Question about 1991 Toyota MR2

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I have a 1991 toyota MR2 when engine is cold idles over 2000 rpm

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OK a fast idling engine is due to too rich a fuel to air mixture. When the engine is cold the ECU enriches the fuel mix initially to ensure a normal fast idle (1000rpm) before leaning the mix to drop back to 700rpm when warmed up. The following spring to mind as possible areas for further checking:-
1) Intake air temperature sensor fault - How to Check? Unplug the device from the air duct and check that it is clean. Whilst out, put a voltmeter across the contacts and measure the resistance due to the ambient air temperature. If you then breathe on it, raising the local air temperature around the probe, you will see the resistance change as a consequence. You are looking for a change in resistance in line with a change of air temperature. If there is infinite resistance, consistent with a broken circuit, or there is no measurable response to change in air temperature then the IAT is faulty
2) Oxygen exhaust sensor fault - How to check? An issue with oxygen sensors is that they steadily lose effectiveness with age and they can under perform for quite a while before they trigger an error code on the ECU. Ensure the connection to the oxygen sensor is robust and clean. As the oxygen sensor only works when hot there is the danger of getting burnt by working on it so a safe approach is to find the electrical connection on the wiring harness remote from the oxygen sensor and to make voltage measurements there. Most garages have systems that can record the amplitude and frequency of the voltage peaks being produced by the oxygen sensor. A less sophisticated means to get some impression of the oxygen sensor function is to use a moving coil galvanometer type voltmeter (analogue needle on dial). Setting the voltage range to 1 volt and by attaching the meter leads across the sensor wires it should be possible to see the rhythmic pulsing and the voltage range of the operating sensor output. If no pulses are seen it could be either a break in the wire or a fault with the sensor itself.



3) Fuel Pressure regulator - How to check? When the engine is at idle, disconnecting the vacuum line to a healthy FPR should result in a temporary surge in engine revs for a few seconds as the immediate loss of vacuum to the FPR causes an immediate build up in fuel pressure. At each injection cycle a little more is injected causing the engine to build up revs. In the next few seconds the gradual ingress of unmetered air into the plenum, also caused by the disconnected vacuum tube, begins to upset the stable idle of the engine. The engine will suffer from rough idle until the vacuum line is reconnected. Most of the injection inputs will be crudely governed by signals received from the MAP sensor and feedback from the oxygen sensor.


4) Engine coolant sensor fault - How to check? Most often the coolant sensor is quite separate to the temperature sender, so a correct read-out on the dash board does not necessarily indicate correct sensor function. Using a voltmeter the resistance across the electrical terminals on the sensor can be measured. By removing the device from the car and putting the end of the sensor in a pan of hot water it should be possible to see an immediate change in resistance, it does not matter so much that the resistance goes up or down but that there is a discernable change with change in temperature. Generally high resistance equates to cold temperatures and vice versa. If there is no resistance change commensurate with temperature change then the sensor is at fault. If there is simply no resistance measurable (open circuit) then the sensor is at fault. If the sensor is working correctly check the connector, the wiring and the wiring insulation for faults and possible shorting
5) Low coolant - When engine is cold put interior heater setting to MAX and fan to OFF. Fill the coolant reservoir and leave the cap off whilst you switch the engine on. Squeeze top radiator hose to encourage air movement. After about a minute replace the coolant filler cap firmly. 6) Throttle position sensor fault - How to check? The socket for electrical connection with the TPS has 3 pins, one for 'ground', one for 5 volts 'reference' and a third (generally the middle one) for 'signal' output. Back probe the signal pin in the connector to the TPS. Attach the positive lead of a voltmeter to the probe and measure the voltage output as the throttle plate is rotated. If working correctly the meter should show a voltage consistent with the throttle position from approximately 1 volt when closed and 5 volts when fully open. What is looked for is smooth voltage increase with throttle change. If there are drop outs in the transition or that there is no transition seen, the TPS is faulty.

Posted on Nov 10, 2010

  • i_am_th3_gam Dec 14, 2010

    Actually 2000rpm is ok at cold start., mine revs upto 2500 sometimes.
    It mostly depends on how cold it is and what sensor signals the ECU is recieving.
    If you find that your engine is reving higher than 1000rpm it may also be a problem.
    Most of the time when timing these motors you MUST set a BASE TIMING which means you need to short out terminals E1 and TE1 before timing the vehicle. After you set this the ECU will take over and adjust to engine load, throttle position, etc..

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