high fuel trims in the negative %

05 Chevy Impala LS vin K 3800. LTFT of -25% and STFT of-7% and at times of -3%. This is at idle and high rpms. Also has a miss on and off, like every 3 to 4 seconds. Almost, like if turning the ign. sw. off/on at a fraction of a second. Gives me a code, PO300. I hooked up my DVOM on the voltage scale and the seconds it's running smooth, it reads 14.1 volts, but, when it starts missing, it reads 12.2 volts. It just keeps repeating this. I replace the heads, vlvs leaking. All new gkts from the heads up, new upper plenum, and just about all the sensors from the heads up, including the KO, both upstream and downstream O2s. Even blocked the EGR. All the ignition sys. is all new. Repaired some frayed ckts. After done all this, problem is still there, even got worsed. It seems like I'm a parts changer, but with over 155k miles and 90% of the parts I changed were o.e. Oh, I also, pressure spray cleaned the inj. and a new pump and filter. Looks like these is enough info for now. Your help will be greatly appreciated. Thanks.

Is this thing flex fuel??

Did you test the fuel pressure regulator and check vac port for leaking fuel?

Did you also ohm test the injectors and get a waveform for each injector?

Yes, I ohmed all inj., 12 ohms each. I don't have access to sophisticated machines. I work at my own home. Like mention before, pump is brand new, new filter, flushed all injectors. Pump holds pressure after turning key off. Besides, no trace of black smoke and fires right up. There is times it runs smooth like there is nothing wrong for about 3/5 seconds, sometimes more,then, acts up again. About the only things I haven't replaced are the cat, inj. fpr., and cam sensor, yet. That is why I want somebody else opinion. Thanks.

Ok with the fuel pressure regulator there is a vacuum port on it. Run the car for a few minutes then pull off the vac hose and see if there is fuel (wetness) in the vacuum connection and the port tube of the fpr should see it if it is leaking. The other thing to check is the idle air controller and the maf sensor.

With the iac an easy way to tell is to look at scan data for commanded rpm and actual. If there is a difference in rpm of 100 or more that will need replaced.

With the maf sensor remove it from car and clean it with maf sensor cleaner and a q tip or acid brush (tin wrapped bristle brush ) spray cleaner inside where wires are and on qtip/brush and clean off the wires very gently. Spray the wires inside maf again and let dry. Reinstall and start car does it idle better and fuel trims get better (closer to zero)?

Let me know

all that you mention have been done before I posted this problem. Maf is brand new, IAC has been clean thoroughly and tested, and FPR testing has been performed at least 3 times already and no change. Problem still there. What I am going to do next is to remove the CAT and check the fuel trims. It won't rev. above 4000 rpms, starts bouncing back and forth.

No, it's not flex fuel

Ok sorry for the late reply...
You must be hitting rev limiter which will do that in park or neutral.

Does the scanner you have do live data ie can graph the upstream and down stream o2 sensors.

If so i want you to start car and let it get into closed loop. Then get scanner set to graph o2 activity ie the millivolts reading. Now with car idling unplug a vacuum line and watch scanner to see if o2s react quickly short term will react the fastest then downstream should react shortly after that. Voltages should fluctuate for upstream and down stream till you disconnect vac line. Do both peg out high .9 millivolts? Or is one stuck at .2 millivolts? Let me know sometimes a upstream o2 will fail causing fuel to be dumped in but will know for sure after you do this.

Now after that test plug vac line back in and give engine another source of fuel (propane works or a aerosol can of carb cleaner or brake clean) see if fuel trims go richer or if they stay where they're at. Let us know

I will try this with my new scanner. I just got it today. OTC #3209. I'm getting familiar with it. As soon as I feel comfortable with it, I'll give it a try and let you know. Thanks.

Well, 1st, I pressure test the Cat. Gives me 1 psi max. at idle and at 3k rpms, so, I believe that is good. Next, I ran it at idle, and trying to keep it from dying. HO2S1 shows slow CCTS from .10 to .90. LTFM reads -25. Seems like if I introduce more fuel, it's going to get much richer. So, what's next? By the way, O2s are a week old.

Did you unplug a vac line when checking stft reaction? And what was the stft reading (in millivolts and trim numbers)?

Also what was fuel rail rest pressure koeo? 42psi or was it higher after the initial 50-60 psi buildup?

Also are o2 sensors oem Bosch?

Fuel pre at idle 46-48. At rest with koeo goes up to 54 psi and holds. One O2 is Denso and the other is by Walker. Bought them from Rock Auto. Another thing, hooked a vacuum gauge and my readings at idle, not steady at times, is between 14" Hg to 30" Hg and vibrating real quick. At WOT, it reading from 10" Hg to max 30" Hg. Like I said before, I smoked the system and didn't see any smoke out to the atmosphere ,so, no vac leaks. Is there a possibility that maybe injectors are sticking open, interrupting the pulse width, only when engine is running? I'm just saying. I previously cleaned and ohm them, they all showed 12 ohms.

Ok with what this looks like is the fuel pressure regulator is stuck closed. To confirm it is stuck use a hand vac pump (mighty vac hand vacuum pump) and with key on engine off fuel pressure at ~54psi start applying vacuum and write down when the regulator starts releasing pressure. Then keep increasing vacuum and find when it stops releasing fuel pressure. If it does not release pressure then it is bad and the reason for the fuel trims being rich.

Now with the bouncing heavily between 14 and 30 inches hg this can be a valve timing issue. But we will dive into that after testing fpr.

I send you a message yesterday, 9-15 -20 and today, I can't seem to see it. Seems like it didn't go through. So, today, I'm going to send it again about testing the FPR. I cycled the key a few times just to get an accurate pressure of max 50 psi. Fuel pressure started dropping at 5"Hg and stop dropping at 40 psi and vacuum reading at 22"Hg. You, also, mention valve train problem like maybe a jumped timing chain. The scan shows 20 degrees adv. There is no valve train noise either. Chain is the O.E.

A jumped timing chain will not show on that data pid that is for computer advanced timing. I'll send you a pm with my number so you can call me.

Hello, I'm still breaking my head trying to figure the problem. I think I'm going into the PCM to check all the grounds and all the low reference ckts. I do have a question, is it safe to check the PCM grounds with an ohmmeter at the terminal pins or is there a chance at frying the PCM. What I'm after is , what if I have an intermittent problem, loose connection or corrosion.

Disconnect battery first ie both leads.
You will fry your multimeter checking a live circuit and possibly cook pcm.

Best wire integrity test is voltage drop. Paul has some good videos on this, maybe even search voltage drop vs ohm test.

Engine coolant temp sensor can also cause a rich condition.

Ia there any way the the front and rear o2 harnesses are mixed up? I don't remember your values there but I remember seeing something weird one time that made me think of that.

Timing also comes to mind to me. Especially with recent head work, even though there was a problem before that, you did mention that it seems worse.

Like someone else mentioned, are you able to force the o2 sensor and fuel trims lean if you pull a vacuum line off?

Is there any change if you pinch off the purge valve hose? A stuck open purge will usually cause a lean condition, unless the canister is saturated with fuel.

Just a few more ideas for you to try out before you swap the PCM. I personally wouldn't go there yet. I'm not sure if there are any bulletins addressing this either.

Nissan Technician