Idle lowers in gear

[jleblanc06]

87 TBI with auto th400 tranny. I'm currently trying to get my truck running solid and a trying to figure out the cause of my idle issues. The one odd thing I've noticed is it idles great in park but once i put it in drive it drops quite a bit. Only does this when cold. Once its warmed up it does fine

 

[Honky Kong jr]

All automatic equipped vehicles do that. If it’s excessive that’s a different story.

 

[Rusty Nail]

Is this a trick question?

 

[jleblanc06]

All automatic equipped vehicles do that. If it’s excessive that’s a different story.

Hmm that’s what I’m trying to figure out, if it’s excessive... it barely stays running.


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[Honky Kong jr]

Hmm that’s what I’m trying to figure out, if it’s excessive... it barely stays running.


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Do you have a tach?

 

[jleblanc06]

Do you have a tach?

Nope. That’s on the list though.


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[Honky Kong jr]

Nope. That’s on the list though.


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Ok because you could be able to tell us how many rpm it drops when put in gear. But check your vacuum lines for damage it could be a simple case of a weak engine mount that allows the engine to move and open up a hose and make it idle stupidly. Then closes up when there is no torque on the hose and mount.

 

[Rusty Nail]

Adjust the choke.

 

[bucket]

Adjust the choke.

I see what you did there, lol.

 

[1987 GMC Jimmy]

I'd check for vacuum leaks, as aforementioned, and if that doesn't lead somewhere I may also very gently clean the IACV pintle and make sure the idle air passages are clear of carbon when you look underneath the TBI and where the valve screws in. You may also want to clean the EGR valve pintle for good measure. If you had a scan tool, you could see how many steps the valve was at, which would be good to know. You could also try an IAC reset. What you want to do is jump the A and B terminals on the ALDL (the two top right ones), turn the key on, wait for thirty seconds, unplug the IACV connector, turn the key off, undo the jumper, and reconnect the valve connector. If you want to know more about the homemade scan tool, it was recently made a sticky in the TBI/EFI section, and it's by Georgeb.

 

[jleblanc06]

I'd check for vacuum leaks, as aforementioned, and if that doesn't lead somewhere I may also very gently clean the IACV pintle and make sure the idle air passages are clear of carbon when you look underneath the TBI and where the valve screws in. You may also want to clean the EGR valve pintle for good measure. If you had a scan tool, you could see how many steps the valve was at, which would be good to know. You could also try an IAC reset. What you want to do is jump the A and B terminals on the ALDL (the two top right ones), turn the key on, wait for thirty seconds, unplug the IACV connector, turn the key off, undo the jumper, and reconnect the valve connector. If you want to know more about the homemade scan tool, it was recently made a sticky in the TBI/EFI section, and it's by Georgeb.

Awesome! Thank you for the help, I'll be trying that tomorrow morning

 

[jleblanc06]

Awesome! Thank you for the help, I'll be trying that tomorrow morning

Does the reset make the IAC freak out. Connected the terminals and turned the key on, the IAC just started clicking constantly until I unplugged it. Is that normal?

Finished the reset and the idle is still poop. It starts to surge then stumbles and dies. I have to hold the throttle until it warms up, then it holds.

 

[1987 GMC Jimmy]

Yes, that's normal. I wouldn't worry too much yet. You still need to check for vacuum leaks, and I'd pay close attention to the TBI base gasket and EGR valve. I'd use a baby can of starter fluid sprayed around the base gasket to see if your idle changes. The brake booster can also be a source of vacuum leakage, which is best tested with a Mityvac, but if you hear a loud whoosh when you step on the pedal, that's a good sign it's bad. Check all the lines, and if you suspect one or more vacuum components are giving you trouble, just plug it off at the TBI or manifold and see if the issue improves. While, it'd be prudent to make sure the EGR valve is working, I don't think it's gonna come into play when you first shift it into gear.

 

[Tim K]

You need to check; Idle Air Control, Throttle Position sensor and Manifold Absolute Pressure sensor. Get yourself a MityVac and a copy of the factory diagnostic manual. Both of these are very useful diagnosing issues on the TBI engines.

 

[jleblanc06]

You need to check; Idle Air Control, Throttle Position sensor and Manifold Absolute Pressure sensor. Get yourself a MityVac and a copy of the factory diagnostic manual. Both of these are very useful diagnosing issues on the TBI engines.

Will do! i plan on getting the Mity Vac this weekend