Hey Ya'll, I am still dealing with smoothness issues on my 3.4 automatic.

It idles okay but not great when in park and neutral but when in D, 2 and L the idles drop to right above 500 rpm. It doesn't ever seem like it is going to stall but the exhaust vibrates enough to hear it in the cabin. Sometimes the shifts don't seem to be as crisp as they should either but that is mostly after coming to hard stop and then getting moving again. Finally, when I come to a stop, the truck idles normal for a few seconds but then it falls to the low idle

What I know:
I tested the MAF, TPS and IAC and the resistance is within specs from the FSM on all three. The IAC seems to be opening and closing just fine when hooked up to the battery as well. I also jumped pins T1 and TE1 and the idle does increase to 1000rpm for 5 or so seconds.

What I know need to test
I am feeling like I might have a vacuum leak somwhere so I am going to test for that. While researching how to test, I read that I should "Disconnect the idle control motor". Does this mean I should unplug the IAC while spraying throttle body cleaner on the various hoses and connections?

Other than that, does anyone have any suggestions on what I might test? I am trying to do all that I can before I finally take it to the dealership just because I want the satisfaction of solving the problem myself, especially after spending so much time on this... Thanks in advance!

 

idk why they would have you disconnect the iac, but yes unplug it

check your normal tune up stuff too, air filter, plugs, wires, cap, rotor

 

I guess they suggested unplugging it so that it won't compensate for fluctuations in the mixture....

I do need to check the plugs but only because there are dark rings around the boots. I replaced the gaskets for the spark plug tubes a few months ago so I am wondering if they aren't sealing correctly... The dark rings are hopefully just the dialectic grease that I liberally applied to the boots though.
The wires are probably 5 years old but they seem fine to me.

Also, I used to run an Amsoil Air Filter. Has anyone seen oil cause permanant damage to the MAF? I have cleaned the MAF since switching back to a paper filter and the resistance does check out ok. I do not have any Check Engine Lights either.

 

Do you have the FSM? There is a step by step on this.
1. Air induction system
2. Electric throttle control system
3. PCV valve and hose

I would also check all vacuum hoses. Mine idles at 500 on P,D but goes to about 650 in neutral. I had a loose hose on the intake which I connected but that didn't fix it. I'm going to check PCV next.

 

The FSM I have is a scanned copy, kinda sucks for doing the step by step procedures when you can't easily jump to the diagnostic section. I think I will have to tough it out and see if I can follow the steps in order.

I replaced the PCV valve last night since I broke it while doing the other test It is a PCV from Autozone. I think I am going to buy a Toyota PCV.

I think I just really need to check for vacuum leaks tonight. Suppose I decide I need to replace some vacuum hoses, does Autozone carry hoses of the right type and diameter that I can cut myself? What kind of material should the hose be made out of?

 

If you get a new PVC valve...Get a PCV gromett,they are usually hard as a rock,and should be replaced with an new pcv valve..

I highly doubt that you have a vacuum leak...if you did it would idle too high.The IAC valve is what controls the idle along with the throttle body stop...

Have you removed the IAC from the trottle body and cleaned it on a bench yet?You will be suprised how gunky it can get...

another odd possibility could be the VSV eavp pruge valve is hanging open enough to richen up the mixture,but you would get an evap code with that...

I guess you could move the throttle stop a little bit to open the throttle plate a little more,Also see if someone has messed with it...It should be marked with yellow paint...I would not do this untill the IAC was cleaned on the bench.

 

Take the old hose with you and they should map up the diameter. Get one that is for the engine bay. This is regular stuff any part store should have.

If the PCV work you dont need to get another one. You should be able to blow air to the cylinder head side with ease. Blow on the air intake chamber side and there should be resistance. Thats all you need to check. Put a new hose and you are good to go.

Get a can of seafoam while you are at the part store. That should help too.

 

I am trying to think about what would make it idle down--and the only thing i can think of is maybe ----just maybe a crank angle sensor is a little to dirty....Knock sensors retarding the timing, or the IAC valve.

either way--it sounds like maybe timing is being retarted or IAC needs cleaning

Thats my .02$

 

Thanks, the good replies are really starting to roll in now.

The IAC is clean as it will possibly get except for the port that goes to the Air Assist Hose. That sucker is kinda hard to get at so I just soaked it in throttle body cleaner (without the solenoid attached) over night.

Hmm... I will definitely have to read up on the crank and knock sensors and VSV tonight.

And I am starting to agree about the vacuum leaks not being the culprit as well. I noticed a few times last night that when I would unhook a vacuum line, the rpms would rise which seemed counter-intuitive to me.

Finally, If I get enough free time over the next week or so, I am going to try and swap my buddy's throttle body and MAF from his 98' which runs very nicely to see if that makes a difference.

 

i have the same problem............toyota had to replace my idle control computer thing a couple years ago .....but now on my 99 the rpms drop occasionally.....sometimes in drive thru's it will actually cut itself off . but in the end it runs pretty good and that rarely occurs....let me know what is the culprit please

 

sweep the knock sensors also--i havent done it before--i would have to look it up...Just keep eliminating variables like you have and.....

 

hross, what exactly does "sweep" mean? Is that a technical term?

 

Yeah--like widgets

sweep just means to test them--i think the term comes from when TPS sensors were more important or the voltage of a sensor would "sweep" across the screen

So sweep means test

 

Cool... I figured that's what you meant. Well IjJust got back from dinner with friends and was looking over the FSM for getting to the knock sensors. This might be a lot of work so I'll probably just keep studying the FSM and try to get some quality work done this weekend.

 

dont try and take them out--there is a connector for them at the front of the motor. Find it and you should be able to test continuity and resistance (Ohms).

You can also hook up a scanner that lets you view the sensors and tap the side of the block with a hammer and see if the sensors are picking it up.

Do you see where i am going with my reasoning? Something is retarding the timing?

I could be wrong--but it would help you to eliminate variables.

Oh Check continuity of your connections to these sensors. What that is is checking to see that the circuit is complete...i.e... wire end A is connected to XXXX at wire end B.

 

Don't bother with the knock sensors,you are barking up the wrong tree...will have a bunch of symptoms besides a low idle...Plus the knock sensors are not going to retartd timing at 500 RPM's.car would be shuttering so bad it would not stay running....focus on the IAC..really

 

If the connectors for the knock sensors are easily accessible, that makes me much happier! From what I just read about timing, I think you might be on to something but figuring out the root cause will most definitely be hard for me. The only thing I can do is keep ruling things out, like you said, until I solve the problem.

 

Okay... Plus, the truck runs really well and pulls hard at all throttle positions so IAC does seem like a likely culprit even though it is clean and resistance tests out okay.

One thing I noticed is that in the FSM is says that the throttle plate should be at 10 degrees when "fully closed" but it looks like mine is really fully closed when at "fully closed". Do you think that might throw off the computer's ability to control the IAC correctly?

I really do think I will try swapping throttle bodies with my buddy before I go start testing knock sensors and other things.

 

The only way I know to test a knock sensor is hooking up a scope(graphing meter will work)..then tapping the side of the block with a hammer to recreate a "ping"..but you need a really fast meter to see this....Because knock sensors are "piezio sensors" menaing they need to see a "concusion" to generate low level voltage..ie a "ping"..then that voltage is sent to the computer and as long as that voltage is present the ECM retards timimg...But as soon as there is no voltage the ECM puts timing back to where it was....It cannot keep the timing retarded for very long or else vehical will overheat,cat can be melted down.........Good luck testing one..its hard as hell even for a professional tech at the dealer.......

About the fully closed throttle,its talking about the angle the TPS is seeing,you can see this with a scan tool..Basically if the throttle plate angle from the TPS is under 10 degrees that ECM opens the IAC all the way,The ECM will partially open the IAC if the angle is over 10 degrees,but generally not much depending on rpm,ect...and fully closed at WOT,which for the ECU is about roughly 70 degrees of TPS throttle angle.

That being said if your TPS is reading 5-7 degrees idle might be a little low,but generally fully opening of the IAC can get idle smooth,now 1-2 might be a problem..On the otherhand if the TPS is reading over 11+ degrees at idle the ECM might not be opening the IAC valve all the way ..causing low idle condition..

The real question is have you removed the IAC and cleaned it on the bench? and used a small screwdriver to work the sliding valve back and forth cleaning it?......If not this point is moot,it is the only way the ECM can control idle speed..
The IAC has a very weak motor with a return spring..any small piece of carbon/varnish will cause it to stick w/o codes...causing low idle

 

my 2002 3.4 has the same "issue" and had since i bought the truck with 52k miles. it has 120k miles now and it has never cut on me but its noticable on occasion.. esp at a stop light you feel it vibrate a tad but when you barely touch the gas its all good.. i would lean towards butterfly. but i dont know squat haha... maybe a spring issue dunno