High idle ONLY while moving

Discussion in 'Engine Performance + Maintenance' started by Chillin Dylan, Oct 15, 2019.

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  1. Chillin Dylan

    Chillin Dylan Newbie

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    Hey guys, been a reader on here for years, just forgot my old login stuff so I made a new account.

    Anyway, I’ve got a 95 454, NV4500. I bought the truck a few months back running but not driving. Had to rebuild the transmission and put a new clutch+flywheel on it. Been driving it for about a month now and it’s been real good to me. Slowly but surely fixing up the little dumb shit the previous owner did. He thought he was a mechanic, electrician, and based on the stupid ass stickers on the toolbox lid, a cowboy. LOL

    As for my idle issue. It’s been this way since I got it running. The idle will hang up around 12-1400rpm but only while I’m moving. Doesn’t make a difference if the clutch is pressed or if it’s out in neutral, it does it. It’ll do it even when I’m JUST BARELY moving. As in I’m rolling up to a red light, I’m going so slow that I’m done downshifting so I’m in neutral. The idle will hang right up until I feel the little lurch of the truck fully stopping. As soon as I fully stop, the idle drops right down to about 800rpm with no hesitation. I’ve verified that there’s no vacuum leaks with the good old starting fluid around the sealing surfaces while the truck is idling. I’m thinking that maybe it’s supposed to do it for some reason or another. The only thing I haven’t done is unplug the speed sensor off the top of the transmission to verify that it’s 100% speed data related. Assuming that it’s supposed to do it, is there a way I could make it stop? It’s very annoying to me, particularly when it’s cold outside and the engine has idled down, but the tranny is still cold so the synchros don’t wanna work that great. So I just double clutch and rev match but with that stupid idle thing it makes it a little tricky sometimes. Gimme your thoughts, suggestions, whatever. Thanks guys.


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  2. Schurkey

    Schurkey I'm Awesome

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    Connect a scan tool, compare actual idle speed to the commanded idle speed.

    Verify that the IAC and TPS work properly.
     
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  3. Chillin Dylan

    Chillin Dylan Newbie

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    Unfortunately I don’t have an OBD1 scanner. However, if I’m not mistaken, OBD1 doesn’t control idle though..? I could be wrong, but I thought the idle on the TBI motors was only controlled by the idle screw that’s blocked off, and the coolant temp via the IAC. I have a 95 TBI 350 that was 100% perfectly functional when I pulled it on Saturday, could I swap the TPS from that motor to see if it makes a change on my 454?


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  4. Schurkey

    Schurkey I'm Awesome

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    Idle speed control has been handled by the computer since the carburetor days. Yes, the TBI computer looks at the coolant temperature to help determine appropriate idle speed.

    TPS may or may not be the same. There were several variations--here's a photo of two styles. ("Flat" three-pin vs. "Round" three pin.)
    [​IMG]
     
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  5. DerekTheGreat

    DerekTheGreat I'm Awesome

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    That is normal on a stickshift TBI vehicle. Like Shurkey said, if you connect a scan tool (I recommend a Snap-On MT2500) and drive around with it connected and the TPS voltage & IAC counts visible, you'll see that any time you step on the throttle & bring the TPS voltage up, the IAC counts will also rise. It's part of the logic programmed into the ECM. I had a 1988 C3500 with a 454 & SM465, it did that. Our 350 TBI five speed trucks do the same thing. When it's warmed up, the idle will be slightly lower when moving, but not until you bring the vehicle to a stop and the ECM can command it down to that idle. From that point forward you'll see that while the idle goes high when moving in neutral, it is about 200rpm less than when it was cold.

    Not sure why they do this, as the EEC-IV SEFI Ford stuff I have and have had don't do that, maybe it was to help you shift into the next gear. I just deal with it, not sure if it is something you can program out with a custom chip- that isn't something I know much about at all.

    EDIT: Oh, and before you get the idea of punching that cover off the base idle screw to adjust it- DON'T! As said earlier, the ECM controls the idle and it will always try to achieve that number, big blocks seem to be programmed to idle at around 800rpm. So if you try to lower that manually thinking you'll "help" your issue, you'll only create others. When I got my C3500, someone had that idea and when cold, their tweaks caused it to either hunt for idle until it got warm or stall out.
     
  6. Chillin Dylan

    Chillin Dylan Newbie

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    This is pretty much what I was looking for. I could tell based on the perfect regularity of it that it had to be intentional. I just wanted to confirm it since you guys know things specific to these trucks. Personally I find this to be annoying, but I guess it is what it is and it’s not gonna change unless I put a carb on it. Thank you man for the thorough reply



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  7. DerekTheGreat

    DerekTheGreat I'm Awesome

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    Don't slap a carb on it, no need to go back to the 50's..

    Pics of said truck? If you ever get tired of it, sell it to me. I'm a TBI fan and miss our old '88. Was a regular cab long box SRW truck and I've been looking for another one ever since. Must have clearance lights :D
     
  8. Chillin Dylan

    Chillin Dylan Newbie

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    No I won’t put a carb on it simply to get rid of this function. The only way I’ll put a carb on it is if I swap in my buddy’s low mileage vortec 454 with a cam. I’d like some more towing torque but I know the upgradability with the tbi is very poor, and to retain the vortec setup in a tbi truck is a very long and tedious process... I’m still undecided on what I’m gonna do about power. I might just deal with it since I don’t tow all that much anyway. Could always 12v swap it lol

    I really need to take some more pictures of it but here’s what I have, all outdated. I’ll take some more this week after I get the brush guard on

    [​IMG][​IMG][​IMG][​IMG]


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