IAC Question

[Atrais]

Just my two cents.

I bought a 99 montero that had been sitting a while. It had a similar issue. idle bounce. Once the vehicle warmed up though the bounce pretty much cleared up entirely. From a cold start the idle would go to around 1200rpm and slowly drop as it warmed up as it should... but once it got down to about 900rpm or so this idle bounce would start.

I checked so many sensors one by one. Of course the IAC was early on that list. I got on the phone with an engineer in Texas who rebuilds IAC motors and computers for these Mitsubishi's and a few other manufacturers. His technique for testing the resistance of the IAC was to test from the center pin to the left and center pin to the right on each row. Then compare that number to the resistance on the outer two pins of each row (skipping the center pin). The outer pins resistance should be roughly double. These can also fail mechanically but apparently this is much less common.

I tried a new IAC anyway just to be sure and it didn't help at all.

In my case it ended up being a MAF sensor. I tried to go cheap and get one off ebay but it too was off (cleaning didn't help). Ended up with a rebuilt one and it works perfect.

Check your fuel trims to see if a clear pattern presents itself. My old MAF was stuck reading lean causing extra fuel to be added and thus an ugly bounce. Fuel trims got a little better under load. Points to a vacuum leak but I ruled that out.

Maybe this helps or maybe just incoherent gibberish.

Either way best of luck!!

Ah, this is actually a great point. I've been focusing a lot on the IAC as it seems to be something of an issue on these platforms but I hadn't thought to check the fuel trims. A MAF issue may actually make more sense in my case now that I think about it. Mine feels more like a miss at idle than any kind of idle surge (there's no discernible change in RPM according to the tach when it happens). I'd ruled out a vacuum issue with a smoke machine and the ignition side of things is new, but a MAF reading incorrectly could definitely do it if it was adding too much ST fuel trim. Thanks for the heads up on the ohm readings too, I'll have to check that tomorrow as well!

Sounds like Painted Mountain may be on to something. I doubt the IAC would be that "off" at the readings of 44 ohms, particularly if the outside pins are roughly double what the inside-to-outside pin readings are. Mine was about 44 ohms as well. I cleaned (carefully) the passages in my throttle body and reinstalled my old IAC and left it alone (95 SR with the 3.5 DOHC). I too would be looking more at the MAF since it has so much control over most aspects of fueling when the vehicle warms up.

John B.

Agreed, I'm definitely going to check it now before I change anything.

 

[Salonika]

Yes, I'm talking about scan tool base idle lock, which allows you to properly set the idle air bypass screw. My scan tool won't do it. Also, it was my understanding that with OBD2 systems using both a MAF and MAP sensor, the MAP sensor is only used to verify that the EGR system is functioning. I don't think the MAP sensor data applies at idle, but I'm not 100% sure. My fuel trims were way off at idle (they would peg at 25% positive after about 1-2 mins). I have a spare MAF sensor that I swapped in during my troubleshooting and it behaved exactly the same way. I ruled it out.
My rough idle condition was not visible on the tach. My engine wasn't "hunting", it was running at about the spec RPM, but it was shaking a little every now and then. It would shake the 4x4 shifter knob a lot.

 

[nwoods]

So have you cleaned the IAC yet? I cleaned mine (very easy to do) and it was a big improvement.

 

[Atrais]

Yes, I'm talking about scan tool base idle lock, which allows you to properly set the idle air bypass screw. My scan tool won't do it. Also, it was my understanding that with OBD2 systems using both a MAF and MAP sensor, the MAP sensor is only used to verify that the EGR system is functioning. I don't think the MAP sensor data applies at idle, but I'm not 100% sure. My fuel trims were way off at idle (they would peg at 25% positive after about 1-2 mins). I have a spare MAF sensor that I swapped in during my troubleshooting and it behaved exactly the same way. I ruled it out.
My rough idle condition was not visible on the tach. My engine wasn't "hunting", it was running at about the spec RPM, but it was shaking a little every now and then. It would shake the 4x4 shifter knob a lot.

Hmm I'll have to check that then, that's almost EXACTLY what mine's doing. My friend has my scan tool right now so when I get it back I'll check the trims and if need be reset the idle.

You are correct that the differential pressure sensor is only used for EGR flow monitoring. I deleted the EGR on mine after having multiple issues with the system and I use the output from the DPS (modified with a PWM sin wave oscillator) to keep the ECU from running the monitor to verify if the EGR is working correctly (and thus setting a code for "EGR insufficient flow").

So have you cleaned the IAC yet? I cleaned mine (very easy to do) and it was a big improvement.

Just cleaned it along with the MAF today (also tested the MAF and it's good). I reset the ECU and am letting it warm up and supposedly learn the idle right now before taking it for a drive. I also have a body lift and a snorkel to install so this is kind of killing me haha.

 

[nbishop]

How did this pan out? I am experiencing a similar rough idle once the engine is warm. Checked the IAC readings, they were right at 39 ohms.

 

[Atrais]

I cleaned it out and replaced the O-ring. It improved a little in that the frequency that the rough idle has decreased but when it does the roughness is the same. Again, it's odd because the rpm's never fluctuate but when it happens it feels like a miss.