My first question is this....I checked the fluids in my 99 Jimmy and noticed the coolant overflow reservoir was empty and the rad was just a tad low. The reservoir looked as though it had the GM Dex-cool so I topped it up with some Prestone Dex-Cool approved premix I had in the garage. When I topped up the radiator I quickly foud out that somebody had flushed and filled it with the regular green stuff. Since I didn't actually use GM Dex-cool and just the Prestone pre-mix that was Dex-cool approved will I get away with this or should I have it flushed and filled to be safe?

This is the product in question.

The 2nd question is where on the temp gauge should this truck be running? I find it running 1/4 but it seems a little cool?

 

You should be ok. The Prestone coolants are friendly with both the old green (even though current green is not the same as old green) and with Dexcool.

As far as the gauge position, it should be about straight up which corresponds with around 210*F at full operating temperature.

 

Thanks for the quick reply! I was worried that they wouldn't work well together.

Sounds like I need a thermostat then as it never goes above a 1/4. This might also be the reason for my poor fuel mileage.

 

If you are worried about it at all, it really wouldn't hurt to do a flush and refill with Prestone Extended Life coolant while you are in there doing the thermostat replacement.

 

That's funny...I just said the same thing to my wife. :-)

I plan on doing the thermostat (plus coolant flush) and u-joints this week but just wanted to be sure I wasn't running on borrowed time with the different coolants.

Thanks again.

 

For that short period of time you should be fine.

My daughter's blazers temp gauge over time kept going down and eventually would only go to about 1/4 even though the thermostat had been replaced and the new one was checked with a thermometer before it was put in. The sending unit was going bad causing the guage to read less than what the coolant true temperature is.

 

Is there any way of telling if the sending unit is the issue? I am guessing a scan tool won't work since it gets the same signal from the sensor has the gauge.

I will have to find somebody with one of those laser infrared temp sensors maybe.

 

You can remove it and test it. I seem to remember posting the resistance values through the sensor as it changes due to temperature. But here they are again:
It should be a fairly linear progression through the range.

CLICK HERE for a delphi document (looks to be missing some things though).

And here is the picture from my photobucket that I remembered posting some time ago:

I had to get it off of my old PC because I had attached it to the forum the last time I posted it (November 2005). That chart works for just about all GM two wire temperature sensors (ECT, IAT, Ambient air temp, HVAC output temperature sensors/switches, etc).

 

Turns out it was the thermostat after all. Changed it tonight and now the truck runs at a line or 2 just below half on the temp gauge.

Also did the u-joints on the rear shaft as they were still original after 10 years and 240,000kms. Was going to do the front shaft u-joint but the diff seal is leaking quite good so we are going to wait and do the seal at the same time.