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My 97 has always had a rough almost shudder at idle, even when it was a year old with nearly no miles on it. I always thought something was not quite right. I have had several of the Vortecs now and they are all the same. I think it has to do with the PCM altering the timing continuously to help trim the idle speed as well as the closed loop fueling. I can disable the idle spark variation and go to open loop and it will idle much more smoothly.
Could be...makes as much sense as anything else. The longer you sit, at a light, in a drive thru, or in a parking situation, the worse it would be.... "Shudder" is a good description; it's like when a horse shakes its head and snorts, like it has something in its nostrils. Normal idle speed in OD is around 500-600 RPM according to the tach.
Its been a long time since I have looked at the programming on a stock one, but it is programmed for like 625 in Park and 575 in Drive with the AC off at normal operating temperature. I believe the ac changes the idle speeds a bit as well. My 383 is set for 800 in Park and 725 in Drive. Low battery voltage, greater than 80% duty cycle to the voltage regulator field terminal can add 150 rpm. I also have it set to increase the idle a bit at higher than normal coolant temps. I have the maximum idle rpm capped at 1,200 rpm in the tuning.
Good to know, thanks!
A substantial amount can be done with the black box. That being said one of the things that helped on my stock engine was actually lowering the WOT upshift RPM. It did not like dragging its way all the way to 5,100-5,200 rpm with the stock cam and springs. This was shifting at like 4,600-4,800 with some timing added to it on 93 octane. This was an older upload so I forget if it had the shorty headers on it yet by this time frame. Listening to it accelerate, the 4L65E was also already flaring a slight bit on the 1-2 shift.
There is a possible explanation for this. First though why an improvement can be had on higher octane fuel. If the ECU can look for detonation and not find it, it can advance the timing and the engine be the better for it. On my smart it took around 4 tankfulls to get to the point where it no longer tentatively probed but went straight to the new timing confident that there'd be no detonation. (MPG went from 45-48 (54-57mpUSg) to 50-55 (60-66mpUSg) and the throttle response was much improved).
Do tell!
That's of interest to me - especially the 2nd to 3rd shift.
I've long heard that a brisk acceleration uses less fuel, but it's really dependent on the torque and your shift points. If you accelerate slowly, but you're below your peak torque, you're not at your optimum fuel usage. If you accelerate briskly and run all the way up to your redline, you're not at your optimum fuel usage - and the guy lugging his engine is getting better economy. If you're accelerating briskly and keeping your rpm near your torque peak, you're getting good fuel economy. Also note that you'll be on the throttle for less time than the guy lugging his engine.
My rule of thumb is to run the lowest octane that an engine can use without retarding the ignition. I've heard that it's because the additives used to achieve a higher octane rating aren't power producers. A lower octane carries more potential BTUs of energy, but running a higher compression ratio lets you use a higher percentage of those BTUs. Thus, a low compression engine gains nothing by running high octane, but a high compression engine can extract more power from high octane fuel.
Maybe if the truck is under powered. My 97 van cruise has always worked well, granted I can get a touch better MPG accelerating a touch over the speed limit as I approach the base of a hill and use the added momentum to help me carry it up the hill. Not that speeding to gain momentum to climb a hill is legal.
