'85 manual IAC valve
Thread Starter
8th Gear
Joined: Aug 2018
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'85 manual IAC valve
I've come to the realization that the IAC in my '85 is completely dead and open, leading to a 900 RPM idle in gear. I bought the valve to fix this, however, I was curious as to what the 4 pins on the valve are used for. Ideally, I'd like to be able to rig up some sort of manual control for it, so I can get that nice lopey idle at 500 RPM when I want it, and 800 for when I actually need to drive. I'm assuming I could do this with some sort of resistor, but I need to know what each of the pins are for first. Thank you!
Safety Car
Joined: Sep 2004
Posts: 3,953
Likes: 550
From: Mission Viejo CA
The IAC is a "stepper motor" and uses a complex digital signal from the ECM to move the pintle in or out as required for a good idle. Having the engine idle at 500 rpm wouldn't give you a "lopey" idle.
Thread Starter
8th Gear
Joined: Aug 2018
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. But it used to idle down that low, until the shop replaced the throttle body, and I'm assuming the pintle was retracted during that replacement. The idle screw is also already adjusted all the way out, so that couldn't be the problem. There's no way to use some sort of 12V signal to adjust it in and out manually?
Safety Car
Joined: Sep 2004
Posts: 3,953
Likes: 550
From: Mission Viejo CA
Thread Starter
8th Gear
Joined: Aug 2018
Posts: 8
Likes: 0
All 8 are sitting at 160 PSI, which seems about right for how much overlap the cam seems to have. A few were a little low, but after a valve adjustment, all of them came up to spec. For 143k miles, the engine seems to be in really good health, with 45 PSI of oil pressure at (the ridiculously high) idle. At this point, I'm thinking I'll have to look into some stepper motor controllers and seeing what I can find out. It's odd, because it idles really low at startup, around 500, and sometimes dies once or twice. But once it reaches operating temp, the high idle comes back. Grounding the pins on the ALDL to reset the min idle does nothing, which is what makes me think that the IAC itself is bad, along with the odd warmup conditions.
Racer
Joined: Apr 2016
Posts: 336
Likes: 93
From: San Antonio TX
I've come to the realization that the IAC in my '85 is completely dead and open, leading to a 900 RPM idle in gear. I bought the valve to fix this, however, I was curious as to what the 4 pins on the valve are used for. Ideally, I'd like to be able to rig up some sort of manual control for it, so I can get that nice lopey idle at 500 RPM when I want it, and 800 for when I actually need to drive. I'm assuming I could do this with some sort of resistor, but I need to know what each of the pins are for first. Thank you!
https://www.ebay.com/itm/Stepper-Mot...!-1:rk:14:pf:0
It would be very "klugey".
Pro
Joined: Mar 2000
Posts: 690
Likes: 7
From: Bradenton, FL
Cruise-In II Veteran
The idle air motor is a stepper motor controlled by the ecm. That idle screw you are referring to sets the minimum air flow with the idle air pintle in the fully closed position. Stock, the ecm sets the idle at 650 rpm. You can adjust this by reprogrammimg the chip in the ecm using TunerPro. One way to accomlish what you want would be to set the ecm idle speed closer to 500 (say maybe 550?). and use the air conditioner request input to raise it when you want a higher idle. When the ac comes on, the idle is bumped by a certain number of rpm. That is also programmable. I also think there is a board available from Moates that will let you run two different programs, switch selectable. You could have one for low idle and one for high idle.
I’m just happy when mine idles near 800....
I’m just happy when mine idles near 800....
Oil Producer
Joined: Apr 2014
Posts: 9,377
Likes: 2,737
been here on the 85.
do the minimum idle adjust. search for it. Set your minum to 550 ish.
ensure you have no vac leaks.
for my 85 i found it was "slow to react" with the iac. Im not sure what caused it. My belief is that the oem stock 85 prom chip was somehow slow to deal with unmetered air.
BUT after i installed the gm 85 chip that was released as a modified updated chip, AND resealed the engine, it ran better than new.
do the minimum idle adjust. search for it. Set your minum to 550 ish.
ensure you have no vac leaks.
for my 85 i found it was "slow to react" with the iac. Im not sure what caused it. My belief is that the oem stock 85 prom chip was somehow slow to deal with unmetered air.
BUT after i installed the gm 85 chip that was released as a modified updated chip, AND resealed the engine, it ran better than new.
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Team Owner
Joined: Jul 2004
Posts: 28,475
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From: Hartford WI
All 8 are sitting at 160 PSI, which seems about right for how much overlap the cam seems to have. A few were a little low, but after a valve adjustment, all of them came up to spec. For 143k miles, the engine seems to be in really good health, with 45 PSI of oil pressure at (the ridiculously high) idle. At this point, I'm thinking I'll have to look into some stepper motor controllers and seeing what I can find out. It's odd, because it idles really low at startup, around 500, and sometimes dies once or twice. But once it reaches operating temp, the high idle comes back. Grounding the pins on the ALDL to reset the min idle does nothing, which is what makes me think that the IAC itself is bad, along with the odd warmup conditions.
Somehow I think you are trying to make the car sound like the old muscle cars without really understanding what is going on. It isn't an "on off thing" that you can easily do. Maybe you can spend the money and have someone design a cam that will sound like what you think will impress the boys around the block? Idealy he should be able to reprogram the ECM so it will work with the cam for that sound effect?
Oil Producer
Joined: Apr 2014
Posts: 9,377
Likes: 2,737
i am assuming you have an automatic. the prom you are after is hlh3200
it stops the rediculous early tcc lockup that feels like stack shifting, holds a better idle, among other things that i do not know. it basically makes an 85 behave the same as the 86 and up. it was by far the best upgrade i ever did.
i had an 87 and an 85 at the time and i couldnt believe how crappy the behavior of the 85 tcc was.
my thread on the subject: https://www.corvetteforum.com/forums/c4-tech-performance/3830279-1985-corvette-early-tc-lockup-shorted-ecu.html
there is a link in that thread to see the dofferent proms and the order thwy were released.
now, i already checked ebay and there are no hlh3200 prom chips for sale.
i think one of you guys who still has an 85 (i dont, sold it, i now have an 86 4+3 and the 87 auto) should call hypertech with thier 85 chip. I AM WILLING TO BET that for the automatic version it bumps up the tcc lockup behavior to match the 87. So i dont think tou will need the hlh3200, im betting you can simply get a hypertech chip for the 85 and all will be well but id be calling them first.
i remember reading sssooooooo many thread of crap idle and "stack shifting" or "early tcc lockup" causing chugging/lugging and i couldnt believe this fix had not been posted.
gm did take care of it with the final release of the hlh3200 and my understanding is that it was only swapped into cars whos owners complained.
it stops the rediculous early tcc lockup that feels like stack shifting, holds a better idle, among other things that i do not know. it basically makes an 85 behave the same as the 86 and up. it was by far the best upgrade i ever did.
i had an 87 and an 85 at the time and i couldnt believe how crappy the behavior of the 85 tcc was.
my thread on the subject: https://www.corvetteforum.com/forums/c4-tech-performance/3830279-1985-corvette-early-tc-lockup-shorted-ecu.html
there is a link in that thread to see the dofferent proms and the order thwy were released.
now, i already checked ebay and there are no hlh3200 prom chips for sale.
i think one of you guys who still has an 85 (i dont, sold it, i now have an 86 4+3 and the 87 auto) should call hypertech with thier 85 chip. I AM WILLING TO BET that for the automatic version it bumps up the tcc lockup behavior to match the 87. So i dont think tou will need the hlh3200, im betting you can simply get a hypertech chip for the 85 and all will be well but id be calling them first.
i remember reading sssooooooo many thread of crap idle and "stack shifting" or "early tcc lockup" causing chugging/lugging and i couldnt believe this fix had not been posted.
gm did take care of it with the final release of the hlh3200 and my understanding is that it was only swapped into cars whos owners complained.