TBI Swapped Ford 351M - Hi Idle Issues after in gear

[dave w]

I once read somewhere, don't remember where, that the IAC resets at 35 MPH. It's only here say information that the VSS is needed to reset the IAC.

Maybe someone else can confirm or deny the VSS is needed to reset the IAC.

The vast majority of gearhead-efi posts related to idle problems are vehicles without VSS.

IAC counts in the 10 - 40 range when the engine is fully warmed up is a good target to adjust to.

dave w

[ony]

park neturel switch? maybe lower temp where it goes closed loop,change iac followers sometimes help if you haven't , one bin I seen had the iac follower at 100 set at 40 helped but mine is std trans

[MQQN28]

Dave
My system does not run a VSS. It is a customised tune on a delco 808. When it was developed in the mid 90's for the flathead it had a lot striped out including the VSS. I have learned that the first users of these were told when they got the first tune to install A VSS signal.

I got my system from a person that sat on it for number of years and didn't use it. So when I finally got my going it was on a generic tune and the idle was over 1000rpm. The initial tune was to get the fuelling right for the diff ratio and wheel size. The idle was ignored. A short time later I got a another tune with the idle set to 700 (c4 auto netural). It was only mentioned at the time that economy would be better if a VSS was installed. The tuner decided to change the IAC steps from 256 to 128, I don't know if this is better or worse.

On reflection the idle seems to be ok, but I have a strange condition that if in neutral and I slowly raise the revs to say 2500 and try and hold it surges low and high. In drive cruising and stoping seems fine.

I have just this week installed a VSS sender and wired the signal up to the ECM ready to change the data. I'm under the impression that on the Delco 808 that if you hold the revs at around 3000 for 6 sec's it will reset.
Maybe that's how this works others are different, any way not far away from VSS and more questions.

MOC

[Speedyzee]

Hey Dave; I thought about the lack of VSS a few times - but I have set any tune values to 0 as far as throttle follower steps, MPH to enable Idle or large IAC error etc.
Not discounting anyone's thoughts, but riddle me this: what it does is - Truck in Park, idling - closed loop, holding desired idle RPM in park (IAC counts around 18 = 750 RPM) - pull it into gear and let the ECU raise IAC counts to get back to desired CL idle RPM (Now prob 34 counts to get back to 750 RPM) I put it back into Park (yes ony - P/N switch works and I can see it change state with my scanner) and then the idle seems to hang at anywhere from 950-1000 RPM - the IAC counts are higher than at desired RPM (usually stay at 34-ish), so the ECU is commanding the new idle speed; since I haven't moved the truck, a VSS signal shouldn't matter - otherwise a stock GM TBI would act the same way if you pulled it into gear, idled for a few seconds sitting still and put it back into park, right?
I did find a scaler in the tune labelled Min TPS % which was set at 25% - now that I'm at 0.50V TPS at idle that puts me at 10% - I changed the value to 10% and will see what happens tomorrow. Thanks for any and all suggestions so far... I wouldn't give a rats a** about the idle issue if it wasn't for the truck having a doubler transfer case with a redic amount of gear reduction, and my tendency to flop the thing on it's side from time to time on the rocks. A nice predictable low idle would be very nice :)

[dave w]

Hey Dave; I thought about the lack of VSS a few times - but I have set any tune values to 0 as far as throttle follower steps, MPH to enable Idle or large IAC error etc.
Not discounting anyone's thoughts, but riddle me this: what it does is - Truck in Park, idling - closed loop, holding desired idle RPM in park (IAC counts around 18 = 750 RPM) - pull it into gear and let the ECU raise IAC counts to get back to desired CL idle RPM (Now prob 34 counts to get back to 750 RPM) I put it back into Park (yes ony - P/N switch works and I can see it change state with my scanner) and then the idle seems to hang at anywhere from 950-1000 RPM - the IAC counts are higher than at desired RPM (usually stay at 34-ish), so the ECU is commanding the new idle speed; since I haven't moved the truck, a VSS signal shouldn't matter - otherwise a stock GM TBI would act the same way if you pulled it into gear, idled for a few seconds sitting still and put it back into park, right?
I did find a scaler in the tune labelled Min TPS % which was set at 25% - now that I'm at 0.50V TPS at idle that puts me at 10% - I changed the value to 10% and will see what happens tomorrow. Thanks for any and all suggestions so far... I wouldn't give a rats a** about the idle issue if it wasn't for the truck having a doubler transfer case with a redic amount of gear reduction, and my tendency to flop the thing on it's side from time to time on the rocks. A nice predictable low idle would be very nice :)

I'll answer your riddle with a riddle. Why did the spark advance change in your experiment "when the VSS signal shouldn't matter"?

dave w

[myburb]

I haven't used that ecm so this is just a wild guess. Could it be with no vss connected the unknown internal logic thinks when you put it in gear it moves to moving vss logic and stays there until the key is cycled.

[ony]

some where if you search this sight there was a man in germany with about the same conversion,can't think of his handle.might find something he learned I think he had this problem too.