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

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● Location: Cylinder head, under the intake runner to cylinder #2. This is the rearmost<br />

sensor (the front sensor is the coolant temperature guage sensor, the<br />

angled one is the knock sensor.)<br />

● Function: Supplies ECU with engine temperature in<strong>for</strong>mation so various engine<br />

parameters can be adjusted <strong>for</strong> optimum per<strong>for</strong>mance.<br />

● Test Procedure:<br />

❍ Replace with known good unit.<br />

❍ Use a DVOM to test voltage at ECT. See “Diagnosing ECT Failures” in<br />

Electrical: Engine Sensor section of 700\900 FAQ.<br />

● Correction Procedure: Clean connector and/or replace.<br />

Turbo Boost Overpressure Switch: Turbos Only<br />

● [Dan Ridenour] The Boost Overpressure Switch is “normally closed” and is<br />

designed to “open” if the turbo-boost pressure exceeds some preset limit. This<br />

function is designed to protect the engine from a runaway turbo or a stuck<br />

waste-gate.<br />

● On my 1988 760 Wagon, the boost overpressure switch is mounted in the<br />

engine compartment, on the right front strut tower, and is effectively “just above”<br />

the turbo. High underhood temperatures can cause this switch to fail and shut<br />

the engine down.<br />

● To confirm switch failure, disconnect the waste-gate controller and wire the<br />

waste-gate fully open, effectively disengaging the turbo. Then short the boost<br />

overpressure switch. If the car starts while hot and runs without incident, then<br />

you've found the source of your hot-restart problem. Don't drive this way as you<br />

may overboost your engine.<br />

Section II: Engine Starts Only After Prolonged Cranking<br />

Hot starting problems addressed in this section are characterized by an engine that<br />

starts only after long periods of cranking and/or depressing the accelerator pedal with<br />

the engine not wanting to hold idle <strong>for</strong> a few seconds to a few minutes after starting.<br />

Possible causes of this situation include malfunctioning fuel pressure regulator, in tank<br />

pre-pump, slow responding Idle Air Control Valve, and to a lesser extent a faulty gas<br />

cap.<br />

Fuel Pressure Regulator<br />

● Location: On <strong>for</strong>ward end of fuel rail. It is a cylindrical vacuum operated valve<br />

that has a small vacuum hose coming out of the front side and a larger fuel<br />

hose coming out of the back side.<br />

● Function: Maintains proper fuel pressure in fuel rail based on vacuum signal

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