Help! Tbi Idling problem!

[espcamaro]

Hello everyone, this is my first time using this forum and it seems like it could be very helpful. I have a 1987 chevy silverado with a 350 tbi. It has a weird idle to it, it idles up and down. I've changed the iac, and map sensors. I've also changed plugs and wires, cap and rotor. I don't want to keep pouring money into on parts it doesn't need. I've checked the timing and it's dead on. Also, when I'm driving it, it tends to bog down for a second at full throttle and sometimes backfires. I have a tbi spacer so could that be some of the problem? I think that everything? Any ideas would be greatly appreciated!
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[Wrecked1982]

you should have scann tool pluged and check for coolant,oxygen sensors, bad egr also check for codes

[espcamaro]

I went to oriellys to have it checked but their scanner was broke.. Just my luck.
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[Jabes1]

Sounds like a lean condition. I'd start by changing the fuel filter. How old is your fuel pump? Check the fuel pressure...

[espcamaro]

Fuel pump is only a couple years old. Ill check the both anyways though. Thanks for the ideas!
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[Sabaka454]

Check for vacuum leaks around the throttle body

[pontgta]

When you changed the IAC, did you perform the learn procedure?.

[espcamaro]

Not that i know of, what's the learn procedure?
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[pontgta]

Posted RFmaster orginally:

http://67-72chevytrucks.com/vboard/s...d.php?t=515419

1. Install a jumper wire between Pins A & B of ALDL diagnostic connector.
2. Turn on the ignition (but don't start the engine)
3. Wait 30 seconds.
4. Leave ignition on and disconnect the IAC connector (four wire plug).
5. Remove the jumper wire from the ALDL connector.
6. Start the Engine. The engine probably will run rough and may stall. Just restart and try again.
7. Adjust the throttle stop screw to set the idle to 500 RPM +/- 50 RPM. - stock camshaft!
8. Turn the engine off and reconnect the IAC connector.
9. Turn ignition on and adjust TPS to proper settings. (0.60 - 0.65Vdc at idle is typical, but 0.3 to 0.9Vdc will work)
10. Repeat procedure until the 500 RPM +/- 50 RPM (stock cam) in step 7 can be maintained while TPS is at proper idle setting. Performance camshaft may require higher idle RPM!

[walkerscott]

backfiring OUt tailpipe or intake

What was the timing set at?

place your timing light on cylinder #1 and check injector spray pattern a dial back is best

its possible that the hose between pump and feed line in tank is leaking the methanol eats the rubber

[espcamaro]

@pontgta


This is my first tbi so sorry for the dumb question, but why must that be done? When I put the new one in its not working correctly?
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[espcamaro]

It's backfiring out if the intake. Timing is set at 0 degrees. I've read that a wire needs to be pulled from the distributor to change timing or the computer will correct it. Is that correct?
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[pontgta]

Originally Posted by espcamaro

@pontgta


This is my first tbi so sorry for the dumb question, but why must that be done? When I put the new one in its not working correctly?
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Well basically it controls your idle.... It needs to know where the needle is set at... I know if you do not do the "relearn procedure" it will not idle or run correctly.

[pontgta]

Originally Posted by espcamaro

It's backfiring out if the intake. Timing is set at 0 degrees. I've read that a wire needs to be pulled from the distributor to change timing or the computer will correct it. Is that correct?
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Yes the wire ...It should be a brown wire near the firewall. Then just set to what your sticker represents ....It will look out of place and should look like this.. http://67-72chevytrucks.com/vboard/s...d.php?t=495662




got form http://67-72chevytrucks.com/vboard/s...d.php?t=495662

[cmax87v3500]

sounds like an issue two rigs i know of had, one is my truck, and one is a friends, both fixed with fuel pumps. does your truck do it on both tanks? i use one tank regularly, and went to replacing sensors until i switched tanks one day and the idle smoothed out and it ran better. turns out my passenger side tank was way low pressure wise. id try that if u haven't already.

[walkerscott]

PONTGTA is right on yes you must reset IAC ..
basically the motor is dumb and does not know were to stop or open
In resetting the IAC the pindle seats into the TBI body and closes off the air bypass for idle fluctation control.

example when you place truck into gear assuming Auto when you turn on air conditioning in both cases the idle speed motor adjust the airflow around the throttle blades to increase idle .. it does many other things but this is just the simplified version


verify if someone has adjusted the air idle screw(torq) the factory Tb would have a plug installed ..
If the screw has been adjusted which most are then you should perform and adjustment of the screw.

DO you have acces to a scan tool to verify the IAC counts .. for the final idle screw adjustment

[walkerscott]

timing at idle would be ~15+ degrees if you did not disconnect the EST plug as shown above