Reviving this... I had the same symptoms. Bypassed the heater for the end of summer and coolant loss went away so I could be fairly certain that it WAS somewhere in the heater. Get to dreaded winterization day yesterday and decide to clip the wires on the pump, pull it out, and have a closer look. There were obvious signs of dried coolant on the bottom of the heater pump. 100% the culprit, same as eder above. I decided to tear into the pump and see what I could see. They’re on backorder from what I can tell.
[[disclaimer]] Although I am a P.E. level Mechanical Engineer, with a couple of decades experience in industrial automation (including vast experience with pumps); I didn’t design this pump, and I had a beer in hand for this activity. I’m guessing at a few things and making a few more up. Following my advice could result in a voided warranty…
First some info. I hope some of this comes in handy for someone down the road. The pump is Heatercraft part number 4838-111-A65. That is a rebranded/private label Shurflo pump – Shurflo makes it exclusively for Heatercraft. The “A” in the part number says it’s for OEM applications. I believe the 4838 in the part number describes it’s temperature and fluid type specification, but not 100%. This pump has both thermal and pressure relief valves built in – as you’d want when running 200 degree engine coolant! I didn’t call my dealer, but heatercraft has a similar model (the part number does not match perfectly) for $150 – it is backordered. Bakes has another similar model, but it seems like its for a shower, NOT engine coolant.
Like I said, I took the pump apart and nearly instantly found 2 problems.
1)There was a sheared origin in the temperature relief valve (the widget on the bottom with the wires running to it.
2)The adjuster on the pressure relief valve (the other widget on the bottom with a little hex key adjuster) was adjusted all the way in, with he set screw now falling inside the housing.
I’m not too sure which one caused the leaking for me, but I fixed both for free (I had an oring kit to match the damaged ring). I wont REALLY know if it’s fixed until spring when I really get to run it, but I reassembled everything, got it up to temp on the hose and there was no fluid coming from the bottom of the pump!
I should have taken pictures, but I didn’t… beer. The pump is a diaphragm style pump with 4 pistons/diaphragms. The 12v motor turns an eccentric cam (I prefer to call it a wobbler) that moves water from the inlet to the outlet. The attached image is of a similar pump. Like I said above, my problem was likely in the relief valves, but I could also see problems occurring because the diaphragms are ruiptured (replace the main body, part 2 in the diagram), or a bad gasket between parts 2 and 4 (the gasket shows is NOT the same as the gasket on the pump on my boat).
Again, I hope my experience helps someone in the future.
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