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I've got a slow coolant leak. We pressure checked the system at the shop today, started at 18psi and went up to 30psi, no fresh leaks found with engine off. When we started the engine, we see a fresh dribble coming from the bottom of the water pump. I want to stick with an OEM style pump, preferable ACDelco. IIRC, I currently have a 'LS2' water pump mated between my F-body accessories and my LS3 block. I know it was replaced in the late 2000's but I don't recall if it was replaced since then. At any rate, I'm searching for replacement parts and I'm slightly confused which is the latest and greatest ACDelco pump & gaskets to use. I seem to recall that the LS3 water pump was not physically compatible. Below are a few I found that claim to be compatible, not sure which to order?
water pump gaskets: PN 12630223 or new PN 12559648
water pump: ACDelco LS2 water pump 89018052 (251-734), 12681185 (251-744), 19195105 (252-846)
My water pump did the same thing last year. I replaced mine with the ACDelco 252-846. My car is a 2000 Trans Am with stock LS1 block so not sure if that pump will also fit an LS3 block?
From: Poway, Ca. where GOD and the sun always shines
I would avoid the AC Delco "Professional" line, my understanding is it is all from China. If your system is old, and you need to remove the radiator to get to the water pump, you should consider replacing the radiator and cap. I had my leaking water pump replaced with the LS2 unit about 2 years ago in Sara (2001 Z28) along with the radiator. So far so good.
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Water pump weeping 
