I have noticed that my turbo has dropped to about 3psi of pressure. I assume that the wastegate is somehow controlled by the computer and the 3psi is the fail-safe pressure. That said, I don't see any computer control on my turbo; the wastegate seems to be controlled by a vacuum regulator hooked straight into the intake manifold. I assume that if the pressure in the intake manifold (boost pressure) gets high, it forces the rod out on the regulator which then opens the wastegate. I don't see any additional computer controls.

What are the steps for diagnosing a turbo issue?

I have checked all of the vacuum lines and replaced the ones with issues

I get the 3psi turbo pressure at around 2500 rpm, and it does not increase with engine speed. If the turbo was seizing, I would think that at lower rpms, it would not respond at all.

There are no weird turbo noises, nor have there ever been any.

There is no smoke; my car passes an emissions test.

I have also inspected the inlet of the turbo; the vanes spin freely and they are not mangled. There is no contact with the housing.

The only weird thing is that in the winter, I get some obstruction in the intercooler; maybe it is a bit plugged.

Found the problem (and explained another)...

The main hose coming out of the top of the intercooler was split on the underside close to the intercooler. I guess as the turbo added pressure, it simply escaped from this leak. This also explains the mess of oil I have on the top of my engine - I had previously thought it was coming out of the top of the dipstick because of the blow-by that I have.

Two mechanics missed this problem. Man, I wish I could find a good mechanic in my area.

Shan,
<< I wish I could find a good mechanic in my area>>

Sounds like you did. Good find. I had the same problem with the 94-850 my step-daughter bought. A trusted mechanic missed that one too. Volvo wanted 45 bucks for the hose. I replaced it with a 2.5", 60 degree bend, truck radiator hose for about $10.
--
Tom 69-1800S, 72-1800ES, 96-850T

unfortunately, I found and fixed it myself. Still looking for a good mechanic.....

I think the hose is 2 1/4" on the intercooler end and 2" on the other end - I could not find a suitable replacement hose, so I am getting 'hosed' by the dealer with a new one... $72 CDN.

Shan

Just a thought, because this happened to me. Check the bottom of the inercooler housing behind the radiator. I wasn't getting much boost and found that the intercooler case was had a hairline crack. Sometimes if you hit curbs and have damaged the under carriage plastic shield, this may be the result. I took the dremmel tool, made the crack bigger and filled it in with quick set marine fiberglass repair kit that had the fiberglass mesh. Voila!! -- fixed in about an hour.

good luck

Just a hunch: presssure check all your vacuum lines and connectors. On my cars (not turbos) they crack in places I just can't see. My '94 850 non-turbo had a couple of failed vacuum connectors and maybe a cracked vinyl vacuum line. Pep Boys (auto parts chain of stores) carries replacements at about 1/3 the price from the Volvo dealer.
--
-- Bruce / '82 244 (315k miles!), '86 745, '87 760Ti, '94 854