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  1. #1
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    2009 H3 5.3L Thermostat Location?

    Hey everyone,

    Happy New Year! I hope you all enjoyed the holidays

    So my baby is fixed in her parking for now my check engine light is on, temperature gauge is not working and I have the AC OFF warning..

    I connected her to OBD2 scanner and got the P0128 code.. so after googling that indicates the Coolant Temperature Sensor or the Thermostat..
    My brother is trying to help me from overseas, as I no longer have anyone to help me with this. (bless him! Hes a car and plane mechanic but we dont have many Hummers in UK so he is unfamiliar with the engine layout, and hes obviously not here to look himself lol)
    I had a small crack in the top plastic of my radiator so I put some of that temporary sealant in it as it was just before I was due to fly home for Xmas.. so my brother thinks that maybe the sealant goop has got trapped in the thermostat and is holding it open.

    Long story short, I have no idea where the thermostat or coolant temperature sensor is on the 5.3L V8 is and Ive been Googling but I can only find it for the I5 engine
    please could someone advise me?

    any help will be greatly appreciated!

    take care and stay safe!!


    2009 H3 V8 - "Baby"

  2. #2
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    Sorry, thought some pics might be helpful..

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    2009 H3 V8 - "Baby"

  3. #3
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    should be right off the water pump - a little domed housing with the thick hose coming off & 2 or 3 small bolts holding it on.

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    Lauren (04-07-2022)

  5. #4
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    The H3 radiator tank is made of plastic and cracks. The sealant is temporary and you should replace the radiator anyway.
    As for the tstat, as Zebra said, but usually it sticks open and the engine stays too cool and takes very long to warm up.

    Sent from my SM-G960F using Tapatalk
    06 H3, Birch White, Lux , Cooper AT3 35s, Borla Performance Cat-back, Bilsteins front w/ 1/2" extended stops, Hunner sleeve, Air Doc w/ K&N filter, Oem Roof lights, 55W HID Headlights and 35W Fog lights.

    09 H3 V8, Arctic Silver, LS376/480 Conversion, 3000 stall, Custom E-fans, 37" KM3 tires, XD Rockstar Wheels 17x9, OEM Sliders, 40K External Trans Cooler, Bilstein 5160s front 5100 rear, O.D. Steering Bracket, 2" Body Lift, Custom Exhaust, Custom intake pipe & K&N filter, Custom PCM tune, Pioneer DVD unit, Aurora 40" LED Light Bar, 50 W HID Headlights

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    Lauren (04-07-2022)

  7. #5
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    Thermostat is easy to replace, just have to spend time refilling and then purging the cooling system of any air.

    Wouldn't hurt to check the sensor first, I know myself and at least one other guy on here have had issues with the wiring to the sensor (located on the driver side head right by the alternator) wearing through and causing a short. I would check with a OBD2 scanner before doing anything with the thermostat. If there is an issue with the sensor or wiring then it's going to read -40 degrees Fahrenheit. If the sensor and wiring are fine then it should read ambient temperature or whatever the engine is at if it was run. The more advanced scanners (ie, ones that don't simply report the code itself) can read the actual value being reported.
    2008 H3 Alpha

    "Live free or die: Death is not the worst of evils." - General John Stark

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  9. #6
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    The I-5 thermostat is on the L side by the intake under the alternator. At least on LH drive H3's. I can't say on the RH H3's. A real 'treat' to get to. lol. Easiest to get to by removing the F/L tire and go in that way. If you have a IR temp sensor, put it on the engine where the top radiator hose is and read that temp reading. If it's running the right temp when hot, or close to it, then it's probably the sensor. If you do need to replace it, buy a good quality AC delco, is what I would recommend.
    https://www.amazon.com/SKP-SK1511073.../dp/B07CXZMHM8


    Like others said, replace your radiator asap and then THOROUGHLY flush the coolant afterwards (and the heater core) to get that sealant crap out. Don't use that coolant miracle coolant seal stuff any more than you need to --- it does a nice job sealing off heater cores too (no heat!)!! Then buy a couple gallons of purified or distilled water (not tap water) and the right DexCool coolant ..or whatever they used in your vehicle. Although, truth-be-told, if you completely flush the entire cooling system, you can probably use another type of coolant. I'd probably stick to DexCool (or whatever they used in the UK) since these engines run hotter than the older engines. Whatever you use, stick to the same stuff (don't mix coolants).

    Let us know how it goes or if you have more questions...
    Last edited by Jeepwalker; 01-12-2022 at 03:07 PM.
    Jeepwalker

    '07 H3 Advent/Lux
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  11. #7
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    Quote Originally Posted by Jeepwalker View Post
    The I-5 thermostat is on the L side by the intake under the alternator. At least on LH drive H3's. I can't say on the RH H3's. A real 'treat' to get to. lol. Easiest to get to by removing the F/L tire and go in that way. If you have a IR temp sensor, put it on the engine where the top radiator hose is and read that temp reading. If it's running the right temp when hot, or close to it, then it's probably the sensor. If you do need to replace it, buy a good quality AC delco, is what I would recommend.
    https://www.amazon.com/SKP-SK1511073.../dp/B07CXZMHM8


    Like others said, replace your radiator asap and then THOROUGHLY flush the coolant afterwards (and the heater core) to get that sealant crap out. Don't use that coolant miracle coolant seal stuff any more than you need to --- it does a nice job sealing off heater cores too (no heat!)!! Then buy a couple gallons of purified or distilled water (not tap water) and the right DexCool coolant ..or whatever they used in your vehicle. Although, truth-be-told, if you completely flush the entire cooling system, you can probably use another type of coolant. I'd probably stick to DexCool (or whatever they used in the UK) since these engines run hotter than the older engines. Whatever you use, stick to the same stuff (don't mix coolants).

    Let us know how it goes or if you have more questions...
    she did mention that hers is the v8, so it's right up front & easier to reach.

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  13. #8
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    Woops ...You're right! I read the I-5 part and skipped over the V8 line. Must be Covid brain-fog getting to me .....lol
    Jeepwalker

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  15. #9
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    10 to 1 it's the sensor, not the thermostat. JMHO. A stuck open Thermostat would still read something on the temp gauge (it would take a bit longer to register, but...). A stuck closed thermostat would cause overheating, but again, still read on the temp gauge.

    The reason that your A/C turns off is that the computer doesn't know what temp the engine is because there is no info from the sensor, so it shuts down A/C as a precaution against overheating. Mine occasionally acts up (2 or 3 times in the past 5 years). Same symptoms; non-reading temp gauge, NO A/C message, Check Engine light, P0128 code. Clearing the code immediately solves the issue. If it keeps coming back, check the sensor wiring or replace the sensor.

    I would, however, put a new radiator on the priority list; I've never seen any kind of sealant last on a cracked radiator.
    Last edited by 650Hawk; 04-12-2022 at 11:20 AM.
    We don't stop playing because we grow old... We grow old because we stop playing.

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  17. #10
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    Hey guys!

    I'm sorry I've been MIA for the past couple months, life has been a little rougher than usual..

    I just wanted to give an update, just in case anyone else has this problem in the future.

    But first, I really want to thank all of you for your advice as usual, I can not tell you how much I appreciate it!
    Shoutout to Mr Acer4Lo for real time guiding me through the 'locate the thermostat and get it out' process! Bless him

    Long story short, all 3 parts got replaced.. radiator, thermostat and coolant temp sensor.

    I already knew I needed to replace the radiator so luckily I pre-emptively ordered that over Christmas knowing that it would take time to get from the States to Dubai along with a 'heavy duty' thermostat.. Good Ol' RockAuto! ..sucks that I can't use my work APO Box address anymore though

    Anyway, changed the thermostat (I can't even begin to describe the state of the old one to you!) and her dash warnings became intermittent instead of screaming, allowing me to carefully drive her to the mechanic, ripped out the radiator and it had 3 huge cracks in it, not just the top one I had originally found..
    and also the coolant temp sensor died.. it looked like someone had thrown it in a bonfire and then a pile of lasagna
    Thankfully someone upstairs was looking out for me and I found a replacement one here.. literally last one left, everywhere else was out of stock!

    So at least I broke it properly instead of half assing it haha! At least the thermostat and sensor are cheap!

    Anyway, new parts in, system flush and new fluids... Baby is now a happy, fat girl again


    2009 H3 V8 - "Baby"

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  19. #11
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    Quote Originally Posted by Lauren View Post
    Hey guys!
    Shoutout to Mr Acer4Lo for real time guiding me through the 'locate the thermostat and get it out' process! Bless him
    Glad you were able to get it fixed!
    2008 Hummer H3 Alpha Adventure + Luxury

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