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Misfire Has Me Going Crazy!

nicr4wks

By nicr4wks,
in Engines & Forced Induction

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nicr4wks Experienced

nicr4wks

Posted
Posted (edited)

Gday guys,

So here's my setup. 1997 S2 R33 gtst, a few weeks a go the engine developed knock which turned out to be a spun bearing on 5th, the engine is being rebuilt but is going to take a few months. On the weekend a rb25de was offered to me for a carton, so I thought what the hell, let's do a bit of an experiment. I stripped the engine back to just the block and head and swapped on my 25det intake, all sensors, exhaust manifold, wiring, turbo etc. It's now basically a high compression engine with all the DET stuff strapped to it, if it blows up then so be it, I'll have some interesting results to share.

The problem, as soon as it hits ANY positive manifold pressure the car starts misfiring, all the way from 1psi through to 10psi. Under load or sitting in neutral it does not matter. The car behaves fine off boost and is drivable.

Things I've tried:

New spark plugs 0.8mm gapped.

Swapping each coilpack one by one with a known good coil (could be more than 1 faulty, I don't have a full set for testing)

Stock DET injectors as well as larger 550cc injectors.

Different stock fuel rail (swapped same time as injectors)

Checked base timing with light, set to 15deg.

Tried with various AFR from 10:1 to 14:1 (via fuel map adjust)

Tried with various timings (via timing map adjust)

Checked battery voltage reported by ECU, 13.8v

Things left on my list:

Get a fuel pressure gauge and check. Fuel lines were hooked up backward when the engine first went in, perhaps damaged something.

Investigate DET vs DE fuel pressure regs, maybe it needs to be higher for the higher compression engine.

Try and borrow a full set of known working coilpacks.

Anything else I should be checking? Running out of ideas here!

Full mod list:

RB25DE+T

Kinugawa TD06

Tial external gate 10psi spring

JECS 550cc injectors

Bosch 044 mounted in tank

United E85

Innovate wideband

RB20 nistune with vct & flex fuel

FMIC

Blitz i-color EBC

Cheers.

Edited by nicr4wks
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scotty nm35 Grand Master

scotty nm35

Posted
Posted

Drop plug gap to .6, or better get some iridiums and gap to .5

Change coil harness, they are known to become old and shitty. Measure power at the coils and make sure it's 12v at least.

Change to a better coil, or replace them with new OEM or Splitfires.

scotty nm35 Grand Master

scotty nm35

Posted
Posted

Get a fuel pressure gauge and check. Fuel lines were hooked up backward when the engine first went in, perhaps damaged something.

Investigate DET vs DE fuel pressure regs, maybe it needs to be higher for the higher compression engine.

You may need the DET fuel reg. Shouldn't have damaged anything, but checking the pressure is handy to rule that out as an issue.

nicr4wks Experienced

nicr4wks

Posted
  • Author
Posted

I'm currently running the DET reg, after some quick research on fuel pressures it looks like that should be fine even on the higher comp engine. Unless the reg has failed I guess I'm probably looking back at spark.

I've got another coilpack loom I'll swap out tonight, check for 12v at each coil and check resistance to ground also. Thanks for the tips.

jay-rod Proficient

jay-rod

Posted
Posted (edited)

Get your injectors tested
I had same issue, if its a constant misfire, if it feels like its running on 5 cyls or something like that but cant locate the culprit.

Had mine cleaned and they were blocked as all shit.

Car would idle fine, but as soon as you put any load on it it felt like it was running on a constant 5 cyls

My fuel pressure was also up in the 60s when they were blocked.

I also initially had my rails plugged in the wrong way, but everywhere i see says it shouldnt matter and wont affect anything.

Edited by jay-rod
nicr4wks Experienced

nicr4wks

Posted
  • Author
Posted

I've tried two sets of injectors, one set pulled from a working engine just a week prior.

As for the tune, I've tried stock DE & DET maps, as well as custom fuel & timing maps, it misfires the same on all of them.

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