bad fuel economy

Hello, I have a 1986 Oldmobile Delta 88, 3.8L fuel injected V6. It has been very reliable. But my fuel economy has become noticeably worse. When i start the car the idle is really high,
even when the engine is not cold. It lowers after a few minutes of driving. The car shakes a bit and it goes away while accelarating, and comes back when i let off the gas. I replaced the O2 sensor three years ago, and did a tune up back then. I have had the check engine light come on a couple of times but the light has not come back on in a few months.
What could be causing my worsening fuel economy?
Again, i have:
High idle speed shaking at idle bad fuel economy
Thanks, hope you can help me out. I care about this car alot.
myc1972
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You need to pull the codes from the computer. It is OBD-I so it is very easy, but on a motor that old these problems could be caused by several things each. I have no way of knowing what is actually going on from your motor from where I'm sitting or how well you take care of it, so I'll mention just a few probable causes.
High Idle: I believe your motor is throttle body fuel injected, so it is most likely an Idle Air Control valve not seating fully (dirty or broken), a cracked base gasket on the throttle body letting air in, or a bad coolant temperature sensor.
Shaking at Idle: Either a bad or disconnected knock sensor, bad plug(s) and/or wire(s), bad ignition module, cap, rotor, etc.
Bad Fuel Economy: Take your pick, everything from any one of a dozen routine maintenance items to something more serious - I have absolutley no way of knowing.
You are going to need much more help diagnosing these issues by someone in person than you will ever get over the internet.
Cheers - Jonathan
--
Jonathan A. Race
Lieutenant, EMS Supervisor
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The car is multi port fuel injection. I take good care off it. Have used synthetic oil and have always changed the oil before 3000 miles. People give me a hard time because i take extra good care of it always. It does not use a dirtibutor (distributorless ignition system). The car has 100,000 miles.
Thanks for your reply, it might be time for another tuneup. It's been three years since the last one. But i did buy the expensive wires and sparkplugs the last time.
Thank you for your reply!
Myc1972
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