Idle in park vs. drive

[Fishfan]

318 Poly, New Edelbrock 4-barrel.

I know it's normal for RPM to be a little higher when idling in Park or Neutral, vs. Drive. The question is what governs this? What actually makes the RPM go down in drive. Is it just a matter of the transmission engaging and slowing the engine down? Is there any way to adjust idle in Drive to bring it up a little closer to idle in park? I assume not.

 

[Ross Wooldridge]

The friction of the fluid in the trans is what brings the idle speed down from park to Drive. No way of adjusting the difference as far as I know.

Idle speed in park should be roughly 650 - 800 rpm, depending upon timing and emissions settings, year/make/model.

For a 318 poly I expect it's supposed to be 650. It should drop to about 550 when in Drive.

 

[traintech55]

Carburator adjustments in drive is a good way to have your car run over you.

 

[Fishfan]

Carburator adjustments in drive is a good way to have your car run over you.

Understood. I would not do it unless I had someone inside the car on brake while I did it. I also have a convenient parking stop in my driveway (the kind they have in parking lots).

 

[CBODY67]

One MAIN issue is the tightness of the torque converter in determining the rpm drop between "P" and "D". A tighter converter will put more load on the motor than a looser converter will. Generally, back then, Chrysler's OEM torque converters were on the "tighter" side of things, which made driving them in the lower gears a bit more like a manual transmission car, by observation, compared to a similar GM or Ford automatic transmission.

On modern, computer-controlled engines, the rpm drop between "D" and "P" can be minimized by said computer controls. Which vary the IAC valve to maintain a particular hot base idle speed in gear or out. "IAC" = Idle Air Control

In our '66 Newport 383 2bbl, IIRC, the in-gear idle speed was about 500 rpm, with the "P" idle speed being about 600rpm. I played with those to seek a bit smoother idle, but to no avail. Just set the idle mixture, idle speed, and ignition timing to factory specs and do not worry about them any more. Enjoy driving the car!

What are your concerns in this area? Just curious.

From my earlier experiences,
CBODY67

 

[Fishfan]

One MAIN issue is the tightness of the torque converter in determining the rpm drop between "P" and "D". A tighter converter will put more load on the motor than a looser converter will. Generally, back then, Chrysler's OEM torque converters were on the "tighter" side of things, which made driving them in the lower gears a bit more like a manual transmission car, by observation, compared to a similar GM or Ford automatic transmission.

On modern, computer-controlled engines, the rpm drop between "D" and "P" can be minimized by said computer controls. Which vary the IAC valve to maintain a particular hot base idle speed in gear or out. "IAC" = Idle Air Control

In our '66 Newport 383 2bbl, IIRC, the in-gear idle speed was about 500 rpm, with the "P" idle speed being about 600rpm. I played with those to seek a bit smoother idle, but to no avail. Just set the idle mixture, idle speed, and ignition timing to factory specs and do not worry about them any more. Enjoy driving the car!

What are your concerns in this area? Just curious.

From my earlier experiences,
CBODY67

Vibration at idle when in D, like at a stop light. It just bothers me. In P or N it idles much better. My Polara has factory air (I should say had because I have updated virtually all the components) and the owner's manual recommends idling in N when running the AC. I basically put the car in N at every stoplight, whether I'm running the AC or not.

 

[jimmyessbee]

Vibration at idle when in D, like at a stop light. It just bothers me. In P or N it idles much better. My Polara has factory air (I should say had because I have updated virtually all the components) and the owner's manual recommends idling in N when running the AC. I basically put the car in N at every stoplight, whether I'm running the AC or not.

It doesn't hurt anything to put it in neutral, but if you're like me that's probably indicative of a rough idle issue that you just haven't yet sorted out. It's exacerbated by the lower idle at a light.

 

[Ross Wooldridge]

My 66 T&C has a good smooth idle drive with the AC engaged. Something else might be a factor in your case.

 

[MJFUR]

My 69 Chrysler 300 with 505ci idles in P and N at 1000rpm, 800rpm in Drive, about 700-750rpm with A/C on. It doesn't like to idle below that when under load.

Hook up a dwell meter or what ever you have to read the RPM, have someone sit in the car with their foot on the brake, put it in Drive and adjust RPM to your liking.

 

[CBODY67]

Very FEW Chrysler V-8s idle "glass smooth" like the FE Ford V-8s I've seen. No matter what you might do. No matter the rpm, in gear or out.

I don't recall the suggestion of putting the trans in "N" at stops, unless it might be that the engine is overheating or similar, so you can raise the rpms to get the engine fan turning faster for increased cooling. Might as well go ahead and put it in "P" so there is no chance of the car rolling unexpectedly, to me.

You might run a compression check or cylinder balance test to see if there might be a weaker cylinder in the bunch, which could cause the idle to be a bit rougher than usual. Like one with a burnt valve that is getting ready go get worse and needs attention. Or something in the ignition which makes for a weaker spark in one cyl and not others, too.

With an elevated idle speed, it will make the engagement "into gear", especially into "R" much harsher and forceful, which is not generally good for the trans, u-joints, or rear axle, to me. I know there were some emissions HO engines in the later '60s that used hot base idle speeds of almost 1000rom, but they also had looser torque converters or were manual transmission cars. Perhaps 750 might be an upper limit for a normal-torque convertered engine with a stock-type cam?

Might also check the motor and transmission mounts to see if they might be transmitting more vibrations to the body than they should.

Just some thoughts and my experiences,
CBODY67

 

[Fishfan]

It doesn't hurt anything to put it in neutral, but if you're like me that's probably indicative of a rough idle issue that you just haven't yet sorted out. It's exacerbated by the lower idle at a light.

The truth is I suspect low compression in a cylinder. I haven't run a compression test on it yet, though. Not sure what I would do if that turned out to be the case though.

 

[The Goose]

This is a common problem with big cam automatic cars. With a stick car it’s clutch or neutral waiting out the light so no big deal. With an automatic car you just pop it in neutral or drive with two feet. One on the brakes the other artificially raising the idle with the gas pedal. Most of us once as teenagers with “hot” cams can relate...

If the AC idle solenoid is working it’s just raising the idle like your foot would.


Main reason I preferred stick cars to autos is the high RPM in Park vs drive idle issues with a big cam, (That and push starting LoL). One fix is to set the best idle possible in drive. Then the simple trick for high idle in park is to turn it off in drive AND then throw it in park. No slamming if not running.

 

[Fishfan]

So, I've noticed that in addition to the lower idle in D that can be attributed to the transmission being engaged, that the brake pedal is also implicated. The harder I press on the brake pedal, the lower the idle. I assume it's because of booster. Is this a sign of a bad booster? It's not that old.

 

[CBODY67]

Send the booster to "Booster Dewey" for a rebuild. Engine should be missing due to an internal vac leak in the diaphram.