small idle speed oscillatons

[3Q Jay]

plenum is all done, car started right away, no leaks, purrs nicely.i had the aac screw out ~2 turns as 'initial' setting, and after full warm up idle was 750ish. so, i discon'd the aac harness and turned the screw in about 3/4 turn to get it to 625. plugged the harness back in and the r's jumped back to about 750 but seemed to settle about 675. once it's idling for a while, there is not hunting.

but, after warm start [say 1.5 to 2 hours after hot shutdown] it will start right away but oscillate from 500-1000 and then damp down after about 10 sec. also, when coasting to a light after releasing the gas, there is a bit of hunting. will settle out nice eventually and idle very smooth.

note that for the entire 4.5 months and 2000 miles i've owned the car it's done this.

i was originally thinking aac gunked up, and when i pulled the plenum, i was surprised that it really wasn't so. i cleaned it up good anyway. resistance measured at 10 ohms [yes, i know its an inductive device--just comparing that to fsm spec which says ~10 ohm].

thoughts?

[FarFetched]

If you refering to Y33, mine does similar thing. Idle RPM (in Drive) never the same but hovers between 500-550) Off gear idle almost always the same!I don't know what it is, I don't care. Id doesn't stumble or hesitate after KS, MAF, spark plugs, intake clean-up (massive clean-up) and other things I've done to my OldLady.Cheers!

[DAEDALUS]

You think the AAC is causing the fluctuating idle? How rapid are the fluctuations?

[3Q Jay]

Petr, it's the G50. FY33 does this intermittently also.

Robert--i don't know, but the oscillation period is about 1.2 sec [best earball estimate]. it is positive damping.

i have no doubts that the aac screw had been 'adjusted' somewhere in the past. plus, i changed the setting while cleaning it out.doing a search, i found a lot of old threads on aac, but nowhere did i find that anyone had changed one.

[maxnix]

Using a Consult I to read rpm? In-car tachometer is not accurate enough.

[DAEDALUS]

I was more wondering if it's the AAC causing it because the ECU was telling it to.

[3Q Jay]

hmmm, yes. the control system does seem to be doing what it's supposed to--and the aac seems to be responding to the ecu, just seems like the system gain is not high enough. sluggish aac maybe--i don't know. are you suggesting ecu anomaly?does aac go bad?

Brian, i know--but i donot have consult 1. even if cabin tach not perfect, the signs of the oscillations--feel, sound, tach needle say it is there.

[3Q Jay]

one other thing--drove the car more today, and the oscillation is better than it was before plenum job, so maybe my cleaning did do something.but it is still noticable.

btw, tech will perhaps chime in on aac duty cycles and such--i can say that when rapidly lifting my foot from cruise, the tach reads ~1400>1200>1000>800. so there is not the crash of near stall.

but, if i lift my foot from say only 1500 revs [slow taxi thru my neighborhood], it will hunt before settling.

[maxnix]

Brian, i know--but i donot have consult 1. even if cabin tach not perfect, the signs of the oscillations--feel, sound, tach needle say it is there.

Understood. But is it 25 rpm, 50 rpm, 75 rpm?

In the past usually attributed to dirty IAC or even sticky (dirty) EGR valve or plenum passages.

Don't overlook minor vacuum leaks.

[RobertsnewQ]

Sounds like a classic TPS adjustment problem.

The TPS isn't settling down to the 0 throttle position when you let off the throttle, so the ECU is in "dashpot" mode.

I'd check that first...

[3Q Jay]

yes, we might be on to something here....it's like the aac which would normally have duty cycle of 10-15% at hot idle is doing a 0-10% [subjective of course!!] duty. so when it shuts off as it goes below the lower end of its control authority, it is open loop until the ecu says 'whoops, too low' and then turns it back on.

will have to investigate the TPS voltages further.

Brian, i did change all of the normally unaccessible vac hose with genuine nissan. admittedly, i did leave a few easy to get to hoses around the egr and canister sections as those are still relatively accessible. they were pliable however.

btw, new iridium [not the IX's] plugs IFR-5e11.

[maxnix]

btw, new iridium [not the IX's] plugs IFR-5e11.

Ooooops! May be a problem. Got OEM NGK to compare?

[RobertsnewQ]

Jay - I'd not get confused with speculation about spark plugs, rpm ranges, vac. leaks, egr valve and whatnot. None of those problems has the symptoms you describe. Any of those things could cause a rough or too high or too low idle, but not likely an oscillating idle.

You've got a funkiness in the AAC control feedback circuit, either because one of the sensors that determines AAC duty cycle (tps, temp, maf, rpm, AC on, PS load, alt. load) is giving bogus info, or because the AAC isn't reacting the way it should.

What happens if you disconnect the AAC? It should idle fine and drive fine if the problem is one of the sensors or AAC. You can drive just fine without one, especially if you bump the idle bypass up a notch to compensate for the lack of load anticipation.

As I said above, the single most likely culprit is the TPS, especially since the problem gets worse with temp and you already cleaned the AAC. After that I would check the CTS, then the MAF and connector. If all of those check out OK, pull the AAC and make sure there's nothing sticking in it, but I would save that for a last resort.

[3Q Jay]

if i discon the aac, the idle drops from ~750 in park ac on to ~625. if i put it in gear it drops [just] below 500. in gear a/c off is about 625 as well. didn;t actually drive with disconnected.

when aac plugged in, switching a/c on barely blips the tach.

[RobertsnewQ]

if i discon the aac, the idle drops from ~750 in park ac on to ~625. if i put it in gear it drops [just] below 500. in gear a/c off is about 625 as well. didn;t actually drive with disconnected.

when aac plugged in, switching a/c on barely blips the tach.

Sounds perfectly normal. The reason for disconnecting the AAC is to isolate the cause to the AAC control strategy. If the idle fluctuation disappears with the AAC out of teh loop, then it is in one of the sensors or aac itself. If the fluctuation does not disappear, the problem is not under ECU control.

[3Q Jay]

ok, i finally get it about driving with aac discon'd for data gathering on ecu control of oscillation. i'll do that eventually.trying to be methodical, and only change one thing at a time, i unplugged the tpSWITCH [not tpSENSOR] harness to check continuity when closed/open throttle.pins a-->b are short when engine warm/hot and throttle closed [as expected], now open when the FIC is engaged. if i push the throttle closed [it's just warmish right now, not stone cold] i can still get the open-->short. so the 'closed' throttle position switch appears to be doing what it's supposed to.open throttle position [pins b-->c] goes from open to short at ~1/2 throttle [i thought maybe it was sposed to be a WOT thing, but seems clearly not], very repeatable, minimal hysteresis.

my driving configuration for tomorrow is: aac connected, base idle set ~625 park no ac; and tpSWITCH harness completely disconnected. i'm looking for the response of warm/hot engine to not seeing the 'closed' signal from the tpswitch at idle and startup. tpSENSOR will remain connected. eventually, i also plan to measure for the 0.44V at the tcu when warm/hot idle.

[Q451990]

I noticed some weird idle hunting in the weeks before my fuel pump (and controller) cratered on Q1. Mostly it was an issue at cold startup though.

Heath

[RobertsnewQ]

my driving configuration for tomorrow is: aac connected, base idle set ~625 park no ac; and tpSWITCH harness completely disconnected. i'm looking for the response of warm/hot engine to not seeing the 'closed' signal from the tpswitch at idle and startup. tpSENSOR will remain connected. eventually, i also plan to measure for the 0.44V at the tcu when warm/hot idle.

Jay - the TP SWITCH is not for the ECU, it just tells the transmission when the engine is idling and when to kick down (WOT). The ECU doesn't even look at the switch(es) unless something is wrong with the variable sensor. It's part of the transmission control circuit - replaces the old school kickdown rod or cable.

Disconnecting it will give you no kick-down and funky shifting.

What you need to check is the throttle position sensor.

The ECU only uses the switch (hard closed throttle switch) when it suspects a problem with the sensor (soft closed throttle switch).

EF & EC 12 in the '94 shop manual.

[3Q Jay]

guys thanks for the continued interest....

here's the lunchtime update:drove to work today, and seemed to have less hunting at idle.didn't go above 1/2 throttle as i wasn't sure what the hard open circuit on pins b and c would do . no perceived issues with the lack of closed throttle position signal from the switch when warm--specifically, no RPM crash when rapidly lifting foot from cruise. only way i could get any 'weird' variation in idle was that when hot going from D to N with ac on, i could get the idle to momentarily drop from ~750 to ~500, but it recovered very quickly, without a damping oscillation, just 750->500->750.

my subjective assessment so far is that the aac response is 'faster' in this config.

hot 5 minute restart was perfect, but was before, too. didn't get to do the hot 1 hour restart yet.

Fuel pump [oem new] has less than 1000 miles, as does filter. pump was changed for minor noise only, never hard failure. FPCU closely examined, re-installed following ground path upgrade.

[3Q Jay]

more data.

hot 45-minute restart had the dreaded oscillation with tp SWITCH discon'd. so, I'm not too plussed with that answer.

next: discon'd the aac, bumped the bypass screw up to ~750 [gnat hair over] in park, no a/c hot. went for a drive, seemed okay, very small amounbt of oscillation in drive at lights but barely noticable. drove fine otherwise. 2 minute hot restart ok. 5-minute hot restart ng, kept stalling unless i fed in some gas to keep it going--then after 5 sec, i could let it up to idle. so, difficult to say if the primary oscillation, that is the hotsoak restart was there or not. i played with it a bit by doing idle shutdown, and then immediately switching ignition back on before completely stopped. tach dropped to 'zero', and then she rumbled back to life with a single small overshoot to about 1000 revs, and then settled back in.

so for the RNQ 'go/no-go' check, i'll tentatively say the aac is under ecu control, and the surge is also under ecu control.

next up, tpSENSOR voltages.....

[3Q Jay]

tp SENSOR:

when engine hot, throttle closed: 0.394V [Pin 34 to pin 35 at tcu].

when engine cold, closed throttle, but on the FIC: 0.468V

with throttle mashed open at pedal--i only get 3.016V. if i go out under the hood and hand crank the tb open the rest of the way, 3.41V.throttle linkage recently adjusted to yield ~3/8 free play vertical motion at tb when hot.

now from all sources, it sounds like my tp sensor is out of limits, but not sure that the slight adjust to bring it up to 0.44V at hot closed throttle is going to get me >4.00V at WOT. not sure this is a problem, though as the kickdown, etc. is proper. trans shifts right as redline 1-2 and 2-3.

[DAEDALUS]

Did you verify the input voltage is 5.00V? What's the voltage at the ECU?

[3Q Jay]

solid 5.12 V from pin 31 to pin 35 at tcu. didn't go over to ecu to measure.

[RobertsnewQ]

TP sensor is mostly used for idle and acceleration enrichment calculations. WOT signal less important to ECU than other load signals (high MAF voltage for one). I'd bump it up and not worry about it.

[3Q Jay]

hoo haa.hot adjusted to exactly 0.440V with engine idling. was curious, as the idle speed went up when i did this and i couldn't get it to idle below ~950 without disconnecting the aac.

on a hunch, i went back and rechecked the timing which i hadn't checked since before the plenum and plugs, and found that it was at 22-23° at 1000 revs!! set it back to highside 17°, and the idle settled back in nicely. i then went back and rechecked the aac bypass, and set the base idle now at about 2.5 turns out [was only about 1 turn out for same before today] for ~625 in park hot no ac. idle with aac plugged in is ~700 per the inacurate cabin tach. 5 minute hot restart good, we are waiting for the 1 hour hot restart, so far i'm cautiously optomistic.

[3Q Jay]

okay, so afer nearly a week and about 250 miles.... it's not perfect.hot soak restart is the most annoying part. will fire up quickly and rise to about 1200 momentarily and then drift down. after 3-5 sec, the hunting starts: drop to 500-->1000-->600-->850-->650-->700, then near stable within say 10 sec total from fire. focusing on this issue first, i speculate that there is vaporisation in the rails ~30ish minutes after hot shutdown. accordingly, i ALWAYS let the pump run before cranking. and then i thought well, more priming of the rails must be even better, so I'd do 2 or 3 cycles of key ON priming before crank, but no progress. then, just to be different, i tried no priming and just going for it. to my surprise, the oscillation is much less, but still there, as opposed to a full fuel pressurisation.

Onto the hot idle in traffic; in spite of the first day driving around with everything adjusted sweetly and the 'running' idle at stoplights etc beeing smooth, there seems to be small oscillations still at idle even after fully warmed up, occuring after the 2-1 downshift stoping at the light. subtle. like 25, 50 rpm tops, still in positive damping after about 4 cycles.

one thought was active pump, and possible 'normal' load fluctuations due to small active fluid pressure variations. seeing another thread about a '97 questioning the alternator, it got me thinking.... the idle variation is about what you'd expect from a sudden PS load or alternator load. maybe my alternator is in the very early throes of failure .

Green--i know you did an oem alternator recently, and you also have the active. does any of this seem familiar to you?

[maxnix]

I am a little leery of the hot soak theory (though it is well reasoned) as I never experience it in TX on the hotteset days.

And I am sure all your vacuum hoses are tight. Alternator may be worth moitoring.