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1.5jz high load misfire

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Evening all,

i have a 1.5jz running ecumasters classic. It is using the on board map, aem Iat, aem wb 3walbro 450lph pumps in a surge tank. The issue I am having is when tuning in 3rd gear the car pulls clean with 18 degrees of timing, .018 gap, .73 lambda, and 26psi producing 585whp but in 4th the car is missing very hard. The cells being accessed in 4th gear are the same as 3rd gear. I checked the sparked counter and didn’t see anything abnormal. I’ve tried retarding the timing to 15 degrees but it did not change the behavior. The knock voltage was under .2v +-.2v. What would be some things to check/change? I’m assuming the stock coils may not be producing enough spark energy.

Have you logged rail fuel pressure for the full run?

What is the lambda change when the misfire occurs - it may show leaner than the true condition, if there's unused oxygen. What do the spark plugs look like - you need to check at the end of the run, not when driven back to the pit, for a true read?

What, if any, ignition changes have you made - wiring, coils, plug type/range - may be worth trying a slightly larger gap, maybe 0.022"?

Turbo-charger setup? A bit of a long shot, but higher gears load the engine more, and if the exhaust springs are marginal, with a restrictive housing the pressure may build up in the manifold to the point of unseating the valves?

The turbo is a Borg warner s300 67 t4 divided flange... I'll have to double check the A/R. The afr does not fluctuate during the misfire it stays around .73-.75 through the run. I have it a bit richer becauseit is pump gas and it is driven on the street. It has supertech dual valve springs, stock ignition coils and ngk iridium 9 heat range plugs. The guage on the rail is reading 72ish psi but the fp sensor in the computer is only reading 48psi flat so the differential pressure compensates is turned off until a replacement arrives.

Sounds like you might have a coil failing. they usually present weakness (misfiring) at high load and heat when they are going bad. If it is a coil you could fully warm it up and let it heat soak a bit and it would probably miss in the lower gears under boost. All my turbo coil on plug cars have done the same eventually.