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From your symptoms, I would guess either the temp sender or idle control valve - but then again, their operation is tied together.
The idle valve, I believe, still works on a "duty cycle." In other words, the ecu sends a pcm signal to the valve. The frequency of the pcm signal determines how much the valve opens. The ecu makes this determination based on many things (throttle position, rpm, etc), but one of the most important parameters is the engine temperature.
The temp sender, I believe, is a negative coefficient type. In other words, the lower the coolant temperature, the higher the resistance. The ecu reads the resistance of the sender and then determines how much the idle valve needs to open. If the temp sender is reading to low of a resistance, a few things happen. Among them is that the idle valve doesn't open properly, AND the injectors do not enrich the mixture enough for the given engine temp. When the temp sensor goes in this kind of situation, its value is too low, so the ecu thinks the engine is warmer than it is. This can lead to symptoms similar to yours because when the engine is warmer, it's not as critical because the engine doesn't need the extra enrichment needed during a cold start. At higher temps, you start to get the opposite problem - the ecu thinks the engine is cooler than it really is, so it enriches the mixture needlessly. You can measure the resistance cold (I don't know what the spec is off hand, but I'm sure a search will find it). Measure the coolant temp then measure the resistance of the sensor and they should correspond. You can check the warm resistance too. If it's constant regardless of temp, or the resistance doesn't match the temp, it may be your problem. You can also check your plugs. If they look like it's fouling, it's more likely the sensor (at running temps, the engine is enriching the mixture too much because the ecu thinks the engine is cooler than it is at running temps.
As for the idle valve, when they get old, they get gummy and/or they can develop a "flat spot." If it's gummy, just cleaning it out may fix the problem. If it's developed a flat spot, it needs replacing. The flat spot occurs because for much of it's life, the valve operates in a narrow portion of it's operating parameter. This causes wear in a small part of the overall swing - kind of like a motor stator wearing only where the brush touch. A gummy valve doesn't allow the valve to react fast enough thereby causing the hunting (long description having to do with ecu overcompensating then the valve catching up, but then the ecu has to compensate the other way, etc. etc). A flat spot causes the valve to not be able to operate properly in the range that it needs to.
posted by 98.229.227...
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