diagnose this rb25 idle problem, if you dare.

[santorty]

this issue started about a week ago. i had a bad pcv valve (it was hissing) and in my retardation of trying to fix it i somehow destroyed my car's ability to idle. as it sits now, when it's cold it won't idle at all. it doesn't struggle, it doesn't chug. you start it, it dies within 2 seconds. when it's warm, it will idle for about 30 -45 seconds, then it will struggle and the rpm will slowly drop until it dies. if you unplug the tps it idles, but very rough. it constantly tries to die but catches itself. plug the tps back in and it dies instantly.

here's what i've tried;

while i had the pcv valve issue i tried blocking it off at both ends (valve cover and intake manifold) still wouldn't idle.

changed the pcv valve. still doesn't idle.

cleaned the iacv. still doesn't idle. (idle control works while the tps is unplugged, seems to make no difference where you set it. it always dies.)

if you unplug the maf it idles for about 10 seconds, gets very confused, and then dies. (as expected)

checked every vacuum hose i can find. no visible disconnects or audible leaks.

checked all the vacuum connections i can reach on the intake manifold and they all seem to suck air.

i've done most of the regular maintenance **** within the last month (new plugs gapped correctly, cleaned the maf, cleaned the coil packs, new fuel filter) the car runs mint perfect aside from the idle so i have no idea where to go from here. i am as confused as i've ever been and i want it to end!

[jinglebell17]

when i got my r32, i got it for a good deal because the engine was doing this same thing and the guy had no idea what was going on. neither did i. i tried alot of the same things as you but my problem was the MAF was super dirty (i have a HKS mushroom intake and its just terrible filtration)

i think theres a thread on here where someone had this problem and went over everything it could be until he found it and it has steps on how to do what he did. id check that out

[Havoc_tt]

We just tried cleaning the MAF, it made no difference, thank you for the response jinglebell17 it was worth a shot. If anyone else has a suggestion we'd love to hear it, thank you!

[R33rr]

Also clean the other valve (on r33 it is below the intake manifold ) not sure on r32

[santorty]

there's another valve UNDER the manifold? that sounds like fun. is that what the hose on the under side of the iacv goes to? i was trying to get a look at that but there's so much **** in the way it's impossible. i'm thinking of just pulling the manifold.

[Skym]

Can idle without MAF plugged in (stock ECU only feature). But need to unplug MAF before switching engine on.

If engine stalls straight away after starting engine, AACV could be faulty. It's what happens when AACV is faulty.

it constantly tries to die but catches itself.

That probably will be o2 sensor + ECU making engine go rich, lean. If too rich (with misadjusted TPS) engine stalls.

Can check o2 sensor by unplugging o2 sensor before starting engine and see if engine idles when started.

plug the tps back in and it dies instantly.

TPS can be part of what makes engine stall if TPS is not touching idle contact (could wiggle TPS plug on my car and engine stalled, but had faulty TPS, o2 sensor and replacing them solved the problem).

On R33 RB25DET I think they changed TPS (removed idle, WOT contacts inside TPS), so idle contact is activated via TPS voltage being at certain voltage, say 0.44v and what voltage idle contact is at is adjustable on ECU side (software).

Also make sure you have cleaned MAF with correct cleaner. If use carb cleaner it damages the hotwire, etc. Only use CRC MAF cleaner.

[santorty]

i forgot to mention we checked the o2 sensor. it does the same thing whether it's plugged in or unplugged.

anyway to test the aacv? and do they normally just die over night like that? it worked perfectly fine one day, then the next it wouldn't idle. no symptoms of anything going wrong at all.

and we cleaned the maf with electrical contact cleaner. my friend has been using it to clean his maf for years.

so it looks like it's either tps or aacv at this point?

[Skym]

No way to test AACV that I know of.

If happened over night, have you checked fuelpump??? I know another GTRC member had a similar problem when engine cooled down and it was fuelpump related.

From what I understand, ECU runs engine runs richer (injectors open more) when cold and with injectors open more, weak fuelpump the fuelpressure drops (low fuel pressure = less fuel through injectors and engine stalls, similar to unplugging fuelpump fuse and letting engine idle until it stalls to remove fuel, pressure from fuel hoses, etc).

Unplugging TPS would make ECU use a default value / number (like when unplugging o2 sensor which does the same thing) which I think makes engine run slightly richer (with TPS it should up voltage to fuel pump), so I guess would make sense why engine idles with TPS unplugged, as ECU is probably upping voltage to fuelpump which raises fuelpressure.

So check fuelpressure via fuelpressure gauge T'd inbetween between fuelrail, fuelfilter. If below 35.5psi at idle, then found your problem. Pulling vacuum hose to FPR should up fuelpressure by 7.1psi or so, so that could be another way to check if it's fuelpressure related at idle.

[Havoc_tt]

so we hooked up an fpr to test the pump. it sat at 40 psi no problem then dropped 20 psi just before it started to die. we tried to up the pressure to see if it would keep it from dying but it did the same thing again. held steady at 60 psi then dropped almost exactly 20 psi before dying. i don't think it could be the pump since it held any psi we could throw at it. haven't checked the voltage to the pump yet, not sure if there's a reason to. not 100% sure what this all means but i know i have more troubleshooting ahead of me lol

[Skym]

Can drop to 20psi with fuel pressure at idle when bigger injectors are installed (stock ECU tune) and not hardwiring fuelpump (if have aftermarket fuelpump installed it requires more voltage to fuelpump). I think stock fuelpump operates in the 10-12V range. Aftermarket fuelpump requires 13.5volts+, so running voltage direct from battery to fuelpump via relay, etc (fuelpump hardwire kit) solves that problem.

DeatschWerks sells a hardwire kit for fuelpump -

http://www.deatschwerks.com/fuel-pump-hardwire-kit

They recommend the hardwire kit if wiring is over 10 years old.

Also combined with a fuel hose leak near fuelpump or in engine bay near fuel filter, rail (on one R32 I know of a fuel hose leaked in engine bay and caused a engine bay fire) fuel pressure might drop.

[santorty]

injectors are stock, pump is most likely stock (haven't looked at it yet)

one other thing i just realized yesterday is that with the tps unplugged, if the engine stays at a cold/high idle (1200-1500rpm) it will idle forever and never searches or starts to die. it will start to stall if it kicks into a warm idle.

that would bring us back to the AAC would it not? doesn't it control warm idle or something like that?

[Skym]

I was thinking when you mentioned the cold, hot part, that it could be coolant temp sensor for ECU (common problem / failure on Skylines). It could explain the stalling if ECU was told to run engine rich when engine was warm by coolant temp sensor. Would need to check ECU for coolant temp sensor error code. Sometimes alot of head scratching can be avoided by just checking ECU for error codes.

AACV = Auxiliary Air Control Valve. It's used to raise idle when aircon, light's, etc are on and I think when steering is turned, controls idle rpm when engine is warm.

IACV = Idle Air Control Valve. It's open when engine is cold and slowly closes as engine warms up. It uses heat from engine, as you put IACV on a hot plate and add voltage to IACV to test it. Then see how long IACV takes to close.

Both AACV and IACV work together (when engine warms up) and share the same air that comes from intake pipe that goes over top of engine (J pipe).

[santorty]

is it sad that i've been too lazy to pull the ecu and check the codes? i'd rather tear the engine apart than look at a blinking light on the ecu lol.

[santorty]

it fixed itself. no idea what happened. i just drove it yesterday and it started working regularly on it's own... i hate cars lol

[Skym]

Can use Nistune tuning software, laptop, consult cable to check for error codes on ECU via consult plug under dash (where Nissan plugs in their Consult diagnostic tool). Found CAS error code twice.