'03 Chevy Sonoma overheating

Discussion in 'Chevy S10 Forum (GMC Sonoma)' started by bigburb, Sep 6, 2012.

  1. bigburb

    bigburb Rockstar ROTM Winner 100 Posts

    Hey all, I haven't been around much the past couple years, as I bought a Ford back in April of 2010. But, I need some help with a buddy of mine's truck. He has a 2003 Sanoma ZR2 4x4 with the 4.3 with roughly 56k miles. He called me up yesterday, saying his truck was overheated on the way back from school.

    I went over to his house today after work and went for a ride with him It heated up as normal to roughly 210, or whatever they normally run at. Then about 5 minutes later, temp started to creep up until it got close to 260 and the "CHECK GAGES" light came on. When that happened, I told him to pull over and pop the hood and leave it running. With the hood open, there was absolutely nothing out of the ordinary. No steam, bubbling coolant, etc... and the motor wasn't any hotter than normal (I've been around plenty of overheated engines, and it's pretty dang obvious when they're too hot). It was running fine, idling fine, and showed nothing abnormal. Coolant level was a little low, but nothing big...

    While we were sitting there, the temp dropped a little bit and the check gages light went off, and then the temp jumped up and the light came back on. We started easily making our way back to his house, and after about a mile, the temp dropped to roughly 235, and the needle kind of jumped back and forth between about 220 and 250 (or whatever you want to call the mark between 210 and 260), and it stayed there until we got back to his house and shut her down.

    Given the fact that it's running fine and showing no signs of overheating, I want to say it's a wacky thermostat or coolant temp sensor. But I didn't want to say for sure that that was the problem, because I wasn't certain either. So I'm asking for some suggestions. What do you all think? I haven't been around GM's for a while, and have never really dealt with the 4.3, so I don't know if there is some specific problems common to this motor or what.
  2. retired2001

    retired2001 Epic Member 5+ Years 5000 Posts

    Welcome to the G M T C ! Good luckin' truck. The 4.3 L is just 5.7 w/2 cyls cut off. If evrything stays the same except for "Temp." reading, I'd bet on a bad instrument. You might get more help in one of our "Tech." forums.
  3. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    Are you sure that the thermostat is not sticking? I have seen them stick and then finally open enough to cool the motor. It is possible that it is not openening all the way. It is a cheap part to swap out and see if it solves the issue.
  4. bigburb

    bigburb Rockstar ROTM Winner 100 Posts

    Thanks! I've been around since '09, but after buying my Ford in April of 2010 I kinda faded off. But, a lot of my family owns GM products so I figure it couldn't hurt to stick around! And I'm definitely leaning towards something simple, such as the temp sender or thermostat.

    It has definitely crossed my mind, but just the fact that it stayed at normal operating temp for several minutes before climbing kinda made me think twice, along with the fact that the temp jumped around a little bit after that. But it couldn't hurt, if it doesn't solve the problem, we're out like what, 7 or 8 bucks?
  5. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    The thermostat is $9.99 at autozone. It should not take long to change. I would NOT buy one of the failsafe thermostats that they sell. I have put them in 2 vehicles and they were terrible on the freeway. The thermostats would stay open the entire time so there would be no heat. I was driving with a blanket in the car until I put a new t-stat in.
  6. bigburb

    bigburb Rockstar ROTM Winner 100 Posts

    So I have a little update. I put a new thermostat (not one of the failsafe ones) in his truck yesterday, and still no dice. One thing I did notice is that when I took the old thermostat out, there was no coolant in the water jacket underneath. Now the truck hadn't been run in two days, so I wasn't sure if that could be an issue or not. Every thermostat I've ever replaced, the water jacket underneath the stat was full.

    He drove it to school this morning, and told me the gauge never got hot enough to trigger the CHECK GAGES light, but he said it still registered warm and was kind of all over the place, anywhere between about 210 and 240 to 250-ish. Any thoughts?
  7. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    I agree with you about the water jacket behind the t stat. There should be some kind of coolant there. Does it blow hot air from the vents inside? Check that both inlet and return heater hose is equally warm on the heater core, as this indicates coolant flow through the heater core.
    If no coolant flow: Check cooling system is correctly filled, taking care to bleed off any air locks from the system as necessary (with heater coolant valve open);
    Check heater core is not blocked if it is blocked you may be able to backflush it. Have you checked the radiator cap? You can rent a pressure tester at autozone for free. If it is not holding at least 16 lbs of pressure then the coolant and water can boil. Water under pressure boils at a highter temp than water not under pressure.

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