I've been having an issue with my idle surging for the past few months. It showed up out of nowhere (not after any sort of work on the truck). Very sporadic; some days it would idle normally like it always has, then other days it would surge at idle with the surges getting progressively bigger until it almost dies.

First thought was the TPS, but I took off the TB and went through the TPS procedure. Everything checks out and there aren't any dead spots in the TPS reading at any point in the throttle.

I got code 2 (AFM) and code 6 (Ignition). I believe the ignition code was from a miss I was having at cold start up, but I will address that below. As for the AFM, I went through the checks as described by the FSM and it did fail one of the tests (E2-Vs showed open when it should be 20-400 ohm). Could this be my problem?

I cleared the codes about 3 weeks ago and they have not reappeared.

I just replaced my plugs, wires, cap/rotor because I was due for them. This fixed the miss I was having (the old plugs were about done). I also double checked the timing and it was fine. While I was under the hood I decided to mess with the idle adjustment screw. I pulled it out and cleaned off all the crud that was built up. After that I was able to adjust the idle very easily and set it to 750 (before it was running at 850). During all this tinkering the engine idled fine and had no surging.

Then I drove it to work today. Every time I let off the gas the RPMs would drop really low and then try to recover and surge around the idle point. Eventually it would drop low enough where it would die. Lost track of how many times it died on my drive. I may have the idle set too low, but when I did it in the garage it was at 750.

I'm not really sure what to try next except replace the AFM. I was told it could be the auxiliary air valve causing problems when there is an air bubble in the coolant. I did go through the procedure of 'burping' the system before. I also checked the temp sensor for the air valve and it gave a correct reading.

Uhg, sorry for the wall of text! Any help would be appreciated!

 

Replace your water temp sender, front under thermostat. 30$ part.

Ive been having that same problem and that seems to be the culprit. Too cheap of a part to not replace..

 

This morning before I started the truck I removed the air hose that goes from the IAC valve to the TB. I then blocked off the port on the TB to essentially take the IAC out of the system.

Starting cold was rough, as expected because there wasn't any air coming in from the IAC valve to raise the idle. It wouldn't idle on it's own so I kept on the throttle until it warmed up to operating temp and then it was able to idle on its own...no surging. Drove it all the way to work and didn't have any problem; letting off the gas the idle always returned to the same spot. A couple times it dipped maybe 100 RPM but it always returned and stayed at the same idle.

So I would say the culprit is something to do with the IAC valve. One thing I am confused about is how the idle can be lowered by the IAC valve since the IAC valve can only let in air in to raise the idle...can someone explain?

Is the IAC valve passive? Or does the computer control if it is opened or closed? I know there is an electrical plug on the IAC valve but maybe that is only for sensing?

If someone could explain how the computer works with the IAC valve that would be very helpful.

Thanks!

 

The idle stop screw is set to a minimum idle spec (see FSM for spec and adjustment) the IAC/ECM controls the idle speed from there. There's also a trim screw on the AFM but is factory sealed. It's under an aluminum plug on the AFM and usually needs to be adjusted after 60k miles.

 

When you say idle stop screw, do you mean the idle adjustment screw? You're not referring to the "throttle stop screw", are you? The idle adjustment screw regulates the air into the TB for idle, while the throttle stop screw just controls the amount of closure of the butterfly in the TB. I am sure you meant the idle adjustment screw, but just double checking.

So it seems to me that the IACV is a passive system, meaning the ECM is not actively controlling it...correct? The coolant runs through the IACV and when it is cold, the valve is opened (high idle). When the coolant gets warm, the IACV closes cutting off the air to the intake (lowers idle). If the ECM was controlling the IACV then there wouldn't be a point for the coolant to run through it.

The electrical connection to the IACV must be a way just for the ECM to sense the IACV position.

If the IACV is passive, how else does the ECM control the idle? The only thing left I can think of is the the ECM controlling the fuel (injectors).

Sorry for all the questions...having a better understanding how the system works as a whole makes it much easier to diagnose.

 

I've been doing a little more research on the IACV and it is making a little more sense. First off, the IACV is not completely passive. The IACV can be passively operated by the coolant temperature (coolant bypass) or it can be operated actively by the heater coil internal to the IACV (this is what the electrical plug is on the the IACV - a heater, not a sensor).

As far as when the ECU decides to actively heat the IACV, I am not sure about that. I can across a thread that kind of addresses it, but nothing was ever figured out for sure. Maybe the heater coil is operated based on engine temp or it is on a timer and then when the coolant gets hot enough the IACV just relies on that to keep it hot.

The IACV only controls the extra air going into the intake to raise the idle. Engine cold = valve open = more air to raise idle. Engine warm = valve closed = little/no air to bring idle down. If the IACV gets stuck open, the idle will always be high and I believe the surging comes from the ECU cutting fuel when it sees a high RPM but a closed throttle. So the actual surging is indirectly caused by the IACV...it's not like the IACV is constantly opening/closing to make it surge.

I'm not sure my problem is an IACV that is stuck open since those cases seem to have high idles (~1500) that surge (which makes sense - open IACV would have a higher idle). Mine was surging around and below the nominal idle...that and swinging below nominal idle after letting off the throttle.

Since my truck is running fine now (with the IACV air hose detached) I still think my issue is something with the IACV, so my next step is to pull the IACV and TB (and maybe the upper plenum) and make sure everything is cleared of gunk and the IACV is operating properly.

Still might be the engine temp sending unit, but I will check that again at different temps.

 

I may have got my apples and oranges mixed up. Usually when there's an ECM controlled IAC, the throttle stop screw or idle bleed screw has to be set at a minimum idle speed. Did you try removing the AFM trim plug and adjusting it?

 

Yeah, assuming the IACV is operating correctly, you have to adjust the idle adjustment screw when the engine is at operating temp (IACV closed). This is the minimum idle speed.

I haven't messed with the AFM at all. If I remember correctly, I discovered the issue with the AFM years ago when I was diagnosing another issue. I replaced the AFM at that time thinking it was my issue, but found out it wasn't. I returned the AFM because it was so expensive and installed the original and haven't had issues since. I'm thinking the reading that was out of spec isn't critical...not like the variable reading that the ECU uses to determine AFM position.

I'm hoping I'll find plenty of gunk and buildup in the IACV and related hoses/tubes. I did a top end rebuild about 40K miles ago (overheat - cracked head) and cleaned the plenum and TB really good. Don't think I ever touched the IACV so there's a good chance it is gummed up or just broke.

 

Quick question so I don't derail this thread.

When the engine is fully warmed up should the engine remain running if the Idle Speed Adjusting Screw (top of throttle body) is turned all the way in?
My engine rpm does slow down lower than I like but it won't die.


Please note that I first incorrectly called the -Idle Speed Adjusting Screw- by the wrong terminology. I called it the -Idle Stop Screw- which is wrong and is located in a different position.
jstluise quoted me and as a result this thread could be derailed with confusion. This is why you generally don't ask questions in someone else's thread.

 

I'm sure someone else will chime in here, but I've read that the IACV does let a tiny bit of air through when it is fully closed; however, I am not sure if that is enough air to keep the engine running after the idle adjustment screw is all the way closed.

What is the lowest RPM you can get with the idle adjustment screw all the way turned in? If you can't get below 750 RPM I would suspect something is up, possibly the IACV isn't closing all the way.

 

From what I've read (no experience with it) when the IACV is plugged with RTV or FIPG it will just make the idle surge. It won't create a condition of it almost stalling out like you reported earlier.

 

This is what I have read as well. A plugged up coolant line for the IACV will not allow the IACV to come up to temperature and close all the way, thus increasing the idle. The ECU sees a warm engine and a closed throttle, but high RPM, so it cuts fuel to compensate. This is where the surging is.

Still not sure how the ECU controlled heat coil in the IACV comes into play...must not play a major role otherwise we wouldn't need the coolant bypass and people wouldn't have the surging issue.

So yeah, I haven't read about it causing things to stall out like my case it. But, since I disconnected the IACV the engine runs great (no surging, consistent idle)....the only problem is starting and running while cold until it comes up to temperature.

 

I don't have a tachometer that I can hook up but it can idle lower than I like. I have the Idle Speed Adjusting Screw turned 1/8 of a turn out from seated.

 

One way to check the operation of the IACV per the FSM is to pinch the air hose once the engine is warmed up. If the IACV is operating correctly (closed when warm), then the engine RPM should only drop by 50 RPM when the hose is prinched. If the IACV is stuck open, pinching the hose will cause the engine RPM to drop significantly and maybe die.

On start up do you have a high idle that slowly decreases as the engine warms up?

 

Yes, warm up seems normal.

 

I would test the IACV per the FSM.

1) At cold startup, pinch off the IACV air hose. The engine RPM should drop significantly and maybe die.

2) At operating temps, pinch off the IACV air hose. The engine RPM should only drop about 50 RPM.

My idle adjustment screw was turned pretty far out to get the correct idle speed. I suspect you should be able to really bring the idle way down and maybe kill the engine with the idle adjustment screw...I would have to play with mine to confirm that.

If the IACV checks out, maybe you have a vacuum leak somewhere that is bringing the idle up.

 

.....

 

I think that's what might be going on with mine. Totally theorizing here...

I'm pretty sure the previous owner installed an aftermarket 180 thermostat instead of the factory specified 190. As such it has a single opening and will cycle open and closed depending on the temperature.

Today I sat in a drive through line for quite a while. I began to notice my engine rpm change from time to time. When the temperature gauge would DROP to a certain point the rpm would INCREASE and when it got warmer the idle speed would decrease. That's the opposite of what I would expect. Keep it in mind this is the newest most advanced lol vehicle I've ever owned.

The rpm's seemed to be pretty regulated because it kept repeating the cycle. Because of these same repetitions I wouldn't consider it a surge but an automated action. I think if my engine had the special 190 degree Two Stage thermostat these cycles would be less noticeable.

The engine would be kept at a slightly higher temperature and the regulation would be more accurate keeping the engine out of the colder idle up stage more often.

When cooling demand is low, cold weather, idling, etc, the big opening in the thermostat would remain mostly closed to keep engine temps up and alot of the regulation would be handled by the smaller opening.

When cooling demand is higher, hot weather, driving, towing etc, the smaller opening would remain open and the larger opening would do most of the temperature regulation.

 

Well, IF it is the IACV causing the swings in RPM you are seeing, then the pattern is what I would expect: when the temp drops, the IACV opens slightly and allows airflow, causing an increase in idle. When the temp rises, the IACV closed and lowers the idle. However, I'm curious if the IACV can even do this with the temperature swings you are seeing. I guess it just depends on the temp at which the IACV fully closes. For example, if the IACV doesn't close until 180F then I could see small temperatures swings affecting the idle. What is the magnitude of RPM swings you were seeing? And what was the frequency of the swings? (every 5 minutes? Ten minutes?)


I wouldn't either. The surging that people see from a faulty IACV happens rapidly and (I've read) can surge from 500 RPM up to 1500-2000 RPM.

Also, I played with my idle adjustment screw a little bit after my drive home tonight. I was idling at about 850 RPM so I figured I would bump it down to 750 RPM just to see how it ran. It took maybe a 1/2-1 turn to lower the idle 100 RPM, and the screw is no where near seated. So, by you saying your idle adjustment screw is 1/8 turn out from seated, I would say something is not quite right. But, if you have the normal high idle on a cold start, then the IACV might be working somewhat. Have you checked out the TPS? I learned a long time ago that the TPS is one of the first things you should make sure is operating correctly, as it can cause a lot of problems that don't necessarily point to the TPS. How about any codes being thrown?

 

I didn't keep track of the rpm deviations on a minute to minute basis. To me it looked to be triggered by temperatures and not time.

The rpm fluctuations were somewhat similar to what you would expect when you turn the headlights on and off at an idle making the alternator work harder. Wish I had a tachometer but I don't want to give some idiot an excuse to break in or vandalize my truck.

My engine has never done the wide sweeping surge that some people have a problem with unless I caused it.
It did it when I first changed the coolant and didn't bleed it enough and it did it when I advanced my timing from stock specifications.
Both were easily remedied because it didn't do it before I made the changes so it was pretty obvious in those regards.

I haven't checked out my TPS yet because I'm waiting on a multimeter to be delivered.

My only code is 42, it's related to the Vehicle Speed Sensor. From what I've read about that fault is that the engine will all of a sudden drop all power and then very quickly recover, then act totally normal again. It has definitely done that a time or two.
I changed my battery leads around a little two days ago so I expected it to clear that code. I've only put about 20 miles on it since then and haven't used my cruise control. Out of interest I just checked my codes and nothing is showing up.