CentralCoaster

Mine seems to hang, at 1200 rpm or higher when coasting. It almost always drops back to 700 when I come to a stop.

This is with the clutch in or in neutral. So there is no engine braking going on or vacuum pulling the throttle blades open. It also doesn't seem to be booster related, I can hold constant pressure on the pedal coasting in neutral and still the idle rpm doesn't drop until I come to a complete stop. Plus the brakes work as good as ever.

The Helm manual isn't clear on how the idle is controlled.

1985 Corvette



Also be sure to check out the awesome deals to the right courtesy of Google!

cv67

Mine the same thing, its programmed that way.

Dale1990

I believe it is called he Idle Follower in the chip - I could be wrong, it's been awhile. I messed with the setting when tuning the 86 but I could never get it right with the 58mm TB and the SR.

mseven

yours is different and has additional hacks available,
iac throttle follower slope gain, idle vs %tps, idle max vehicle speed, which allows more controll in setting it up to have the rpm come down as fast as a carbed set-up.
I believe his may only idle vs temp. and fan, ac ,and pn add-ons, sorry haven't seen an 85 edit.

Sam Lam

The idle speed is dependant on the speed of the car on my 89 Vette. This can be observed by placing the car in neutral and observing the engine RPM as you slowly come to a stop.

3 pedals

My 85 idled like that - it's in the programming. It really irritated me so I changed it. It wasn't too hard - install a newer computer, buy a flash e-prom, buy a laptop, download Tuner Pro RT, download a 1989 program, edit the program to work with the iron head 85, increase the speed at which the IAC is enabled, and reset the minimum idle speed to 700 RPM. Piece of cake! The most important thing - I enjoy driving the car now!

Dads90

Yeahhhhh !!!! mines finally normal then. I got the idle right and it is doing this as well. Thanks CentralCoaster for asking. Mine holds at 1000 RPM then drops when stopped.

Demonic85

Well on my vette it does that but its an auto. My Focus when in neutral, the rpms are the same as idle. So therefore I have no idea

Staehling

Same behavior on my 88 4+3

USAsOnlyWay

Same thing on my 86 4+3 383, I kept wondering if the TB bearing was hanging but as soon as I stop the damn car, the idle drops and thus I can never check!!!

65Z01

I've noticed that my Auto X-ray shows the IAC count value is higher at speed than when at a stationanry idle, so yes idle RPM depends on vehicle speed.

rons85

Yes - it's supposed to do that.
No, there is no way to edit it in the '85.
Well, not directly - you Can change it some by playing around with the IAC settings and the base timing. Moving the timing table around (taking advance out) at low load/low rpm will also make the car more willing to drop down to lower "idle" rpm's while coasting. Doesn't stop it from happening completely, but it helps drop the rpm's more quickly as it's coasting. There is no way to edit or change the actual "threshold" value for speed where the "normal" idle targets take effect - not in Tunercat anyhow. It must be in the PROM someplace, but the mask doesn't pick it up. Try asking T.C. if he can find and add that to a new release of the mask

tequilaboy

When rolling off throttle at speeds greater than the PID enable speed threshold (typically 1.8 mph):

The IAC position will be defined by the warm park position vs coolant temp table + an offset of typically 10 counts. Fan anticipation can add another 5 counts, etc. A/C on will affect things as well by applying a learned offset.

The 85 bins that I've seen have 50 counts in the warm park table for coolant temps above 32 C.

Add the 10 count additional offset for rolling above 1.8 mph and it should run around 60 counts while in a prolonged rolling idle after the throttle follower effects have backed off.

The 60 counts of IAC position is likely resulting in the 1200 rpm idle that you're experiencing while moving.

It would suggest that you have too much min air for a good fit with the warm IAC park position vs coolant temp table.

Ideally both methods (PID and warm park position)should result in similar IAC values for a given coolant temp and result in similar rpm values.

Reducing the min air setting should bring your normal PID controlled IAC counts back up near 40-50 counts at your normal target idle rpm (assumed to be 700 or so) when stationary. This should help to ease the transitions between the IAC control modes.

Throttle position will futher modify the IAC position to provide some dashpot function when you lift the throttle. The IAC will follow the throttle position in both directions with a delay.

ramiles

Here is a brief excerpt I got from the Third Gen DIY Forum that talks to the general Throttle Follower program logic that is the primary factor in idle while decelerating.

... Now the throttle follower logic kicks in. As the tps% increase, so does the IAC steps. Again, this action is lag filtered. As the throttle is then closed, the IAC retracts, lag filtered.

My investigation of how to change or adjust the Throttle Follower logic in the $6E hack showed me it can get pretty involved - as this linked post describes: Throttle Follower

mseven

I agree w/ the above, but the post from Rrob (who has provided a lot of great insight!)was not for the 870 ecm, and the 6e hack would be for the 165 ecm. I haven't seen any hacks for the 870 ecm, even though they might be out there.

Midnight 85

Thank you CC for asking this question, I can finally stop fiddling around with my '85 w/auto. I've been trying to get the idle to return to 750 while rolling all summer. Mine too goes right down to base idle when I'm stopped but will cruise along at 11-1200 rpm otherwise.