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scumlort

RB25 S2 Stalling on decel

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As stated in the title I'm having a problem with my engine stalling when coming to a stop. Sometimes my rpm will drop but find it's way back to 900 rpm, other times it will flat out die. Other symptoms include bogging/bucking at low rpm, low speed (1200-2500 1st and 2nd gear.) and noticeable misfire at idle. When I start the car AFR is 12-13.3, but as I let the idle sit it will slowly become more and more lean (17~19+). On throttle the AFR reaches 13-14 again, but once I get to a cruising speed it sits at 15-16 AFR.

Mods:

Freddy Front facing intake plenum, New gasket used when installed

90mm TB from eBay, New gasket used when installed

D585 LS coil conversion, using Wiring specialties plug n play harness

Maxpeedingrods stock replacement turbo, New genuine nissan t3 inlet gasket, and 6 bolt outlet gasket were used

Genuine and known good Z32 Maf, Wiring specialties plug n play harness

Walbro 255, Hardwired to battery, original signal wire used to trigger relay

ISR Fuel pressure Regulator

Wiring specialties RB25 to S14 Pro series Conversion Harness

Front Mount Intercooler

Tuned by RS-Enthalpy 2 years ago to support mods, problem started happening 3 months ago

No BOV

Some things I have checked/replaced:

Vac/Boost Leak test the intercooler pipes and core. Found leaks at the welds of the old Atmo vent BOV + a silicone coupler was split. Replaced coupler, replaced the whole pipe the bov was on with an untouched piece of 3" intercooler pipe

Replaced Walbro 255 with a new piece

Replaced Fuel filter, Z32 300zx TT Filter was used

Replaced injector O-rings as well as fuel rail to plenum O-rings, reinstalled with engine oil to grease o-rings so they dont get damaged on their way in

Raised Fuel pressure at FPR from 43 to 50 at idle

Optima Redtop Battery (AGM Battery - 770 Peak CCA) Tested and recharged

Removed IACV and Cleaned, reinstalled with new gasket and RTV

Replaced Narrowband O2 Sensor and Wideband O2 sensor

Cleaned engine grounds and Tested alternator to be in working order

Spark Plugs replaced with BKR7EIX gapped down to 0.8mm

Oil replaced with Royal Purple 5w-30, 4.8qt with Genuine nissan oil filter

Today I discovered a coolant leak from what I believe is the thermostat housing/waterneck. I will drain coolant and regasket that tonight althought I doubt that Is where my issue lies.

 

Believe me when I say I've used the search bar and google plenty before I post here. I'm desperately looking for some more suggestions

Edited by scumlort

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What ECU? How critical is TPS adjustment to the idle on that ECU?

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I'm guessing you have an aftermarket ECU of some sort. Is your fuel control set to closed loop?

What is the fuel trim doing once the car is warm at idle when the car is leaning out?

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1 hour ago, GTSBoy said:

What ECU? How critical is TPS adjustment to the idle on that ECU?

its a z32 ecu with a chip tune. car has been ran on a dyno and tuned to run with the given mods. I am aware that when using this ecu one of the knock sensors is not connected to the ecu because of pinout differences.

34 minutes ago, Murray_Calavera said:

I'm guessing you have an aftermarket ECU of some sort. Is your fuel control set to closed loop?

What is the fuel trim doing once the car is warm at idle when the car is leaning out?

I'm honestly not sure how my fuel control is setup on this tune, I am unable to read the fuel trims with the consult as the tuner replaced a nistune chip that was installed for his chip and tuned the car.

I'm beginning to think I need to get my car retuned, maybe the file has potentially become corrupt 

Edited by scumlort

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f**k off the chip tune. It is 2020, not 1999.

Nistune is the superior solution, even with the difficulties associated with using a Z32 ECU on a vanilla RB25. Nistune has come a long long way to making that stuff work better in the last few years too, so would be worth your reconsideration.

Basically, this stall behaviour is the ECU not being able to control the engine speed when the throttle is closed. We normally point to the IACV because it is usually dirty/stuck. And just because you have cleaned does not prove that it is working. But the ECU can also struggle to know that it should be trying to maintain engine speed with the IACV if it doesn't know that the throttle is closed, hence my question about the TPS. I don;t know for sure about the Z32, but seeing as it is the same era as the R32, I would expect that the ECU wants to see a throttle closed SWITCH, not the analogue value (~0.45V) that is required on later ECUs (ie, RB25). So, I ask again, what TPS do you have on it?

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I honestly would have opted to keep the nistune board in it but I'm not capable of performing the tune myself. The TPS used is a stock R33 TPS with two plugs. Attached is a picture of an identical TPS. The TPS was set to read 0.49v with tb closed and 4.2 or something WOT. For reference the TPS is plugged into the left, the right is not connected

tps.jpg

Edited by scumlort

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52 minutes ago, scumlort said:

The TPS used is a stock R33 TPS with two plugs. Attached is a picture of an identical TPS. The TPS was set to read 0.49v with tb closed and 4.2 or something WOT. For reference the TPS is plugged into the left, the right is not connected

I think that's your problem. Compare that with what I wrote in the post above. Then do some research as to what the Z32 ECU wants to see. If the ECU wants a switch, then with no closed throttle switch input to the ECU, there's no benefit to having the TPS set "correctly" to give the right voltage. The R32 ECU does not look at the TPS voltage at all - it only exists for the transmission controller. Hence why I suspect the Z32 is same same. You'd probably be better off with a Z32 TPS and whatever wiring change might be required to hook it up properly.

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I looked. This page says the Z32 definitely has an idle switch

https://conceptzperformance.com/wiki/index.php/Throttle_Position_Sensor

 

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38 minutes ago, GTSBoy said:

I looked. This page says the Z32 definitely has an idle switch

https://conceptzperformance.com/wiki/index.php/Throttle_Position_Sensor

 

 

46 minutes ago, GTSBoy said:

I think that's your problem. Compare that with what I wrote in the post above. Then do some research as to what the Z32 ECU wants to see. If the ECU wants a switch, then with no closed throttle switch input to the ECU, there's no benefit to having the TPS set "correctly" to give the right voltage. The R32 ECU does not look at the TPS voltage at all - it only exists for the transmission controller. Hence why I suspect the Z32 is same same. You'd probably be better off with a Z32 TPS and whatever wiring change might be required to hook it up properly.

Thank you for finding that page for me, I will look into finding a Z32 Ecu pinout diagram and wire the second connection for the hard idle switch to the ecu. What still confuses me is that once the car was tuned, it would come back to idle perfectly fine for a solid year and a half. Only recently did it begin to stall under decel. This leads me to believe that this hard idle switch being open wouldn't be my only problem and something else is left unresolved. When the car was tuned I had the atmospheric vent blow off valve so i'm positive some fuel was added to compensate and keep the car from stalling. In an attempt to rectify the issue i'm having I removed the BOV altogether. I would assume this would produce a rich condition but my AFR is very lean.

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You have R33 TPS. Does it even have the idle switch? If not, you'd need an R32/Z32 TPS.

You can't really interpret AFRs when an engine is stalling. When the fuel isn't getting burnt, analysing the exhaust gas is a waste of time.

As for the then and now scenarios - things change. Leaks develop. Injectors get dirty. Etc. That's why engines have adaptive shit on them (like the IACV and the ECU given the idle switch so it knows when to use it) - to get them past these changes and keep working. When you "tune" an engine to suit a very specific set of conditions (BOV/no BOV, certain level of cleanliness of the TB/IACV, etc) you leave it exposed to changes in those base assumptions. You either attempt to maintain the conditions of those assumptions or you prepare to compensate (retune) for them as the changes mount up.

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In general you are better to retain the stock TB when going FF plenum. You need an adapter plate but then the TPS etc is all stock. They are good for at least 500hp maybe more. If you still have it consider returning to stock.

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