700/900 Hot Start Problem Quick Reference Guide

Volvo 700 900 Hot Start Problem Diagnosis and Repair PDF

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Volvo Maintenance FAQ for 7xx/9xx/90 Cars

Quick Reference Guide: Volvo 700-900 Hot Start Problem Diagnosis and Repair (by Mark Berglund)


AMM Air Mass Meter
ECT Engine Coolant Temperature sensor
ECU Engine Control Unit computer (either fuel injection or ignition)
FI Fuel Injection
FPR Fuel Pressure Regulator
IAC Idle Air Control solenoid valve
TB Throttle Body
TPS Throttle Position Sensor
VSS Vehicle Speed Sensor


This diagnostic and repair tool was created to help individuals identify the cause and repair of the more common hot start problems in 700/900 series Volvos. Hot start problems have been well documented on the Brickboard. It is from this source that most of the information used to create this reference tool was gathered. As such, credit must be given to those individuals whose contributions made this reference tool possible. Regrettably, the names of all the individuals are not available to accomplish this so I can only give credit in blanket form. This work is not intended to be the final word on hot start problems. Rather, it is viewed as a work in progress needing continual additions and refinements to make it more useful to Volvo owners. Comments are welcomed and encouraged and should be directed to


For the most part, the cause of hot start problems can be attributed to a component that is easily fixed and/ or replaced. It is divided into two sections based on the general nature of the hot start problem. The answer to the following questions will direct you to the appropriate section relating to your specific situation. Furthermore, when answering the questions, it is given that the vehicle had been driven for some time, the engine is at or slightly above normal operating temperature and has not been running for 2 to 10 minutes (hot soak) and subsequently starts fine after cooling down.

Question A: The engine cranks but does not start regardless of how long the starter is engaged or how much the accelerator is depressed?

If the answer is YES go to Section I If the answer is NO go to Question B

Question B: The engine spins but only starts after prolonged cranking or the accelerator has been pressed and continues to run only if engine RPM remains above idle speed for 1-3 minutes.

If the answer is YES go to Section II

Section I: No Start After Crank

This type of hot start problem is almost always caused by intermittent failure (due to high temperature) of the Fuel Injection Relay (FIR), RPM/Crankshaft sensor (if equipped, RCS), Hall Sensor (if equipped), Radio Suppression Relay (RSR) and to a lesser extend the Engine Coolant Temperature sensor (ECT) and Power Stage. Statistically, the FIR, RCS, RSR and the Hall Sensor account for the majority of hot start problems. For this reason, it is suggested that diagnostic efforts concentrate on or eliminate these items first before moving on to the ECT and Power Stage. For the average do it yourselfer, exact identification of the defective part is limited to replacement with a new or known good unit. With more experience and a Volvo Green Manual, advanced diagnostic techniques can be used to eliminate different components before spending money to replace them. If the vehicle experiencing the problem is a high mileage vehicle, it is best to replace these parts. Given the well documented nature of 700/900 Volvos, it is only a matter of time before they give up the ghost. The exception to the previous statement is the FIR. The FIR often fails because of cracked solder connections on the relay's circuit board. Below is a part by part guide to location, function, testing (if possible) and correction procedure.

Fuel Injection Relay

  • Location: Relay tray, center of dashboard console behind ashtray. It is the tall white Hella relay on left in back row.
  • Function: Supplies power to main fuel pump when key is turned to the ON position or position II.
  • Test Procedure: Place finger on relay and turn key to ON position or position II. A click should be felt and the main fuel pump under the car, beneath the drivers seat, should cycle for approximately 1.5 seconds.
  • Correction Procedure:
    • Replace with known good unit.
    • Resolder. See Testing or Repairing Bad Fuel Injection Relay heading in Engine: Fuel Injection section of 700/900 FAQ for instructions on resoldering the FIR.

RPM/Crankshaft Censor (if equipped)

  • Location: On top of bell housing below the distributor (B230f and B230ft motors).
  • Function: Tells the ECU where the crankshaft is and how fast it is spinning so correct ignition timing and fuel flow can be computed.
  • Test Procedure: Replace with known good unit.
  • Correction Procedure: Replace with known good unit.
Radio Suppression Relay
  • Location:
    • Between power steering reservoir and strut tower. If located here it will be next to the cooling fan relay which is sometimes identical.
    • Various locations on passenger side of engine compartment.
    • Inner fender wall, driver's side between battery and power steering pump reservoir.
  • Function: Supplies power to fuel injectors when key is turned the ON position or position II.
  • Test Procedure: Replace with known good unit. On some models, the RSR is identical to and located next to the cooling fan relay. If this is the case, the cooling fan relay can be substituted for the RSR to test.
  • Correction Procedure: Replace with known good unit.
Hall Sensor (if equipped)
  • Location: Inside distributor underneath rotor.
  • Function: Tells the ECU where the crankshaft is and how fast it is spinning so correct ignition timing and fuel flow can be computed.
  • Test Procedure:
    • Replace with known good unit.
    • Use a DVOM to test voltage at Hall Sensor. See Testing Hall Sensor in Electrical: Ignition System section of 700\900 FAQ.
  • Correction Procedure: Replace sensor or distributor. It is advisable to have the Hall sensor replaced by a professional due to the way it is secured to the distributor with steel rivets or replace the entire distributor.
Engine Coolant Temperature Sensor
  • Location: Cylinder head, under the intake runner to cylinder #2. This is the rear-most sensor (the front sensor is the coolant temperature guage sensor, the angled one is the knock sensor.)
  • Function: Supplies ECU with engine temperature information so various engine parameters can be adjusted for optimum performance.
  • Test Procedure:
    • Replace with known good unit.
    • Use a DVOM to test voltage at ECT. See Diagnosing ECT Failures in Electrical: Engine Sensor section of 700\900 FAQ.
  • Correction Procedure: Clean connector and/or replace.

Turbo Boost Overpressure Switch: Turbos Only

  • [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 this way as you may overboost your engine.

Section II: Engine Starts Only After Prolonged Cranking

Hot starting problems addressed in this section are characterized by an engine that starts only after long periods of cranking and/or depressing the accelerator pedal with the engine not wanting to hold idle for a few seconds to a few minutes after starting. Possible causes of this situation include malfunctioning fuel pressure regulator, in tank pre-pump, slow responding Idle Air Control Valve, and to a lesser extent a faulty gas cap.

Fuel Pressure Regulator

  • Location: On forward end of fuel rail. It is a cylindrical vacuum operated valve that has a small vacuum hose coming out of the front side and a larger fuel hose coming out of the back side.
  • Function: Maintains proper fuel pressure in fuel rail based on vacuum signal from intake manifold. Excess fuel is routed by the FPR back to the main fuel pump.
  • Test Procedure:
    • While engine is running (exercise extreme caution when performing this test to prevent explosion and fire) After hot engine has been setting for approximately 10 minutes, remove the FPR vacuum hose where it attaches to the intake manifold. If gasoline is present in this hose, the FPR is defective.
    • Remove fuel return line from the rear of the fuel rail. Attach one end of a long length of hose to the end of the return fuel hose coming off the end of the fuel rail; place the other end of the hose in a suitable container away from the car. Start the engine. Fuel should immediately come out of the hose in substantial quantity. If only a little fuel is coming out, or in sporadic spurts, the FPR is defective.
  • Correction Procedure: Replace with known good unit.
In tank Pre-Pump
  • Location: In fuel tank.
  • Function: Supplies fuel to the main fuel pump.
  • Test Procedure:
    • With engine running, remove gas cap and place ear next to opening to fuel tank. If the pre-pump is working, a faint or not so faint buzzing should be heard.
    • Gain access to fuse tray by removing ashtray assembly. Start motor. While motor is running, remove fuse for the pre-pump. When fuse is removed, the engine should run rougher and main fuel pump noise should noticeably increase.
    • Connect fuel pressure gauge to fuel line between intake pre-pump and main fuel pump. Start motor. Fuel pressure gauge should read somewhere between 5 lbs. and 8 lbs. If pressure is below this range, the in tank pre-pump is not pumping enough fuel due to wear, leaking rubber hose connecting pre-pump and float assembly, or the filtering sock on fuel pickup is clogged.
  • Correction Procedure:
    • Replace pre-pump with known good unit.
    • Replace rubber hose
    • Replace or clean filtering sock.

Slow Responding Idle Air Control Valve\CIS Motor

  • Location: Under Intake Manifold. It is a cylindrical metal part with a 5\8 inch to 3\4 inch hose connected to the forward end, wires connected to the rear facing end and another 5\8 to 3\4 inch hose coming out of the lower side that faces the motor block.
  • Function: At idle, the IAC valve opens and allows the necessary amount of air past the closed butterfly valve in the Throttle Body so the engine will idle properly. Note: It is possible that the IAC is responding slow due to a faulty signal from the ECU.
  • Test Procedure: Replace with known good unit.
  • Correction Procedure:
    • Clean air passage in IAC with carburetor cleaner.
    • Replace with known good unit.

Gas Cap

  • Location: Behind fuel door at rear of vehicle.
  • Function: Prevent fuel from escaping tank, prevent outside materials from entering tank and maintain proper pressure inside tank.
  • Test Procedure: After hot engine has been turned off for several minutes slowly remove cap. If a hissing or rushing air sound is heard then the vent mechanism in the cap is defective.
  • Correction Procedure: Replace with known good unit.


DVOM Digital Volt Ohm Meter
ECT Engine Coolant Temperature Sensor
ECU Electronic Control Unit (engine\fuel injection\ignition management computer)
FIR Fuel Injection Relay
FPR Fuel Pressure Regulator
IAC Idle Air Control Valve/motor (also called CIS motor)
RCS RPM/Crankshaft Sensor
RSR Radio Suppression Relay

Volvo Maintenance FAQ for 7xx/9xx/90 Cars


©Jarrod Stenberg 1997-2014. All material except where indicated.

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