Rough Renix Idle - Wrong Iac?

[OneOverZero]

Ran into a puzzler with my newly-running '90 MJ with a rebuilt '94 HO.  Engine currently has 32 miles on it after cam break-in, and aside from a lifter rattle, all appears to be good aside from a stubborn unsteady idle.  It often lopes along at 400RPM, consistently surging to 600 or so every couple seconds when hot, and will nearly stall when the clutch is disengaged suddenly (approaching stop sign, etc).  I didn't have any idle issues when I pulled the old driveline in late 2012, though basically everything in this truck is new, so I'm starting with a massive list of unproven components.

 

Renix TPS (on HO throttle body with Hesco adapter) is adjusted perfectly at 17% of FSO, air/fuel ratio is steady at ~14.6-14.7 at all times (immediately jumping to 22.4, max gauge range, when off-throttle), and all sensors aside from MAP are new and test properly.  Fuel pressure is perfect with a new FPR and lines.  Grounds are all clean with no detectable electrical issues (though my turn signals did stop working on the way home today, hmm).  I'm going to test the MAP sensor (though I don't see where I can cleanly probe it) and throw a vacuum gauge on the intake tomorrow to ensure that I don't have a massive leak.  I also have a Mopar-reflashed ECU in the garage, though I'm holding off on installing that until I'm confident that all else is stable - my understanding is that the reflash only helped with cold idle issues.  EGR solenoid has been removed since I'm running the '94 intake, though all evap lines, canister, rollover valves, etc have been replaced.

 

I'm hoping that the lifter rattle isn't indicative of something that could result in vacuum fluctuations, though I'm probably going to end up pulling the head to investigate sometime next week.

That said, I'm using the donor XJ's HO IAC valve in the HO throttle body.  I'm willing to bet the valve is at fault, though is the HO IAC valve compatible with the Renix ECU?  I know the HO valve mounts to the block adapter, but I believe the valves themselves have the same bolt spacing and are visually identical - should I be using a Renix valve in the HO adapter?  I see different PNs for '87-'90 and '91-'97 valves, so I'm tempted to dig out and clean off a Renix IAC.  Thoughts?

 

Thanks!

Bob

[HOrnbrod]

Yes, I would stick the Renix IAC back on it. The Holley 543-105 Idle Air Motor is a known substitute at about 1/3 the cost of a Mopar IAC, and only works on the HO engines. So that lead me to assume the Renix and HO IAC motors are not compatible.

 

To test the MAP sensor output (middle pin) you can use a large safety pin and slide it into the back of the weatherpack connector shell where the wires exit. The engine needs to be running during the MAP output voltage test. Also check the 5VDC reference voltage with the connector off the MAP across the two outer pins with the ignition on.

[1987Comanche]

I believe the MAP 3 pin connector is identical to the TPS 3 pin connector.  In this case the TPS patch harness you've been using can be plugged into the MAP circuit, giving you the perfect spot to hook up a multimeter.