Cylinder 6 Misfire

So, I believe I posted earlier about an issue I was having regarding cold starting issues (starter replaced, cranks at normal speed, but takes a while to fire). The plugs and coils have been replaced on all cylinders and injectors have been replaced. The injectors were a no-name brand from Amazon, but I have some new ones from LN ready to go. I was getting a fuel pressure loss after shutdown, but did have approximately 48psi during engine run and about 30 psi prior to engine start. I will occasionally get a cylinder 1 or 2 misfire, but that seems to have transitioned to a constant cylinder 6 misfire under all engine loads. I have confirmed this is the only misfire while running in PIWIS. The car has been sitting a few days while on vacation, but I am tackling this tonight when I get home. Manometer reading after driving home was approximately -3 inH2O. The troubleshooting steps I am going to perform are as follows: Smoke test Check spark plug condition Check Coils condition Move Coil to different cylinder Move plug to different cylinder Replace all fuel injectors and perform pressure test Any other thought/steps before I start later this evening?

It's a good list. If moving coils & plugs doesn't fix it, you could also move the injector. But, since you have new injectors, it would only add time. Good luck with the work!

I didn't get as far as I would have liked to tonight, but here are the results: Smoke test generated a leak in the intake area. Reseated IPD and it appears to have stopped. Manometer (digital) indicates about 1.4 inH2O Definately more oil residue in the intake than I would like Still confirmed misfire on 6. PIWIS injector shutoff testing confirms no engine change when shutting off 6, but there is a diffrence when shutting off the other cylinders I'm leaning towards AOS issues, but would that cause the misfire on 6, or is that just coincidental? I'll pull plugs tomorrow and inspect. Merry Christmas, I guess?

Before completely going down the rabbit hole of bad AOS or bad ring seal (pistons?), I wonder if the manometer values are affected by the misbehaving cylinder?

Low manometer values are either vacuum leak or excessive blow-by. Likely there are others. Hopefully smarter people will chime in. I would continue on. The plug in 6 will show if there is a big problem there that may be leading to blow by.

Quotes source post: 20371813

True^^^ My #6 was completely bridged with burnt deposits due to excessive bore scoring. I'm hoping for a better outcome for the OP.

I attempted to use my cheap borescope, but couldn't get a good view. I tested the resistance of the ignition coil on the 2 outside terminals and it reads 0.3 - 0.4 ohms depending on the meter. The general condition appears fine. The spark plugs and coils were all replaced approxmiately 2235 miles ago. Plugs and coils are from FCP Euro. Plug part # Bosch 79173 - FGR5KQE0, Coil Part # - BERU-ZS178 Compression on Cylinder #6 after approxmiately 8-9 pulses is 120psi and holds for over 10 minutes. I have not checked other cylinders at this time, nor have I attempted to swap coils yet. Attached is the number 6 plug. The residue is dry, no oily feeling.

Replaced number six plug with a temporary new one from the parts store. Still have misfire on cylinder 6. Removed plug and inspected, light coating of fuel on plug. Manometer now read ~4.5 inH2O with one port open (dual port manometer). If I plug the open port, it reads ~5.5 inH2O. I still think that this is an injector issue. ChatGPT is leading me toward AOS failure. Disregard the prior post about the compression tester holding 120 psi, I forgot that is the normal function of the tool.

The mano reading is now in spec. 4 to 5 maybe 5.5 is good. I don't think you are supposed to cap the second port. The question is what happened to change it from 1.5 to 4.5? Blow by in a cylinder doesn't change, so something else is going on. Agreed the plug looks like a rich mixture. Which could be the injector. I would say to check another plug on bank 2 to see if it is light or dark. Can you get a reading of the fuel trims from the computer? If one injector is dumping fuel, the trim should go negative to balance out the 3 injectors on bank 2. Also, please swap the coils. A resistance reading won't show this problem.

Is #6 firing at all? With the engine stone cold start it up for 30 seconds. Then touch each exhaust runner. If #6 is cold then you have a fuel or spark problem. From what you said I would suspect the injector or the wiring to the injector.

I will swap coils from 5 to 6 tomorrow, but will check trims and exhaust temperature first.

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Maybe I had an additional leak that reseating the intake components fixed? Or maybe I didn't have the cap on properly. I also performed some PIWIS testing and Camshaft Adjustment of Banks 1 and 2 both respond with Negative Control Unit Response: conditionsNotCorrect or requestSequenceError (7f 31 22) But that could be user error on my part and the PIWIS clone. The valve lift test causes the engine to stall.

Update: Before swapping coils exhast ports from 1-5 are around 400 degrees F after about 30-60 seconds. Cylinder 6 started climbing to the low 100s, maybe from absorbing #5's heat, but maybe due to incomplete combustion. Swapping coils now to see if it follows. Update 2: Swapped Coil to Cylinder 5, still misfire on 6, readings about the same as before. So now we know the coils are good. I know we have air, so only thing left is fuel. Update 3: Changed injectors 4-6 (all three, since I have the parts, and I have to get in there anyways, still lost a fuel injector clip somewhere, but good thing I have spares) and there appears to be no misfire. New plugs arrive tomorrow and I plan to change all the plugs and remaning injectors then. Manometer hovers around 4 with the misfire resolved. UAOS at clutch time or when AOS fails, whatever comes first. I'll update once everything is complete, since I know everyone here is holding their breath with anxious anticipation of the final outcome. I hope this helps someone else down the line and as always, I am greatly appreciative of the wealth of knowledge and desire to help here on Rennlist.

Good job man, sounds like you resolved it. If the fuel injector was dumping excessive fuel, it may have contaminated the engine oil.

UPDATE: Job is completed, plugs and injectors replaced. Still have a slow to start (normal cranking speed - new starter installed this fall) when cold and the engine stumbles to idle but doesn't die. It then rises to 1000rpm and holds for a few potatoes and goes to a normal (sub 700 rpm idle). No vacuum leaks, per the smoke machine, new MAF, only thing I can think of is possibly coolant temperature sensor or check valve in fuel pump, allowing pressure to bleed off. No smoke on startup. Zero issue after the first start.

Nice! Thanks for posting the follow up. It's nice to see it's better. Excellent calm disciplined approach. A puzzle that may or may not still exist is the one mano reading you have of 1.5. If that was a real value and you didn't find any leaks, it may be an intermittent leak that will come back at some time. I doubt it's blow-by as you've made normal readings since then. The other weird reading was the PWIS. I have zero experience with the PIWIS. I would say just pay attention to mileage and oil consumption. And drive it well!

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. Sounds exactly like a leaky check valve. 996.1 cars the pressure regulator/check valve is on the fuel rail on the engine passenger side, on the 996.2 cars without a return line the pressure regulator is part o the fuel sending unit housing...

I also have a slow start if I turn the key right away, from cold.if I wait the 5 seconds for the oil measurement, then it starts normally, symptom of the above comment (fuel building pressure).

Sorry to bump the resolved thread, but my 996 4s is experiencing very similar symptoms. Misfire in bank 6. Replaced the coil and spark plug with new ones (should have done the swap with cylinder 5 before throwing new parts at it) no changes. I am also leaning towards the injector or injector wiring. Can you please share how you tested the leads going to the coil? What were the settings like on the volt meter? Lastly, how I too am leaning towards an injector issue. How long did it take you to get to #6? Thanks, Mark

I did not test the injector leads, as I knew I had put cheap injectors in prior. But you can get injector testing kits from Harbor Freight and it should hook up to the injector connector and light up when it fires. I don�t have an at home test to see if the injector is properly firing, so if you have any doubt, based on the cars mileage, it may be a good time to replace it/them. If you do 1 you might as well do all 6.

Thanks for the reply. How long did it take you to pull the injects off of the passenger side?

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I work somewhat slow, so I would allot a few hours to the job. Make sure to order replacement fuel injector clips prior, mine was missing a few. Lowering the engine to the cross member allowed additional room for the injectors on the back of the engine. The o-rings are tight, so don't be afraid to pull on the old ones, and forcefully push the new injectors in. The guide on Pelican Parts was actually fairly helpful. Good luck!