Neg % high fuel trims in idle and high rpms and eng. miss.

[jesg45]

I have a 05’Chevy Impala LS 3800 vin K. Has an engine miss and just got worsed to where it’s not safe driveable anymore. Pops code PO300 and the fuel trims are high all the time. LTFT of -20% whether at idle or at high rpms. STFT in the -3, -7%. I’ve been playing with this for over two months. I’ve have replaced a few parts already, as a matter affect, too many, not that it didn’t them, since most were O.E. All the ignition sys. is being replaced, from plugs to ICM, F/P, both O2 sensors, MAF, MAT, CKP, rbld heads, vlvs leaking, upper plenum, all new gkts, flushed all inj. and read 12 ohms. Eliminated EGR, blocked it off. Has good fuel pre. and holds with key off. Also, notice with my DVOM hooked up to battery, when running smooth for maybe 5 or 10 seconds DVOM reads 14.1 volts, but when it’s missing, reads 12.2 volts and lights dimmed. It almost seems like if I turn key off/on for a fraction of a second. Your help will be greatly appreciated. Thank you.

[jesg45]

on 9-3 2020 I posted a problem but, so far, I haven’t had any response. Thank you.

[jesg45]

Hello out there, haven’t heard from anyone, yet. Seems like no one has come across a problem like mine, yet. Maybe I confuse everyone the way I express the problem. Maybe I need to rewrite it as just one little simple problem.

[jesg45]

Hello, anyone. Does anyone have any idea about my veh. problem? Like to here any response, any opinions from someone else or maybe I am at the wrong page for asking for any help.

[Fleet mechanic]

Sounds like a vacuum leak to me.

[Apex]

Certainly not a vacuum leak.

Does the engine misfire all the time, or is it only misfiring when your voltage drops to 12v like you mentioned? Is this the same with the rich condition, or is that also only at -20% LTFT when it is misfiring?

The voltage dropping definitely interesting and I would probably start with what could be causing your voltage to drop like that if it is occurring at the same time as your misfire, though it may be a symptom of whatever is causing the misfire and not the cause. However, I would gather more data first:

Is there anything that consistently causes that to happen? If that voltage drop is a reproducible variable you can start tracking down why it’s happening. What is actually happening during the misfire, is it a loss of spark, mistimed spark, fuel injector dumping fuel? What do your LTFT and STFTs look like during the misfire?

Also, The Garage (Repair and Diagnostics) would be the proper forum to post this in as opposed to The Litter Box (General Discussion).

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