Trick to top-off coolant on L5P Duramax Coolant Reservoir/Overflow tank

RonnyRaygun · December 3, 2021

As many of you probably know, topping off the L5P Duramax Coolant Reservoir/Overflow tank is not really as simple as it may appear. Even if you add coolant to the overflow compartment of the "bottle" the fluid does not always make its way to the pressurized part of the unit. There are a bunch of videos on the web to get the coolant into the pressurized area of the tank, but I found a little (if time-consuming) way to do so that is quite easy.

Now why do we find ourselves in the position of needing to do this in the first place. In my case, coincidentally right after a battery replacement, I got the dreaded "Low Coolant" message on my Driver Information Center display. Upon inspection of the coolant tank I found that the system was indeed low. Not realizing that the reservoir has two compartments (one pressurized like an old-fashioned radiator top/side tank, and another non-pressurized "overflow" container) I simply added coolant to the most easily accessed left-hand threaded fill neck. After that, the message did not come up on the display for a few days and then it showed persistently. That necessitated a change in the whole "bottle" (as termed by the local dealership's service manager "We do 2 or 3 a week." - GM 84257137 Radiator Surge Tank - find it here https://www.amazon.com/gp/product/B07JBBDW9Z/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1 ).

But that's for another thread. That is because the integrated coolant level sensor failed and cannot be removed and changed-out - thanks GM. Even my wife's 2006 AMG cls55 even had a changeable one!

Anyway, this topic is just for a simple way to get coolant into the system's pressurized tank on the unit. After watching an online video (sorry I can't remember the guy so I can't give him proper credit ) I learned that there is a vent tube molded in the tank that allows expanded hot fluid to flow into the overflow portion of the tank.

Keep in mind that if you simply add coolant to through the left-hand threaded cap at the top of the tank, it may not get onto that pressurized part of it. OK, enough with the suspense - read on.

You can add coolant to the tank by doing the following:

1. Get a large syringe - I used 100 ml ones after I took the pictures attached to this post ( https://www.amazon.com/gp/product/B0768FKX9Q/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1 ).

2. Remove the left-hand threaded cap from the top of the overflow portion of the tank.

3. Remove the right-hand threaded pressurized cap from the lower side of the tank near the fender below the hood hinge. Yep, there is a out-of-view second cap tucked in down there that has only a tiny weep hole behind it.

4. Using the syringe, add coolant into the little vent hole in the top neck of the tank just under where the left-hand threaded goes. Push it in quickly so it flows in to the tube and doesn't back-flow into the overflow portion of the tank. Don't worry if it does back-flow, as you can just draw it out of there and put it into the tube.

5. Repeat the injection of the fluid into that little top tube until the coolant reaches the weep hole behind where the right-hand threaded cap goes on the lower part of the bottle. You may need to use a mirror or the camera on your smartphone to observe this level. Once the fluid starts to flow out of this little hole you are done putting fluid in the pressurized part of the tank.

6. Add or remove coolant from the overflow portion of the tank to set the level to the top of the indicator arrows molded into the tank.

7. Re-install the caps an tighten them appropriately.

8. Use your garden hose to rinse the area and the tray under the tank.

Now some of you may quibble with this method as being time-consuming and tedious, but I have found that it works adequately for topping off the system when low or after installing a new tank (to fix the faulty sensor induced "Low Coolant" message). If you are changing the tank, be sure to re-check the level after a run/cool down cycle or two to ensure as much air is removed. When doing the change-out just be careful to keep as much fluid from getting out of the system. I used a piece of small oxygen-generator tubing I had laying around to siphon as much coolant out of the tank prior to removal - again time consuming but effective so be patient. That coolant was properly discarded because it had some flaky black stuff in it from several years of operation.

Using this procedure does not require any fancy air purging devices as long as care is taken to minimize the introduction of air into the system. And, you will have that giant syringe for other non-sterile fluid aspiration/dispense projects in your shop.

Happy motoring!