So, when you are coming to a stop with your 4+3 car, either downshifting or throw it in neutral and coasting does the idle speed stay elevated until completely stopped and speeedometer says 0 or does it immediately go to 800rpms? Mine stays @1000-1300rpm until I am stopped and speedometer goes to 0, then drops to 800rpm. Base timing is correct, minimum idle and tps has been set, new iac, throttle body is not sticking (new bbk 52mm). The shaft on the original was very worn.

Just curious if this is normal, or if I need to hunt down another issue. Thanks!

 

My experience with me car is you throw neutral it holds a little high then creeps down

 

Yes, it was an emission control "feature".

 

Thanks guys. I may have to look into getting that "feature" tuned out. Its kind of annoying...

 

This is not unique to 4+3 cars. Idle speed PID control is only enabled below 1.8 mph.

In my own car I have idle speed PID control enabled below 40 mph so it works a bit like a low speed cruise control while idling in any gear and it also provides some early anti-stall protection when the clutch is engaged. Admittedly, this setting is a bit excessive, since 800 rpm in 4th gear O/D is only 28 mph with my gearing and tire size.

Note: my VSS signal can be a bit noisy due to RFI, so this setting also provides for stable idle speed control if the VSS is dancing around while stationary. My battery cables run inside the driver's side rocker panel near the VSS signal wires which makes it more prone to interference.

Here's an example log of an 88 4+3 car (which happens to be a Callaway) coming to rest with closed throttle and clutch disengaged:

https://datazap.me/u/tequilaboy/1988...2811-2613-2663

Note the IAC motor position with respect to the vehicle speed and engine rpm signals. It takes quite awhile (5-10 seconds) for the idle to settle down once PID control is initiated at zero mph in this example.

 

Tequilaboy, yes! Thank you for that info. My car behaves just like that map. Maybe not 10 seconds, but close to 5 for sure.

Great explanation! Thanks!

 

I do like DFCO, but like most features it can benefit from some tuning in order to make it more predictable and effective.

In maf cars, DFCO (or Deceleration Fuel Cut Off) is triggered by the load (LV8) falling below an activation threshold of about 30 counts with some minor year to year tuning variation. This is a very light load condition, that may never occur, or take some time to occur depending upon the rpm, tps and IAC behavior along with the MAF flow (and calibration). This load condition must be fulfilled continuously for 1.75 seconds before the fuel is cut.

I like to raise the DFCO activation (LV8) threshold to something like 40-50 counts in order to make it more likely to activate and be more predictable when lifting the throttle. The 1.75 second delay timer is just about right allowing for crisp entry and increased engine braking without being overly digital and annoying, which can happen if following the throttle too quickly. A 1 second delay can be annoying in normal driving, so I've settled on the factory setting of 1.75 seconds.

Here's an old log showing a DFCO event example: https://datazap.me/u/tequilaboy/driv...32-11845-11856

Interesting to note is how long it takes for the O2 sensor signal to respond to no fuel...about a 1 second delay before going lean, so keep in mind that the O2 will always be a bit behind what's happening in the cylinders.

 

The fuel cutoff is not as bothersome to me, like barkingdogs said you anticipate when it will happen and push in the clutch when slowing to a stop.

Would a heated O2 respond quicker? I had to put one in my 86 because of random, well kind of a predictable code 13 under light load cruising when it cooled off too much.