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P0128 (engine running cooler than expected) -> Replacing Thermostat (found out I had none) -> now overheating

186 Views 3 Replies 3 Participants Last post by  Flat6er
Hey guys, new here :)

We have a 2009 Tribeca with 94k miles on it (yes, we don't drive much) and recently got wifey a new car and I'm using this massive Subaru.
On a recent trip to work I had a check engine light with code P0128. I checked here and decided I need to replace the Thermostat.

Went to an auto shop (I'm too fat to fit under the car haha) and surprise surprise - there was no Thermostat there. I don't know why but I imagine that's the previous auto shop's way of keeping the car engine temperature low (car was overheating in the past).

Got new one installed with air bleeding and everything done. they had the car bleeding air for some 30 minutes and did the 2 fans on-off cycles and all looked awesome. but after a few minutes of driving it goes above the triple line thingy. dang!

Shop already closed so I did the whole coolant burping thing with the funnel myself (thru the radiator pressure cap) - coolant went up and down the funnel and I got an occasional air bubble. I let it run for 40 minutes. I did notice the coolant is barely hot.

Went on another drive - 5 minutes and it's overheating again.. even went 2 steps after the triple lines to a stage where AC won't blow cold air anymore. horrible! tried giving the car hard trouble to see if it will go even hotter but it didn't. it did play between 1-2 extra lines after the triple lines but as I idled car would quickly cool back to the triple.

One more thing - after that drive I turned off the car and opened the bonnet and heard kind of a boiling sound coming from the engine or maybe it's the radiator hoses.. trying to give any piece of info I can..
And another thing - guy in the shop said it's the same Thermostat as Subaru B4 uses. I thought maybe it made some sense both are Subarus but - is it? could a bad/mismatching thermostat explain why it works great when standing still (revving the car, idling it, everything works great when standing still) and not when driving?

What can it be and what can I do about it?
I imagine the car shop hadn't used the original thermostat (I saw the box, it wasn't that white subaru oem box)
I'm wondering if I should buy the oem thermostat and give it a try or if I should just go on and tell them to remove the thermostat altogether. car hadn't overheat for some years now.

Also.. how hard is it to tackle myself? (got a jack, 2 stands and some good will to save money) - I am usually pretty handy with all the non-crawl-under-the-car tasks.
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Are you losing coolant? Sounds like it's not a problem until the pressure starts building in the system, did any of the shops do a dye test for a leak?
No the 2nd shop I've been to are actually cooling experts so I am not worried about them finding and fixing the problem I went there in a hurry asked them to replace thermostat and go on on my way so I can't blame them for not being thorough enough.

But I don't want to spend any overly amount of money so wondering if there's any strong lead.

So you're saying when driving pressure builds up in the system and that's why when standing still even when revving for many minutes the system isnt as pressurized as during drive? Interesting.
1) error code is P0128 and FSM suggests to check coolant amount and radiator fans.
2) judging by your immediate overheating - I have a few fantasies:
2.1) I think that there is serious issue with coolant pipes/hoses... some main channels in "small loop" are bad.
2.2) you have overheated car and head gaskets are bad. you should test your cars coolant - whether it has CO/CO2 in it. also compression test would be good idea.
2.3) issues with water pump not performing enough?
3) is the car in your posession from brand new?any big repairs/events with it?considering that your wife drove it mainly... how much does she know about cars? or it is just "I need to get to point A, B and C"?
4) I would get yourself some obd adapter and would monitor temperatures
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