Idle RPM question
 

While using P&G (neutral coasting) in my Taurus, I've noticed something odd. That is, the idle RPMs seem to depend on the speed I'm coasting. Coasting at 45-50, RPMs are around 1100 or so, and only drop down to 700-800 when the car is completely stopped.

Is this normal? If not, any idea what is wrong?

That is normal for some automatic transmissions. Mine doesn't do that, though it does take some time to settle down to 550 rpm, whereas while parked it goes straight down.

Possible explanations:
- Neutral doesn't perfectly disengage the engine from the driveshaft, due to viscous power transmission through the fluid or bands/clutches not perfectly disengaging
- Computer keeping idle speed up to make sure transmission stays cool (or lubricated -- that's a whole other discussion), since the transmission pump runs off the input shaft

Most engine management systems have a strategy that keeps you in High Idle above a certain speed so that when you come to a stop, your RPM's don't drop past idle causing a stall.

OK, thanks for the reassurance!

yea even some manual trannies do this, its just from the viscus oil in them that even tho its in neutral and not mechanically connected to the driveshaft, the fluid is still spinning in the same directions as if it were.

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?

My car idles between 900 and 1100 rpm while driving on the highway in neutral and about 700 or 800 at lower speeds in neutral. At a stop it idles at 500rpm. Honda Civic VX 5 speed manual.

That breaks every explanation I can remember that's been proposed to explain why cars with automatics behave that way...and I can't think of any explanation for it at all. Can anyone else?

Was it originally an automatic, and still tuned as one?

Can you measure throttle position in each of the described conditions?

Here's a guess as to why it can be safer to have a higher idle speed while rolling.

We can all agree that this Honda has a really low idle speed at a stop- maybe a little too low if the plugs and wires needed to be replaced...

Perhaps Honda engineers were worried about the engine dying when neutral coasting down a big hill- leaving the driver with only a few pumps of power brakes. So they figured they'd keep the idle bumped up a little the faster the car was rolling to try to avoid it dying and subsequently losing power braking, while coasting in neutral.

This is just a guess on my part- I'm not trying to start a rumor or anything...

The hypothesis I came up with while coasting down the hill on the Mass Pike today (btw, go Red Sox!) was that if it stayed at 500 RPM it would jerk considerably more when re-engaging 5th gear as compared if it's at 1100RPM. Most people don't bother to Rev match. But maybe other VX owners can weigh in on this? What do other VXers idle at at 65mph in Neutral? The other question I had was the lowest RPM you can accelerate at. For my car it is 1000 RPM. 900RPM if I accelerate feather footed, but if I give it considerable throttle, then it lugs at 900rpm. I remember someone saying their VX was happy at 800 RPM in 5th gear. My car cannot cruise at this speed--it's not making enough power and it lugs. It can cruise at 950rpm just fine if it's flat and I don't accelerate much. But 1000 seems to be a good starting point for acceleration in my experience.

Your ideas are no more likely than the lame one I came up with. :( Erik's is only slightly better. Why would it second-guess driver input for the sake of smooth shifts then but never any other time?

Also, it's probably not lugging, just "growling". That's still worth avoiding but it's not going to destroy your engine.

Not sure of my idle speed, never checked it, but it is very low. On the tach it is just above the small mark above the 0 mark. Not really smooth, slow enough where you can feel each combustion event. I would guess its below 500, maybe closer to 400.

Mine will drop to the same idle speed as long as no accessories are engaged.

I can feel the difference when you let the clutch out in neutral, even when you apply the brakes when sitting still, the load of the brake light bulbs affects the idle. First blower speed does not increase idle, but second poistion will kick it up to about 750.

My VX has no radio either.

As far as speed in gear I generally do not go below 1000 RPM, 30 MPH in 5th gear. Even with the AC on (rarely used) it will easily accelerate from 30 MPH in high gear.

It will drop to the low idle speed at any vehicle speed, but it does take a few seconds.

regards
gary

Okay, thanks for the clarification. My dad told me I was lugging it when it was "growling". So lugging it would be trying to accelerate in 4th gear going 12mph. But one mechanic I went to said that it wasn't good to accelerate even at 1000rpm in 5th. Most people I drive with seem to think I should down shift when I'm trying to accelerate at those low rpms. I don't think I'm convinced 800rpm in 5th isn't bad for the transmission.

The shift indicator light tells me to upshift to 5th at 30 MPH. No reason for Honda to do that if it was damaging the car. The gear i choose to accelerate is the highest gear that is adequate for the desired rate of acceleration.

regards
gary

Actually I explained it already. The VX has a strategy that takes into account VSS and uses the IACV to hold the idle a few RPM higher above a certain speed. Then when your speed drops below a threshold, you'll notice the RPM smoothly drop to 600.

No VX and no D15Z1 was ever mated to an auto from the factory.

No, probably not, unless it was an old car that couldn't detect knock and adjust spark timing. Lugging would be a really terrible ping/detonation problem (which could be caused by the driver trying to accelerate in 4th gear going 12mph, but probably not on a modern car).

I'm no expert (then again, many mechanics aren't either), but I don't think you're damaging anything if it's growling; you're just causing extra vibration. Extra vibration, of course, could rattle something loose if it's almost rusted through, or could put more wear on something, or could prematurely fatigue a material that has to flex during vibration.

I try to point it out when someone talks about lugging not because I would encourage driving like that, and not even to be semantically pedantic (that's a mouthful), but because the word "lugging" is associated with the danger of causing damage that would not be caused by what's actually happening.

Ah, I missed (or forgot) that. Did you say why it was designed that way?

Also remember that tachometers in cars aren't generally all that accurate to begin with. My Cressida used to tach an in-gear hot idle of 400-450rpm but with a real engine analyzer on it the idle was really 550. My current car is similar. According to the tachometer I hit fuel cut at exactly 6500 rpm(the red line) but when it's hooked up to the dyno using a Smart-Tach or OBD2 streaming I actually hit fuel cut at 6655. Just remember to hook up a REAL tach before you go messing with your idle.

As for the lugging. The only reason that's supposed to be 'bad' for an engine is it can cause overheating. In an air cooled engine as long as you are at speed it's alright but a water cooled engine has to rely on engine speed to get water really flowing. Also, on older engines they had a mechanical fan without a shroud so you had to REALLY get the engine moving to get good air flow. Like in the picture below, the fan stands by the engine with no shroud and about 4" of space between the fan and the radiator.
https://www.fuelly.com/attachments/fo...38c073fd51.jpg

Another note on EFI vehicles, you don't need to mess with your idle. The ECU will adjust the idle to where it is supposed to be. If you move a set screw or anything it will still idle the same unless you mess with the IACV or you've adjusted the set screw(if there is one) so much that the IACV can't adjust anymore.

Because at 65mph the tach would lurch to 2100rpm whereas at lower speeds of say 30mph, there is basically no lurch because shifting into 5th gear from neutral at 30mph means the rpm would jump to 1050rpm. No reason to have the idle be as high as 1000 at 30mph....

dkjones that flathead engine compartment looks like my 37 Ford.

regards
gary

That actually is a 37 Ford!

You are forgetting about keeping the cat warm - at high coasting speed the cat is being cooled off and the engine idles faster to warm it up. If you stop it warms up - I think this is what happens to my xB - I stop at a stop sign for about 20 - 30 seconds and the idle drops to 650rpm from 850rpm.

Well, that explains more thoroughly why it would second-guess the driver for that type of shift at that speed, but my question remains...why wouldn't it second-guess the driver for the sake of smooth shifts during OTHER types of shifts?

If the ECU programmer had the foresight to know that the driver would neutral-coast at 65mph and then shift into 5th...he would also know that the driver would neutral-coast at 30mph and then shift into 2nd or 3rd. Sound weird? Well, so does neutral coasting at 65mph, if you're not a hypermiler. Much more normal, very common and done by everyone, is downshifting from 5th to 3rd to accelerate (often for a pass); I would expect it to second-guess the driver that time if it's going to do so for other times. Why put in special programming for something people will never use, but then not put nearly the same program in for something everybody uses?

The question of keeping the cat warm, that one makes the most sense yet; it's essentially a "cold idle" behavior. Let's compare it to cold idle... Does the car idle higher for a few minutes when it's first started (but not driven away), or just for a few seconds?

Here is a picture of mine, sold it 3 years ago. Got better mileage than my Tundra with a V6. All original 50,000 miles, still 6 volt. Adjusted the mechanical brakes with a digital thermometer, measuring temps at each wheel.

I could stop the car with my thumb.

regards
gary

HeHe the coil is not original.

regards
gary

mechanical brakes are a wonder, misadjsted and u cant stop worth a crap, spot on and wowza can they lock em up and stop quick!!!

shoulda kept it! people instantly disregard old cars as having decent fuel economy because well thier old and not all computerized with fancy sensors. i for one would rather have an old car anyday!

VO I have found sentimentality to be a burden at times. It was fun at the time, but after 3 R&Rs of the tranny to find out old NOS parts are usually crap, I got tired of doing things more than once.

You should have seen the 49 Plymouth Businessmans Coupe on a Nissan pickup truck frame, sectioned and channeled, with a 240 Z engine and 4 speed transmission.

regards
gary

That is too cool!

I guess they didn't vary between cars and trucks under the hood. The picture I posted was from a truck. I'm wanting to take one of those flat-head v8 Ford engines and slap it onto a motorcycle. Sounds cool and not a suicidal amount of power.

idles near 2000 when cold and takes a good minute or two before it will start coming down and 5-8 minutes before it's at 500rpm. I agree keeping the cat warm is the best hypothesis yet.

I had a flat head 6 in my first car - 1965 Rambler American - talk about a smooth running torque generating engine only mine had a rebuild after my first summer and ended up with slightly higher compression(195psi), head polishing, SU carburation and water injection to keep the carbon from building up.

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.

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.

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

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.

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?

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.

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.

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.

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

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.