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

the engine, you blow black smoke/overfuel. Could also be fuel pressure regulator, but unlikely
since it idles, apparently. [Woody Sulloway] My engine was cutting out when the turbo was over
50% boost. As the boost increased the car began to surge- almost like a prolonged miss;
accelerate, bog momentarily, accelerate, bog momentarily in a rather rapid sequence. It turned
out that a rotten AMM-to-turbo inlet hose was collapsing under high vacuum, limiting air flow into
the turbo. [HenryC] In my case, the hose from the AMM to the turbo inlet was soft and had a
small rip in it. The turbo sucks air in through this hose, so it is in vacuum all the time. As a result,
the mixture was lean as well as constrained by air flow.
Diagnosing Intake System Leaks. [Tip from Randy Starkie] If you take a plug of the correct size-
the inside diameter of the turbo hose- and install a schrader valve you can use compressed air
to check for leaks in the system. I use a master cylinder cap with a schrader tire valve to bleed
my brakes using a bicycle pump. Turns out the cap is the correct size for the turbo outlet hose. I
use a hose clamp around the outlet hose and turn my regulator on my air compresser down to
15psi. I use that to pressurize the system and the leaks are then apparent. I find leaks I miss
with visual inspection.
740T Has Weird Deceleration; Anti-Stall Valve Hose. [Symptoms:] Our '87 740 turbo has
recently begun behaving very strange. Acceleration is fine, normal running is fine, but if you lift
off the throttle after acceleration (with boost), the car jerks once and a "hiss" can be heard from
the engine. [Response: John Binford] '87 7xxT's had an anticompressor stall valve......about a
foot beyond the turbo, close to the fan. If the hose to the valve is off, it won't release and you'll
get compressor stall and maybe the excess pressure can 'hiss' somewhere. The diaphragm in
the valve can also die/hole, in which case it won't release either. Check the valve for proper
hose to it and operation.
No-Hot-Restart Problem: Boost Overpressure Switch Failure. [Dan Ridenour] The Boost
Overpressure Switch is “normally closed” and is designed to “open” if the turbo-boost pressure
exceeds some preset limit. This function is designed to protect the engine from a runaway turbo
or a stuck waste-gate. On my 1988 760 Wagon, the boost overpressure switch is mounted in
the engine compartment, on the right front strut tower, and is effectively “just above” the turbo.
High underhood temperatures can cause this switch to fail and shut the engine down. To
confirm switch failure, disconnect the waste-gate controller and wire the waste-gate fully open,
effectively disengaging the turbo. Then short the boost overpressure switch. If the car starts
while hot and runs without incident, then you've found the source of your hot-restart problem.
Don't drive the car this way as you may overboost the engine.
Compressor Bypass Valve Diaphragm. [Inquiry] My Mitsu TD-
04 with integrated CBV has a torn CBV diaphragm, but no one