ST Fuel Trim troubleshooting

[Overlander23]

Occasionally, I've been getting a P0172 code thrown (bank one too rich). I'm wondering if a vacuum leak could cause this? I searched and found references to vacuum leaks and P0171 (too lean), but could this also throw a P0172?

Monitoring short term fuel trims at startup, I'm getting ~-22% which seems way rich. Blipping the throttle zeros out the STFT, and once the engine is warm the trim stabilizes at around -9% to -5%... but it's not sitting at the typical +- 3%, and it never sits in the positive range.

Is it common to be so rich on cold startup?

Thanks!

[®ob]

what brand of fuel injector?

[Thomasio]

How do your plugs look? How do the coils look? Could be a low-RPM misfire that clears up with higher RPM. Look for water around one of your coils or signs of arcing.

Re-read your post... I'm doubting my diagnosis.

[Overlander23]

Bosch...

Originally Posted by ®ob

what brand of fuel injector?

[kverges]

The fact that the ECU is subtracting so much fuel trying to get to stoich makes me wonder if fuel pressure is too high or a fuel injector is stuck open. Given the fact that you have aftermarket (probably larger flow rate) injectors, then you probably need to tune the duty cycle with either a remapped or stand-alone ECU or piggyback to get back to stoich. That big a corrrection makes me worry that the ECU may change the overall injector size scalars and make the car go lean open loop, so I'd treat this as more than a nuisance.

[charliex]

in my experience a vacuum leak makes it run lean, you'd see positive fuel trims ( in the +17-25 area)

my theory is that too rich and negative fuel trims are exhaust leaks, but i've yet to get to try it in practice, but its only a matter of time.

however if it only does it at cold startup, its meant to run super rich since the elise doesn't have SAI, after warm up it settles down to -4 to +4 its fine, if it doesn't and continues to oscillate much higher and the rpms are wobbly too, as well as it being large value trims, i'd be looking at leaks then. -9 to +4 isn't horrible and that might be related to the version of relash you have, or a small leak.

to make matters worse sometimes exhaust leaks clear themselves up when everything gets upto temperature and the leaks self seal, so it can appear to be ok, when its not.

look for soot and heat damage on the clamps, clam , check where the header connects, as well as all of the joints and the cat for leaks.


i doubt its injectors, you'd have misfires and bad running too.

[Overlander23]

The car idles fine... I checked the exhaust system for leaks (Stage 2). I've got signs of soot at the 180 bend before the tips (the bend in the left side well). That's after the O2 sensor, though. It shouldn't cause a problem?

Car idles well, but if cold, when I very lightly tap the accelerator revs will dip and hunt in a cyclical fashion for a couple of seconds. Kind of strange, but I guess this is the ECU making adjustments up and down, hunting for the right O2 voltage.

Also wondering if it's related... when cruising at 2500 rpm, if I trounce the accelerator there's a pause before acceleration... sometimes a second or so. Like lag. Or like an accelerator pump on a carbed engine isn't quite working. This behavior does not manifest itself from rest. Full on acceleration from a stop is smooth and consistent. It's just that state change between cruising and full acceleration that suffers sometimes.

[charliex]

sounds like an old flash to me with potentially an exhaust issue.

if its idling, its not a vacuum leak.

if its not warm or left for at least 30 seconds after start, it'll go rich if you tap it and bog down, if it does it after its warm/30 seconds , leak.

the acceleration enrichment might have been adjusted on later flashes, i can't recall off hand.

the exhaust leaks could potentially affect it, depending on how much of a leak it is, if it alters the mix at the primary o2 it'll be off.

if you have access to an obd ii reader that can read the firmware id or CVN ( 4 digit number) i can tell you if its an old firmware and you need to update it, or the dealer obviously can.

[RCHAD]

keep in mind that at idle, with a larger exhaust, the flow of exhaust gasses is moving slower than it would with smaller pipework. it sounds like you may have a problem...it has been my experience with forced induction cars that if your plugs and o2 sensor are good, then look for plumbing leaks on intake side after the maf or exhaust leaks upstream of the sensor...what mods are on the car and what are the fuel trims at a cruise?

btw, imo, bosch anything is not a good choice for japanese engines!