Diagnosing Rough Idle Hell

[DylChrFla]

It would try to idle down but it would “cough” and idle back up. I’m unsure that I will be able to diagnose this on my own. Is anybody competent enough to read my datalogging? I am going to run compression tests on each of my cylinders tonight and see where that puts me too.

 

[DylChrFla]

There's a fine line. If done right, the IAC will close accordingly to maintain your idle. What's happening is your throttle blades are too far closed so, the IAC is allowing more air through than it should have to.

Which, BTW, may cause a rough idle with the fuel puddling on the throttle blades and then dripping into the intake manifold instead of staying atomized and flowing smoothly, evenly into the combustion chamber.

When I got my IAC position near 0, the idle was way too high.

 

[DylChrFla]

Big update: Actually found something that would affect my idle. I noticed the top of my injector pod was saturated in fuel, meaning the gasket isn’t holding a seal. I will replace the gaskets and report the results.

 

[MIHELA]

Another often overlooked part is the pickup coil in the distributor. They dont fail as often as the old vac advace distributors do because the wires don't move but they still get baked over time.

 

[Schurkey]

...and the magnet for the pickup coil is a known failure point.

Note crack in magnet where the rivet passes through.

 

[DylChrFla]

...and the magnet for the pickup coil is a known failure point.

Note crack in magnet where the rivet passes through.

No dice with the gasket change, I don’t know how old this distributor is but it is probably an older unit.

 

[DylChrFla]

Sure enough, one of the magnets was badly damaged, I swapped my distributor last night and idle quality is definitely improved, I will take a closer look tonight and report my findings.

 

[DylChrFla]

Still having issues and I’ve been messing with the IAC more, but I took a 20 minutes drive through stop and go traffic and highway. Doubled checked my base timing (0°). And my datalogging showed this:

TPS Voltage @ idle: in the .55 to .59 depending on set screw adjustment
IAC position @ idle in park: 15
IAC position @ idle in gear: 75
Idle RPM in park: 875
Idle RPM in gear: 825
Knock Counts: 67 (seems pretty high)
Coolant Temp: 195-200°
INT/BLM: Both stayed near 128 in closed loop.
Park/neutral switch working

When put into park after driving, rpm would shoot up to 1300 and slowly go down until IAC position returned to 15. This seems like a large gap for the IAC to make up. Other than the idle weirdness, truck seems to still drive okay with light throttle all the way to WOT. I’m near my wits end with this, I’ve thrown quite a few parts at it with no luck, by the end I will probably have all the components for a new motor haha.

 

[DylChrFla]

I just noticed my TPS % shows 30% with no input and 220% at WOT. Hmmmm
My idle also seems to rise whenever I plug in the ALDL cable.

 

[man-a-fre]

Knock counts are way high mine are 0. When it see knock it pulls timing. Tps voltage at idle should be .50 to .60 you might double check that also.