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Change the oxygen sensor. There I've said it what.. three times?
Of the following: oxygen sensor, frequency valve, frequency valve relay, ECU, .. the oxygen sensor has the shortest lifespan (except for maybe a fuel filter), especially on a normally aspirated engine where nothing else is taxed very much.
An oxygen sensor that has stopped responding will register a steady voltage, and a steady voltage will cause the frequency valve to not toggle on and off.
Change the oxygen sensor and go from there.
If you're hell bent on keeping your oxygen sensor, at least use an analog meter to verify its voltage. A digital one will tend to average out the fluctuations and so may give you useless readings. IIRC the failure method of an oxygen sensor is that it becomes less and less responsive to any changes and would rarely set a check engine light (which of course only applies to the newer cars that have check engine lights.. your 240 does not).
The oxygen sensor is cheap, and on a normally aspirated car probably not too difficult to change.
Sure any of the parts you've listed COULD be bad, but they're not common failures (in fact if you've got cold running issues, they are most likely not related to the lambda sond setup at all). The oxygen sensor is a wear item, there's a reason Volvo had a 30,000 mi counter rigged to an idiot light for them.
Also keep in mind that the oxygen sensor's output is what controls the ECU's tweaking of the frequency valve et al. A bad reading from the O2 sensor will mean funky results anyhow.
- alex
'85 244 Turbo | ~144.9k miles
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