The A4 is a thermostat BYPASS engine .... meaning if the thermostat is CLOSED (bypassing the flow 'around' the engine) ... the entire flow of water is bypassing the block. When the thermostat is OPEN, water enters the engine block. (corrected/edited)
A more common water ***blockage*** problem in the A4 is the Universal Cast Iron Exhaust manifold ---- slab rust !!!!!!!! Huge 'platelettes' of rust break loose from the internal water passages of the exh. manifold casting and thus block the water outlet. (Sometimes these 'slabs' will temporarily shut of the flow and when the engine is stopped, they fall back to the bottom of the inside of the manifold ..... waiting to later block the flow at the most inopportune time. Easy way to check: remove rubber inlet hose and see if there is flow (3 gallons per minute at ~1500 rpm), then reconnect the inlet and disconnect the outlet. If no flow at the outlet, remove the exhaust manifold and turn it upside down .... and use a stiff wire and insert in both the inlet and outlet and BREAK up all the 'slabs', then shake the manifold until they ALL come out .... there will be some slabs that have 'cemented' themselves to the bottom of the manifold. DO NOT use ACID to clean out the manifold (or engine).
BTW - take a dental pick or other 'probe' and count how many treads are left in the engine casting (manifold bolt holes) when you remove the manifold and its bolts. If you can count 4-5 threads remaining, then the A4 still has enough 'meat'; 3-4 or less threads remaining .... you will soon need a new engine.
For more precision to validate that the manifold doesnt have pin holes (communicating water to the 'gas side' and into the cylinders), seal either the inlet or outlet, apply a 'nipple' to the other and with a gage and hand pump (*with gage and gas-tight shut off valve) inflate the manifold with at least 10-15 psi of air pressure, and shut off the valve. Watch the gage to 'hold' pressure - shows that the manifold is still 'intact' internally.
Never ever ever, drain the cooling water from an A4 and store it 'dry' as this will accelerate the formation of 'slab rust'. Always 'store' or lay-up the engine with fresh water + antifreeze with **RUST INHIBITOR**, never 'dry'.
Other: The A4 usually never wears out!!!! it usually rusts away on the 'inside'. The best way to prevent internal destructive red ('ferric') rust is to 'run hell out the engine' and be sure that the engine/manifold gets HOT regularly. The HOT temperature will 'convert' red (ferric) rust to black/blue (ferrous) rust - which is 'protective' to cast iron (the US Navy 'never' shuts down its cast iron engine systems for this very reason). When being made in a foundry, etc., cast iron when it cools down in it mold when made, stratifies - like an onion with many 'layers'. The formation of red rust between these stratification layers generates huge pressure and breaks the layers apart (slab rust), keeping the protective (ferrous) black rust intact will prevent such destruction - by use of rust inhibitors or keeping the engine HOT. If you want your A4 to 'last', run the hell out of it.
Last edited by RichH; 11-11-2009 at 02:05 PM.