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Testing the AIC is difficult for the DIY mechanic because all of the formal testing requires a bidirectional scanner to command the valve to various positions. Also, this is a stepper motor with two coils and two control circuits sending out square wave signals. You can monitor for activity on the control lines with an led test lamp but you probably have that if unplugging the AIC causes the rpm to change, IOW it is being commended to at least some degree by the PCM. These valves go through a reset at each key off and on cycle where the valve finds the fully closed position before being commanded for the idle position. You can watch for this sequence although I have never done this myself to describe the behavior. Another thing you can do is measure the resistance of the two coil windings to see if they make sense.
I am not familiar with how to adjust the idle screw back to factory specs. Was the metal cover over the screw violated?
Just for GP, this is a thread I did about 5 years ago from the IAC on my truck that shows what the waveform looks like on a scope and explains what's happening during a key on/off event. Note it is from a 4.3.
Had a thought. If this high idle is the result of the idle screw having been violated and the IAC is working correctly then the IAC valve should be in the fully closed position trying to wrestle the idle back down to target at idle. It is curious that you are not getting a P0507 code for high idle which gets triggered when the actual idle is 200 rpm above target. That rpm of 900 that you quoted, is that from the dash gauge, scanner or both?
You really have to convince yourself that you do not a vacuum leak so that you are not chasing ghosts here. Your fuel trims suggest no leak but you should make sure: brake booster, HAVC, 4wd, etc, pvc, LIM gasket, etc.
the 900 +/- is from the scanner. I tried unplugging the IAC again, and it did not make a change in the idle. The cold stumble still seems to be there. These TBs don't have a cap over the adjustment screw. At least the half dozen I looked at on ebay don't.
I'm not going to put the time or money into testing the IAC since it seems questionable to me at best. I'm just going to do the swaptronics and see what result I get.
I want to share a bit more bit of information that I did not mention earlier because I attributed it to the truck being old and the cleaning of the injectors that I did. The issue is that the truck runs poorly for about 10 seconds when cold at the first start of the day. After 10 seconds, everything evens out and it runs fine. Also, when I turn on the A/C, I get a big dive in the RPMs before they catch up and bring the engine back to idle. The truck is new to me and I just got it running, so I don't know if this is "normal" for the vehicle or how long it has been going on.
With the TPS replaced and no vacuum leaks (I checked a third time), I'm guessing the IAC must be the problem (or a problem) since its about the only thing left in the circuit. I'm wondering if it is weak and can't move fast enough or far enough to do what its supposed to be doing. When I had it unplugged this last time, it was for around 30 seconds and the idle did not change. When I unplugged it last week, it took maybe 15 seconds before the idle shot up. I'm hoping that it was just in the process of failing, but I really don't' know. I presume its active when the car is started and when the AC gets turned on and if so, the evidence seems to indicate its bad or going bad.. Or I may have its function wrong.
Last edited by jacked_72; Jul 10, 2023 at 09:15 AM.
The TPS replacement was reasonable because of the output glitch you saw mid travel. Anytime you have trouble with a sensor and its function it can be the sensor, power, ground, the module it talks to and/or the associated wiring. I would go back to the TPS sensor and measure the reference voltage across the 5v ref and conditioned ground and make sure thats solid throughout travel and that the output has smoothed out.
Regrading the AIC, given that your confident in no vacuum leaks, that sensor and its circuits are definitely suspect. As mentioned earlier, complete testing for the shadetree mechanic is difficult on this sensor. I see two options. One is trynostic and the parts cannon as you mentioned. Sometimes thats just the play, especially if you dont have a $4k Snap On scanner. The other is to grab an led test light and make sure that you at least have some pulses on the two wire pairs that control the stepper motor. I don't say this often but an IAC replacement (parts cannon) is reasonable.
If that does not solve your problem then I would watch the IAC at key off and key on to make sure that movement is commanded, then start the truck and if the idle is still high, pull the IAC connector while its still running so you freeze its position, then take it off and if its fully closed then adjust the idle screw to taste.
George
Last edited by GeorgeLG; Jul 10, 2023 at 01:01 PM.
I don't have the issue solved yet, but it is definitely something to do with the IAC valve. The new one yielded the same result. Messing with the throttle screw does nothing unless I crank it so the throttle plate is open. If I plug or mostly plug the port where the air input to the IAC chamber enters, I can manually get the truck to idle in the 600 rpm range. There is a pretty strong vacuum in this area. The pintle definitely moves, so the computer has control of it, just not enough to seal the air bypass port sufficiently. Again, I'm seeing no change in rpms by soaking the area with brake clean so I don't think it's the o-ring on the IAC.
I figure that the computer is commanding the TPS into a negative degree value to close the throttle plate as much as possible, which it is doing. I also figure that either there's something wrong with the throttle body itself and I didn't get it clean enough or the pintle on these IACs are incorrect for this throttle body. I see that they're selling one with the top hat looking pintle, but I've measured the bore and the size of the pintle and they should seal. The FSM material that I have indicates that there are different size and shape pintles and to replace the IAC with an identical size and shape, but I have no idea if this thing has ever been replaced. I think the next move is probably to pull the throttle body off again and to give it a good once over in this area.
Beyond that, I'm not going to throw another IAC at it, but may pick one up at the next junkyard trip just to see what other vehicles have.
Edit: I'm wondering about my map sensor. With the engine at idle it reads 40kPa. At key on engine off, it reads 100 kPa which is close to atmospheric pressure of 101. I see from the internet that 27 kPa is a good reading at idle. I have no idea if 40 kPa at idle is in the ballpark.
Also I found a PID for the IAC counts. It ranges between 20 or so to 70 depending on engine heat and whether AC is turning on or off.
Last edited by jacked_72; Jul 15, 2023 at 11:02 AM.
This is the damndest thing. This car just wants to idle at around 900 RPM. I have confirmed that the IAC can close. I have taken the throttle body of the car and videoed the pintle fully seating. I adjusted the idle screw with the TPS in and with the engine off. I adjusted it to zero, it had been more than -3. This opened the throttle plate slightly. When the engine computer took over, it put the IAC counts down near zero, meaning it was closing the IAC fully. Still 900. I put in the other TPS and it shows an even more negative number. So I try to adjusting the throttle stop screw while the car was running and still 900 rpm. I am sure there's no vacuum leak under the hood.
I'm posting a couple of pictures showing what things are doing if anyone is interested. I'm wondering if the downstream oxygen sensor is a little weak since it doesn't seem to fluctuate as much as the upstream, but the fuel trims generally look good. I haven't put in the new map sensor yet. It's almost like this car's idle is set in the computer at about 900. I suppose somebody could have flashed it or something. But I'll be damned unless I stick my finger in the IAC Port if I can get the item below 900 rpm.
The question is whether the computer is asking for this idle or if it’s asking for a normal idle but that cannot be achieved. Here is a good article related to some of these questions. If the computer cannot achieve the desired normal idle then the throttle plate would be commanded fully closed and the AIC count should indicate a fully closed pintle. If the this is not true then the computer most likely wants this idle.
Computer cannot achieve proper idle:
Are you sure that the aic pintle seat is clean and in good shape?
Sounds like your convinced that the computer is commending different pintle positions
If you fully close the idle screw do you still have the problem?
If you disconnect the throttle cable and let the plate fully close do you still have the problem?
Sounds like your convinced of no vacuum leak.
Sounds like you have ruled out the idle screw.
Computer wants this idle
Does the ECT agree with ambient on a cold engine, key on engine off?
The map sensor under the same conditions sounds reasonable.
Does any of this change if you unplug the maf sensor and then start the engine?
It sounds like the system responds properly to AC on and off?
I need to look at if this system looks for a power steering input for idle control but I’m on the road today
Are you sure that the aic pintle seat is clean and in good shape? Yes. I can even see a ring on the pintle where the seat has made an impression.
Sounds like your convinced that the computer is commending different pintle positions. that is my current thought. It seems that if the throttle plate is slightly open, the IAC is fully closed. If the throttle plate is fully closed, the IAC opens. In one case IAC has 0 counts and 20 counts in the other.
If you fully close the idle screw do you still have the problem? yes. On mine, the counts were going a little less than 20 when the idle stop screw was adjusted so that the tps was set at about -3.5 (at this idle screw setting, it was not touching the stop on the throttle spring lever, meaning throttle plate was closed as far as it could be). Adjusting tps to 0 (which was a couple of turns) opened throttle plate a little bit and iac went to 0 counts, meaning the computer fully closed it. Still had 900 rpm idle.
It is possible that making the tps adjustable by grinding off locating pin I could set the tps to 0 and still have the throttle plate fully closed. I'm betting the computer would open the iac and still make it run at 900 rpm.
If you disconnect the throttle cable and let the plate fully close do you still have the problem? same result. Throttle cable is currently disconnected.
Sounds like your convinced of no vacuum leak. i Am. Fuel trims seem to support this. There is nothing under the hood that is leaking. Possibly EVAP could be leaking under the truck somewhere, but I did plug the ports one by one earlier and no change. I'm going to plug all at the same time and see what happens. Edit: if I go through intake side of engine with propane, I can see a 1-3% change in short term fuel trims for a second at times. It does not make any discernable change on engine speed.
Sounds like you have ruled out the idle screw. I think so. See above.
Computer wants this idle
Does the ECT agree with ambient on a cold engine, key on engine off? it is close. It looks like it's reading 5-7 degrees high
The map sensor under the same conditions sounds reasonable.
Does any of this change if you unplug the maf sensor and then start the engine? I'm sure you mean MAP. No maf on this engine. There is anitial jump in rpm then it evens out and change to idle if unplugged.
It sounds like the system responds properly to AC on and off? idle jumps up to maybe 1300 or so when it is turned on or off and then goes back to 900.
I need to look at if this system looks for a power steering input for idle control but I’m on the road today. I doubt that's it. I pulled the DLR relay thinking it might be a current draw issue and there was no change.
Have you done a compression test recently? yes, it was good. Vacuum at TB vacuum port was about 20 in-hg.
Last edited by jacked_72; Jul 16, 2023 at 12:15 PM.
What happens if you plug the inlet to the purge solenoid for the evap system?
Do I understand that if you remove the IAC and plug the IAC port at the seat you get the desired RPM? If so, does that not imply that teh IAC is not fully seating even at the correct IAC counts? Have you made sure that you have pulses on all four IAC control wires?:
