Dominic Sorresso

Title pretty much says it all. I've done the SA and VE flattening etc, but would love to eliminate this annoyance all together. Played w the TF a bit. Any one have some other suggestions?

TORN

iTrader: (17)

Throttle position sensor @ 0% ? I was getting a surge because it was sticking at 1 - 2% while coasting.

Dominic Sorresso

Yes its closed. I make sure of my Min Air setting and TPS V before I do anything else.
BTW, this seems to be a characteristic of the LT-5 not just my motor.

84Elky

Dominic --
Some questions before responding:
- Using S_AUJP?
- Occurring at any MPH?
- Do you see IAC movement in Logs when this is happening?

Dominic Sorresso

I should be more specific. This is occurring as you decel but im neutral. As this is an LT-5, no not using AUJP.

I'm thinking the lower speed IAC steps need to be raised just a bit to keep motor from dropping rpm too quickly. IOW, ramp down the IAC a bit more slowly.
Have not logged anything yet but this is a characteristic of the LT-5 btw. Especially, modded ones.

RBob

iTrader: (1)

As for being modded, have the others with this issue also removed the secondary butterflys?

RBob.

Dominic Sorresso

RBob,

Most who upgrade power will delete the secondary port throttles from operating. Some, like myself, have removed the mechanism all together.
The idle oscillation, which stops once you reach the mph where Idle SA takes over, is present in most modded cars secondaries or not. Porting the top end alone appears to introduce this. Stock motors don't exhibit this from what I can tell.

RBob

iTrader: (1)

> Porting the top end alone appears to introduce this.

This is interesting as I have nearly the same issue with a car that has had the plenum enlarged. A little different but on a warm engine at a stop, shift into P/N and the engine does a slow oscillation.

It isn't proportional gains as it is in open loop at this time. When left in or placed back into drive the idle is steady, no issue. Took a while for it to show up as I usually key-off shortly after placing the trans into park.

Further testing shows it to be worse if coming to a stop with the trans in neutral. Which is similar to what you have found. Reducing the idle compensation SA helped reduce the magnitude of the RPM oscillation.

Made some IAC changes but haven't tested that BIN yet.

RBob.

Dominic Sorresso

To your point Bob, I've reduced the Prop Gains significantly from stock. My idle still runs in C/L. The last bin I did seems to have mitigated the issue noticeably. The IAC TF and Min TF, I brought the IAC step values closer to what the Idle or 0 mph steps are set at.
I bumped them again today but can't test it out since my alternator went on the fritz. Getting it rebuilt today so should be able to test it out later this week.

RBob

iTrader: (1)

Yes on the IAC TF, keeps the RPM from dropping too low while slowing in neutral. Which is similar to what I'm now seeing, although not a TF issue as the car is not moving, just idling. Yesterday I tested a BIN that had the IAC's reaction to high idle RPM changes slowed down. This helped show what is going on, that is the RPM dips a little the IAC over-reacts and causes an overshoot to the high side.

But with the high idle RPM changes the IAC now steps closed a little slower. Now as a long as the RPM doesn't dip the idle steadies out. I'm thinking that the idle SA is too low which is allowing the idle to dip.

I also discovered that when the electric fan comes on that is enough of a load to also help steady the idle. Not as stable as when the trans is in gear, but it does help.

RBob.

Dominic Sorresso

RBob,

I found a similar reaction to the electrical load a fan throws on. As I posted earlier, once the mph reaches the point of using Idle SA, the swinging stops. Currently, Idle SA is at 20d. I have also matched that with the, in my case, Closed Port Throttle SA in the idling region of the table ie
45-55kPa/800-900rpm. Minimized the SA comp for hi and low idle too. Those changes are what have led me to the IAC controls. Modded LT-5s tend to be "unstable" in that they react to changes more rapidly. Slowing things down from the stock settings helps. Overall, the motor idles very nicely especially for one that's cammed.
I may also try raising the mph where it goes into Idle SA to see if that helps.
I recall having a similar issue w my Xfire, but finally having it so the idle would just sit like it would at a stop. But I don't remember what I did to achieve that. Been a while

Dominic Sorresso

RBob,

Got the alternator in and took car out w new cal. First of all, I was mistaken about the parameter I had changed. It isn't the Closed Idle SA MPH. Its the
Min MPH for TF. Once the car hits that, the oscillating goes away. One other observation from today. With raised TF steps, the oscillation was subdued but it would seem to increase in amplitude as the mph decreased until it hit the Min MPH for TF. Then it stops.

Dominic Sorresso

One other item was lowering the TF Offset.

RBob

iTrader: (1)

Time for an update. Did a BIN with the idle SA at 26* BTDC, up from 20*. Then blended into the main table for a smooth transition. This provided a large improvement in idle stability.

And it also helped in off-idle driveability. The engine is smoother and more responsive, so this engine likes it.

Then continuing with this BIN the P/N idle speed was bumped up 50 - 75 RPM in the warmer to operating temperature range. This had very little effect on the idle dips.

Getting lots of data of this idle dip with the above BINs, I did notice something. That when the idle dips, many times the IAC would open by 1 maybe 2 steps. This caused a surge in the RPM, with a slow recovery back to the proper idle speed as the IAC slowly stepped closed.

This suggests either a very small vacuum leak (unlikely, but not sure). Or, the IAC adapter on the bottom of the TB has an issue. This I believe is a greater possibility as it is either custom or aftermarket.

The pintle to orifice dimensions may be allowing too much of a change in airflow for minor changes in position. And since the IAC pintle can move a little on it's own, it just may be shifting enough to reduce the airflow and causing the RPM dip.

RBob.

Dominic Sorresso

RBob,
'How did u finalize on a 6* uptick on idle SA? Also, did u happen to try free wheeling decel?

RBob

iTrader: (1)

It isn't really finalized, just a number I plugged in after doing some research. I started looking at stock calibrations for this engine and found that GM ran at about that value. Surprised me, I was going to bump by 2*'s at a time and try it. But seeing what GM used I just went to the 26* right off.

I did, there was less oscillation while still moving and it settled into an idle with less nonsense.

RBob.

Dominic Sorresso

Interesting. The stock idle SA for the LT-5 is set at ~ 16*. Mine is at 20*. The motor is ported from airhorn to the valve plus it has more aggressive intake and exhaust cams. I've tried timing somewhat lower and a few degrees higher. 20 seemed to be a smoother idle w rpm ~875-950. MAP is in the 50-55 kPa range.