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

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
Orange County (FL) Fire Rescue Department

(This message may contain personal opinions and/or information not related
to my employment or employer)
<myc1972@netzero.net> wrote in message
news:1110065544.916676.227460@z14g2000cwz.googlegr oups.com...
> 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
>

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

I would check for a broken vacuum line

<myc1972@netzero.net> wrote in message
news:1110065544.916676.227460@z14g2000cwz.googlegr oups.com...
> 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
>