SR20DET Idle AFR and RPM fluctuation

[KoukiMonsta]

I have been trying to sort this out for a while, figured I would finally get a few other brains involved.

Some engine background. This build has around 1k miles. 0.020 over, rings, bearings etc. It runs and drives fine @ 12psi. accelerates smooth. NO bucking etc. The issue is when I come down from cruising speed to idle (red light) - Idle will rest at 850 rpm's and AFR's will hover around 14.7. All is good for ~5 seconds, then idle will jump up to 1100 and AFR will drop to 12's. I can hear the motor change pitch right before this happens. It's like clockwork that this is every time.

What is the root cause hear? Does idle jump cause fuel dump? or does additional fuel cause idle to increase?

I would think for those first 5 seconds I am running closed loop, then its flips to open loop and something (sensor) says to add fuel and then idle increases as a result.

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Redtop SR20DET
Enthalpy ROM Tune
GTX28
Toyota Supra Fuel pump
NIsmo FPR
SARD 850CC Injectors
N62 MAFS

Things I have checked:
Boost leak test - no leaks
TPS - 0.45v @ idle and 4.5v @ WOT
Brand new O2 sensor (NTK 24002 - 89 300zx)
Removed, disassembled and cleans IACV (note I have greddy style IM, when I pulled the IACV it had quite a bit of fuel/possibly oil in it.)
new water temp sensor
Cleaned MAFS
Fuel Pressure with Vac disconnected in ~43psi
As part of testing I have removed and blocked off BOV ports
I do not have a catch can. PCV setup: PCV valve to intake manifold. Rear of 'T' to crankcase. Front of 'T' to pre turbo/post MAFS intake.
See pics of BKR7E below w/ 350 miles on them.

I do have a consult cable and ecutalk software. This means I cannot data log but can read values.This allowed me to enter timing mode and set timing @15 deg

I have not found good info on values I should be seeing (such as IACV) I can easily verify TPS and water temp are correct readings.

At this point I am out of ideas. What can be causing this shift in idle every single time. Besides throwing new parts at it (which I have for O2 and water temp...Not TPS or IACV yet..) I need some input.

Thanks Gang.

[Kingtal0n]

Anytime the engine changes significant idle speeds on its own, is because of additional airflow.

Sounds your IACV is kicking up the idle for some reason. I would unplug it first to see the problem vanish.

[KoukiMonsta]

Anytime the engine changes significant idle speeds on its own, is because of additional airflow.

Sounds your IACV is kicking up the idle for some reason. I would unplug it first to see the problem vanish.

I tried this last night.

With IACV unplugged, under same conditions of coming down from cruise speed to red light.

The idle no longer jumps from 850rpm to 1100rpm. It stays steady around 850.

BUT the AFR's still do the same thing. They stay around 14.7 longer but ultimately go rich.

One thing to note is keeping my food on the brake pedal and AFR at 14.7. Let off brake pedal and AFR drops. [brake booster pulls vac and things change...not a big surprise. But this change is not automatically corrected]

This also happens if I stay on the brake pedal the whole time.

[Kingtal0n]

brakes are vacuum assisted and cause the engine to run leaner, the air is unmetered by the maf. pump the brakes and watch the engine get leaner.

air fuel at idle is affected by many factors, including,
wideband temp, batt voltage, injector programmed delay vs actual delay per unit voltage, maf/injector resolution, active fuel reservoirs available to the intake manifold or cylinder walls/valve areas, camshaft profile, and others including the fuel map itself (if it is not factory then it could be jagged giving poor performance)

For example, a high vacuum engine with an OEM-like valvetrain and OEM style manifolds will not have much problem with fuel puddling or sharing with an OEM injector and maf sensor. However, even in such a high vacuum OEM like engine, use of a larger fuel injector and larger maf will lower resolving fuel incremental values, especially at low speeds, and in fact the lower the speed, (IDLE) the more variable these incremental fuel values become.


The idle no longer jumps from 850rpm to 1100rpm. It stays steady around 850.

glad you found your problem

[KoukiMonsta]

glad you found your problem

I wouldn't say I found the problem.

Cars been sitting for a while but I finally drove it and it does the same thing still.

I am unable to maintain AFR / idle RPM while stopped - I am still under the impression something is 'wrong' and causing this.

[TheRealSy90]

Maybe contact Enthalpy and see what Martin says? Seems like you have your ducks in a row as far as troubleshooting and proper engine setup. It's possibly something in the tune itself now.

[KoukiMonsta]

Maybe contact Enthalpy and see what Martin says? Seems like you have your ducks in a row as far as troubleshooting and proper engine setup. It's possibly something in the tune itself now.

That's basically my next step. I've kinda lost interest in the thing but now that fall is coming I want to get it in good shape for some nice weather cruising.

[ImThatGuy]

Bump for an update.
I have nearly same build and same issue