Computer says lean, I say rich

[putzman02]

Ok, I have a 99 Eclipse with a 420a. It is relatively stock, besides crane stage 2 cams, adjustable cam gears, and an msd coilpack. The engine was just rebuilt, but the following was present before the rebuild as well. After initial startup, the lovely little CEL came on. The DTC that I retrieved was PO171, meaning lean. The funny thing is every definition of PO171 ive gotten tells me to clean the MAF. I dont have a MAF, but rather a MAP. Even more funny is the exhaust is pretty black at all RPM's. Can anyone explain to me why the computer thinks the engine is running lean, when it is apparently running rich? It has a slight lope at idle, which i originally figured was caused by the cams, but it almost sounds like a misfire. The thing drives fine, and has plenty of power, but when coming to a stop, it stumbles down to around 250 RPM's and then catches itself. My first guess would have been maybe o2 sensor, but there's a DTC for that. I cleaned the MAP sensor and the o2 sensor before putting the engine back in, and the crank and cam sensors, the coolant temp sensor and tps are brand new. I'm stumped. My buddy just finished UTI and has nothing. I asked 2 techs at my local Mitsubishi dealership, AND...still nothing. Any help would be appreciated.

 

[Team Addiction]

The interesting thing here may be that in a misfire a lean condition will actually be read in the exhaust. Consider what a wideband sensor is actually; An 02 sensor right and its actually measuring the presence of oxygen in the exhaust.
In a misfire there is an excess of fuel also but the sensor is not actually designed to read fuel so it reads the unburnt oxygen from the misfire and actually indicates lean condition.

"capiche" Or whatever

May this possibly be what you are experiencing?? Im goig this route considering your "RICH" claims.

 

[putzman02]

This very well could be what I am experiencing, but which direction should I go in to look for the cause of the misfire? The computer hasnt thrown a code for a misfire at all. And how does almost stalling while coming to a stop play into the equation?

 

[Team Addiction]

Yeah I Blabbed a nice theory but remain somewhat boggled as to the actual condition in place. Fuel filter replaced?? Possibly a faulty regulator??

Typically yes cleaning the MAF has solved this issue in many different makes. But if ya dont have one A vaccum leak could be looked at as well.

 

[putzman02]

The car only has 48,000 original miles, so i didnt opt to replace the fuel filter yet, and ill have to get a fuel pressure guage on the line tomorrow and see what its running at. Wait , what is stock pressure for the 420a?

 

[kergilman]

The stock fuel pressure is right around 40 - 50 PSI (idle and under load respectively).

 

[Locke]

What is your vacuum at idle?

Hypothesis: Cams with more overlap that stock can cause a loss of vacuum at idle. The map sensor reads this as engine load and adds fuel. Front O2 reads the rich condition, and adjusts STFT. The ECU tries to keep STFT amplitude to a minimum, so after some time (cars spend quite a bit of time at idle) LTFT's are adjusted to keep STFT close to 0. With a rich condition at idle, but proper AFR while cruising, LTFT goes lean.