Cold Start Valve Woes

[ARC240]

Hey guys, well, almost a year later and the engine still runs beautifully after the build, however, I've always had a problem of high idle, ever since the engine's been in the car. I'm starting to get annoyed by the high idle so I decided to start chasing it. Engine likes to idle between 1100-1300RPM. The car has always had a hard time starting when cold, but fires up first crank if you just BARELY crack the throttle, which led me to believe that the cold start valve is no good, or that there is a problem with the valve's electronic circuitry. To clear things up, I've also tried to adjust AACV as per the nissan manual (doesn't help much...also points to a cold start valve problem), I've gone over the engine & i/c piping with a fine-toothed comb 3 times to find any vacuum leaks (had none), adjusted my TPS & idle switch as per the manual...only to find that none of these were helping. Today, I decided to start concentrating on the cold start valve, I pulled off the connector and attempted to measure voltage at the connector...the ground has continuity, the + wire has 0V...yes, 0V lol, that can't be right. Can anyone tell me what the voltage is supposed to be at the positive terminal of the blue 2-wire cold-start valve connector? I've looked at the engine manuals, nothing is specified for RB20, only 25DE and 26DETT and the ECU pinout doesn't indicate what pin the voltage for this thing comes out of. Any help is appreciated, sorry for the short novel !

[ARC240]

*also forgot to include that im running a stock RB20 ECU, however it's from an auto...don't know if that could have anything to do with it?

[RSi]

ECU wont matter.

[Skym]

Starting problems can be due to startup fuel tables being too rich or ignition timing is wrong, especially when engine is modified. Or IACV is sticky. Or there's airleaks / faulty BOV.

IACV (Idle Air Control Valve).

IACV (cold idle valve or known as a choke on pre 80's Nissan's) can stick due to being dirty or fail and I think stay open. IACV should be fully open and close slowly close as engine warms up. Nissan call IACV a "Air Regulator". IACV is like a automatic version of a manual choke that's controlled by ECU.

I think IACV does start at 0 voltage and voltage increases to IACV as engine warms up. From what I understand, the way you test IACV, is apply car battery voltage (- and + on battery to IACV valve) and see if valve inside IACV closes within 7 minutes. When coolant??? temperature goes over 60 degrees, IACV should be fully closed. If IACV doesn't close properly or at all, IACV is clogged with deposits or faulty.

Also inspect IACV via looking down hole to see if there are any oil deposits from blowby or failed turbo preventing valve from closing (should check AACV, IACV valves after replacing turbo). It's the hole on IACV where hose from T joint on vaccum feed hose to AACV connects to. Not the other side of IACV which goes to bottom of top half of intake plenum.

Can spray cleaner down that hole and check valve movement with flat head screw driver.

[ARC240]

Thanks Skym, yeah, I was reading up about what you mentioned in the manual yesterday, but what stumps me is that I cleaned all of this stuff really well when I was building the engine, so I can't really see it being because of things being gummed up or dirty. Like I said earlier, I measured voltage at the IACV's connector and ended up with continuity on ground (OK) and like 0.02V on the + (which I have NO clue of how normal that is). Do you know where the IACV gets its + voltage from and how much is supposed to be delivered to the unit?

[bobbo]

Keep in mind (atleast with the RB26) there are 2 idle air controlers. There is the adjustable one and there is another one in "never never land" under the intake manifold. A 30 year Nissan teck told me it "probably" uses a wax based chemical to control initial start up idle speed and adjusts idle as the coolant warms up.
The idle air controller that is adjustable is designed to compensate for WARM idle to compensate for loads such as A/C and power steering demands.
The cold start idle air controllers are prone to fatigue and are hard to get at, the intake manifold has to be removed. The good news is they are available and about $140.

[ARC240]

well i figured as much, seeing as there is no pinout on the RB20 or 26 ECUs for the cold start valve (meaning ECU doesn't control it), so there must be some sort of external trigger (like in thermostats with the wax-based pellets), however, having barely any voltage going to it is making me raise my eyebrow. I know that this IACV unit sits specifically on top of one of the coolant passages right near the runners in the intake manifold and uses that temp to determine idle speed. The Nissan manual fails to specify how much voltage is supposed to be fed to the unit. I think that I should just replace it all together, I don't have to remove the intake manifold, just the TB, so it's a considerable option I guess. Does anyone know the amount of voltage that is supposed to be fed to the unit?

Thanks for the help thus far!

[Skym]

ECU should control voltage to AACV. Isn't voltage mentioned in manual for IACV???

Nistune and I think Nissan's Consult can do a test to check IACV, AACV valves via ECU. I gather this is where Nistune tuning software is worth the $$$ to have???

[ARC240]

ECU gives voltage to AAC (and my AAC is fully functional, just doesn't do much as both AAC and IAC run off of the same vacuum line, so even if I adjust aac, makes a slight difference, but air is leaked past the IAC control slider "stuck open" condition and makes its way into the engine anyhow...thus causing the high idle). IAC (cold start) isn't fed voltage by the ECU, manual says nothing about testing IAC via electronic scan/consult. The only test it suggests (for RB25DE, does not specify for RB20) is a manual one by completely removing the IAC and heating the bottom mount to 20deg.C and look for movement of the air block off slider inside the valve within 7min. That's all, says nothing about where it grabs voltage from, actually, there isn't even a wiring scheme for the thing. Seems like Nissan kept the whole system a cryptic secret LOL.

[ARC240]

I just found the problem...I decided to pull out the IAC valve and was thinking of testing it by putting it the freezer (open all the way) and then heating it with a heat gun, except when I pulled it out, I heard something rattling inside, so I pulled it apart and found that the pin that holds the little actuator rotor(slider) had somehow come loose and fallen out and the spring that is attached to the rotor had pulled the rotor all the way open and was jammed, thus allowing all of the air to leak past the rotor. So the culprit is a faulty IACV...looks like I'm off to Nissan to find another, happy it was just something insignificant and fairly easy.

JUST A NOTE: for anyone attempting the same thing (on RB20), it IS possible to remove the IACV under the intake manifold/throttle body rotor WITHOUT disassembling the throttle or intake manifold, look me 10 mins to remove without disconnecting anything. Removing any part of the intake manifold and/or throttle body is completely uneccessary, just remove the rearmost 3 vacuum lines (except to AAC) towards the rear of the engine, just after the throttle body. done and done!

Thanks for the help guys, appreciate it

[Skym]

Good to hear that you found the problem.

I always wondered how you could heat IACV when IACV is removed, but on a stove would work. Or one of those hot plate looking machines you can test heat affect on wastegates, etc.

Doesn't manual mention resistance in Ohms and 2 pinouts, going by the wiring to IACV??? I think pin 1 is ground and pin 2 is power. From what I can remember black wire is ground??? I think it's pinout 2 for + and then to ground is - on multimeter after you disconnect harness with ignition off. Also was thinking, this could be done when engine is cold and after engine has warmed up. Especially if you can monitor coolant temp via Consult or Nistune.

Also I think voltage goes to fuelpump relay, then to IACV. Fuelpump relay should be pin 18 on RB25DE ECU, going by pinout diagram. So I assume if you cut voltage to fuelpump via Consult, you cut voltage to IACV.

But the interesting thing, Nistune shows them as IACV / AACV, so I assume are affected at same time or maybe some peope call IACV / AACV the same part???

[bobbo]

JUST A NOTE: for anyone attempting the same thing, it IS possible to remove the IACV under the intake manifold/throttle body rotor WITHOUT disassembling the throttle or intake manifold, look me 10 mins to remove without disconnecting anything. Removing any part of the intake manifold and/or throttle body is completely uneccessary, just remove the rearmost 3 vacuum lines (except to AAC) towards the rear of the engine, just after the throttle body. done and done!

Interesting. I'm not sure that it's that easy on a RB26 but I'm certainly gonna take a crack at it. I hate high idles!

[bobbo]

I just spent about 10 minutes looking under the intake manifold trying to lay my eyes on that damn iacv and couldn't even see it. I looked in the manual and on the nissan4u diagram to help locate it with no luck, the diagrams are too vague.
Any more information on where to look to find this little prick? Maybe it is different on the RB26?

[Skym]

RB26DETT idles at around 950rpm vs RB20DET at 650rpm.

Because it's not under the intake plenum, only the AACV is, as shown in photo below when separated from intake plenum -

http://www.booztd3.com/Pictures/RB%20Build-up/RB26DETT_R33_55.JPG

IACV is mounted under cylinder 3, 4 throttlebody's -

http://www.booztd3.com/Pictures/RB%20Build-up/RB26DETT_R33_82.JPG

Should be able to reach IACV to remove IACV once plenum is off. Or might have to remove AACV to get at it.

Found photo's here -

http://forums.240sxone.com/showthread.php?p=83547

[ARC240]

^ exactly, RB20 it's pretty easy, for a 26, it's way under there, so I think taking the plenum off is the best way and will make life a heap ton easier for you. Have you tried adjusting idle via AACV yet? The sure fire way to tell if a high idle is produced by the IACV (if you've already tested for vac leaks, but can't find any) is if you try to adjust the AACV and has little to no effect on idle when engine is warm.

Sorry dude, added the "if anyone is attempting the same thing (on RB20)" now, forgot to include that the first time.