SOLVED! Coolant reservoir cap has me feeling stupid

[Elizabetty]

Today I was replacing the radiator cap and I thought while I was at it I would check the coolant reservoir cap. It fits pretty loosely and I noticed there was dirt around the threads of the reservoir where the cap would go on as well as inside the cap. The guy at the store asked me why I was replacing that cap. I told him I was basically just doing it to do it since I was working on my cooling system in general. That made me think maybe I was making a unnecessary decision but it was cheap so I proceeded. Upon putting the new reservoir cap on, I noticed it fits much tighter and not just by a little bit but a lot. When learning about the system in general, it seems the reservoir isn't ever under a lot of pressure and I read things saying that cap isn't serving any function other than keeping debris out and doesn't have to be a tight fit. Between that and the guy at the store, I'm wondering if going forward, I should ever care about a loose fitting dirty reservoir cap. Wouldn't a cap being too loose be the cause of the dirt and debris and since that's the purpose of the cap, a tight fit would matter? The part listed as compatible for my 2002 Trailblazer is 82594 which is the dorman help coolant cap. However, I noticed it says non-vented which makes me believe there's a vented option. On the back of the package, it lists a bunch of vehicles but not specifically the trailblazer. However, a part search said this is what I would use. Maybe this is tight fitting because it's non vented and my other cap is loose because it's vented? I don't want to build up pressure that needs to be releasing somewhere by having the wrong non-vented cap.

 

[Reprise]

As you've learned, the coolant bottle cap isn't an integral part (doesn't hold pressure). Actually, as I think about it, having an 'extra tight-fitting' cap might (?) create a secondary (negative) pressure, and that may be why the OEM fits loosely (mine will 'snug'; it isn't super-tight, but it's not 'falling off', either). Just a thought; don't know how valid it is.

I'd replace just about everything else before that cap (e.g.; upper & lower hoses, t-stat, radiator (pressure) cap, clamps for the hoses, the coolant, the radiator... and even the coolant bottle itself (many do eventually spring leaks) as needed, before I'd worry about the reservoir cap. But if you can't / don't want to return it... like you said -- it's cheap. If you ever have to replace the reservoir bottle (they crack at the seam), I think you'll get a new cap with it, anyway. But if not... you'll be set.

Welcome to Club #PartsShotgun. There's a lot of us here, so no worries...

(on edit): 'as needed' means exactly that -- not 'replace all of that list, wholesale'

 

[Mooseman]

On this platform, it doesn't need to be perfect. It doesn't hold pressure, sorta like the windshield washer tank. Unless it's broken, it doesn't need to be replaced. The radiator cap, however, is supposed to hold pressure and release when over the set pressure (~15 psi) and can be the cause of overheating if it doesn't hold pressure or leaks. It can fail but not a common occurrence. Easy way to tell is if the engine is hot and the hoses are not hard.

On the full size trucks, the tank caps are pressurized, same as radiator caps, since they don't have a radiator cap.

Edit: @Reprise beat me by 1 minute

 

[movietvet]

As was originally stated, there was dirt around the original cap threads. That in and of itself is enough reason to replace the cap. As it ages and does loosen, air under the hood carrying dirt will swirl enough over time to get dirt past it and in to the system. Just the act of unthreading it can pull dirt up the threads of the bottle and then drop in there when pull the cap away. IMO, there is a reason for the cap to fit snug and I would not dismiss the importance of that.

 

[budwich]

might be reading too much into this "design". I wonder if you need to screen off the "blowby tube" on the side of the tank since its totally open to the air and allowing all sorts of bad things in. It is a car and engine compartment, not an surgical operating room.

 

[Mooseman]

Reading way too much into the dirt thing. These usually last the life of the vehicle, seldom ever replaced. If the dirt is a concern, rinse it off, wipe the bottle's neck and be happy.

 

[Elizabetty]

Reading way too much into the dirt thing. These usually last the life of the vehicle, seldom ever replaced. If the dirt is a concern, rinse it off, wipe the bottle's neck and be happy.

Ok. I guess I got a little concerned after I was putting the new cap on and saw some black stuff inside the tank itself, and I wasn't sure if grit in there would be a problem. It doesn't appear to be oil or anything like that. I've already done my hoses and big fixes so I was on to the easy cheap cap ....the "just in case" type things and I thought well let's do it to do it cuz dirt in liquid can't be good but maybe I worry too much.

 

[mrrsm]

When time permits... Try this Experiment to see precisely WHY the Radiator Over-Flow Reservoir is Vented to Atmosphere:

(1) Using an Empty Plastic Water Bottle...
(2) Fill it with VERY HOT WATER for a few minutes...
(3) Then turn it upside down to rapidly empty the contents...
(4) Immediately TIGHTLY CAP the Plastic Water Bottle...
(5) Then Stick it right inside of your Freezer...
(6) Wait Five Minutes and then Check on its Progress.
(7) The Results will explain WHY the Overflow Tube Feeds the Reservoir from the Bottom. :>)
(8) In a CLOSED System... After the Engine & Radiator & Cap COOL DOWN...

...The Partial Vacuum Draws Coolant right back down inside of the Radiator.

...and for anyone in the market to R&R this ENTIRE Reservoir in the GMT360 SUVs sporting the LL8 - 4.2L Engines... for all kinds of reasons ...such as Flushing Out Old Debris or perhaps even chasing down any mysterious leaks, etc. Here you go:.

 

[movietvet]

Ok. I guess I got a little concerned after I was putting the new cap on and saw some black stuff inside the tank itself, and I wasn't sure if grit in there would be a problem. It doesn't appear to be oil or anything like that. I've already done my hoses and big fixes so I was on to the easy cheap cap ....the "just in case" type things and I thought well let's do it to do it cuz dirt in liquid can't be good but maybe I worry too much.

Well, again IMO, keeping dirt out whenever you can, is a plus. I check those caps and replace as needed. The dirt will not get past a tight cap but can on a loose fitting cap and that, again IMO, is an easy fix and inexpensive as well. I am anal about these things. I ran shops for 35+ years and have seen what dirt/debris can do to the inside of engines and cooling systems.

 

[Elizabetty]

Hey guys. I think I have what I need answered here. I tried to see if I'm able to close or mark it solved myself but I'm not able to as far as I can tell?

 

[movietvet]

On another forum I am on, you could have gone back to your very first post and in the upper right corner above the post window, there are 3 dots you click on and the options that open, allow you to edit the thread title and put the word SOLVED. I looked here and that option seems to not be available. Glad that it seems to be no longer a concern. Good luck.

 

[Mooseman]

I don't think that ordinary users can edit their thread titles or posts after a ceetain amount of time. I have added the Solved tag for you.

Anytime anyone would like something changed, edited or deleted, you can hit the Report button and put in your request that way. Any moderator can assist then.