Hi, I'm trying to figure out why my 402 is randomly going lean. I noticed that my Long Term Fuel Trims are going from 0 to -15 at WOT and this is when the car starts to go lean. You can actually watch the injector duty cycle go from 66% down to 53% when the LTFTs drop. How can I fix this?

I can email anyone the graph of this taking place as a powerpoint file if this would help. Thanks, Brian

 

Negative LTFT means engine is running rich and PCM is reducing fuel.

What are your HO2SB11 and HO2B21 voltages at WOT...?

 

-15 trims at WOT is abnormal, it should clip at zero. I've
never seen this myself, seems like the computer has lost
its mind (or still thinks it's in closed loop).

 

I agree with jimmy....

At WOT, if LTFT is negative PCM uses zero, if LTFT is positive PCM keeps positive (or richer of LTFT and PE).

 

Yes, this sounds more like you are never getting to open loop at WOT. You should log the FTC at WOT and make sure that it's 15 or 22 (more likely, the latter). If not, then you need to dig into what switch is keeping it out of open loop (e.g., TP%, Engine Temp, etc.).

 

I looked at all the sensors that I can think of would cause this and they look great. What gets me is that when this takes place all the sensors stay the same so it makes me think it is tuning related, but I don't know. I notice my lights dim when I go WOT. Could this have anything to do with the negative LTFTs? I think it is from my belt slipping on the alternator.

 

Have you confirmed that you are in FTC (fuel trim cell) 15 or 22 at WOT? This is not a sensor value; it represents what fuel lookup table your car is in.

The dimming at WOT, I think, is a separate issue (unless, of course, the controller supply droops below 4 volts, but I think the car would just shut off then). That's odd... in and of itself. Perhaps a slipping belt.

Please check the FTC at WOT.

 

Check your tps% at WOT. If the TPS is messed up it may not know your at wot.

Also check the PE enable table. Always possible a wrong value has been entered forcing it to stay in closed loop.

 

Ok, I think the dimming is coming from the alternator slipping because there is rubber next to the pulley on the alternator, but it still does drop to 11.4 volts as soon as I get on it. I logged it last night and the cell it was in for the most part was 22 at WOT.

 

Weird.

That qualification is a little troubling. If the TP% showed 100, you should expect the FTC to be 22. You do need to be careful in interpreting the data from some scanners. If you are just running synchronously (ask for PID, get it, ask for next PID, etc.), then there can be some latency in the readings (especially during WOT when things are changing quickly) depending on the data rate the scanner is operating under. Therefore, it's possible that at the top of a gear at WOT, during a shift, one may close the throttle slightly before going to the next gear. Depending on the mode and speed of the scanner, it's possible to catch a TP% reading of 100, while by the time it gets to logging the LTFT it's already showing -15 due to the throttle closure on a shift. But, perhaps I'm repeating something you already know.

If you have a contiguous stream of data with TP% at 100 and the FTC is not steady at 22 (without being in a "transition" region, i.e., shift), then there's something that is overriding the PE Enable TP% vs. RPM table. That's probably the first place to check. Have you done something with the tune, recently, to cause these issues? However, if you have a contiguous stream of data with TP% at 100, FTC at 22 and LTFTs at -15, that's a problem that would extend beyond the capabilities of any tuning software, that I know of, to deal with. There may be a problem with the PCM. I hope that's not the case.

I hope all this rambling is helpful and not confusing. Good luck!