Lars87

I installed new injectors (FIC), new 52mm TB and a PCM4Less tune to match. Car is a LT4.

Car runs really good, but I am struggling with a idle problem.
When you slow down, and come to a stop the idle dips down below 500 rpms or so, to the point where it almost dies. Every time its then like the PCM "catches" it and it idles fine after wards at 800 or so..

At steady idle the car idles fine.

TPS reads 0.71 Volts with closed throttle, and 4.6 volts at WOT.


I put on the entire coolant passage bottom from the old TB, so I have not touched the IAC, and as I understand it the IAC is like the TPS self adjusting, so what can I really do here?

slickfx3

you didn't need a tune for that stuff, sounds like the tun( idle category), throw in the old chip and let it idle...

and the volts sounds like it is reversed should be higher as you open the throttle.

you can reset the IAC...as well

Lars87

I did need the tune since I moved for bigger injectors too.

I dont think its the tune, since the car idles fine once it settles.

As I wrote the I have 0.71 volts closed and 4.6 volts at WOT, that = increasing as throttle opens, not sure what you mean?

slickfx3

you are right i missed the decimal point

there is stall saver parameters where timing is pulled in stages to help with that, but sounds like something is making it fall.. is this cold or warm or hot behavior?

Lars87

It dosent really matter if cold or hot.

How can i check/rest my IAC?

slickfx3

mine tpi you lt4

search...clean while u r in there

i don't think it is the iac i think the idle to temp has to be checked out

ask alvin at forless

96GS#007

Seems like it's one issue after another with this car

I doubt it's the IAC. Typically something like this is in the tune. There's a parameter in there for the IAC and with enough changes to the car sometimes that needs to be tweaked. Some other people have had to change some of the injector parameters to really get ones sold by FIC to work. Once the tuning is fixed, they work well. Going from memory it was likely the injector offset they were changing.

My 383 did this with the initial tune and I had to go in and modify a little bit of the closed throttle spark advance table to fix it.

Lars87

Well, if dont want to work on the car I should just stop moding it, lol. The car itself has proven really reliable..Knock on woods.

I have the Autotap with GM parameters now, so maybe I should log some data and send it back to PCM4LESS for a look.

I was thinking that maybe I should adjust the idle screw on the new TB a little. I read somewhere on here that the IAC only supplys 75% of the idle air, and the rest is from the TB. Right now the TB is fuly closed at idle.

Weav's Vet

Lars, you can adjust the throttle body blades so they are just slightly open by turning the screw you mentioned. Remember though you'll have to readjust the TPS every time you adjust that screw. I have found mine idles best with about .61 volts at a 900 rpm idle on the TPS. It will vary from car to car though which reading is best.

If you have a BBK 52 TB they can be difficult to make idle correctly.

If adjusting the screw doesn't help it probably something in the tune like Jim mentioned.

96GS#007

The idle will be the same, but that will reset the minimum idle air. To do so, turn the ignition key to run but don't start the engine. Wait 10 seconds and then disconnect the IAC electrical connector. Now start the engine and adjust the idle speed such that the engine just runs. Done.

The other (easier) alternative is to fire up Autotap and adjust the tb screw until the IAC is between 15 & 30 counts (steps). Sometimes with modified engines and aftermarket throttle bodies (especially the BBK) you just can't get there. If that's the case then you have to get into the tune.

Lars87

So I can do this "LIVE", with the engine running and Autotap monitoring ther IAC counts?

Seems like the easy way to see if the the tune needs to be redone.

96GS#007

Yep

dirtcheap74

Im fairly new to the LT4, how exactly are you adjusting the idle speed with the IAC disconnected? Mechanically? Would like to know for future refrence.

JrRifleCoach

I've been through the tuning process with Alvin twice now.
The tune you have from PCM4less is a baseline. You need to go out and get a few datalogs for him to review.

My runs included a 0-100 MPH wide open throttle, and another that showed normal driving with steady state throttle at your typical road speeds. For me that was 45-55-75 MPH.

Make sure you offer a lot of info to describe how the engine was responding during the runs and any problems your experiencing.

My experiences required three to five different versions until the engine was running perfect. Just remember he cannot hear or feel the problems you may be experiencing so your descriptions must be complete.

GL

96GS#007

The operating idle speed is not being adjusted. It's controlled by the PCM. The amount of air being passed via throttle blades vs the IAC is what's being changed. If the blades aren't set properly, the IAC will/can be out of it's control range resulting in idle issues as the PCM commands the IAC to do things it's mechanically unable to.

dirtcheap74

I see. That is what was thinking as I read through my FSM but I thought maybe there is another way to set minimum idle.

I have another question for you as it seems you have a good grasp of this stuff.

In the FSM page Book 2 page 6-672 at step 7 it talks about resetting the IAC pintle position after hit has been installed. Lists 4 steps. Is this only for a brand new IAC valve or would this also need to be done if your PCM is "tuned"?

96GS#007

Only for a brand new IAC.

The only time you need to reset the minimum idle air (99% of the time) is after significant mods...heads, cam, etc....or a new throttle body.

Lars87

UPDATE:

I started by hooking up Autotap. At idle I had a IAC count of 71.
I then adjusted the idle screw on the TB to add some air from the TB at idle.

I then fired it up again and my IAC was at 2-5 counts.Its not steady at one fixed value, but changes between 2 - 5 counts, I suspect that be normal and shows that the IAC corrects the idle to get the desired idle rpms.

I took it for a spin, and the car did not bug down when stopping like before.

Back in the garage I backed the TB idle screw a bit back, to get the IAC counts up a bit. I believe its needed, to put the IAC in a position where it can control the idle by both adding and removing air. With my counts at 0, the IAC cant remove any more air.
I ended my with a count circling around 14.

My TPS voltage is 0.9 with this setting. It seems a little high, but the PCM seems to account for it, because the throttle angle is 0 degrees at idle still. Would there be any gain in slotting the TPS sensor hole to get the voltage at idle down?

96GS#007

ECMs and PCMs from 1990 and on "self zero" the throttle position if the voltage is less than 1.0v at idle. There's no benefit is messing with the sensor.