I noticed on my Tunercat program that there is a function , Idle RPM adder with AC on. I have a 383LT1 with a 230/236 cam, and am wondering will this increase the engine rpm when I turn the AC on? Or what. My Vette will not idle in gear with the AC on, and I am wondering if I increase this Adder up a couple hundred rpm if it will raise the rpm so my Vette will idle in gear with the AC on? Thanks.
1993 Corvette LT1 383cu in, 700r with 2800 stall, LT's etc etc.

 

DP

 

I had a long reply, but the site bombed out...

Anyway, the idle adder terms raise the desired idle speed. This means that the PCM will target a higher idle speed. It may help, but odds are that its a problem with the PID loop calibration itself. If the PCM cant recover from the immediate transient load, then increasing the desired idle speed probably wont do anything.

By the way, does the car ever achieve a stable idle? From what Ive found in the truck PCMs, I *think* the computer wont update the stored steady state integral terms unless the idle speed remains within +/- 25 RPM or so for a period of time. Also, the computer prevents them from getting too far apart, so if there is a large difference in the ammount of IAC flow needed with the A/C on, the computer wont let the integral terms go to where they need to. With a lumpy cam, the PCM may never learn the idle. This is especially a problem if the PID calibration is way off. Resetting the PCM by disconnecting the battery for a few minutes may also be worth a shot.

Other then that, fixing it would mean recalibrating the idle PID, and I doubt TC covers all these terms in the definition file.

 

Thanks. The idle does eventually steady out, once warmed up it idles around 900 rpm. When cold it hangs around 750-800. I need to start logging earlier as I never get it to idle with out me useing the throttle to keep it going, till warm, around 150 degrees.
But, once warm she is fine, even in gear. But the AC really sux her down at idle, and I was hoping it was a simple fix when I saw that portion of TC.
Thanks, Steve
I am just starting out. I just bought tunercat, but I have been datalogging for a while with TTS Datamaster

 

IIRC, from the truck PCMs, there is also a table of added transitional flows for when the A/C is turned on. This may help, assuming the engine isnt bogging down already.

Depending on what your ECM/PCM has, a wholesale adjustment of the idle speed and intial IAC flows would help with the larger cam to make sure the engine has a faster idle at all times. There are probably a good deal of things that can be adjusted, but if these arnt covered in the tuning definition file, then it doesnt help much

Also, have you tried tinkering with the engines tune in those areas? If there isnt enough/too much timing or the fueling is off, then the engine wont idle as well as it can and will be prone to stalling.

 

I was told by my tuner to go ahead and do the AC RPM adder by 50rpms, and see what it does.
I am going to take your suggestions and take small steps, well baby steps, as the idle is my most important issue now. It is real good when warmep up, but getting to the warm up temps its a little choppy. IACs are going up and down .

 

The added desired RPMs will help if the engine never smooths out when the A/C comes on. It will also ultimatly increase the integrator term, so there will be more commanded IAC flow. This may not help the engine wanting to stall when you first flick the A/C on, but after its been running, it will probably help keep the engine out of the trouble zone.

Also, the engines tune at low speeds is critical for a stable idle. If the timing and fuel are off at low RPMs and med. to high MAP values, then the engine will puke when you put a load on it and the idle will surge up and down uncontrollably.

 

I think you definitely understand what my Vette is doing. It runs great when warmed up, but when the ac was turned on it bogs down and doesnt want to stay at the desired rpm. SO, I put it in Nuetral when stopped at a light etc.
I have allot of data to start working on. I will eventually try different things to stable out the idel while running AC. Thats my goal.