So I noticed oil seeping from the oil filler neck on the 88 to the point where it was pooling in the block 'pits'. Being a newbie, I couldn't see how to get at the oil filler gasket except by taking off the intake or removing the thermostat housing. The thermostat housing looked like it would not come out without removing the filler pipe. After some research, I found a picture of the thermostat housing removed while the filler neck was still in. That was all I needed to get started...and I was looking forward to some quality under the hood time with the S4.
So for the other newbies that would like to perform this job, I've included some pics of how I did it. Not too hard.

First, here's the gaskets I ended up replacing - thermostat housing to heads (2), thermostat housing to block o-ring, thermostat o-ring, thermostat sealing ring at back of housing, new thermostat, oil filler neck gasket - so it's best to get these ahead of time.
Name:  Gaskets replaced.jpg
Views: 83
Size:  28.4 KB

First, Jack up the car and support it with jackstands. Then, remove the cross brace. It takes an 8mm allen wrench.
Name:  cross brace 1.jpg
Views: 85
Size:  64.0 KB

Next, drain the radiator and the engine block (2 13mm bolts one on each side of the block). Radiator fluid looked OK, the previous owner changed it last in 2004.
Name:  Drain Fluid.jpg
Views: 83
Size:  40.6 KB

Next, I removed the hoses from the thermostat housing. There are 4 - block to radiator, radiator to block, bleeder, and heater return hose. I replaced all 4 hoses since they were all original and 20 years old.
Name:  Remove hoses.jpg
Views: 83
Size:  76.5 KB

I then removed the fuel pressure damper. I took the plastic cover off the passenger fuel rail so I could access the hex mount to counter hold the connection - then disconnected the hose.
Name:  fuel line 1.jpg
Views: 80
Size:  76.9 KB

Next, came the remaining 2 connections - one at the driver's side fuel rail and the other at the bottom of the damper. Also remove the vacuum connection at the top.
Name:  fuel line 2.jpg
Views: 83
Size:  76.3 KB

Then I removed the thermostat housing allen head bolt (6mm) that also holds down the damper bracket.
Name:  fuel line 3.jpg
Views: 83
Size:  78.7 KB

The damper assembly comes out.
Name:  fuel line 4.jpg
Views: 82
Size:  70.8 KB

Next, I took the remaining thermostat housing allen head bolts out. The tricky one is the driver's side rear next to the intake. It's next to impossible to get a socket extension in there straight so I used a regular allen wrench with an adjustable wrench on top to turn it. This allowed me to get a straight and solid fit with the wrench and avoid stripping it out.
Name:  housing bolt.jpg
Views: 82
Size:  68.8 KB

After the bolts are removed and the two temperature sensor wires disconnected from on top, the housing comes out but it's a tight fit. By rocking it back and forth and pulling up and toward you at the same time, it eventually comes out.
Name:  housing removed.jpg
Views: 81
Size:  74.7 KB

Once the housing is removed, it's a good idea to stuff the openings with something to prevent debris from falling into the openings while working on getting the filler neck out.
Name:  stuff openings.jpg
Views: 82
Size:  78.0 KB


Next, to remove the filler neck, I disconnected the two hoses attached to the base.
Name:  oil hoses 1.jpg
Views: 78
Size:  71.0 KB

Then remove the two 10mm bolts that hold the filler neck to the block.
Name:  oil filler bolts.jpg
Views: 77
Size:  86.2 KB

Remove the filler neck...
Name:  filler out.jpg
Views: 76
Size:  63.5 KB

And plug the hole to prevent debris from entering the block.
Name:  plug holes 2.jpg
Views: 77
Size:  83.4 KB

With the thermostat housing out, I began disassembly. First I removed the thermostat cover plate.
Name:  thermo bolts.jpg
Views: 77
Size:  61.7 KB

I made note of the orientation of the thermostat. Notice the 'up arrow' on the thermostat oriented at the top as well as the o-ring on top of the thermostat.
Name:  thermo and oring.jpg
Views: 77
Size:  77.2 KB

I then removed the thermostat sealing ring at the rear of the housing. I used a screwdriver to bend the edge in toward the center enough to get a grip with some pliers then pulled it out with the pliers being careful not to damage the housing.
Name:  sealing ring.jpg
Views: 77
Size:  86.2 KB

After removing the remaining gaskets and o-ring from the housing, I began putting in the new seals, gaskets. First the sealing ring at the rear of the housing. This just presses in.
Name:  sealing ring in.jpg
Views: 78
Size:  62.2 KB

The sealing ring sits flush...
Name:  sealing ring seated.jpg
Views: 78
Size:  45.6 KB

Next, I installed the housing to heads gaskets.
Name:  housing gasket.jpg
Views: 77
Size:  66.4 KB

Then the large o-ring for the connection from housing to engine block.
Name:  large oring.jpg
Views: 79
Size:  55.6 KB


Next came the thermostat and o-ring. I placed the thermostat in with the arrow oriented at top followed by the o-ring.
Name:  thermo in.jpg
Views: 75
Size:  60.8 KB

Then tighten the thermostat plate back on.
Name:  tighten thermo housing.jpg
Views: 75
Size:  68.2 KB

Here's a pic of the clearance between the thermostat and rear seal when the engine is cold - recirculating coolant through the engine until the thermostat closes against the seal and simultaneously allows water to flow to the radiator.
Name:  seal clearance.jpg
Views: 76
Size:  41.6 KB

Next the parts are ready for installation. I installed the filler neck first (after cleaning it off and installing the new gasket, of course.
Name:  oil filler in.jpg
Views: 76
Size:  69.8 KB

Then comes the thermostat housing. Installing it the same way it came out - rocking and pushing down and away from you. After some maneuvering, it will go back into place. Before putting the thermostat housing back in, you could fill the engine block with coolant to minimize the bleeding. However, I left it empty and needed to spend a little extra time allowing it to bleed off the air in the block before I would get normal temperatures.
Name:  housing back in.jpg
Views: 74
Size:  76.6 KB

At this point, finishing the job is a matter of installing the thermostat housing bolts back in (14 ftlbs - the manual recommends the 2 step approach - 7 ftlbs first then 14 ftlbs next). Remember to install the fuel pressure damper bracket back in place before tightening the driver's side front allen head bolt. Finally, re-attach the crossbrace and tighten down before lowering the car. Here's everything back to gether again...
Name:  all done.jpg
Views: 74
Size:  84.7 KB