Deceleration at high speeds

[444]

Hello all. New here and new to the Z car. I have been driving a 76 280z with auto. trans. Everything works great, love the car.

 

One thing I noticed. If I'm driving about 70 and take my foot off the gas, the car noticeable decelerates to about 55 and then you can feel the car sort of just smooth out. Although it is still slowing down, just not as quick.

 

Just wanted to know what this is and the purpose.

 

 

Thanx

[jfa.series1]

That's probably your emissions controls at work.  A rapid deceleration can dump unburned fuel and other bad stuff into the exhaust whereas a slow decel. allows the combustion process to complete and make for cleaner exhaust gases.

[Captain Obvious]

It's an RPM controlled "fuel cut". Take a look at page EF-12 of the 76 FSM. It describes the operation there.

 

Basically, if you take your foot completely off the gas when the RPM's are around 3200, the ECM stops injecting fuel. Once the car slows down to the point where the RPM's drop below about 2800, the ECM starts injecting fuel again. That's the "smoothing out" you feel.

[ramsesosirus]

On 12/6/2015 at 7:49 PM, Captain Obvious said:

It's an RPM controlled "fuel cut". Take a look at page EF-12 of the 76 FSM. It describes the operation there.

 

Basically, if you take your foot completely off the gas when the RPM's are around 3200, the ECM stops injecting fuel. Once the car slows down to the point where the RPM's drop below about 2800, the ECM starts injecting fuel again. That's the "smoothing out" you feel.

The Captain is right, just studied this for my Z.  It's the TVS (Throttle Valve Switch) (Some call it the TPS, which is what it is on some cars) on the side of the throttle body  (?).  It has 3 pins on the connector.   Some great info about it here:

http://atlanticz.ca/zclub/techtips/tps/index.html

IF it is working correctly, it enriches the mixture a) at idlie  and b ) at or near WOT .  Like the Cap'n said, it also cuts the fuel around those RPMs, sensing that you are trying to slow down.  I just adjusted mine, it seemed to go directly from the idle contact to the WOT contact.  I read that it wasn't supposed to contact either side between about 1400 rpms (or 5 degrees) and 2/3 or 3/4 of WOT.