I found the source of the leak. It turned out to be the ECU Temp sensor which I had replaced a couple of months ago (with the Volvo factory replacement).

The Cause:

The Volvo replacement sensor had an aluminum sealing ring which had an OD that was too small. The difference between the ID & OD on the radius was about 2mm. This small radial sealing sealing surface, coupled with the thread relief on the turned brass fitting of the temp sensor, allowed the skinny sealing ring to drop (due to gravity), off-center into the thread relief during installation, thus further reducing the sealing at the ring on one side to a very small area of width approaching about 0.5mm! The brass fitting of the ECU temp sensor did not have a very good surface finish either, having what looked like swirl marks you would see from end-cutting milling even though I would think the part would be turned???

The Pain of Diagnosis:

Since the region in question behind the cylinder head is so blind, just to even see back there required removing the exhaust heat shield; dropping the down pipe & removing the exhaust manifolds. Then build a pressure leakdown tester to go on the coolant overflow reservoir and pressurize the system to 21 psi and study, study, study the back of the cylinder head with flashlight & mirror looking for the 'smoking gun' weeping hole; fitting or whatever back there. I never find it of course, so I have to make my best estimation of parts and get them ordered at a cost of ~$180 with a few odds & ends added in.

It was only after I removed the two heater hoses back there that I started zeroing in on the culprit. I was not inclined to suspect the ECU temp sensor as I had replaced it, and the thing WAS tight in the cylinder head. Only when I had removed the temp sensor and studied it very carefully did I see what was going on. Then I was a bit discouraged at the lack of competence of Volvo for shipping this ECU temp sensor with this joke of a sealing ring. I then remembered thinking that the ring looked a bit small when I got the part, but hey, it came from Volvo; who was I to second-guess their engineering? . . .

The Fix:

Skim the sealing face of the brass ECU temp sensor on a lathe to make a good finish sealing surface. Get an aluminum sealing washer with much bigger OD. The c-bore in the back of cylinder head is fairly large, so I got a sealing ring with about 5mm sealing area on the radius. Put the whole thing back together with new heater hoses; new exhaust manifold gaskets; a couple of new down pipe studs; new washers & nuts; new exhaust manifold flare nuts. . . about $120 worth of new parts, all because of the inappropriately sized sealing ring.

So, buyer beware; one must scrutinize even parts received from Volvo. If it doesn't look quite right, it may not be right.

Another humorous irony is that when I ordered the parts, I knew I would need some new sealing rings for the ECU temp sensor, so I asked for some, but was told that they could not cross-reference that as it was not sold as a seperate item; it only came WITH the ECU temp sensor!

Another adventure comes to a close. At least I didn't have to remove a cylinder head.