Air lock in home made EWP system
In August I added a post about making your own EWP (electric water pump) system (in this case installed in our diesel PT Cruiser). At the end of it I added a little note about the car still overheating once or twice and it being due to a faulty fuse, well I was wrong, it was a very specific form of air-lock and here is the fix (well, at the bottom, skip ahead if you want).
Just to also be clear; I have been using this fix for 6 weeks and it does solve the problem. What problem? The problem of the two electric water pumps pulling air down from the top of the overflow tank (via the overflow pipe) and locking it into the top of the radiator. Eventually the entire radiator was so full of air that the pumps were sucking air, not water, and then they began to cavitate. When I thought I was fixing it by wiggling the fuse what I was really doing was making the pumps stop long enough to get water back down to them.
I noticed the problem whilst bleeding the system; you always have to bleed the radiator after you have changed the coolant, but when I bled the system the overall level of coolant (post bleed) was unchanged, it should have dropped and more coolant should have been required to replace the evacuated air.
In this case the coolant was always too high in the overflow tank (even when the car had cooled down overnight) before bleeding, it was never lower.
Since the coolant level was never changing, and when I opened the radiator cap the system was not pressurised, it meant that the system was sealed; no extra air was getting in and no coolant was leaking out, instead the distribution of the air and water was changing.
Since the only place the air could be coming from was the top of the overflow (catch) tank it was pretty easy to reason it was being pulled down via the overflow pipe into the top of the radiator (the overflow pipe ends in the thermostat housing, just next to the top of the radiator).
Once I realised that the fix was simple; add an 8mm (tail size) non-return valve to the overflow pipe. This fix took minutes, just wait for the car to cool down, open the radiator cap (just briefly to check the system isn’t still under pressure), cut the overflow pipe (I used the serrated blade on my Charge TTi), insert the non-return valve, and use some jubilee clips to secure it.
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