350 thermostat questions

[Mikester]

hi folks,
I just replaced the thermostat in my 95 GMC K1500, which is not yet the Vortec engine.
I am more a hobby mechanic rather than a pro so Im not sure if its working correctly.
Here is what it does with the new 195F Thermostat:
Start engine and idle for ca 5 minutes: temp shows around 150-169 F.
When reving up the engine for some time the temp gets as high as 200 then drops again fairly quickly to 150/160.
What I dont get is why the upper hose from engine to radiator collapses when temp is below 200 when hitting the gas??
Also when opening the rad cap the coolant squirts out big time.

Then I installed the original thermostat back, also 195F and all of a sudden I dont see a temp value in the gauge anymore even after driving around for half an hour. That would mean the temp is below 110 all the time which cant be good, right?? And this because the old thermostat wouldnt be closing??

I added enough coolant maybe a finger below rad cap.

What is wrong with the system or is the first scenario absolutely normal?

Thanks for your hints!

 

[sntrym]

sounds like you have air in the cooling system. When you fill the radiator, you should let the engine run with the radiator cap off until all the air bleeds out. You'll have to add coolant from time to time as the bubbles escape. Once the bubbles stop, you're good.

 

[black90sport]

Yep. You have air in the system. You need to bleed/burp it. Report back after looking that up on youtube if you still have issues.

 

[mars2878]

as said above, radiator cap while running, don't be too quick to top off as sometimes it'll go back to full looking then drop again. I run the heating system on full blast.

 

[TylerZ281500]

as said above, radiator cap while running, don't be too quick to top off as sometimes it'll go back to full looking then drop again. I run the heating system on full blast.

proven to not do anything but i do it too lol

 

[mars2878]

yeah, its how I was taught. def doesn't hurt.

I think the mindset is if the ac is on full blast when charging, then same when doing the heating system.

 

[Mikester]

Hi and thanks,

we did indeed follow the open cap procedure, maybe just not long enough. Will try again.

Ive seen videos where lads drill a small hole in the outer ring so bubbles can escape, does that make sense?

I know now definitely that the old thermostat does not close anymore by the way

Could another reason be a blocked coolant bypass channel?

 

[TylerZ281500]

dont drill any holes.

how to check for a blockage, pull thermostat out, pull off lower hose and drain coolant. grab garden hose run water from waterneck into block and itll all come out the water pump.

as far as burbing the system, start the truck with the radiator cap off, let truck idle and let waterpump circulate coolant. pour in coolant as necesary. at some point youll stop filling let it drain down fill a small bit let it drain etcetcetc. once your at this point id suggest putting your radiator cap back on. (by the way this is with the thermostat reinstalled) let the truck idle til it gets to temp, in your case about 200* with a 195 thermostat. keep an eye on your temp guage (which i hope works) it should hit 200 ish and then the thermostat will open and drop to say 160 or so. this is normal function.

if you still have issues after that then we can deduce further, but method stated above is absolutely fool proof, is what many or your house hold garage mechanics have been doing for ages.

is there a machine to bleed the system for you? yes there is, it works well, but if your like many folks you wont have access to it.

*other things to plausibly consider* what color coolant came out and what color are you putting in? some coolants do not mix and form what looks like the center of your favorite jelly donut (mmmmmmmmmmm.......donut)

 

[Mikester]

Hi Tyler,
forgot to mention that we had indeed bled the whole system and flushed it with water as per your instruction to get all the rusty stuff and old coolant out.
The coolant Im using is red/pinkish and it looks still fine Id say by looking at it in the overflow container.
For my understanding...shouldn't the system suck back coolant from the container once the thermostat is closed again? Is this when the hose collapses? I mean if the overflow hose was blocked for some reason...would that make sense.
Couldnt try anything and wont be able to the coming week as I cant do all this in my driveway and there is no access to the garage I use to do all the things...

 

[77Impala]

The system only sucks back coolant 1) when the engine cools down after being shut down and 2) if there is no air in the system.

I drill a 1/8 hole in all my thermostats that I replace if they don't have a bleeder valve built in. As you pointed out it does help bleed air pockets out but is not large enough to cause cooling issues.

Our trucks have a awkward heater core in that the inlet/outlet tubes are below the core so when changing the core or just the coolant air will get up into the core.

As pointed out in previous post I also keep the cap off and run the heater fan at full blast till the engine gets up to temp. I am old school mechanic taught and one reason to have the fan on high is to help prevent the engine from overheating while we wait for the thermostat to cycle. I also over fill the coolant overflow bottle to take up any needed coolant as the engine cycles during the next several drive cycles.

Also with these trucks I prefer to get the front end up higher such as on ramps because of the heater core tendency to trap air. But it also helps to get any air in the block up to the thermostat which at that point is the highest point of the block.

Hope this helps you.