high idle when hot

[mistercrow]

hey guys some of you may have read my last post with my car having issues, i fixed mostly everything. exept the idle
i have read throughout the forum that iac's and aac's can be faulty, maf's and vaccum leaks.

i used to have an up and down problem, witch was due to exaust timing being 1 pin off.
retimed the engine cams and crank, and ignition to 20 btdc,

as is, when cold, the engine idles very smoothly @ 1100 rpm, once it gets hot, i would suspect it to go back down to 900 where its supposed to be but it doesnt i goes up instead, to around 1250-1350 rpm.

i have switched off the IAC to known functioning one, problem persists, i have blocked the aacv line that goes from intake to aacv, the problem persists (maybe i have to block the other hose to check if it works correctly?) i know i do not have a vaccum leak, as i went though all the lines possible (smoke, pressure, brake cleaner), and when i simulate a leak, the idle lowers about 100 rpm.

i have also switched the maf's to know working ones.

the engine has been through a full rebuild with new gaskets everywhere, and the bov recirc piping has been deleted (was never plugged anyways).

is there another way to check the aac?

any help would be apreceated thanks!

[Skym]

Check for MAF error code on ECU via consult. When MAF fails (could be wiring related or ECU still sees MAF error code) ECU can make engine idle around 1500rpm when cold or hot.

Also check coolant temp sensor, as when faulty ECU can stay on warmup tables which result's in engine idling higher. The 1250rpm-1350rpm idle makes me think this is the cause, as is nearly exactly what's on warmup tables.

[mistercrow]

thanks pal, i will check it out this weekend and give news.

[mistercrow]

forgot to mention, also the iac screw is all the way in, to have 1250 idle. if i unscrew it it goes to around 2k

[Skym]

When adjusting idle rpm you put ECU into idle adjustment mode via diagnostic screw / pot on side of engine ECU (or via Nissan's Consult / handheld diagnostic computer). Then unplug AACV (stops ECU adjusting idle rpm) and adjust AACV to correct idle rpm (as seen accurately on Consult) then plug AACV plug back into AACV and see if idle rpm stays the same. If it doesn't stay at same rpm, then repeat unplugging, adjusting AACV, plugging back in AACV plug until it does.

IACV is the automatic choke (can drive car while engine warms up) and there is a valve inside that's activated by engine heat, voltage. You'll know it as the manual choke / knob on dash as found on pre 80's Nissan's or old school muscle cars, etc that you press in once and while as engine warms up. IACV provides extra air on top of what AACV does when engine is warming up and is not used when engine has warmed up. High idle can be due to intake airleaks (including intake manifold gaskets), faulty IACV (valve inside is stuck in open position), but eliminate the common causes which is coolant temp sensor, MAF's.

[mistercrow]

thanks for the info. i just plugged my wideband and saw the mixture at idle was 17.1, ill start by correcting that, may be the cause of the extremly high idle. i may have a vaccum leak around an injector seal, only spot i didnt look. once everything is checked if nothing good i will repost. as of now, i will go get my consult and try to ajust idle via you ways.

[Skym]

You're welcome. If that lean it can be a intake manifold or exhaust leak (wideband picking up oxygen entering exhaust via leak). If it's a GTR (even GTS models have this problem when boost levels are raised) the intake gasket's are known to blow. That's why Tomei and other tuners have metal gasket kits (intake, exhaust, headgasket).

[mistercrow]

alright so heres the update on this weekend: figured out tps was reading .34v @ closed throttle and o2 sensors were picking up 0v?
i raised the tps voltage to 0.5v as said in shop manual and still had 0 reading @ closed throttle. if i put the tps @ 0.42v i do get a reading at idle and the afr is perfect. no codes are present at the time.. i cancelled the ground on the manifold as i wrapped them, do the o2's ground there? i wouldnt think so as i do get a reading with a little more voltage on the tps.. as for idle, i still havnt found out.

[Skym]

On Consult or in Nistune tuning software the idle contact button should light up. The problem is the contact pin can bend over time, so might not be where it was when new (0-5V part of TPS does wear out and contact, WOT pins bend). On later model R33 (series 2), R34 NEO they removed idle contact pin and made it voltage activated with software on ECU side.

From memory 0V should be very lean, and 1V very rich. Unplugging o2 sensors makes ECU use a preset value for o2 sensors (this value can be seen via turbo timer if it has a o2 voltage feature) and engine should idle smooth. The common missing at idle can be the o2 sensors + ECU making engine go too lean. Sometimes it's intake airleak related.

Exhaust is grounded to chassis (at least it is on GTS models) via a ground from turbo to chassis.

[mistercrow]

alright so with that said, if for example the idle contact pin in the tps is faulty, i'm guessing it could cause a high idle, as the tps doesnt send the correct information to the ecu? i also have a mines ecu in the car, and exaust temp sensor always goes on, dont know if it has a meaning.. and yes the car runs super lean @ idle when the o2 sensors show 0v. would it be super bad to run the tps @ .42v as i do get a reading at 02 sensors when tps is at .42v?

[mistercrow]

I am getting good voltage from the TPS, but im pretty sure the idle contact in there is going dead. simple, when i turn the car on, and ajust tps to standard voltage, i get a perfect AFR, but a sosoish idle. once i let the car sit for a night, the morning, or even the afternoon i start it again, the afr is crap, going hyper hyper lean.. think a faulty idle contact can cause high idle? also, any canadian motor have the same tps as rb26? thanks guys.

[Skym]

Faulty / bent idle contact inside TPS (not touching idle contact when fully release throttle) makes ECU not select idle stabilisation tables (adjust's ignition timing at idle to match o2 sensor that makes ECU make engine go rich, lean), use kickdown tables (kicks rpm up to prevent engine from stalling when under load with aircon, light's, etc on and going up hills, etc), etc (many features it affect's on stock ECU).

TPS is for transient fuel enrichment. When you suddenly accelerate (step on throttle) engine goes lean and get hesitation before engine accelerates, as MAF's are too slow to react to sudden changes in airflow. The TPS (+ software on ECU to suit) was invented and TPS attached to throttlebody (where the sudden change in airflow happens) to help prevent engine from going suddenly lean. You'll know TPS (electronic version) as an accelerator pump if have worked on carb engines.

More TPS voltage = engine runs richer. If voltage is too low it could make engine run lean, rpm rise a little bit, get hesitation before accelerating, etc. The TPS just fails, I have been through 2x TPS. Only way to eliminate TPS as a problem is via running a Motec M600 ECU with drive by wire feature, sensor to suit.

Also it should be rich when warming up. Initially when warming up, o2 sensor is not looked at by ECU, so can eliminate o2 sensors as a cause. Other things (apart from coolant temp sensor failure) that cause engine to go lean can be dirty hotwire on MAF's (clean with CRC MAF cleaner) as produces less voltage = lean, faulty IACV (stuck open and needs to be cleaned or replaced), intake manifold gasket or exhaust manifold (broken studs in head) gasket leak, incorrectly adjusted TPS or faulty TPS (WOT / foot to floor voltage can drop by up to 0.4 volts or so).

This is a GTS-T TPS voltages for TPS from Consult printout, but gives you and idea of how the WOT / depressed voltage drops vs new TPS -

TPS failure voltages as below-

DEPRESSED : 3.60V
RELEASED : 0.42V
RANGE : 3.18V
SPEC : MORE3.0V

TPS correct voltages as below-

DEPRESSED : 4.02V
RELEASED : 0.44V
RANGE : 3.58V
SPEC : MORE3.0V

Released is no throttle (on idle contact), depressed is to the floor (WOT contact). Also the WOT is 0.02V too high, should be 4.00V (that TPS failed).