Fuel related issue

[SuperImpala00]

Alright, got a noob here seeking the wisdom of the 60 degree gods. 2000 Impala base, 3400. MPG has been lacking lately, not severe but enough to notice. When watching the fuel trim readout on my scanner at idle fully warm, my LTFT reads -18 and STFT reads 0. When increasing rpm's, LTFT goes 7 while STFT hits -7 and stays at those values while holding 2500 rpm. Also, if left sitting a few days when starting, it barely turns over as in it feels like a dead battery, but catches and fires right up no problem. All electrical checks out, battery tested fine, alternator, starter and cables. Any ideas?

[SuperImpala00]

Thought I'd give a bump with update. Rented a fuel pressure kit and confirmed pressure at key on-engine off and engine on. Pressure was good in both instances. It's really giving me a headache, best I can come up with is the O2 sensor is getting very lax with it's readings but not enough to send a DTC yet.

[pocket-rocket]

That very well could be, though I've no experience with fuel trim readings in OBD2. How many miles is on the O2?

[SuperImpala00]

271,000 miles, mostly all original parts

[Purple pit]

Oh wow the O2 is way overdue. They are said to last 40k and after that they are going down hill.

[SuperImpala00]

Yea, I kinda figured replacing it should be done regardless but after observing live readings off of my scanner I'm just not convinced it's that bad enough to send a false rich signal to the PCM.

[pocket-rocket]

Dear gob...

Yeah, I would start with an O2, plugs and wires if not done recently, clean/replace the air filter, change the fuel filter and clean the MAF.

Before you change plugs I would run a can of GM Top Engine Cleaner through it to decarbonize the engine.

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[Purple pit]

What was the range?

By the way stay away from Bosch O2's.

[SuperImpala00]

What was the range?

By the way stay away from Bosch O2's.

I'd have to go look again, but I believe it ranged from as high as .900 to .400 on the low. I noticed when increasing rpm's the waveform started off sluggish before bouncing back and forth normally while holding at 2500 rpm's.

[Purple pit]

Yeah the range is not too bad, just a little high/low.
I'd still do what Pocket-rocket said. After that the car should run normal.

[pocket-rocket]

Sluggish O2 readings is a sign of an old worn out sensor.

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[SuperImpala00]

Well, didn't think it was possible but replacing the o2 sensor and BAM, rich readings disappeared. But....after doing that I was using an inspection mirror to look at the starter and happened to touch the BAT terminal and got rewarded with sparks on contact. i don't believe that's suppose to happen, any ideas on this new gremlin guys? Car was off by the was and had been sitting for awhile.

[pocket-rocket]

There is a direct connection from your battery to the starter. The solenoid on the side of the starter completes the circuit when you turn the key, so really it's no gremlin.

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[SuperImpala00]

There is a direct connection from your battery to the starter. The solenoid on the side of the starter completes the circuit when you turn the key, so really it's no gremlin.

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I do know that it stays hot all the time, but I've never had it spark like that. The car was off by the way, nothing in ignition. Just to be clear that stud was where you connect the positive battery cable to the starter. So it'll spark when something metal touches it just like that, as you say complete the circuit?

[pocket-rocket]

Yes. It's a direct connection. It is not switched anywhere and there is no fuse; it's hot all the time as long as your battery has power.

When you bumped it it created a direct short to ground through the metal stick fir the mirror. You would get battery voltage there if you checked it with a meter.

The lump on the side of it (where that stud is) is the solenoid, also called a bendix. It serves two purposes. Kick the starter gear out, and complete the circuit to the actual starter motor to power it.

If you were to take a screw driver and cross it between the small stud and the large stud on the starter it would trigger the starter and start turning the engine over.

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