3800 ****urgent*** fuel problem?

[sgtp]

I have a problem where I have a rough idle, even after replacing the plugs. I noticed while scanning that the STFT goes to -14.85 when the roughness starts. Is this negative # mean lean or rich? If it means lean Im replacing the fuel filter. If that doesnt fix it completely I think its the fuel pump, which I suspected being on its way out because its pretty damn loud.

Let me know what the negative stft means, also is this negative number a cause for the bad idle, or could it be the pcm adjusting fuel BECAUSE of the bad idle?

Click to read more...

 

[sgtp]

oh, right after startup the STFT is at 0 and idles great till the RPMs settle down to 750-800 and the STFT goes neg. STFT is used to adjust idle fueling right? sorry Im noob to fueling type scanning

 

[sr71_rgl]

Originally posted by sgtp@Jun 8 2004, 04:26 PM
oh, right after startup the STFT is at 0 and idles great till the RPMs settle down to 750-800 and the STFT goes neg. STFT is used to adjust idle fueling right? sorry Im noob to fueling type scanning

Im not exactly sure what the stft value represents but any fuel trims in the negative means the PCM is sensing a rich condition and is attempting to correct it by pulling (decreasing) the amount of time the injectors and spraying fuel.

 

[imported_Ron Vogel]

Neg is pulling fuel. However, the STFT is going to fluctuate a lot, and it's not really what you should look at. LTFT (long term fuel trims) are an average of the STFT's, and will give a more accurate read of what's going on. It' sounds to me like it's the 02 sensor though. If the problem happens when the car is out of "cold run" mode, it's switching from reading the maf to reading the 02's for engine adjustments. You may have moisture in the connector, or a problem with the wiring. Look at the crosscounts on the front bank 02, the crosscounts should be pretty quick, if it takes several seconds for the voltage to switch fron low to high, it may be the culprit.

 

[sgtp]

well, I rescaned with my ls1m... heres what Ive got:

at idle:

closed loop status: 0 mostly, but jumps to 2,3,4,5,6 at random then back to 0
LTFT: peaks at -22.6
O2: steady at 1105 (yeah I know at wot is the only time its accurate)


at wot:

Super rich o2: 1105 again

I dont understand crosscounts.......

As far as I know the o2 sensor is original and there is 72k on the clock.....

 

[sgtp]

am I answering my own question posting these way rich O2s on an old sensor? What brands or 02s are decent? I know bosch plugs are crap, what about their 02 sensors?

 

[imported_Ron Vogel]

Only get the AC replacement, Bosch sucks. Casper's has them the cheapest anywhere. And you are way too rich, It's prolly causing a bunch of KR.

 

[sgtp]

well, I did find a tiny bit of moisture in the old sensors plug just like you said. I ended up going with a bosch sensor. The delco can be had for 116-126 at the dealer, unfortunately I had no chance in getting it before they closed the parts counter. I gotta got to work tomarrow(my 2nd day at my new job, wheee ) so I shopped around. A bosch at napa goes for 65 with a 12 month warrantee, but would close b4 I get there so I went to AZ and paid 70 with a 3 mo warrantee which sucks. But sure enough I pop it in and the problem disappeared.

So essentially what was going on was the o2 was stuck at 1105, the pcm saw a rich condition and pulled massive amts of fuel at idle and the engine started missing, not much unlike when you are about to run out of gas. However, when accelerating or cruising, the pcm went into closed loop(or is it open loop, anyway it showed "1" on an ls1m) and I assume followed a fuel trim coded in the pcm. Does this sound right?

Thanks ron for showing me the light, I might just have to commision you for a tb job in the future...

 

[sr71_rgl]

FWIW..the O2s are rarely accurate on our cars especially at WOT. A wideband O2 on a dyno is the preferred way of tuning but on the street LTFTs will be more accurate than O2s at WOT.