94 3800 won't idle up

I've tried the search, but it's not very good on this site. I have a 94 TransSport with the 3800. It's got about 150k miles, smokes a bit, and is ugly. I bought it to let my aunt drive as she tears up any vehicle she gets in. It has several problems. This is the most aggravating. It will start when cold and usually idles if you don't do anything. If you try to drive, it drops RPM and dies. I can keep it running by holding the throttle slightly. My aunt can't figure it out. Once the temp reaches 180 or so it will usually run okay, but occasionally it still stalls. I have tried cleaning the IAC and I thought it helped, but it didn't. I just pulled the throttle body to clean everything, but I haven't put it back on. What can I check before I put it back together? Where should I look to fix my idle situation? How do I adjust the IAC pintle position?

Simply retract the IAC pintle fully so it doesn't get damaged when it is installed. The ECM will step the pintle to wherever it has to go to control idle. It is also important to clean the idle air passages in the throttle body before installing the cleaned IAC.

Removing the pintle and spring, cleaning the rack gear, then lightly lubricating everything with clean engine oil is usually all it takes to restore the IAC. Make sure the spring length is correct before assembling the IAC pintle into the stepper motor. IIRC, the spring should have a free length of at least 2-3/16".

Trottle Positin Sensor can be a factor in this problem. It has been so in several cases. Do you have a code set? If TPS is the culprit, you will normally loose 4th gear, TCC and cruise control. Can you verify that?
TPS is a variable resistor and since it is part of the throttle body, you can verify with an ohmmeter. YOu sould have a continuously variable resistor value from a ferw hundreds ohms to near 10000 ohms..without jumps or misses. It is a common problem.
www.avigex.ca/xport/tps.jpg
www.avigex.ca/xport/tpsloc.jpg

Simply retract the IAC pintle fully so it doesn't get damaged when it is installed. Removing the pintle and spring, cleaning the rack gear, then lightly lubricating everything with clean engine oil is usually all it takes to restore the IAC. Make sure the spring length is correct before assembling the IAC pintle into the stepper motor. IIRC, the spring should have a free length of at least 2-3/16".

How exactly do I retract the IAC pintle and remove it? I had someone tell me I can just push it against a hard surface to retract it. That didn't seem to work. Right now it seems to move freely, but less that 1/8" from a contact point to a contact point. But then it just stops. The pintle position doesn't really change.

As far as the TPS, after lots of reading on this forum I've seen that it is a common problem. I have all that stuff apart right now for cleaning. I'll check the TPS for smooth resistance range. I've got a DVM handy, but I'll try to dig up my analog meter. I assume that I could see the sweep better.

I hope you didn't push very hard, since that's a good way to destroy the IAC. I also hope you don't rely upon that same source for medical advice...

The IAC is stepper motor which controls the pintle position. It consists of a pair of rack gears that rest against leaf springs. The rack gears are slightly staggered in their positions. The pintle has a mating set of teeth along its length. The two rack gears are actuated outward by the solenoid coils in the motor, and returned by the springs. If the coils operate alternately, the pintle is moved in a direction depending upon which coil/rack gear is operated first. To reverse the direction, the coils are operated in the opposite order. This is what the ECM/PCM does to control the IAC pintle position, and therefore, the idle RPM.

You can move the pintle by simulating the alternate operation of the stepper coils. If you are trying to extend the pintle, pull very lightly while you move the pintle side to side. This will alternately engage and disengage the rack gears and allow the pintle to "walk" out over them. It takes about 3-4 minutes to remove a completely retracted pintle that way. Installing the pintle and retracting it into the stepper motor is done the opposite way, but lightly pushing the pintle inward while rocking it side to side against the rack gears. Again, it may take 3-4 minutes of this to completely retract the pintle.

This is a crude diagram showing a similar (single rack) stepper drive:

http://www.youtube.com/watch?v=sq9fRpJ1U4g&NR=1

It becomes plain to see why pushing hard is not a good idea.

Thanks for the advice at resetting the pintle. I'll give it a shot. Is there a way to confirm that the IAC works electrically?

I've asked 2 of my trusted local mechanics about retracting the IAC. They both said that they aren't aware of any way to do that and I should just replace it.

THe IAC motor on the transport is rotary, not linear. As shown in this diagram
www.avigex.ca/xport/fuelcontrol_iac.jpg,
the stepper motor is acted on by alternately applying voltage pulses to windings one after another and in combinaison of voltage polarity to decide which way it rotates...somewhat complicated to do manually ...like a,a+b,b,-a,b-a,-b......takes a computer. some illustrations here
http://en.nanotec.com/steppermotor_animation.html
http://en.wikipedia.org/wiki/Stepper_motor
You can hardly push to change position as it would be equivalent to slide a nut straight on a screw . A stepper motor rarely fails electrically, but it can be mechanically blocked by dirt or corrosion.....or the commands to the motor are not forwarded for external reasons like other flawed sensors that give false info to the ECM.

LMP, Then how can I reset this thing? It acts like the IAC isn't controlling the idle. It may be something else, but I want to do all I can while the throttle body is out so that I don't have to take it out again. I've cleaned it with carb cleaner and it's all nice and pretty now.

Is it advisable to change the throttle screw setting at this time? I guess I could get it running by doing that, but I don't want to open a can of worms by messing something else up.

Here is what the GM book says :
Before installing IAC valve, measure the distance that the valve is extended. Measurement should be from motor housing to end of cone. Distance should be not greater than 28 mm (1-1/8”). If the cone is extended too far, adjustment is required as damage may occur to the valve when installed.
Adjust IAC by manually compressing the pintle to achieve the correct length. (they show just pushing with your thumb. my remark: so obviously the motor steps are coarse enough that it does not resist outside force. My mistake in previous post. Then if it cannot be done, then there is some blockage ; if it can be done and your idle speed is not OK, then there is a problem with step 3 below))
1-Install IAC valve to throttle body. Use O ring. Tighten to 27 lb.in.
2-Install electrical connector
3- The ECM will reset the IAC when the switch is turned ON then OFF.
4- Start engine.

IS the "check engine light" turned ON ?

UPDATE: The IAC didn't rock or push. It SCREWED in and out. Now I feel like a dummy. I screwed it all the way in, which may have been a mistake. It ran, but the idle was high. Sometimes as high as 2200-2500RPM. It would start an idle at about 1100 and drop to 900 in gear. But as it would sit there and idle, it would gradually increase.

Anyway, all seemed good enough for a test drive. I drove it several miles and had no codes/service engine lights, had overdrive but it seemed to shift late. Then all let go. I stopped and checked under the hood expecting to find the old cracked belt flopping around. That was good, so I started it back up. Knocking and clattering, it sounded awful. It never had the best oil pressure, so I figured the engine was a bit tired. I guess a bearing finally let go or even a rod or piston. Lots of blow-by now. I can remove the oil cap and the idle even changes...and I create a nice oil mister!

What's the procedure for pulling the engine?

..mmm...too bad it turned out that way....I do not have the full procedure scanned already....but here is the result you should expect....
www.avigex.ca/xport/IMG_3219s.jpg
(picture thanks to Tom Van Hooft, in the Netherlands)

So jack up the body and drop the subframe? I suppose that's the quickest and easiest. I've got a friend that will sell me a used longblock for $100. He bought it to go in a buick regal or something, but there were so many little things that were different other than the wiring harness and throttle body. The van isn't great, but I'm wondering if my best bet at this point is to save the money, scrap the van and put it all toward a better vehicle for my crazy aunt. $400 scrap+$200 savings from not messing with it anymore+$200 in her shoe=$800 to go to a better vehicle.

I'm also in a time crunch. Remodeling a kitchen, bedroom, and the wife is having another baby before mid December. I'll have to stick the aunt in one of my other vehicles.

THe IAC motor on the transport is rotary, not linear.

That's odd. The IAC in my 3.8 Buick SFI is linear, as evidenced by the keyed ways for the pintle to ride through:

http://www.wwdsltd.com/files/IACCompare.jpg

Then again, it's a '95, not a '94. However, every IAC I've had or worked on in GM cars from 1984-2001, including a '93 3.8, had a two-phase linear stepper as the IAC (except the throttle-by-wire systems). I doubt that GM would have changed the IAC in 1994 then changed it back in 1995, especially since my '94 B-car is also a linear.

That's exactly what mine looked like. The only difference I see is that the keyed part for the pintle was worn. I guess I really messed it up. I screwed the pintle right on down! Keyway turned to round hole. I'm still debating the engine fix. This morning I bought an S-10 for my crazy aunt to drive. I've got so much else going on that I really just need to cut it loose and haul it to the scrapyard. I just hate to see a car go that way!

.mm..indeed...the threads on the stem are straight, not screw type as I thought. My mistake then. I was confused by the symbolic in the diagram and the shape of the actuator that matches that of a rotary step motor...

.mm..indeed...the threads on the stem are straight, not screw type as I thought. My mistake then. I was confused by the symbolic in the diagram and the shape of the actuator that matches that of a rotary step motor...

Nope. My threads were a screw type...like you thought. It's hard to tell in that pic, but I THINK it looks just like mine. I screwed mine in. Lefty loosey, righty tighty. However, it never bottomed out. I don't understand why. I think that I should have adjusted in to the specified length and then let the PCM do it's job. Instead, I was having to deal with the high idle. I don't really think that had anything to do with me blowing the engine though. It had low-ish miles, but always smoked just a bit, had pretty low oil pressure, and I always thought I heard a slight knock.

I bought my aunt a little truck. I guess this is going to scrap. Is there anything I should keep? Nevermind, I just remember a late last night call. A buddy with an engine and several GM 3.8L vehicles is willing to give me $200 and a late 70s Ford van to take to scrap.