- Two days ago my truck’s cooling system died. I come to find out after inspecting it completely that the radiator needed a new core (this didn’t surprise me), the hoses needed replacing (no problem there), and the reservoir is cracked on top.
- Now, the only place to get a new reservoir is the dealer; the auto-parts stores only sell a “generic” one that won’t fit in the space the OEM one will. In addition to the “fill hole”, the reservoir has a big open vent hole in the top, about 3/4" across, and the fill-cap fits loosely. My question is, do I even need to replace it, or put water in it at all? I always guessed that it was supposed to have coolant in it, because there’s a fill line about halfway up, but having taken all the pieces out and looked them over I don’t see any way for coolant to get from the reservoir into the radiator. The connecting tube connects to the top of the reservoir tank (not the bottom), and the other end connects to the radiator cap neck above where the cap seals, so there ain’t no way I can see for the line to suck coolant back into the radiator; only blow it out. - All the coolant in the reservoir did was grow gray slime in it. I guess that the reservoir is there to catch the boil-over if the water pump fails, but does the reservoir do anything the rest of the time?
-(this is an '82 vehicle, so there’s no computer lurking anywhere in there) - MC
MC
The truck will run without that reservoir.
The only thing is you will have to check the cooliant level more often and you will have to open the radiator cap to do it.
As the coolant heats up it expands and pressurizes the cooling system to about 14PSI.
The radiator cap has a couple of valves in it, one vents hot coolant to the reservoir when the pressure exceeds 14PSI and one allows the vented coolant in the reservior to return to the radiator as the system cools and the coolant contracts and creates a vacume in the system.
YES
LION’s response is correct but if your car or truck has a functional reservoir it should have water in it. (Well, it should have the same thing the rest of your cooling system has in it – presumably a mixture of water and antifreeze.)
If you find that your reservoir keeps going dry in normal operation it’s an indication that you have a cooling system leak which is a non-trivial problem. A slow leak that you’re aware of can be handled by frequent replenishment but it’s a good idea to have it tested and repaired because small leaks frequently turn into large ones.
Alright, alright, alright enough is enough you people make this so muddled. To start out with I am a qualified auto mechanic with a degree and I also am the king of jimmy rigging stuff on the side. It’s real simple MC heres how it works ready???
A. If you radiator is operating perfectly no you don’t need a water resovoir to persay and yes you would need to check it more often.
B. However, it is better to have something than nothing so here is what you do: Ready??
Since you don’t want to cash out the mulah for a OEM part or can’t find it or whatever the next best thing is search the Laundry detergent aisle of your nearest large grocery store. Find a bottle that will fit down in the whole that you have in your car. The advantage to detergent bottles is they are hella reliable and durable and they almost always have a see through side stripe to see your coolant level. milk jugs or the actual antifreeze jug works also but you can’t see in the antifreeze jug and milk jugs are week as hell. Next, secure said jug in the spot with bailing twine or some such stout wire. Drill, cut, or poke a hole in the top. Remove top fill with half and half antifreeze/water and put top back on and insert hose. Works great and doesn’t cost a scent since you can use the same little drainage hose on your radiator and you needed the laundry detergent anyway 
oh yeah and for the really dumb THOROUGHLY WASH OUT BOTTLE BEFORE USE!!!