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On a 1989 740, non-turbo, 90,000 miles I have been consistently getting codes 2-3-1 and 1-1-3 about 10-20 miles after clearing them. Engine fully warm evarhthing fine; the light just appears...no running problems at all. This is a very predictable pattern.
I have a new O2 sensor, new AMM, cleaned throttle body and IAC, set TPS, cleaned and tightened most grounds--car runs fine...
There is occasionally on starting at about 40-50 degrees temp a fluctuating idle--after 15 seconds at 1,000 rpm it begins to swing from 500 and almost stalling then surging to 1,300 and then back down. This lasts for a minute or 2 until it starts to warm up and then settles to about 800 rpm, very quiet and nice. This pattern is intermittent, about 1 evry 4 starts
What i've read on Brickboard points me to the engine temperature sensor, which I have not yet gotten to to check. My theory--I am new to this--is that the sensor which is normally not a part of the controls picture during warm running occasionally malfucntions enough (because it sometimes doesn't work in controlling the idle on the cold starts) to make the engine run rich and set the code. Nothing else is going on except this minor hiccup.
Does that theory make sense? Anything else going on here?
If the engine temp sensor is a supect, do I have to remove the manifold in order to get decent access to it? What do I check for?...just follow the FAQ on the topic for readings? Are there other critical connectors in the wiring harness to check? Would it show an abnormal reading if it was not in one of its malfunction modes?
And if I need to take off the intake manifold, should I plan on doing other jobs under there that are easier when it is removed?
I'd appreciate any advice.....
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