1993 4.3 TBI emissions questions - S-10 Forum
 
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post #1 of 10 Old 04-25-2019, 12:47 PM Thread Starter
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1993 4.3 TBI emissions questions

Best I can tell my IAC is not functioning properly. I put in a new OEM unit last November. It ran OK but not great -- sometimes it was great.
Things I have done over the past 6 months since I had no history on the truck and it was no crank no start when I got it.
1) TPS
2) IAC valve
3)Distributor
4) Plugs, wire, rotor, dist cap. module under dist. cap, coil under dist cap, and battery
5) Engine coil
6) EGR valve
7) Knock sensor
8) Water temp sensor (both)
9) Water pump, thermostat, radiator hoses, belt
10) Changed oil, filter, trans fluid and filter, diff fluid
11) Fuel tank and pump, filter and rubber fuel lines (weren't alcohol friendly)
12) Rebuilt TBI
13) New harmonic balancer
14) MAP sensor
15) Heater core
16) New air filter and PCV valve
That's all I remember at the moment.
I have tried to set the base idle several times but I cannot get past the "give it 30 seconds,(KOEO) with A and B terminal shorted together on the OBD connector" for the pintle to withdraw. I set my pintle at 1" -- installed it, turned the key on then Key off -- jumped the connector, key on for 90 seconds, unplug the IAC valve, pull out the jumper and started the car. I don't have a tach but my guess is it idled between 1600 and 2,000 rpm.
Did this three times. Each time I removed the IAC and measured the pintle and each time it was 1 and 3/32 inches.
Is the IAC valve bad? Is there some way I can check the 4 wire plug for voltage or ohms or ??? When I plug the IAC in the truck idles normally -- I guess about 800 rpm's or so. At times it will "surge" or buck enough to be irritating. Sometimes at low speeds, sometimes highway speeds.
I would appreciate any insight or ideas.
Doug

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post #2 of 10 Old 04-25-2019, 02:54 PM
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Re: 1993 4.3 TBI emissions questions

you can check iac coil resistance. Base idle is computer controlled




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post #3 of 10 Old 04-25-2019, 04:08 PM Thread Starter
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Re: 1993 4.3 TBI emissions questions

Thank you. That helps me tremendously.
Maybe I called it by the wrong name. I was trying to identify the set idle procedure for the IAC. I had to remove that little plug over the screw on the front of the TB and with the IAC disconnected (and a couple of other steps), adjust the idle with that screw to 550-600 rpms if I remember correctly and then plug the IAC back in. It is a 4.3 auto.
Thank you,
Doug.
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post #4 of 10 Old 04-25-2019, 07:41 PM
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Re: 1993 4.3 TBI emissions questions

To the best of my knowledge there is no way to set or adjust idle on that truck and looking in the service info I have available to me there is nothing that indicates otherwise. There is an idle speed screw but this screw is adjusted by the manufacturer and is not intended to be tampered with. Idle speed is 100% computer controlled. To force the computer to reset the pintle valve, simply turn the ignition on for 10 seconds then turn the ignition off. The computer will find and learn the full closed position of the iac pintle and takes full control idle speed all on it's own.

Have you checked it for vacuum leaks? Was the surging problem present when you first got the truck running or has it showed up since then? What about carbon build up in the throttle body and iac passages? Check for that as well

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post #5 of 10 Old 04-25-2019, 08:13 PM Thread Starter
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Re: 1993 4.3 TBI emissions questions

I rebuilt the TBI twice and no carbon last time -- three weeks ago.
Here is the procedure I have been referring to:

The GM TBI (Throttle Body Injection) system uses a pair of low pressure fuel injectors mounted above the throttle body opening. Unlike a port fuel injection system, that sprays fuel into the intake manifold runner just above the intake valve, the injectors in this system spray fuel directly into the throttle body above the throttle plate. The air gap between the throttle body and the throttle plate is calibrated to allow enough air to pass by it for the engine to run at base idle speed. The IAC (Idle Air Control) valve utilizes the on-board computer to fine tune the idle speed according to engine conditions.

Step 1
Remove the wing nut on top of the air cleaner and lift the air cleaner off the throttle body to access the vacuum lines. These are attached to the bottom of the air cleaner. Twist and pull the vacuum off of the bottom of the air cleaner and lay the air cleaner aside. Plug the vacuum lines with golf tees or other suitable item.

Step 2
Remove plug over the idle screw located on the right front of the throttle body by using a hammer to drive a punch into the plug and then using the punch to pry the plug out of the throttle body. Discard the plug. Start the engine and let it run until it warms up.

Step 3
Turn off the engine and leave the ignition key in the "Off" position. Unfold a paper clip and bend it into a "U" shape. Push the ends of the paper clip into the "A" and "B" terminals of the ALDL (Assembly Line Diagnostic Link) connector, also called the diagnostic connector. In GM vehicles that use this TBI injection, the connector is a black plastic rectangular connector located under the driver side of the dash. The "A" and "B" terminals are located on the top right of the connector.

Step 4
Unplug the IAC valve located on the passenger side, rear of the throttle body. Start the engine. Insert a T-30 size Torx driver into the Idle adjustment screw that is located behind the plug removed earlier. Turn the screw to adjust the idle speed to 650 RPM +/- 50 RPM.

Turn off the engine and disconnect the negative battery terminal from the battery using a wrench. Reconnect the IAC valve. Reinstall the air cleaner and remove the paper clip from the ALDL connector. Reconnect the battery terminal after 5 minutes and test drive the vehicle

Haynes manual has very similar instructions.
Thank you for your help.
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post #6 of 10 Old 04-25-2019, 09:21 PM
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Re: 1993 4.3 TBI emissions questions

That's the minimum idle air rate adjustment and it only applies up through 1991 TBI engines iirc. Chilton and Haynes manuals are full of info that doesn't always clarify which year of vehicles it applies to. I threw out every haynes and chiltons I had because of this.

This is what ALLDATA says about your truck and minimum idle air rate adjustment. They buy their info from the manufacturer same as chilton and haynes, but its more specific to year, model, engine size etc. instead of the haynes/chiltons condensed versions in those books.



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post #7 of 10 Old 04-27-2019, 10:24 AM Thread Starter
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Re: 1993 4.3 TBI emissions questions

Any ideas why with the IAC plugged in the idle is perfectly normal -- probably 700 or so but if I unplug the IAC it goes to 1500 or more (no tach)? If I had a vacuum leak I would think the plugged in would be very high as well. Just cannot figure this one out.
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post #8 of 10 Old 04-27-2019, 01:52 PM
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Re: 1993 4.3 TBI emissions questions

Am I reading this wrong? If everything is hooked up as it should it idles fine, but when you disconnect the IAC it idles too fast? Am I missing something?
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post #9 of 10 Old 04-27-2019, 01:56 PM
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Re: 1993 4.3 TBI emissions questions

I'm thinking if it's not broke...don't fix it.

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post #10 of 10 Old 04-27-2019, 02:42 PM Thread Starter
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Re: 1993 4.3 TBI emissions questions

Well, I am feeling like I missed the joke. I was under the impression that with the IAC unhooked it would idle too slow and would need to be bumped up by the screw under the plug on the front drivers side of the TBI. By the comments you guys left, and I appreciate your leaving them, it is acting correctly. If that is correct, thank you. I thought there was something wrong with that.
Not being a technician (mechanic in my day) I have to rely on youtube, Haynes, and my copy (3 volumes) of the Chevy light truck manual. Sometimes not everything is in there. Luckily you guys are willing to fill in the blanks for the rest of us.
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