Thanks, clogged CAT is an interesting suggestion! That's a totally new angle that I haven't considered. What is the best way to inspect for a clogged cat? I've never encountered one before. Should I just open the exhaust and do a visual inspection or is there some way to see evidence in my scantool data?

Additionally I'll mention that I've set up and begun using JLR SDD 130 instead of Autoenginuity. Autoenginuity is fast and simple but SDD gives me access to real-time PIDs from different modules at the same time. I'm planning to take it for a drive tonight to monitor fuel pressure and MAF under load at high RPM. Is there anything else I should monitor while I'm at it? I'll record / snapshot if I see anything interesting.

Depsite setting the multi-cylinder misfire code, I do seem to trip cylinder 3 and cylinder 8 pretty frequently. What is interesting is that both of those cylinders were missing at idle when I first bought the car. Changing the coils eliminated the #3 and #8 idle miss but I'm starting to wonder if I don't have a weak primary signal on those coils. Perhaps the new coils simply respond better to a poor primary ignition signal. Prior to replacing the coils, I swapped them around the engine and there was no change with OE units, the idle miss was still there. I thought that was weird at the time but I assumed I had just lost track of which cylinders' coils got swapped.

Unfortunately I don't own a portable oscilloscope but I can hook up my bench scope at idle and look at the primary on 3 and 8 vs. known good. Cylinder #4 has been well-behaved since I bought the car so I sort of use it as my benchmark.

Cylinder 3 and 8 are consistently misfiring at high RPM. Compression in both seems normal, no evidence of oil or water contamination on the plugs, if anything they look like they are running lean. I swapped the coils around yesterday and cleaned up the fuel pump inline harness connector FP2 which got wet under the seat along with FP6. Fuel rail pressure rises ~15psi to 65psi right around the time that the misfire occurs. The problem is its hard to tell if that happens due to computer intervention to shut down injectors or if it is normal for WOT under load.

Regardless, I am still getting the same exact behavior. Manifold absolute pressure looks fine, fuel trims look fine, all of the transmission codes have cleared out. The only thing I can think is that I'm not getting a good primary ignition signal at high RPM from the ECM. LT and ST fuel trims seem awfully low at high RPM for a car that consistently misses on two cylinders. Regardless of whether I have a fuel or ignition misfire I would expect the trims to indicate a lean condition resulting from incomplete combustion. Hopefully I can disconnect the O2 sensors without sending it into limp mode. I'd like to take it on an "open loop" test drive to see if the behavior changes.

Cats may be damaged due to this lingering misfire condition and all of my test driving but I don't think the cats are clogged enough to cause a misfire. There is no sign of a clogged cat in the MAP data and occasionally it will pull hard without misfiring. Injectors and coils have all been swapped around, 3 and 8 are missing consistently. On a rare occasion I pick up a misfire on cylinder 7 but SDD shows intermittent and it might happen once every 4 or 5 test drives.

All brand new SMP UF519 coils and coils have been swapped around six ways from Sunday, misfire does not follow the coils... replaced injector 3 to rule out an injector issue, swapped injector around, doesn't follow injectors. Short of any further ideas I am going to start digging into the primary ignition and injector driver circuits.

For my diagnostic test drive tonight I am going to plug ALL of the vacuum accessories. No PCV, no EVAP and no Power Brake Booster. If I'm still missing I will pull the fuel rail pressure sensor vacuum line (plug the port) which will trick the computer into running rich. That should improve the problem drastically if I have some unspecified air leak via manifold, duct, etc.

I had misfires at high RPM before when my catalytic converters were shot. The clog allowed enough exhaust flow to get through at low RPM to where it wouldn't cause misfires, once it got higher though, the exhaust gas would get backed up and superheat the downpipes and cause random misfires.

Have you checked that cats?

I fabricated a threaded plug to go in place of the upstream O2 sensor that gave me a hose barb for testing back pressure. I installed a very sensitive analog gauge that ranged -15 to 15psi. From Idle to rev limit at ~4200 rpm I have absolutely no back pressure in the exhaust, in fact it pulls a slight vacuum at high RPM as expected (exhaust rushing past the gauge port.) Pressure pulses at idle are ~1/4 psi max and appear very consistent on both banks. No obvious dead spots to indicate a sticking exhaust valve or anything unusual happening with the engine. No difference in back pressure behavior from bank 1 to bank 2 and 99% of the time my misfires originate on bank 1. If it were a clogged catalytic converter I would have expected to see a major difference between the two banks. What other method would you use to check for a catalytic converter problem? MAP pressure / vacuum behavior looks picture perfect on the scan tool. No vacuum pulsation to indicate an intake timing problem or intake valve issue. Vacuum at idle is ~19 inHG, I'm not sure what a normal baseline idle vacuum would look like for this engine because I didn't conduct this extensive testing on my previous 2004.

The only thing I noticed that seems unusual to me is that my MAP sensor reports 13.92 psi with the key on / engine off. I would have expected to see something closer to atmospheric 14.7. My altitude is ~950 ft above sea level so I should be really close to 14.7. Granted the MAP sensor calibration may be poor but why would that only cause a problem on bank 1?

It doesn't look like I could visually see the catalyst honeycomb on these cats if I pulled the pipe down because there is a bend just upstream of the converter. I would need to buy an inspection scope to look down in the tube and I could only justify that random expense if I observed some back pressure during testing. Again, the problem is concentrated on bank 1 and mostly Cylinder 3, therefore it is hard to justify buying brand new catalytic converters without some way to verify.

This problem is exhausting (no pun intended)... I have absolutely no other trouble codes on the system now, I've cleared all of the electrical issues. Transmission is clear, all network faults are resolved, absolutely nothing else wrong. When it misbehaves, cylinder 3 misfire is the most common but I occasionally pick up cylinders 1 and 7 as well so it is 99% related to bank 1. Maybe two or three times in all of my extensive testing I have pulled an intermittent code for cylinder 8. I would say 90% of the time it is cylinder 3, 99% it is on bank 1.

What am I missing here? This is the most challenging car problem I've ever faced. The fault is NOT intermittent. It bombs out and misfires every single time I romp on the gas, usually it will hit about 6k RPM and drop into limp mode instantly. The problem is that I can't reproduce the failure in my driveway, it has to be under load. I beat the daylights out of it the other night, just pulling hard, triggering the misfire, and repeating, over and over again, hoping to catch some coolant loss or something to give me direction. Absolutely nothing wrong. I watched every sensor I could think of, nothing unusual. Coolant level is totally stable, no odor or residual pressure in the jar. Running compression is consistent across all 8 cylinders. No other codes triggered. Vacuum looks great, exhaust back pressure is normal. All 8 coils are replaced. I'm 99% certain that the problem doesn't move with the injectors but I'll verify one more time tonight. Crank and Cam signals look consistent. Fuel trims look normal through the whole RPM band up to where it fails.

Tonight I'm going to take it out and drive with the O2 sensors unplugged which should eliminate the trim system and rule out any issues related to the O2. I'll then try unplugging the MAP sensor for another drive, not sure if that puts in limp mode or not.

:icon_bricks: HELP

Okay interesting result last night... unplugging O2 sensors made no difference in the misfire behavior. I also confirmed that cylinder 3 is still showing a miss despite swapping the injector to #4. I had a misfire code for cylinder 8 as well, which happens occasionally.

Grabbing at straws, I connected my analog vacuum gauge to the fuel pressure sensor vacuum reference and found ~17.2 inHG at idle, in agreement with the MAP sensor, but seemed a bit low to me given that I had previously noted 19. I was getting irritated because the fan was running constantly while I was trying to poke around so I reached inside and shut the AC off. When I came back to look at my gauge I had over 20 inHG! AC switched on drops my engine vacuum ~3 inHG at idle. Various electrical loads have a big impact as well. This seems unusual to me... not sure if it means anything but I'm going to investigate further. Tonight I'll pull both O2 sensors off and see how the vacuum compares to idle with sensors installed. If I see a big change with sensors out then that gives me justification to tear down the exhaust and investigate.

I'm also going to watch battery voltage during my high speed runs to see if it could be an alternator problem. I'll probably order a PicoScope today so I can try to catch the coil primary and fuel injector PWM in the act while it is actually misfiring on the road.

After months of struggling with this problem I've finally had a breakthrough! The high RPM misfire became consistent enough to trigger restricted performance mode with a quick snap throttle to 4k RPM while in park in the driveway. This gave me an opportunity to try a bunch of things quickly to see if anything changed the behavior. Lo and behold, unplugging the fuel rail pressure sensor electrical connector eliminates the problem! Not going to lie, I jumped up and down a little and had a celebration in the driveway. Enthusiastic test drive confirmed that the high RPM misfire is totally gone when the fuel pressure electrical connector is disconnected!!!

Here is why this is a weird result (and totally unexpected):
- Early in my diagnostic process I swapped the fuel rail pressure sensor with a used spare part that I had in my parts box from my previous '04. High RPM misfire didn't change at all. The used sensor was always presumed good, I just got it with a used fuel rail assembly.
- Early on, I tried unplugging the vacuum feedback hose and plugging the throttle port which would have commanded excess fuel under all circumstances (rich condition) but it had no impact on the high RPM misfire.
- Even though the retention tab is broken on the harness connector, the female pin sockets feel tight (tested with a male terminal) and don't show any sign of corrosion, seal is intact. I zip tied the connector down to hold it firmly in place due to the broken tab. No change in behavior.
- Pressure feedback appears to be normal and stable on the scantool when the sensor is plugged in, ~55 - 60psi throughout the RPM band. Fuel pump PWM driver ramps up as expected to maintain pressure.
- At first start, fuel pump primes the rail with no issue and the engine starts enthusiastically. Idle to 4k RPM is buttery smooth with no hesitation with the sensor plugged in.
- Throughout the RPM band up to 4k RPM the fuel trims remain low and steady. Immediately before I get the restricted performance message, trims jump to ~10% but it happens so fast it's hard to tell if it's a cause or a symptom.
- With all of my intake sealing work we are at 0%/0% LT/ST at idle on both banks! Totally sealed, no air leaks at all.
- O2 sensors respond quickly and seem to be accurate. Unplugging the O2 sensors (front and rear, either bank) has no effect on the misfire condition.

Action Plan:
- I am going to install a brand new, Jaguar OEM fuel rail pressure sensor (ordered one on eBay)
- Ford sells a replacement electrical connector under Motorcraft Part #WPT118, I'm going to order one, extract the terminal pigtails and crimp new terminals and wire seals onto the harness wires. I always hit each terminal crimp with a tiny bit of solder. Will perform a zero splice repair.
- Replaced the fuel filter Saturday but no change in the behavior.
- I'm going to trace all of the fuel pump wiring to see if there is any chance I have a bad electrical connection to the fuel pump.

At least I have a solid diagnostic path now!

Update: I ordered the AMP connector kit from Ballenger Motorsports https://www.bmotorsports.com/shop/pr...oducts_id/3418
My preference is to crimp (and solder) new terminals on the old harness wires whenever possible and install a new plastic connector body. This approach avoids a splice repair using pigtails. $8 for the BM connector kit vs. $50 for the Motorcraft pigtail kit.

*** RESOLVED FINALLY ***

After months of battling the intermittent high RPM misfire I finally resolved it by replacing the fuel rail pressure sensor with a brand new Jaguar OEM part and replacing the associated electrical connector with a kit from Ballenger Motorsports https://www.bmotorsports.com/shop/pr...oducts_id/3418. I crimped and soldered all three female terminals and applied a little bit of silicone grease to each of the wire seals.

All in all, proof positive that sometimes it is something really simple and cheap but extremely difficult to identify. The breakthrough came when the problem became consistent enough to be reproduced in Park in the driveway by simply revving the engine. In hindsight my only clue should have been a brief spike in short term fuel trim values just a fraction of a second before reaching 4000 rpm under load. I'd get a blip of high trim values and then the ECU would go into reduced performance mode.

If you are experiencing a misfire under load, don't automatically assume that you need to replace the catalytic converters. I was becoming really convinced that the cats might be to blame but I'm glad I held out a while longer because it would have been gut wrenching to sink 1k into converters only to start it up and have the same issue. Visual inspection of the cats gave me confidence to rule them out. I've seen damaged catalytic converters before and it is usually really obvious that they are melted, clogged, or broken. Mine looked completely normal. An overnight soak in lacquer thinner revealed no dissolved carbon buildup or loose chunks of catalyst. If you think it might be cats, probably worth pulling the old ones off to have a look first.

Congratulations on a job well done, keeping at it will eventually solve the problem.

Not an easy task, but necessary, unless you have the money to pay to have it done.

Phew!

That's a very rare issue so no wonder you've had trouble finding it.