Hi all,
I'm still trying to figure out the cause of the slight hesitation off idle in my 1999 Altima. I'll skip the history and the details of what I've tried so far, but suffice it to say that all the regular tuneup items have been replaced, EGR valve and passages have been cleaned, coolant temp sensor is new, etc etc. I've posted in detail before if anyone wants to know more background.
Anyway, I've got an Autotap interface for my laptop now so I'm able to monitor a few of the OBD paramaters (when the software isn't crashing). Everything looks as I would expect as far as sensor readings; the TPS, MAP, MAF, coolant and intake air temp sensors all seem good.
I have a question about the short term fuel trim though. When I first started the car with the scanner connected (it was warm, and had only been off for maybe 5 minutes) the short term trim was reading around 0%, and the hesitation was at its worst (as it always is when the car is just started). When I blipped the throttle slightly, which is when the hesitation manifests itself, the short term trim dropped to around -10% after the engine recovered from the miss, and then after returning to idle, the short term trim was back to about 0%.
After a few minutes, the short term trim worked its way down to around -8% at idle, and would still drop a little lower when I blipped the throttle. The hesitation at this stage was less apparent though, as it normally is after a few minutes of running.
So, I'm wondering if the ECU is compensating for an over-rich mixture, and that's over-richness is what is causing the hesitation.
Interestingly, the long term trim is actually close to + 10%. So is the short term trim just a fine tuning either side of that long term value, in which case maybe there is no problem here? Or is there something fishy going on with the two values working in opposite directions?
Thanks in advance for any insight!