5.4 low rpm misfire..Stumped
 

I'll try to keep this as short as possible.
2011 e350 5.4 purchased with a bad misfire, and CEL saying cylinder #2 is misfiring.
Found that the spark plug was totally melted and the female threads were destroyed.
So for cylinder 2: Time-sert, new spark plug, new coil. This got it running much better.
BUT still has this annoying low RPM misfire and idles at 600. When I'm going between 35-45 feathering the throttle and it shifts into overdrive dropping the RPMs to around 1000 it bucks like a SOB
So I replaced,
#2 injector
All coils with new
All spark plugs with new
Scanner showed -30% Short Term trim bank 1 when sitting at warm idle.
Pulled Fuel rail and hillbilly'd the injectors to the fuel rail with zip ties. No leaking injectors with fuel rail pressurized. Tested each injector with a 9v battery and alligator clips. Each one had a good spray pattern add shut off nicely.
Brand New catalytic converters because of a thief with a sawzall a couple weeks ago.
Compression tested cylinder #2 cold @ 119psi,
Fuel pressure is at 58 psi pretty steadily from idle to 3000 rpm. Which seems high.
O2 sensor B1S2 is giving weird signal. Just sits around .825 V at warm idle. No fluctuation like the other bank or my other van that runs well.
Sprayed carb cleaner everywhere to find a vacuum leak, with no luck. Disconnected brake booster and the red vacuum line and capped. No luck.

Sorry for being long winded
Any tips?
Thanks guys!

The compression PSI is a bit low it seems--should be around 155 or so dry?

I wonder if this could be a bad power or ground connection to the injector? If the plug was melted that could suggest a super hot cylinder like you'd get with no fuel being added but the COP keeps firing.

Naturally that -30% STFT is scary.

Hope this is nothing too serious although at this point that's not of much use. :)

Yeah the next thing I'm planning on doing is to start digging into the electrical. I was thinking weak/inconsistent spark or bad injector signal.
I hooked up an old school spark tester to the #2 COP and then to the #4 COP, the spark looked the same and they seemed to spark at the same timing

I'll be interested seeing what this issue really is---again hoping its nothing serious or $$$$Costly$$$$.

If I were you I'd be looking into that wonky O2 sensor. It sounds to me like that is a fairly likely culprit.
I would also check over my time-sert to make sure its holding well and all.
But please come back and let us know what caused it. It's an interesting issue.

A downstream O2 sensor won't cause misfires. That one is probably pegged from the misfiring #2 cylinder.

Go back and check compression on the rest of the cylinders on bank 1 along with #2 again. Taken individually, the #2 reading you provided is out of context and you can't know if it's "good" or not without knowing what the rest of them are.

Personally, given what you've told us about the original issue and repairs leads me to suspect mechanical damage to the #2 cylinder or valves. Perhaps precautions weren't observed when installing the TimeSert and a valve got knicked or the original failure damaged the valves, piston, or rings.

Couple of questions. How did you 'determine' Fuel pressure, via OBDII scanner?? (Should be ~ 40 RELATIVE TO IM VACUUM). You can't just measure it with a pressure gauge relative to atmospheric pressure. If you did, actual (relative to IM vacuum) would be ~ 70 psi. That would sure make sense of your -30 STFT.

Also significant NOTES: Proper ignition (NO Misfire) is actually determined in the PCM's high speed processor by reading the crankshaft rotational speed acceleration right after commanding ignition to a COP. If the acceleration rate is 'below' a factory programmed spec, it equals a 'Misfire'. It IS POSSIBLE that the spark plug 'sparked', but failed to ignite the fuel/air mixture, or the explosion just wasn't strong enough. What's important about this in the case of your symptoms it might be wise to get a quality 'power balance' test done in light of that #2 compression reading.

ALSO: The PCM 'strives' to protect the CATS. If misfires (actual electrical or lack of sufficient ignition) are occurring above a certain level - the PCM will alternately shut off injectors (and FLASH the CEL) for a short period in an effort to "PUMP" fresh air through the exhaust attempting to flush fresh air through the CATS. Many individuals (and seasoned mechanics) perceive this as a horrible occurrence of misfires often described as "BUCKS LIKE A SOB".

Thanks for the knowledge guys.
Tonight for *****s and giggles I'm going to put a OE spark plug in #2 just to be safe.
I'll Compression test other cylinders to see what the spread is between them.
In regards to the fuel pressure, I had a fuel pressure guage hooked up to the port on the fuel rail. I'm assuming the fuel pressure regulator is above the fuel tank? That pressure seems pretty high for a bone stock engine. Do y'all think I should dig into that further?
I'll keep y'all in the loop
Thanks again! This is my first real fuel injection diagnosis, so I'd like to get to the bottom of this. I've wrenched on a decent amount of carb'd stuff so this is somewhat new to me.

Uh, what spark plug did you use that you're going to replace it with an OE?

You don't have a fuel pressure regulator. FP is managed by the closed loop control of a pressure sensor and the pump motor controller (FPDM).

55-60 psi is normal.

It had cheap autolite ap103s replaced with bosch double platinum. Heading to NAPA now

Ugh on the Bosch plugs. They've been repeatedly implicated in misfires, even when new, over the years. The Autolites are actually a far better choice.

Agreed on the Bosch ignition parts as used in these engines--too often we've read about their sensors being problematic too. Odd from such a usually high quality manufacturer but the anecdotal experience just here on FTE is enough to steer people from that brand.

I'll be interested to see what your compression tests reveal.

So a little update,
Replaced the bosch spark plug in #2. Torqued it down to spec this time instead of giving it an 'arm torque'. No change

Tested compression vs #4. It was within a couple psi. It was reading just over 120. I got lazy and didn't compression test the other cylinders yet.

Removed valve cover bc I came up with a theory that maybe the cam has a flattened lobe. Cam Looks goods with no obvious signs of oil starvation.

Oh yeah and changed the oil with high mileage synthetic blend with
https://cimg5.ibsrv.net/gimg/www.for...4a3d63c208.jpg
https://cimg3.ibsrv.net/gimg/www.for...04363e3fa7.jpg
marvels mystery oil for the hell of it

Forgot to mention my next steps

1. Compression test other cylinders
2. Leak down test.
3. Throw the towel and take it to the mechanic if it passes those 2 tests

I just re-read this whole thread and you got a bunch of savy, sincere members trying to help. Don't throw the towel in.

I am struck by a couple of points. You seem like a savy guy also. Through all this you have given descriptive diagnostic diagnosis (obviously from a scan tool) - but never a code. What code are you reading that you say is a 'Misfire'? If it is a P0352 - shut this rabit trail down and start looking the other direction toward the PCM. I confess, in my prior reply I also assumed, and focused my reply on a P0302 - and failed to ask.

Also (for full disclosure, I'm just a 73 yr old driveway mechanic - experience limited to tearing up lots of stuff [both by abuse & wrench] as a kid), but your compression test... ??? On cyl #4 now BY ITSELF?? That suggests to me that you are doing compression test with all other spark plugs "IN PLACE"... ??? It has ALWAYS been my understanding a compression test is done with starter motor while all plugs are removed {once dry, then once with a squirt of oil}. If I'm right, we still don't know if compression on number 2 (or 4 for that matter) is good. Although 119 or 120 should not, IMO, result in a constant P0302.

Don't think I'm trying to beat up on you. I, like my fellow members here, just want to help.