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Overheating Issue - Mystery to Solve

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Old 02-12-2018, 10:11 PM
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Default Overheating Issue - Mystery to Solve

OK here we go!

The vehicle is 1996 A6 C4 2.6 12V Auto - Australian RHD - approx 200,000 km's

Original problem was over heating and coolant blowing off through the expansion tank cap.

Whilst I was fixing (or trying to) the overheating problem decide to do the cam belt and lifters.

A few broken parts were found along the way and I have progressively replaced all of the following

Cambelt and tensioner
Cam seals
Rebuilt the lifters
Replaced camshaft position sensor
Water pump
Thermostat
Radiator
Thermoswitch at bottom left of radiator
Bypassed the fan resistor (was blown - waiting for new parts)
Replaced fuel pump (just died sitting in the tank)
New expansion tank cap
Serpentine belt
Flushed cooling system, drained engine etc

Fans work when hot wiring at the switch plug
Dash temp gauge seems to be working - only ever gets to about 91-93C
Have not tested expansion tank caps (i have 2) - waiting for an adaptor

Have repeatedly pressure tested cooling system and no leaks and holds 1 bar for at least 24 hours.

So - here is the problem.

Start the car.
Wait for thermostat to open (currently testing without any thermostat, will reinstall when fixed)
No bubbles at all in the expansion tank
Have attached a pressure gauge to expansion tank - pressure rises steadily - no jumps or rapid increase. Although I have not left gauge attached to full temp as I don't want it to get it full of coolant.
Temp on dash gauge rises to about 90C-93C
Can just still put hand on top and bottom radiator hose
Radiator around the temp switch (bottom left) a bit cooler
Coolant blows off through expansion tank cap (dash temp still 90, fans have not started as not hot enough around the switch, meant to come on at about 90, have tested and it works on the bench)
Stop engine and swear

The only thing I can think of now is that this is a blown head gasket, cracked head or cracked block.
BUT
The coolant pressure test doesn't leak down and there are no obvious bubbles in coolant, no oil in water, no water in oil.

Has anyone seen a cracked head / blown gasket where a pressure test still passed as OK.

Am waiting on a leak down tester to see if that finds anything

Is there anything anyone can suggest before I rip the heads off

Any help really much appreciated
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Old 02-13-2018, 09:10 AM
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Your problem is currently testing without any thermostat, will reinstall when fixed.

On the 12v the thermostat is a coolant diverter controlling the 'by-pass" or radiator flow. Without it the coolant will not flow correctly.

Do install a thermostat and bleed the system to do your repair or diagnostics. If you think you have a bad headgasket or cracked head..get a block tester from a local parts store and test for exhaust gases in the coolant.

Last edited by Rusty Spokes; 02-13-2018 at 09:12 AM.
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Old 02-13-2018, 01:24 PM
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Thanks for replying - much appreciated!

OK. Will put the thermostat back in and see what happens.
I assume you mean the liquid chemical test kit?
Have you seen a cracked head / blown gasket where a coolant pressure test still passed as OK?
That's a great diagram of the cooling system - what publication is it from?
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Old 02-14-2018, 06:55 AM
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Originally Posted by zartoop View Post
Thanks for replying - much appreciated!

OK. Will put the thermostat back in and see what happens.
I assume you mean the liquid chemical test kit?
Have you seen a cracked head / blown gasket where a coolant pressure test still passed as OK?
That's a great diagram of the cooling system - what publication is it from?
Yep..the block tester kit where you pull the gases from the coolant tank into a container with a rubber bulb and look for a change in color of the fluid inside the container. About $40/$50 at NAPA. Works well finding combustion gases in the coolant tank..but I don't think you have that problem.

Usually when you have a blown headgasket or cracked head the coolant pressure test will show a leak. Another test is the rubber glove test on the coolant tank. If you remove the coolant tank cap (engine cold) and use a rubber glove with a rubber band secured in its place..start the engine and watch the glove. If the glove inflates in about 30 seconds you have a head gasket problem. Test for less than a minute as the glove will inflate when the coolant warms up..that is normal. I have not seen a coolant pressure test not find a bad head gasket or bad head.

VWVortex.com - inflatable glove head gasket leak test question


The image is from the old 12v.org website which is idle now. Impossible to really get deep information unless you google your search words with the 12v.org in the search panel. Here is a link to the site..and there is more information deep inside the website but impossible to navigate to.

12v Pages - Home
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Old Yesterday, 10:10 PM
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So ......

Reinstalled the thermostat.
Got a vacuum coolant refill kit

Made absolutely sure I purged the coolant system of all air.

It works!!!!!!!

Hooray - not a head gasket (so far)

So my current problem was caused by leaving the thermostat out and having air in the cooling system.
Previously when filling the system I didn't purge the air properly and that was what was causing the coolant to blow over the expansion tank cap.

Couple of points.
With all new radiator, water pump and thermoswitch etc this car will run and the fans will cycle without the expansion cap on and not boiling with only water.
If when it warms up you see your coolant level rising quickly = that is air in the system = leave the cap off. If you have overfilled the reservoir than its going to over flow. Too bad. Better to leave the cooling system 1 to 1.5 litres empty and slowly top up as the air purges.
The vacuum kit for the coolant helped but there was still about a litre of air in the system after filling.

Still going to do a leakdown test and block test just to be sure but seems ok so far.
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