Volvo Maintenance Hints for 7xx/9xx - Bill Garland's Nuclear ...

Various Vacuum or Intake Hose Leaks:
[Response 1:] The problem turned out to be caused by a VACUUM LEAK in a rubber tube under
the intake manifold. The tube only leaked when it was hot, and subject to vibration/movement of
the motor. My mechanic has been elevated to "saint of motors." The bright side is we now
have all new ignition, wires, ECU, fuel pumps, etc. There is a moral here: "Go ahead and get
your hands dirty...really dirty." Unless you know the car's history fully, remove every rubber
hose and inspect it closely, even if they are hidden below the intake manifold and throttle body
(yes Virginia, that may be a very oily area). A minor vacuum leak there seems to have major
consequences with the injection system. The way-high boost TB guys have already found that
the main boost hoses can give out and cause poor driveability. I am here to tell you that even
with stock boost getting to the local grocery store can be risky if the various minor vacuum
hoses develop hidden cracks. [Tip from Chris deCourcy-Bower] If you notice low turbo
pressure, reduced performance, hissing noise when accelerating, then check for a split or delaminating
hose. Check ALL of them including the short piece where the air leaves the
intercooler. If it turns out to be a damaged intercooler you will need to visit the junk yard. They
are not cheap. See the discussion in Special Tools about a homemade leak tester for the intake
system.
[Response 2:] I had a poor driveability problem with my 87 760t on take off. Turned out to be a
vacuum leak on the big hose from the air box to the intercooler.....and a big dent in the
pocketbook since the thing is a preform with a couple other hoses molded into it......$130 or so. I
just replace all the vacuum hoses every 5 years or so.
[Response 3: John Sargent] The turbo on our third 740T was not putting out enough boost. I
used a pressure gauge (installed with a tee fitting under the dash) to verify what the boost
gauge was telling me, the level of boost was too low, about 1.0 to 1.5 psi. I checked all of the
hoses from the turbo on, but all were okay. The turbo spun well and did not contact the housing,
and it was quiet. The car seemed to drive fine, and seat of the pants performance was okay, but
less than the other cars which have the Turbo Plus. There was no black smoke to indicate a too
rich mixture caused by a leak after the AMM. The wategate was connected, and not seized. The
catalytic convertor was not plugged. It didn't make sense, everything to check was fine, but the
boost was still too low. Finally I pulled the rubber line that connects the intake manifold to the
boost gauge, and blew on it. Well, you have probably guessed that the rubber line had a hole in
it where it passes under the intake manifold. The hole would mostly seal itself under vacuum,
but would go wide open under pressure. That was the cheapest turbo overhaul I've done!
Always be sure of your tests, and don't jump to conclusions. [Ernest Smith] When my turbo
stumbled under boost, my mechanic first put the complete intake under pressure. He found a
couple of bad gaskets at the BOV valve, and made new ones to fit.
Tips for Checking Intake Hoses. [Randy Starkie] I had one intake hose leak that I couldn't find
until I applied 15psi of compressed air on the end of the hose that connects to the turbo. I made
a plug of the correct size and added a Schrader (tire) valve for this test. The plug I use is the
cap off of a master cylinder that has a schrader valve in it for flushing the brake systems on my
cars. Put the plug in the hose after disconnecting it from the turbo and apply the air. Listen for
the leak. Splits are hard to find sometimes without simulating boost with compressed air. [Tip] A
crack in a hose may only show up under boost. I found a leak in a guy's hose by having him sit
in the car with all brakes on full and in drive. I opened up the throttle under the hood till it was