Idle RPM question

[itjstagame]

Originally Posted by ziddey

sorry to threadjack a little, but on the topic of idle rpm...

How about lowering idle rpms for fuel economy? Stock, my car is around 850 or so. But I can get it down to 600 and still have it be smooth. However, I have a problem with setting the mixture. I can adjust the idle mix screw, but it seems like the system compensates over time, and I end up having a pig rich idle at low rpms, or it'll eventually have occasional issues with wanting to stall unless I have gas when idling. FWIW, I have a digi2 vw. I've got the idle air boost disabled so it won't add more air when idle drops below 800. Guess the system is trying to compensate with more fuel when rpm is below 800?

I would say probably yes. Most IACV are placed right near the intake after any MAF or anything else so they usually can't calculate air being added. They 'know' based on how much signal they give and therefore how 'open' the valve is, but no direct meter I don't think. So depending on how you disabled it, maybe it's assuming it's open and there's excess air and still increasing fuel as if you were running at 800RPM.

You best bet would probably be to get someone to flash the chip to a lower RPM, I think you'll only hurt yourself trying to slowly disable/figure out a working system.

[itjstagame]

Oh, and as for idling questions at speed. My cars all seem to drop to same as if idling at a stop light. The Lexus is the only one with an actual tach though :P, it drops right to 800 as soon as I shift to neutral, same at a stop or 55MPH down a hill.

The Festiva has no tach, but I can tell where idle is because I can tell when it's under load at idle or below idle. But now I want to ask a new question which confuses me about this car.

Stopped in neutral: fairly lofty idle, runs great, no load, 750-800 I'd guess
Stopped in neutral with brakes on you can feel the load, I think mostly there's a vacuum leak in the brake booster that only shows up when the brakes are applied, still smooth just a bit lower tone, maybe 650-700
Stopped in gear (any gear) with clutch disengaged (so neutral): much lower idle, still smooth with no load, <600 ?
Stopped in gear (any gear) with clutch disengaged and brakes applied: very lumpy, not smooth, almost stalls, hard to pull away in

So clearly my brake booster has an issue, but what I'm more curious about is the lower idle when I'm in gear. At first I thought the clutch wasn't disengaging all the way and I was getting dragging or something, but if that were true I would think it would feel different in each gear due to different resistance. Also I can disengage clutch, stay in gear and EOC and don't feel any dragging or notice any RPM on the engine.

Still if anything I would think it would be designed opposite, lower rpm in neutral and higher in gear because you'll be accelerating away soon.

[R.I.D.E.]

According to the shop manual to adjust the idle speed on a 37 Ford you set it so the car would go 7 MPH in high gear. That worked out to 350 RPM.

I would not try to defeat computerized idling systems, but i would definitely set the idle at the lowest speed that works, if it is practical to do so in your car.

regards
gary

[JanGeo]

Lower in gear so you get more engine braking and it is not going to stall in gear because you are moving. At Idle in gear and clutch in could be to slow the clutch plate from spinning - try not pressing the clutch all the way to the floor? A higher idle makes it easier to take off or hold the car on a hill while moving your brake foot to the gas pedal. You also are pressing on the end of the crank when you press in the clutch so you are adding crank shaft load and maybe the end bearing is not lubricated properly. Being in another gear still results in no movement of the clutch plate however in neutral would be the indicator and then putting it back into gear if the clutch is dragging a little the clutch disk would be spinning in neutral and cause a little grind when pushing into reverse (no syncro).

Applying brakes uses some of the vacuum in the chamber but air is bled in when the brake pedal is released - this gives you braking assist when you apply them and if you keep them applied when the engine is not running. However if you pump them the vacuum goes away and you loose vacuum boost. If you have a constant leak when applying the brake and holding them then something is leaking or worn.

[ziddey]

itsjstagame, I believe IACV air is still being metered. It just routes around the TB.

As for the rpms changing when you have the clutch depressed and are in gear... hmmm. any noises too? That doesn't sound right. Throwout bearing? Hydraulic clutch? If so, it uses the same fluid as the brakes? Maybe something related?

[VetteOwner]

Originally Posted by R.I.D.E.

According to the shop manual to adjust the idle speed on a 37 Ford you set it so the car would go 7 MPH in high gear. That worked out to 350 RPM.

I would not try to defeat computerized idling systems, but i would definitely set the idle at the lowest speed that works, if it is practical to do so in your car.

regards
gary

yea sounds about like my model A, should be around 60 rpm idle (hence the distinctive packita packita packita)

now my chevette has a mind of its own, i set it for 300 rpm(sposta be 850-900 stock) and it generally stays there yet some days its at 500 rpm others its at 200 (a few times its been 100 and dont know if its running lol)

whats odd is theres no excessive shakeing, its smooth as can be and doesnt stumble when you press the gas.

[JanGeo]

Sounds like my xB when I go up my friends really steep driveway in first gear no gas turning about 400-500 rpm it just goes nice and smoothly up the hill. Of course with variable valve timing it really makes thing run smoothly. Be interesting to see what the ignition timing is doing as well as the engine LOD reading of the SG.

[1993CivicVX]

TheHolyCow: I was rushing to a play today and was in 4th gear going 70mph up a rather big hill (not that steep but about a mile long) when I got to the top of the hill it goes down for a bit so I put it in neutral--even at that speed the tach dropped straight to 550rpm and stayed there. It didn't bounce or anything--stuck at 550rpm. So to me this indicates the heat generated going up the hill meant the catalytic converter was hot from going up the hill so the idle didn't need to warm it up.

[R.I.D.E.]

Originally Posted by ziddey

itsjstagame, I believe IACV air is still being metered. It just routes around the TB.

As for the rpms changing when you have the clutch depressed and are in gear... hmmm. any noises too? That doesn't sound right. Throwout bearing? Hydraulic clutch? If so, it uses the same fluid as the brakes? Maybe something related?

Perfectly normal for idle speed to change when you push in the clutch pedal.

Most of the gears and shafts in the transmission are spinning anytime the clutch is engaged.

In RWD Nissans the imput shaft and counter shaft are spinning. The counter shaft is in the bottom of the tranny case and is immersed in fluid, which creates some noticeable drag when you let the clutch out in neutral. Not a lot, and not because of any mechanical problem.

IN my VX I can feel the difference, although slight it is noticeable.

regards
gary

[dkjones96]

You are assuming that there is a temperature sensor in the cat so the ECU would know it didn't have to idle up to keep up the temperature.

Honestly, with a heat shield and everything I don't see the cat really cooling down THAT much. Plus, emissions tests never test high speed neutral coasting emissions so I don't see them putting something like that in.

Most likely the fast idle at speed is to keep the hydraulic pump in the auto trans going and they just use the same engine management code for the manual trans engine management. It could also be that the same exact ECU is used for auto and manual versions of the car, especially if the auto trans isn't electronic(or the ECU and ECT are separate units). I know Toyota was still using a fully hydraulic transmission in the Corolla in the early 90's.