Poor starting, have to tap gas then fine??

[Big6ft6]

I put a rebuilt throttle body on my truck (89 5.7L) from an ebay seller. Everything is great except I have to tap the gas pedal when starting the truck, if I don't it barely starts, then stumbles and dies.

If I tap the gas pedal while cranking, she fires right up (although there appears to be no high idle?) Once started then it idles and operates fine in every operating condition.

I want to be able to reach through my truck window and turn the key and have the truck start up and run, I don't want to have to get in and push the gas when starting.

Any ideas?

 

[ryoken]

sounds like an IAC issue..

 

[GMC Truck]

They might have messed with the minimum idle screw, is the plug missing??

 

[Big6ft6]

Plug is there, but like you say could be not adjusted perfectly. I thought about that, but if the minimum idle was bad wouldn't it have trouble idling every time I took my foot off the gas? This is only at start-up, once I get through start up it idles fine when driving stopping etc.

But you're the second person to suggest this idea.

 

[smier]

X2 on the IAC... When you tap the throttle you allow more air into the throttle body, not more fuel. So, in theory, your taking the place of your IAC valve and giving it that bit of air it's craving to get started. Your IAC will also have an impact on your high idle at startup. I'd check that before I start playing with the idle settings... Please correct me if I'm wrong...

 

[Code3K5]

IAC !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Remove it, clean it, replace it ! Easy to clean, get some sea-foam and saturate the crap out of it. Or just replace it. It gets dirty from carbon deposits but easy to clean with sea foam. Reinstall it and have a beer when it fires back up at high idle

 

[Big6ft6]

Ok...I hear a strong "IAC" from the audience. But why does it even out as soon as I stab the gas?

When I start it without touching the gas pedal it will stumble along for quite some time before finally dying. If the IAC was sticking wouldn't that give it enough time to open up?

Also once I took my foot of the gas, if the IAC was sticking wouldn't it go back to the crappy idle?

Just trying to soak up as much of your guys' knowledge about how these things work as I can!!

 

[eagle mark]

If they rebuilt it properly then they put new bushings in the throttle shaft. This will change the min air setting. Remove the plug and open up 1/4 turn and try! If not good 1/4 turn and try... if you have a laptop, cable and WinALDL then set the IAC to like 25 at warm idle.

 

[Big6ft6]

Thanks for all the advice. I'd really like someone to confirm these ideas could be the problem EVEN with the fact that AFTER it starts it idles just fine?? Remember it will start without stabbing the gas, it just stumbles and dies after a few seconds of trying to stay running. But if I stab the gas it "wakes up" and seems fine after that.

It seems if any of these setting/adjustment were bad, it would have poor idle all the time eve after it was started. But in my case the problem is only if I try to start without stabbing the gas. Once started it idles just fine.

It helps me get off my lazy butt and face my wife's scolding for working on the truck again, if I can have complete confidence these things could be the problem. But if you all agree that it is unlikely to be these things if it idles fine after starting, then I won't waste my time trying these things.

 

[bigblock72]

I'm going to guess the IAC is ok and the problem is the throttle body itself. I'll bet the throttle plates are adjusted just tight enough that the engine can't suck the fuel past the throttle plates but can when the engine is actually running. The cure is simple enough; just remove the plug and adjust the throttle stop screw a quarter to half a turn. I generally prefer to adjust the stop screw with a scan tool to around 10-15 counts with the engine fully warmed up and all loads turned off. Opening up the throttle plates will usually smooth out the idle a little by helping fuel distribution.

 

[Big6ft6]

Big Block! Thanks, that makes sense. What is the easiest way to pop the plug out?

Also I've read other places that I need to have the A B holes jumped in the scan port before I adjust the idle screw, is that true?

Lastly, what do you mean by "10-15 counts" (I don't have a scan tool, but sense this is a learning opportunity)

 

[bigblock72]

Big Block! Thanks, that makes sense. What is the easiest way to pop the plug out?

Also I've read other places that I need to have the A B holes jumped in the scan port before I adjust the idle screw, is that true?

Lastly, what do you mean by "10-15 counts" (I don't have a scan tool, but sense this is a learning opportunity)

There really isn't an easy way. You can try drilling it but some plugs are hardened and won't drill; in which case you'll just have to use a small chisel on the throttle body to push the plug out. I wish I knew of a cleaner way but just haven't found anything better.

No need to jumper the ALDL, just adjust the screw a half a turn and see how it acts.

On a scan tool the IAC position is referred to in steps because the IAC is a stepper motor, it usually has a range of 0 to around 150 steps. By adjusting the throttle stop you'll effectively be increasing the usable range of the IAC system and can cure drivablility problems when the engine is under heavy loads (AC on, idling in gear while turning steering wheel) and also improve fuel distribution at low engine speeds. The downside is the adjustment needs to be made with a scan tool so you don't cause more problems.

 

[Big6ft6]

Thanks for taking the time to reply Bigblock, that helps!! And I learned something too!