Car Stuck! Engine Coolant boiling over - Update 1

[SJ]

I received numerous inquiries regarding the reservoir cap and keeping system
pressurized, etc etc.

Many think that the pressure cap is the culprit.

A reply to a poster:
"Keep in mind that the reason for pressurized system is to prevent
boiling of coolant at elevated engine temperatures . . . when the engine is
working hard and getting really HOT.
I was testing with cold engine, this is long before the pressure cap starts
to function.
There is no way to test for gas leaks if you seal the system. You wont be
able to see anything because you cannot replace/add hoses with clear tubing
in order to watch the coolant/water flow.
The small tubing from the radiator nipple to the jar of water prevents
sealing the system. But this is what you watch to see gas escaping.
I'll run the same test on my other audi and time it, and watch the temp
gauge. I expect to get some bubbles . .maybe from the impeller cavitation,
but nothing like what I have witnessed on the problem audi."

-------------------------

To make sure that I was not overlooking anything, I tested my 88 audi and
retested the ailing 90 audi.

The setup:
- disconnect the small line at the radiator, I think this is the air purge
line for the system. The other end goes to the top of expansion tank. I blew
into it to make sure the end inside the tank was in the air space and not
under the coolant level. It emptied into the air space.
- attach clear tubing to this radiator nipple, other end of tubing goes into
large jar half filled with water. This will be my visual indicator of
bubbles.
- cap sitting loosely on tank (no sense making it tight since the air purge
line is open to atmosphere)

88 5kq - test vehicle, no problems with engine.
0.00 - engine start
- heater set to economy, temp to HI ( this opens the valve to the heater
core)

0.04 - heater hose starting to warm up
- upper radiator hose starting to warm up
- no bubbles
- temp gauge just off the peg

0.09 - temp gauge at second mark (about 20%)
- no bubbles at idle or high rev

0.13 - temp gauge at 25/30%
- upper rad hose hot
- no bubbles at idle or high revs
- shut off heater

0.18 - temp gauge at 30%
- a few bubbles in tube . . .coolant flow into jar (coolant expanding)
- fan kicks in
- Shut off engine


Perform same setup on the ailing 90 100q

0.00 - start engine
- heater control to economy, heat to HI
- gauge pegged at bottom
- no bubbles idling or revving

0.01 - some bubbles seen
- water heater pipe getting warm
- gauge still bottomed out

0.03 - lotsa small bubbles when engine revved
- temp gauge coming off the bottom
- upper rad hose warm(no thermostat)

0.10 - temp gauge at third small line(10%)
- small bubbles when revved

0.13 - temp gauge at first major line(25%)
- steady bubbles at idle
- more bubbles at rev

0.17 - temp gauge at 35%
- upper hose is hot
- lotsa bubbles when revving
- no bubbles at idle
- shut off heater

0.18 - temp gauge at first major line(40%)
- bubbles at idle
- lotsa bubbles when revving
- radiator fan kicks in
- shut off engine
- bubbles still flowing . .then subside . .water from jar flows into tank


Conclusion
1. You dont need the reservoir cap to test system at lower temperatures of
coolant.
2. I have a leaking head gasket.

I'll be posting a query regarding head removal as soon as I finish reading
the manuals and looking at the FA pix. The manual is not too helpful in
detailing the procedure.


SJ
85 Dodge PU, D-250, 318, auto
85 Audi 4k - - sold but still on the road
88 Audi 5kq
90 Audi 100q