High Idle, part II

[s13sr20chris]

ok good thats fairly simple. try out the smoke and get back.

[Meaux]

Wes, did you ever get this solved?

I guess I did it this time.......

I cleaned my MAF and TB for the heck of it yesterday,and now my idle in N and P is 1000rpm steady. Haven't been for a ride yet, keep gettin distracted with other projects, people, but it seems like I'm getting too much air at idle now.

To get down to 1000rpm, I have to adjust the TPS all the way counter clockwise.

Hell, I was only trying to solve an off idle stumble.........

[PoorManQ45]

Wow, why are your cars idling so high? When my Q was running, it would idle at ~600~800 as stated on the tach. With the ac on it would be ~1k.

Is there something wrong with your cars? Or mine?

[Meaux]

It's almost like mine "thinks" the A/C is on all the time......

Won't be long and it will........Idles at 800rpm with the A/C on.....and, 1000 with A/C off....... Guess I better get an IAC gasket.............

[elwesso]

If you didnt clean the IAC, you probably wont benefit with a new gasket...

The way I see it, it would take a large vacuum leak to cause the RPMs to increase 400 RPM.... At 80% duty cycle IAC, the engine runs at like 1200

Let me know what you come up with meaux.... Theres another guy (cnickk) who has a J30 with the same problem...

He replaced the coolant sensor and nothing changed.... Im really hoping this is something stupid and easy....

Eventually Ill have the nissan dealer locally adjust the TPS to .44 volts as seen by consult, and probably have a few other housecleaning items done at the same time....

My old Q would always idle lower with the AC on (dirty intake), and my new Qs idle was oh so nice when i first got it, it was a little rougher than I wanted, so thats why im here now... Nothing pisses me off more than doing something right and causing another problem, since you cant blame anything else!

Seems that its somewhat reverse of what people say.. Whenever Ive disconnected a vacuum line on the Q, the idle speed seems to lower rather than increase.... Also, vacuum leaks are also somewhat known for hunting....

In a sick perverted way Im glad someone else is sharing in my tribulations!!

[Meaux]

Wes says, "In a sick perverted way Im glad someone else is sharing in my tribulations!!"

Thats funny......:-)

Well, I'm not going to remove the IAC for cleaning and testing untill I have a gasket to replace it with.....Thats why I said I might as well get one.....I noticed inside the TB that the two little holes under the throttle plate were really dirty, and now after cleaning, it seems like I'm gettin too much air.

I still think something isn't right with the TPS........I hope the culprit is a dirty IAC, but we'll see. I'm also certain that changing the 02 sensors wouldn't hurt.

I wish the Q had a self diagnostic system like the Porsche 968, it uses the "check engine light",... key on, throttle to the floor, after 13 seconds, one long blink, release the accelarator pedal slowly, count the blinks and read the code for what sensor isn't right. This procedure also sets the TPS. Sure saves me alot of trouble and money. Funny how that system would be used by Porsche and not Nissan....err...Infiniti......:-)

[elwesso]

Meaux... I know exactly what your saying, but I really dont think its the TPS... Ive already gone screwed mine up (at least its close now, perfection comes later).... Reason i say this is Ive had no change when i swapped in a known good one... Hell, i took the TPS OFF THE CAR, set it to .44 volts as read by my VOM and it still idled funky....

Just so you konw, a dirty IAC tends to make the idle lower (!) because when its dirty, less air passes through.... So expect it to idle higher when you clean the IAC, and youll probably have to back in the screw all the way as well...

However I dont think its a vacuum leak because of what i decribed earlier.... Plus if it were that big you could probably easily trace it with a steth... I need to go have mine checked, but the shop is very busy.. One of my local independents (who did my suspension work on my first Q) has this nifty vacuum leak detector... Its this smoke machine (similar stuff to the smoke they make for stages and performances) and you hook a hose up to the brake booster port.... Then turn off all the lights and have a flourescent light in the engine bay, and smoke will come from wherever the vacuum leak is... I think its kinda nifty.....

If you want to read your TPS voltage, do this... Assuming you have a DVOM (with that many german cars, i hope you do ), get a straight pin, and on the TCU (sounds goofy i know, but if you look at the circuit diagram it makes sense.... TPS voltage goes to the TCU and the ECU)... Probe terminal 34 and 35.... Look in your FSM to see exactly where they are, i cant remember. Its in the foldout section, right under the ECU pin terminal locations.... If memory serves its the 2 pins on the top row, the last two pins closest to the dashboard (higher vs lower)... Me trying to describe it probably made you more confused so check your FSM... As always if you dont have one just email me and ill get you one....

By god we're going to defeat these damn cars and make them idle right!

[Meaux]

Well, I went to the store today, about 20mi. trip, I let the car idle in the driveway @ 1200 rpm cold (normal) for a couple minutes, forgot my wallet. Everything seemed normal, as far as accelerating goes, and cold running rpms.

By the time I got to town, about 10mi., the idle had dropped to 900rpm steady, warm! (it was at 1000rpm+ and hunting the other day, warm) Is this dang computer reseting itself? It's still about 100 or so rpm off, but it's better. I did nothing different, didn't even raise the hood...nothing.

I'll be going for another ride tomorrow..........It would be nice if this car would "fix itself"........ :-)

[Meaux]

Well, now today it's at 950, warm.

I'm assuming with the IAC unplugged, it dosen't allow any air thru to the TB.

I'm gonna unplug that, and see if I can set the TPS a little better.

My "off idle" on the TPS happens at 25% of throttle travel. (so says the onm meter) Seems like too much travel to me, but when I set it to work @ "off idle", I'm up to 1200rpm.

We'll see.

[DAEDALUS]

With the IAC unplugged it doesn't allow any air through the IAC.

[elwesso]

I guess that means I have a vacuum leak...

im also seeing much decreased gas mileage... I got about 15.6 MPG on my last highway trip.......

EDIT: Im gong to have them put on the smoke machine tomorrow if they can squeeze me in.....

[Q45tech]

With the IAC connector unplugged the ecu cannot add air. The idle air is then controlled by the amount slipping under the throttle plate [when closed [the the precise TB cleaning to shinny perfect every year or 6 months or 3 months as the engine ages [blowby from PVC] AND the amount that flows past the IAC bypass screw [turned in 2.5 turns off bottom].

Off course an error in MAF voltage [0.1 to 0.05 volts against 1.2] at idle can make ecu adjust advance to try to fit what the MAF says is happening........leak after MAF means lower than normal MAF voltage so ecu wants to increase idle speed [gets confused with trying to maintain 675 rpm and what MAF says is flow for 675 rpm [hunting] to find what the reference normal should be.

Remember the CAS provides the exact rpm to ecu master clock, consider that an abnormal idle rpm is telling you something is wrong!

[Meaux]

Wes, Did you ever get this straight?I went out this morning, warmed up the car, and set my "on-off" part of the TPS, with the IAC unplugged, started her up and she idles fine now...I expected for the idle to go up when I connected the IAC, but everything is fine now...go figure.

I'll have to "road test" later, but it's not at 1000rpm+ now.

I haven't messed with this in awhile...been in guitar building mode lately...now I will be in Porsche A/C repair mode for awhile............

[elwesso]

Hmm, so you basically reset the TPS with the IAC removed... I will give that a try sometime but it really doesnt bother me much anymore....

[Meaux]

The last time I tried it was with the engine running, IAC hooked up, and had to rotate the TPS fully CCW to get the Idle to 1100rpm.

This time I unplugged the IAC, started the car, warmed up to temp, shut it down. unplug the TPS, put my ohm meter on the top 2 pins, adjusted it CW till continuity dropped off when throttle plate was moved "off idle". Plugged the TPS and IAC back in, and started her back up. Maybe the "sequence of events" had something to do with it......who knows?

[elwesso]

WEll I went out and tried it.. I wanted to do the volt method.. PIN 38 on the ECU (block to the right of the screw, 1st block, 3rd pin from the right, white wire) and I couldnt get a cursed reading!!!

I then tried to ohm it and I couldnt get a reading either! It kept showing an open circuit. If it REALLY were an open circuit it would have thrown a code...

Meaux, if you get the change, id liek for you to try the ECU method.. If you have my FSM (i think you do?) turn to EF-EC 211, and then go to the next few pages... It should be set at .44 volts... please do this and see what you get... Adjust to .44 and see what it does!!!

[Meaux]

Wes, I tried to download the FSM, but my POS computer can't read or "un-zip" the download. I'm gonna order the hard copy, or check e-bay (I was a few minutes late on one) for a copy. I've been refering to my 89 Maxima FSM to follow certain steps, or should I say,"line of thinking at Nissan", as to what parts do what...check this, check that, I know they are 2 different cars...but, hopefully a TPS is a TPS...Heck, I don't even know where the ECU on the Q is...

I still have to "road test" the thing and see if she shifts OK.

I'll let you know....

[elwesso]

Meaux, email me again and if you already have it downloaded, i think i know whats going on... no reason to pay for something you already have!

[ponzy]

this has been too long ago, but can anyone remember what was the cause of high rpm idle? i may have a different problem but i could use this as an idea. since i removed a TPS when doing the plenum job ( i knew i am not supposed to but did it anyway). i am having a fluctuating rpm after i put back everything (plenum job), and suspecting a vaccum hose leak or a piched o-ring(would it make that symptom-fluctuating harsh idle from 1500 rpm - 500 rpm) or an air packet on the cooling system ( did the knock sensors)........can someone pls reply, ..... my angst went down and want to try to fix it again tomorrow.

thanks...

[sdkhalsa]

I was chasing down a high idle problem on my 94' Q for a long time and finally have it under control. The TPS needed changing anyway for other reasons, and that was not the problem. I checked the IAC for sticking, cleaning it well, and that was not the problem. Took a vacuum gauge to confirm there were no leaks and had a solid 18.5 inches. My consult software indicated that the mixture looked good, and the idle was very even, so that convinced me that there were no leaking injectors. Finally, what should have been the first step, was to check the timing which was off. One note: bad knock sensors can throw the timing out. Meanwhile, car idles at perfect 750 now.

[Q45tech]

The solenoid valve passage is bypassed by the manual screw.

2.5 turns assumes the IAC is clean as brand new AND the throttle plate and housing is as clean as brand new.....................at 650 rpm idle the engine consumes almost no air 5-8 gm/sec and even less fuel considering 255 gm/sec is ~~300 hp.............the injectors don't even have time to fully open before the power is cut.

Disco IAC connector, set TPS at appropriate value HOT [as the hot wax idler] will move the throttle plate open until even 5 minutes after it reaches 176F.

Many Q will have defective thermostat and will not warm to 176F [in shop at 70F ambient much less outside in colder temps............so sitting still with hood open you may NEVER get the hot wax to disengage 100%.

Idles are best set in Summer when you are 100% sure everything is at operating temperature.

My point is even though Consult reports coolant at 176F the throttle body assemby may not be 176F [with hood open].

I set IAC duty cycle [via bypass screw] to be 10,11,12,13,14,15% depending on resultant rpm and smoothness..........higher in Summer to offset the duty cycle gain when compressor is on to have a smooth idle in drive...........each engine/compressor is probably different .............don't take 2,5 turns out as an abosolute might be 2, 2.5, 3,3.5 depending.

Sometimes I have 3.5 in July and 2.5 in May and October..........depends on how handy the long screw driver is.

Reminds me time to cloth wipe the pooled oil [a few drops] under the TB plate, it only takes 6 months to build up and change idle rpm.