her down).

I'm going to try and replace the thermostat and flush/fill the coolant soon, I've read a few success stories here so we've got our fingers crossed.

Any recommendations or tips?

~Rae~