The A4 thermostat system is a BYPASS thermostat. Running WITHOUT a thermostat will shunt the cooling water around the engine, the engine will NOT be cooled. A BYPASS thermostat system only cools the engine when the thermostat is 'closed', the thermostat closes and then sends the water flow INTO the engine block. To send FULL FLOW of water into the block, squeeze down on the bypass hose with a vice grip, etc.
More Importantly and a higher potential cause of 'overheating' on an A4:
The exhaust manifolds on Universal Atomic 4 engines are 'notorious' for forming '
slab rust' internal to the manifold; the slab rust will eventually block the flow of water!!!!!!!! ..... sometimes the flow will completely block, sometimes not, sometimes when you shut down the engine the slabs that block to exit nipple will fall to the bottom of the manifold and everything is fine .... for a while.
The formation of 'slab rust' is especially common in manifolds that had their water drained when the boat is long term stored. The best way to long term store an A4 is drain the water and then refill the water passages in the engine AND exhaust manifold completely with 50:50 antifreeze with RUST INHIBITERS; never store an A4 or its exhaust manifold 'dry'. When prepping for long term lay-up, run the engine at full throttle for a long time to develop 'heat' so that the engine internals become HOT and change/convert any destructive 'red' rust into 'protective' black (ferrous oxide) rust, ..... then add the antifreeze with rust inhibiters.
•Manifold examination:
Firstly, with explanation - when cast iron cools in it mold when being made/cast it cools in 'stratifications' like an onion. The zones between the stratifications are 'weaker' than the base metal .... and are more subject to 'rusting' than the base metal. Once the area between the stratifications begins to rust it will form 'red' or destructive red rust and red (ferric) rust is less dense than the base metal and thus 'pushes' the stratifications of the cast iron apart to form 'plate rust' or 'slabs of rust'. These 'slabs' usually fall to the bottom on the manifold but if large enough will be 'disturbed' by the water flow and will (occasionally) 'choke' the outlet nozzle at the rear (top) of the manifold. Usually when the engine is shut down, these slabs will again drop to the bottom of the manifold .... only to 'wait' until 'stirred up' and thus 'available' to again and suddenly block the outlet nipple on the rearward water exit nipple. The slabs will never come out of the manifold, because the inlet and the outlet are on top of the manifold .............. . Cast Iron when kept HOT as much as possible will form black/blue ferrous oxide internally in the presence of water which is protective. Slowly over time unless the engine is run to full operating temperature, the black rust will change back to destructive red rust, the low density red rust will start 'pushing' the stratifications of the casting apart. Alway run an A4 for as long and as HOT as possible (T'stat at ~150° if raw water cooled, and 180° for FWC cooled) to keep a well formed layer of black (ferrous) rust 'intact'. This is the reason that Navy ships with cast iron engine components virtually never are shut down --- it keeps the 'protective' black rust intact and prevents the 'red' rust from forming; and, is why for small boat engines and manifold you should always fill with antifreeze with 'rust inhibiters' for long term shut down.
Procedure: Remove the manifold, and 'invert it' (turn it upside-down) so that any 'slabs' will fall to the top of the manifold. You will then be able to 'shake out' most of the slabs. Most 'rust slabs' can be shaken loose or sometimes you can insert a 'stiff' wire to break them up. Then 'pickle' the manifold to remove any 'fouling' (carbonates) - best is to use an 'inhibited' boiler descaler compound such as RydLyme (
http://www.apexengineeringproducts.com/products/rydlymehome/), etc. The use of muriatic acid (HCl etc) or other non-inhibited means will only make the slab rusting 'worse'.
Then
PRESSURE CHECK the manifold to validate that it is still intact, and to determine that the manifold has not developed a 'pin hole leak' between the 'water side' and the exhaust 'gas side'. If there has developed a pin hole, the engine when it cools down will draw water into the cooling combustion chamber ... and you will rust the piston rings to the piston ring grooves and the cylinder walls !!!!!!!!!!!
How to do a pressure HOLD test on an A4 exhaust manifold ------
• Once any slabs are removed, fouling cleaned out and the manifold 'pickled' ----- block either the inlet or outlet nipple, use a hose and apply a 'cap' etc. over the end to completely block any flow for the test .... .
• Apply a Tee with a pressure gage, a valve between the pressure supply and the 'pressure gage' to the 'other' nipple, all connected to the manifold
• Apply water (or air) pressure to about 20 psi and
close the valve.
• If the pressure stays constant (does not decrease over time) when the valve is closed, then high probability that the manifold is 'intact' and without pin holes between the 'gas side' and the 'water side'; if pressure decreases then high probability of 'pin hole' inside the manifold and you MUST replace the manifold.
• If intact, then reassemble .... and promise yourself to NEVER again long term store the manifold with the engine 'dry' - only filled with 50:50 water/antifreeze with rust inhibiters.
The A4 cast iron exhaust manifolds are notorious for developing 'slab rust' and if the slab rust is severe will quickly develop 'pin holes' and you'll eventually get 'water in the cylinders' !!!!!!!!!!!!
hope this helps. ;-)
