IAC steps

[FrankieD61]

Hello all,

So, my truck is a 91 Chevy k1500, regular cab, originally had a 4.3l put a 5.7l in it. It is a 5 speed man. The computer is a EBL - Flash 2 system. The motor is bone stock except for shorty headers, no cats, and no EGR.

The truck has a new distributor (Davis), new coil (Napa Echlin), rebuilt throttle body, new intake gaskets, old fuel pump (262,000 miles..), new knock sensor, new CTS, new 02 sensor, new ESC module, new PCV valve, wires, plugs all new ACDelco.

So, I've noticed that my IAC counts are at 0. The EBL computer came with software called What's Up Display, which shows various sensor data. These trucks typically, when running correctly, have IAC counts for 20 - 22.

I have winALDL and the adapter. If I need to use it I can.

I've checked for vacuum leaks. There are none. Fuel pressure hasn't been checked. I've looked into the IAC reset procedure, but don't want to do that unless I know there are no underlying issues. I'm at a loss here... Truck idles fine. Just don't know why I'm at zero counts at idle.

 

[Schurkey]

Has anyone ever tampered with the base-idle screw?

What throttle body--the one that was on the 6-popper? What injectors?

What is the curb-idle RPM? What is it supposed to be?

Can you command the IAC to raise and lower the idle speed?

 

[PlayingWithTBI]

Has anyone ever tampered with the base-idle screw?

^^^This.

Some folks will adjust the IAC count by screwing in/out the min idle screw until your counts get to ~10+ so, if the screw is too far in, the ECM is trying to lower the idle below what you're at which, BTW, can burn up the stepper motor in it.

I have the EBL too and the WUD is very accurate on these things.

 

[PlayingWithTBI]

I've checked for vacuum leaks. There are none.

I had a vacuum leak and it idled at 875 with IAC at 0. When I turned on the A/C, the idle went down to 750 (where I had it set) and the IAC went to 4. If your min idle screw is set right then, you do have a vacuum leak, normally the base gasket from TB to intake manifold.

With no A/C (note MAP too)

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With A/C on

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[FrankieD61]

Has anyone ever tampered with the base-idle screw?

What throttle body--the one that was on the 6-popper? What injectors?

Idle screw hasn't been tampered with. The plug is still intact. The TB is off of a 95 GMC Sierra with a 350! It had a auto trans. The injectors: they are for a 350. I had that concern as well, so I took the numbers off the top of each injector and found that they are the stock injectors for a 350. I don't remember the specs. 50 lb/hr??

What is the curb-idle RPM? What is it supposed to be?

My truck is supposed to be 600 to 650. It idles at around 570ish.

Can you command the IAC to raise and lower the idle speed?

Can I do this with the MT2500 brick? That would be awesome.

 

[FrankieD61]

I had a vacuum leak and it idled at 875 with IAC at 0. When I turned on the A/C, the idle went down to 750 (where I had it set) and the IAC went to 4. If your min idle screw is set right then, you do have a vacuum leak, normally the base gasket from TB to intake manifold.

What is the best way to check for a vacuum leak? I've always used brake cleaner and my ears.. they say propane works too. But brake cleaner has always worked for me. Maybe I'll just say screw it and replace the gasket. It's brand new, though.

 

[tayto]

you want 5-15 counts for TBI. i shoot for 10 on a fully warmed up engine. make sure base timing is set and engine up to operating temp. to remove the tamper plug i use a die grinder and make a flat spot in the middle,then i drill through with a #7 drill bit. remember to stop once you go through the cap there is a screw head you dont want to chew up. then I use a 1/4-20 tap. once the tap starts to bottom it will push the cap out. i usually do this on The bench but it can be done in the vehicle. usually I double check the counts after getting home from work when i know vehicle is hot and do any final adjustments.

ps: i check my idle in gear with parking brake set. stock should be 550-600 rpm.

 

[PlayingWithTBI]

I don't remember the specs. 50 lb/hr??

Stock injectors are 61#/Hr (65 on a cop car) +/- up to 15% so, unless you have a matched pair, you don't really know what you have.

What is the best way to check for a vacuum leak? I've always used brake cleaner and my ears.. they say propane works too.

Yeah, a lot of brake cleaners aren't as volatile as they used to be. I use an unlit propane torch.

i check my idle in gear with parking brake set. stock should be 550-600 rpm.

Yeah, in the EBL you can set it to whatever you want. Now, I have mine set at 750 in neutral and 850 with A/C on for better air flow with an 11 blade fan and in the A/C system as well.

Here's an old .pdf for setting your base idle and TPS. Again, another way is as mentioned above.

 

[Schurkey]

DO NOT use aerosol brake cleaner. Aerosol carb cleaner, or propane, or starting fluid.

Brake cleaners are not all flammable. Some brake cleaners produce toxic gasses when burned.

 

[FrankieD61]

I had a vacuum leak and it idled at 875 with IAC at 0. When I turned on the A/C, the idle went down to 750 (where I had it set) and the IAC went to 4. If your min idle screw is set right then, you do have a vacuum leak, normally the base gasket from TB to intake manifold.

With no A/C (note MAP too)

You must be registered for see images attach

With A/C on

You must be registered for see images attach

I got this figured out awhile ago. My throttle blades were open too far.

But I do have a question just because I find this stuff awesome and interesting: why is the MAP reading significant in this case. You said to take notice of the MAP reading in the first picture, why?

Just curious.