Part throttle all cylinder misfire

[vinylscratchp0n3]

I have a 1998 Legacy 2.5GT 5MT with everyone's favorite engine, the EJ25D. As the title states, it misfires on all 4 cylinders at part throttle, mostly on the highway. Almost never actually throws any codes for it and when it does it's only p0301 through p0304. Idles fine all day and runs great at wide open throttle every time. Started a few weeks after I did all 4 wheel bearings, struts and springs, front control arm bushings, rear trailing arms, SPC adjustable lateral links, diff outrigger bushings, and changed the transmission and rear diff oil. Trying to figure out what the cause is is kicking my butt, tired of firing the parts cannon and it's been a long time since I've gotten to drive it. The drivetrain is not modified save for an MSD 8239 coil pack, UEL header, Tsudo high flow cat, and a grounding kit.

 

New parts this year:

- Upstream O2

- battery

- fuel filter

- copper NGK spark plugs gapped to .055"

- BG 3 step fuel system and throttle body/intake cleaning service

- exhaust manifold (old one was cracked) and all new gaskets and donut

- all small diameter vacuum hoses (replaced one at a time to prevent accidental mix-ups)

 

Recent new-ish possibly related parts it has from the last couple years:

- hitachi knock sensor torqued and angled to FSM spec

- NGK plug wires

- OEM MLS headgaskets, 45k miles old (heads were resurfaced professionally)

- OEM coolant temp sensor

- STi Group N engine, dogbone, and transmission mounts

- Professionally rebuilt and rebalanced driveshaft

- OE reman front axles

 

Swapped from my 96 Legacy Outback (has a 97 engine), known working:

- MAF

- MAP

- coil pack

- ignition ignitor ("power TR unit" above the dogbone mount)

- ECU

- EGR backpressure transducer

- TPS (adjusted properly to exactly half a volt on the center pin)

 

Freeze frame from one p0303 event gave me -11.6% short term fuel trim and -3.8% long term. 26.7% load, 3063 rpm. Before this it had been occasionally throwing p0400s (EGR flow) which had me suspecting something in the EGR, but it still stumbled with the EGR unplugged. It hasn't thrown that code since it started stumbling. The EGR valve does stay all the way closed at rest. I've been meaning to swap over the fuel pressure regulator from the Outback to it but haven't found the time yet.

 

Any ideas? I just want to drive my car again.

Edited December 30, 2021 by vinylscratchp0n3

[silverton]

the short trims aren't as telling as the long. since your long is under 5, that's great. with a freeze frame showing negative 11, it's removing fuel at that moment so an incomplete burn seems to be what's going on.

 

the gap does seem large, but you do have an aftermarket coil pack and I don't know what they recommend for that unit, but I do know MSD's can punch! Anecdotal, but I had a full MSD ignition set up on an S12, and iirc, I ran something stupid like .060. So... I'm wondering if your voltage is dropping and the coil pack can't make that jump with your big toe on the throttle.

 

If it's easy, I'd put a factory coilpack back on it and gap the plugs to oe spec and see if that takes care of your problem. You may also just try .050 on the plugs and see if that helps.

 

What brand upstream O2 did you go with? These car's are pretty finicky with O2's and MAF/MAP sensors.

[vinylscratchp0n3]

the gap does seem large, but you do have an aftermarket coil pack and I don't know what they recommend for that unit, but I do know MSD's can punch! Anecdotal, but I had a full MSD ignition set up on an S12, and iirc, I ran something stupid like .060. So... I'm wondering if your voltage is dropping and the coil pack can't make that jump with your big toe on the throttle.

The old NGK laser platinum plugs that were in it when this started were gapped to the factory .044 and about 45k miles old (changed with the headgaskets), which it was still doing this with. I did try a stock coil while the old plugs were still in it which did not fix it. .055 is what another 2nd gen nerd recommended to me with that MSD coil after he'd played around with the gaps on his for a while, runs strong when it's not misfiring with that gap.

 

What brand upstream O2 did you go with? These car's are pretty finicky with O2's and MAF/MAP sensors.

It's a Bosch, the old one was Denso and also only a few years old like the plugs were. No change in how it ran after.

[silverton]

With that information, I would start with one step colder than stock platinum or iridium plugs, gapped at 050 and see what happens. coppers are great, but they don't last very long..especially if you're throwing a higher power coil at it.

 

If you are still having issues after that, I would put stock components back in the car, not necessarily new but known good; say from another running car. Not your old stuff.

[vinylscratchp0n3]

One update: I've cleaned the EGR valve which cured the issue at highway speeds. Unfortunately, the stumbling remains from 30-40ish, still only at part throttle. I'm considering pulling the valve covers to check the valve adjustment as I've heard these like to tighten up their exhaust valves over time.