Rough Idling HO

[Minuit]

My '91 is running a little rough. Has been for a while, probably should've sorted it out by now. The idle is rough and usually a little low. It idles at about 680-750 RPM when my MT2500 says it's aiming for 720. The vibration goes away instantly with any throttle. Above idle it runs smooth. Also (possibly related?) it feels like it has VTEC - after almost exactly 3000 RPM there's a noticeable increase in torque.

 

I've checked a few things, but I'm running out of ideas. Here's what I know:

 

- Plugs and wires have about 20k on them. They look normal and are gapped properly. Cap, distributor, and rotor were replaced about 5k miles ago, but the truck was running rough before then.

 

- The spark is purple-orange and not very consistent, regardless of whether it's at the coil or at the plugs.

 

- Coil was slightly out of spec at 1.8Ω primary 10.3kΩ secondary. Replacing the coil improved things slightly ( 1.2Ω primary 12.8kΩ secondary), still not great. The coil to distributor wire measures about 2kΩ. I can't find a straight answer as to what it should be.

 

- The coil is getting 12V when cranking, and the ground is good. 

 

- Timing advance at idle is 12-14 degrees with occasional jumps to about 17.

 

- Vacuum readings are consistent at about 18 inHg. The vacuum line going to the MAP was replaced recently and looks and feels fine.

 

- TPS reads exactly 0.5V when the throttle is closed.

 

- I've checked for vacuum leaks and found none. The manifold bolts are tight.

 

- Injectors are the originals.

 

- If I listen by the exhaust I can hear an occasional misfire, but not very often.

 

I've done all of the things people usually say to check. The spark is my main suspect, but I'm not sure what's going on since the coil is in spec (admittedly not Mopar) and it's getting 12V. I'm kinda at a loss  :dunno:

[omega_rugal]

idle stepper motor? the only air the engine gets when idling is throught this port...

[Minuit]

idle stepper motor? the only air the engine gets when idling is throught this port...

 

My scanner has a test where you can set the engine idle with the IAC and it responds to the test, so I'd assume the IAC at least works. I'll have to check to make sure the o-ring isn't torn up.

[HOrnbrod]

Try some NGK ZFR5N plugs. Those and the ZJ ignition coil / harness TSB I did really smoothed out my idle.