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TBI throttle

carpeonnel

carpeonnel

1/2 ton status
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Never thought much about it but my 89 idles in park at about 1300rpm approx. It runs great and it's been like this since I bought it 4 years ago. Never had a problem with it and it runs great, but is a TBI supposed to throttle that high?:confused: If not, how do you turn the idle down?
 
A TBI should idle between 500-550 rpm's with the transmission in drive which is more like 650 in park. There is a procedure for setting base idle. Basically you need to ground the A and B connector in the ALDL plug then turn the key to the on position and wait for 30 seconds (this seats the IAC pintle) then while the key is still on disconnect the IAC connector. Now remove the jumper from the A and B connector and start the vehicle and adjust the base idle with the adjuster screw that's on the drivers side of the throttle body (might still have the plug covering it). Remember to have the vehicle in drive and adjust the idle to 550 rpm's. Once you're done shut the vehicle off and reconnect the IAC connector and start vehicle.
 
yep, check for vacuum leaks first. Adjusting IAC with a vacuum leak would throw off everything else. Give the entire engine bay a very good visual inspection, touch any vacuum lines, make sure they are flexible, not cracked, etc.

Pretty rare for someone to mess with idle in a TBI rig, so unlikely previous monkeying with is the problem. Possible, but unlikely.
 
I don't have any leaks that I can find. I assume by grounding out the A B connector in the ALDL you're talking about the trick with the paper clip, what you do to find a check engine code? But what is the IAC connector I have to remove? Pretend you're talking to someone who has no idea about TBI stuff, cuz I don't
 
carpeonnel said:
I don't have any leaks that I can find. I assume by grounding out the A B connector in the ALDL you're talking about the trick with the paper clip, what you do to find a check engine code? But what is the IAC connector I have to remove? Pretend you're talking to someone who has no idea about TBI stuff, cuz I don't
Click to expand...

Well then he wouldnt be pretending now would he :D

I'd also like some more insight on this seeing as how my 87 350/sm465 idles at like 900-1050. I wanted to turn it down to 800 but I'm glad I broke that screwdriver since the adjustment was flogged up ;D
 
There's a connector on the firewall towards the passenger side of the truck if I remember correctly. This is the part he's talking about...or I might be thinking of timing...
 
carpeonnel said:
I don't have any leaks that I can find. I assume by grounding out the A B connector in the ALDL you're talking about the trick with the paper clip, what you do to find a check engine code? But what is the IAC connector I have to remove? Pretend you're talking to someone who has no idea about TBI stuff, cuz I don't
Click to expand...

The IAC connector is on the passenger side of the TBI unit furthest back and is a square connector with 4 wires in it.
 
4X4HIGH said:
A TBI should idle between 500-550 rpm's with the transmission in drive which is more like 650 in park. There is a procedure for setting base idle. Basically you need to ground the A and B connector in the ALDL plug then turn the key to the on position and wait for 30 seconds (this seats the IAC pintle) then while the key is still on disconnect the IAC connector. Now remove the jumper from the A and B connector and start the vehicle and adjust the base idle with the adjuster screw that's on the drivers side of the throttle body (might still have the plug covering it). Remember to have the vehicle in drive and adjust the idle to 550 rpm's. Once you're done shut the vehicle off and reconnect the IAC connector and start vehicle.
Click to expand...

I haven't adjusted my idle but tomorrow I'm gonna try and set it. But had a couple questions. First off, my truck is a manual now, so how do I set it in drive? Can I just do it in park? I can't find a leak around the TBI but I'm gonnna pull it and replace the gasket just make sure before I start messing with it. All my lines look top notch and can't find/hear any vacuums leaks. I really have to set it now cuz it's idling at 1500-1600, so high that it's preventing me from shifting easily (if at all) from neutral to any gear
 
For a manual trans set the base idle at the higher setting. If your idle is starting to go up then you definately have a vacuum leak somewhere that is getting worse.
 
well I don't think it's gone up. I just underquoted the RPM earlier. But it's been like this for 5 yrs so I don't think whatever it is, is getting worse. And I can't find a vacuum leak. I've checked every line heading to and from the TBI. Anywhere else you can think of that vacuum leak may reside
 
Are the throttle plates closing all the way? Any damage to the plates, linkage, or throttle shaft?
 
More importantly (so we know if anyones been into the idle adjustment before) is there still a plug/cap over the adjustment screw?
 
not sure. Where's it at? I've never messed with it or anything on the TBI to tell you the truth. I just know carbs, this new fangled TBI is greek to me :D
 
BGKYK5 said:
Are the throttle plates closing all the way? Any damage to the plates, linkage, or throttle shaft?
Click to expand...

Yeah, everything seems to be working fine the other day when I was setting the rocker arms. All the plates and linkages look good. THe only thing I've got hooked up right now is the throttle linkage. Obviously my kickdown cable is gone now and I disconnected the cruise control linkage. I thought maybe the linkage might be too tight, but it's fine. The bracket is returning all the way to stop when the cable is at rest so I assume that's what you want. I do have a throttle body spacer in there. Think that might be doing something?
 
The idle screw is located on the driverside just like a carb and in just about the same place. Have you removed the throttle cable completely to make sure it's not causing your problem? I installed a TBI spacer once and it pulled on the throttle opening it up some.
 
Went thru every vacuum line I could find and finally found one that was just about worn apart (the one disconnected in pic on back of manifold). Its a pretty big line so I figured that had to be the problem. Went ahead and put my new Accel wires on and went to fire her up and --- nothing:confused:. Wouldn't do anything. Finally got investigating and a power wire on the firewall where the hot wire connects are had rubbed in 2. Replaced it and it fired right up. Still idling at like 1400 when I cranked it up, but it did slow down to about 1200 after a minute. Gonna look again when I get home Tuesday and see if I can find anymore vacuum leaks, but probably gonna reset the idle. Anyway I'm gonna enjoy my 6 days I have off:eek1:, in a row, by sitting in cold tree and hopefully shooting stuff with antlers:D. I'll let you know if the idle works out. Thanks again for your help

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Forgot, posted the other pics so you can point out the adjustment screw to me. I don't see anything that looks like it can be adjusted. Also, Sprayed carb cleaner all around the base while it was running and nothing. Went ahead and cleaned the TBI and removed the throttle cable to see if it might be too tight. Cranked it and still runs the same. Unless there's something I'm missing her, I guess I'm gonna reset the idle. Just kind of gun shy about doing that now, since it seems so odd that mine is idling this high
 
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