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Erratic Idle on Cold Start - Idle Air Control? 700 1989

My '89 765T has idle problems when it first gets started after sitting for ~8+ hours (overnight or after work). The tach will oscillate from ~500rpm to ~1500rpm over 20-30 seconds until something gets warmed up, on occaision the car will stall if I don't give it a little throttle. After it settles down and I can start driving there is also some slight hesitation for the first 1/2 mile...after warming up the car runs like a champ, no idle problems or hesitation.

Over the 20-30 seconds that the tach is oscillating, the amplitude will slowly work its way down to 750rpm and flatten out.

Idle control valve?
Injector not firing optimally on one cylinder?
Ignition power stage?

All ideas welcome! Thanks.

Bean
--
'80 242GT 93k, '94 945T 139k, '89 765T 68k (new '94 B230FT)








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    Erratic Idle on Cold Start - Idle Air Control? 700 1989

    I have a '92 740 turbo with similar symptoms. When it's cold (particularly after a really cold night) it will oscillate in idle until it's fully warmed. Idle RPM varies from about 500 to 1200 rpm, with a period of about 30 seconds. I've speculated the air mass meter is having a hard time with the really dense air that cold weather brings. But it's only speculation. I haven't seen anything inthe FAQ that addresses this problem. Sorry I'm not much help...I'm watching to see what else develops on this too!

    Good luck!
    Barra








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      Erratic Idle on Cold Start - Idle Air Control? 700 1989

      I have seen in the 700/900 FAQ that there was an ECU upgrade that compensated for the use of oxygenated fuel (MTBE). The car came from CT which uses the oxy fuel, it now resides in RI which doesn't. I highly doubt that this is the problem but it's one thing to consider. They probably fixed the problem for the '92 model year too...

      That's a good theory that you have, my car is LH2.2 which allows me to set my timing and also tweak the duty cycle of my O2 sensor via a potentiometer on the AMM (commonly known as CO adjustment). I set my CO adjustment a few weeks ago (using an LED set up as per the FAQ) and the LED was flashing with a 50/50 (on/off) duty cycle. When I checked it again a week later, the LED no longer flashed 50/50, it was full on. Had to turn the pot to lean the mixture out a bit in order for it to blink again. Then today, for the heck of it, I checked it again and it was full off, had to richen the mixture for the LED to respond... I don't think this is normal and am thinking the AMM may be flaky. Funny though that the car runs like a champ when warmed up.

      Let's see if other's respond.

      Bean
      --
      '80 242GT 93k, '94 945T 143k, '89 765T 68k (new '94 B230FT)








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        Erratic Idle on Cold Start - Idle Air Control? 700 1989

        Hi Bean,

        I'm curious about your LH 2.2 50/50 duty-cycle setting: Do you have the LED rigged up so that you can watch it while driving (you can do this by connecting the LED between pins 22[-]and 18[+] at the ECU connector) ? If so - once you've set the AMM pot at idle to achieve the 50/50 DC, is this DC maintained even while driving ? Reason I ask is that I too have a 2.2 with the LED permanently rigged up, and I find that the DC is maintained only at idle: while driving the LED is always off. It flashes back on briefly if you take your foot off the accelerator. Others on the board with LH 2.4s dont see this behaviour (DC is always maintained even during driving) and was curious to find out whether this is a peculiarity of the 2.2 or something else.

        Thanks !

        Noel








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        Erratic Idle on Cold Start - Idle Air Control? 700 1989

        I would check the following:

        - Engine Coolant Temperature Sensor (embedded under the Intake Manifold - it's the sensor closest to the firewall). Check the wiring as well as the sensor's resistance at different temperatures. This is a key input to the ECU. You can find more info here (Scroll down after clicking the link to see a table containing the resistance at different temps).

        - TPS: Verify that it is properly closed at idle before adjusting the AMM pot. More info here.

        Verifying the resistance/continuity of both the above sensors at the ECU connector end is the best way - since that also factors in any wiring-harness issues you might have. Refer to these wiring-diagrams for the correct ECU-connector pin numbers.

        Noel







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