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I'd like to flush out the heater matrix on my 2007 Hawk STi.
I can disconnect the rubber hoses at the firewall which will expose the two aluminium matrix ports. The ports sit next to each other left and right, rather than one on top of the other.
I'm likely to add Holts Speedflush neat to the matrix via a PVC hose / funnel. My idea is to add the Speedflush neat and leave to soak for maybe 30 minutes, But which port from looking at the bulkhead in front of me is the inlet to the matrix and which is the outlet? I ask so I can backflush with water etc.
I tried soaking the matrix with distilled malt vinegar for an hour last week, a few bits came out but not much from a water flush, I think the matrix is scaled up. When refilling with coolant from the left port, I started seeing coolant come out of the right port - but I had only added around 350ml of coolant which would suggest the matrix is heavily restricted. Not forgetting the volume in the aluminium pipes would have been using up some of that 350ml.
This shows my matrix set up 72130FE000 but I'm not sure if after the flush it would best be filled from the bottom up (so add coolant to the left hand port)....or....fill with coolant from top to bottom using the right hand port, I want to minimise air locks.
I've added above what I think is inlet / outlet, but does anyone know if it's bottom fill or top fill ideally? Is it better to bottom fill it with coolant from the left port so that any air will bleed out the right hand port as it fills?
Last edited by Hawkeye D; Mar 27, 2026 at 06:54 AM.
Really just because I found the overflow pipe nipple on the radiator hard line was totally blocked with junk, I know the passages in the matrix are narrow.
It's not been great for a while, and now made worse by the RCM Cyl4 mod recently fitted.
I think maybe my water pump has done it's time, it's the original factory pump, 2007 and 120k miles on it...
I've had a couple of cambelt / tensioner services over the years, but looking at the invoices, both invoices only mention water pump gaskets, not the actual water pump.
Is 120k miles about right for a water pump to become lazy?
Heater Matrix
