Truck won't idle up when voltage drops too much

[87ChevyR10]

Ok, so if I run my headlights and defroster, the volt meter drops to less than 12 volts at idle. The truck simply won't idle up on its own to compensate. It will idle up some when the AC pump kicks on.

I have not seen an idle-up solenoid around the TBI, so I assume the IACV does this? If so, my IACV is kinda acting up anyways. When I give just a slight amount of throttle, the engine RPMS go up and down some. Once fully off idle, the engine smooths out.

So, if the IACV does not idle-up the engine when needed, where else should I look?

Thanks!

 

[87ChevyR10]

Did I ask too hard of a question?

 

[PrairieDrifter]

12 volts is pretty low, is your guage accurate or not really?

 

[87ChevyR10]

Yeah, it works fine. It usually reads near 14 volts long as I don't have anything else on. If I run the headlights and the defroster, it drops to around 12 volts when the truck is idle.

 

[PrairieDrifter]

have you set your minimum air rate on your motor before?

 

[87ChevyR10]

have you set your minimum air rate on your motor before?

No, never heard of such a thing.

 

[PrairieDrifter]

When the butterfly plates are at idle position the IAC allows air to flow in a different path to fine tune the idle, the motor should still idle if the IAC is out of adjustment but will most likely run rough or at high idle.

I can't remember if you have to make the IAC fully extend or retract in the throttle body, but after that you unplug the IAC motor so it stays in place then you start the motor and set the base idle to the spec you need

 

[Itali83]

They will not idle up to keep voltage at 14 or so volts. Not how it works. You've either got a weak alternator or inaccurate volt gauge. The motor will only idle up if a load is placed on it like a/c pump kicking on or putting it in drive if an automatic. My '87 read around 12 volts at idle with factory gauge but read 13 or better with a volt meter on the battery. Kind of the nature of owning a 28 year old rig.

 

[87ChevyR10]

Mine reads near 14 volts at idle. It only drops when I use headlights and defroster/ac. I would guess the alt is weak and not keeping up.

 

[PrairieDrifter]

Do you have a multi meter to test what voltage you're actually getting?

 

[firebane]

Mine reads near 14 volts at idle. It only drops when I use headlights and defroster/ac. I would guess the alt is weak and not keeping up.

Do not go based off the gauge. Grab a volt meter and check the voltage at the battery not running. Then start the truck and check the voltage again. And finally with the truck running turn on everything high draw.. heater, high beam lights and take another reading.

At rest your battery should be in the 12.7 or so range. When running you should be 14v or higher.

My truck at idle sits around 14.6v

 

[Irishman999]

Pull your idle air control valve and check it out. Its been a while since I purchased mine but I feel like it was around 50 bucks. They do wear out and stop moving properly, mine actually shot the entire plunger out.

You cant just pop a new one in either, there is a process to program the new one but I cant recall exactly how I did it. It involved disconnecting the battery and jumping a couple terminals at the OBD1 port. After reconnecting the battery with the jumper wire in place the check engine light would flash and you could adjust some **** or something hell I cant remember.

 

[87ChevyR10]

Do not go based off the gauge. Grab a volt meter and check the voltage at the battery not running. Then start the truck and check the voltage again. And finally with the truck running turn on everything high draw.. heater, high beam lights and take another reading.

At rest your battery should be in the 12.7 or so range. When running you should be 14v or higher.

My truck at idle sits around 14.6v

Ok, checked voltages:

Engine off @ Battery: 12.47v
Engine On no Accessories: 14.78v
Engine On w/ Wipers, Defroster and High Beams: 14.5 to 14.6v

And I mis-spoke when I said volts drop too much at idle. What I intended to say they drop too much with truck in gear sitting at a light.

I wish my truck had a factory tach. It doesn't seem like it idles down enough; but, that could be due to the IAC. I'll have to see if the chilton book I have has the replacement procedure for the IAC when I get a chance.

 

[PrairieDrifter]

Pull your idle air control valve and check it out. Its been a while since I purchased mine but I feel like it was around 50 bucks. They do wear out and stop moving properly, mine actually shot the entire plunger out.

You cant just pop a new one in either, there is a process to program the new one but I cant recall exactly how I did it. It involved disconnecting the battery and jumping a couple terminals at the OBD1 port. After reconnecting the battery with the jumper wire in place the check engine light would flash and you could adjust some **** or something hell I cant remember.

Yep! that's the minimum air rate, there is a bolt on the front of the throttle body to adjust the base idle it is a 6 point but it is a female bolt head, and you jump the terminals with the engine off and that moves the IAC then you disconnect it and start the vehicle and set your base idle.