Re:1996 Ford 5.0 Truck diagnosis (massive negative LTFT)

1996 F150 OBD2 Old body style, 5.0L, 2wd, 110k miles (YES, OBDII)

Current concerns:
-Engine idles rough like it has a missfire.
-Feels short on power pulling hills
-Under load, pulling a hill, there is a distinct spark knock sound from passenger side. The knock does not jump around...it seems to be very localized knock/ping/detonation sound. The knock can be silenced by lifting off the throttle a little. Only under load can it be heard.
-flat out driving, mostly under lockup in overdrive, you can feel a hiccup or misfire
-light throttle acceleration from stop (closed loop) it struggles and bucks like it is out of gas

All sincerity it feel like ignition issue or fuel delivery issue…however the Autel says opposite!!
This is where my crazy confusion comes in.

Scanner shows:
-Massive negative -20s LTFT & negative STFT also (BOTH BANKS) – also throwing code
-O2b1 & O2b2 showing normal average of .45v, and so is O2b1-2 downstream
-FP is 35ish regulated, and slightly higher with vacuum like pulled off
-FP slightly rises with throttle on tank1, and drops to a crappy 20 on rear tank (known weak pump in this tank)
-New fuel filter
-TPS .8 at idle, goes up to about 3.5v cranking the throttle arm (unsure if I got to WOT)
-No blockage in air intake/filter
-New ICM (was bad, replaced with BLACK version for 5.0)
-Plug wires are routed correctly – unknown age but appear very new along with plugs.
-Passenger side exhaust manifold leak, repaired, but still slight leak due to broken stud on #4
-EEC pulled and inspected, looked perfectly brand new, no capacitor leakage

On my to do list:
Test MAF return voltages with DVM

I’d enjoy any comments you can throw at me. I am running out of ideas

Questions:
Can a cat be plugged up and still have the proper o2 ref post cat?
Can misfire (ignition issues) create enough raw fuel to show rich?
Any other data on the scanner help? 1996 does not have the entire host of sensors or data available like todays cars.

FIRST POST
Thanks for the help…been an mechanic of all things 90’s for my whole life, but only recently diving into data driven scan diagnosis. It’s a 5.0!! There can’t be that much wrong with it!

After another 20 minute drive I graphed the secondary o2 post cat and found it was way way down in the .0x voltage range and only spiked infrequently.

Any help would be great.

Can you post a pic of the front and rear o2 sensors graphed? With engine rpm and temperature. From the sound of it the .45 might be a bias voltage like gm uses. Not sure if that vintage of ford used bias voltage. Also it might not be a. Bad idea to actually scope or use multi meter to check the actual oxygen sensor voltage Incase the pcm isn’t interpreting it right. From the sound of it sounds like 2 bad oxygen sensor in the front because you said the rears were dead lean

Thanks...the Autel can't show me more than 2 graphs, but I will get a combo of pictures at least then. Something is telling it I have 25% more air coming in. I plan on use the DVM on the MAF this weekend.

as for the .45v, I am just suggesting that is the average I am seeing as it floats from .1 to .8 when it switches.

I also need to pull the fuse and let it re-learn for a better picture.

It does also have a canister purge code...solenoid replaced twice now. I need to check wire integrity - there are no other sensors to suggest CAN P is bad other than the EEC verifying there is ohm value at the solenoid...so if two units fail, I assume there is a wire issue.

I have way to much stuff on my brain to check...I am the final last person who will ever work on the truck as the previous owner gave up with it in the shop multiple times. The previous diagnosis was "burnt valve or broken ring"...which I find hard to believe. So a confirmed compression check is on my list also. Replacing the ICM made significant improvements.

If I pull the spout and the MAF connector it would be stay in open loop no spark advance, and read a pre-set table of fuel curves based on o2's still right? Will the o2's still function and graph proper?

I have ran without the MAF connected and it still ran like crap.

next images

My EEC is pulling fuel hard.
CODE PO172 and P0175 popped up today

I AM NOW SERIOUSLY QUESTIONING THE AMAZON INJECTORS FLOWING TOO MUCH!

If that is true, I am super confused why it runs like it is out of gas when I am providing too much?

What do the o2 sensors say on the freeze frame data? I'm with Cheryl thinking that low rear 02 voltage indicates that the front sensors may be lying. Do a if the sensors respond if you introduce a vacuum leak or propane? Is there any way the front and rear sensor wiring was accidentally reversed?

Sent from my moto g fast using Tapatalk

If the rear 02 is reading low, isn;t that what it is supposed to do? That means there is no o2 and the cat is doing its job?

If I introduce a vac leak I should see the 02sensors richen up to compensate for more air than the MAF has calculated right? I should see it start to add fuel. I will try that.

Lastly, I heard a noise yesterday I had not heard before. I hear a gurgling or almost a grinding sound in the intake beyond the throttle body...it was really strange...more so when cold...once it warmed up I could not hear it anymore.
ALSO
Explain how a broken ring or burnt valve plays into this equation? That is what I have been told by two other mechanics who previously worked on it. If that were true, how does that create negative fuel trims?

To answer my own question:
If the compression is low, excess raw hydrocarbon will be in the exhaust stream. The O2 sensor will read this as rich, and the computer will lean out the mixture (causing negative fuel trim) to compensate. An excessively lean mixture can cause misfires.
If the spark is not healthy (weak) then excess HC will be in the exhaust stream, same effect as previous.

I introduced a vac leak.
1) I pulled the CAN PURGE line (1/4" hose in front of the throttle body, but after the MAF) and my idle stft and ltft climbed up into the positives....o2 bounced like normal...appeared to hang at the upper voltages; as the top of the curves were broader than the valleys. Holding the throttle at 1500ish they fall again and start to go negative. On a 20 minute drive the averages on the trims were we much less negative. LTFT hovered around -8 and st was moving + and - . If it idles for too long it will peg the +25% ranges asking for fuel.

2) replaced the hose in question and ran another 20 minutes with the FPR vac hose (much smaller maybe 1/8" line directly to the upper plenum, after the throttle body obviously). The resulted in pegging the STFT in the first few minutes with O2 hanging at .8-.9 and after several minutes the fuel trims just went 0.0% and the 02 stayed pegged high. I assume it just went open loop and stopped reporting. I plugged the line back in and off again a couple times. plugged up, everything went negative...unplugged it would peg and stop reporting again.

3) Detonation or rattle under load did not go away with more fuel, possibly less, but for sure still existed.

So, from all my scans, I can trick it into pushing more fuel with a vac leak EASILY.
Plugged it all back up and immediately starts going negative again.

So what did I learn?

You reported in your first post that B1S1 and B2S1 02's are oscillating 100-800mv as well as are both post cat 02's.

This is concerning. Those post cats should not be mimicking the pre cat 02's. At idle with HOT cat's, both sensor 2 02's should be steady around 600-700mv. At high RPM they would go low. Again with hot cats. Absent any cat codes I think your post 02's are bad. Bad cats will cause lack of power and oscillating 02's. 02' are less expensive than cats I would explore the post 02's first.

Hope this helps. Look forward to seeing further posts.

I only have 1 post cat. It stays low with not many spikes. Doesn't this tell us that the cat is working and it is burning lean....which the computer is telling it exactly to do. I do not believe the system is running rich at all....with the vac leak it runs better!

Yesterday I pulled the MAF and the SPOUT connector - i ran like crap, still went negative on the fuel trims but did not detonate or rattle when loaded up.

While the kids was at soccer I did some ear timing on the distributor. Retarded it to where it would idle but barely rev and heard an exhaust leak I did not know about. There is a 1/4" hole right in the Y pipe area that was puffing pretty loud. When I advanced it back to where I started the leak stopped making noise...just strange...but I have a small hole in the exhaust just before the cat I didn't know about now.

Im about to throw everything I have been told by previous mechanics out the window because now I even question the timing being set properly.

I am going to run a comp test and leak down combo and examine the spark plugs.

Driving it around with 1/4" hole vac leak up at the throttle body; other than the detonation that makes your skin crawl under load, you just back out of the throttle a little, it drives very very well!