TBI surging idle

**C2500** · 07-09-2021

Whats the cure for this?

**steveo** · 07-09-2021

one thing you run into with TBI is that the fuel pools on the plates then dribbles into the throttle body. this means any adjustments your ECM makes to fuel or IAC or whatever can be really delayed so it's really fighting with itself

to minimize this, the first thing to try is to make sure most of the fuel is actually spraying through the throttle plates rather than puddling up on them. this usually means you start cracking the idle stop screw until your IAC counts get very low (but not zero ideally) at HOT idle.

most people find that the lower the iac count the better the idle on TBI engines.

other than that give it a bit more (or less?) timing and a bit more fuel

a lean condition or incorrect timing advance can give you a really unstable idle, and the ECM will just make it worse trying to correct for it.

**C2500** · 07-09-2021

THANKS !! I did open the throttle plates some but I was not sure how much I can go with out disturbing something else in the process..I will take if further and see what happens…
Anyone try to drill a small hole in the plates? We did this in carbs with big cams to still have a handle on the T-slot opening and let enough air in to get the idle constant…

**steveo** · 07-09-2021

i'd try to open the plates until your hot IAC counts are around 10 or so

i know that some ecms get complainy when you move the throttle plates around (as it thinks you have your foot on the gas) so sometimes TPS readjustment/relearn or power cycling the ECM would be necessary before it'll go back into 'idle mode'

**PlayingWithTBI** · 07-09-2021

This .pdf may help, it shows how to set your min idle and then calibrate your TPS.

**C2500** · 07-10-2021

Explain how the IAC works.. I know it opens to allow idle but does it completely close with the throttle open wile driving? Will there be an issue with it with the counts that low? Dumb question I guess as I’m sure you have done it and it works..
Gotta give that I try..So if the TPS is above ? At idle where is it acceptable and think is should be at.50 volts…
Thanks