AFAIK the valve is in the vacuum line from the throttle body to the canister. My 87 doesn't have a valve though.

 

I'm not sure if unplugging the IAC should kill the engine but you should notice a change fore sure. First think I would check if your wiring is OK.

 

you may have exhaust leak somewhere in front of o2 sensor, and thats why its saying 172 system lean,

 

If your 172 code is still in memory, run your diagnostic test either KOEO or KOER and when it starts running the test turn your reader off or pull the paper clip. That should clear your memory codes. Test again and see if it is gone. If so, run your truck to at least operating temp and test again to see if it is still gone. If it is, you should start seeing better gas mileage. I get a little better than 10 mpg with my 460. So, you are not running lean. Also, unplugging the IAC and it not changing idle is suspicious. Sandy

 

I unhooked positive battery cable for about 30mins before driving. Will this clear codes as well? When I ran KOER test code 172 and 411 was active. I will try to clean iac and see what happens. Could iac cause truck to run rich?

 

I believe you need to disconnect the negative cable. I wouldn't clean the IAC yet. What is your current idle RPM? Sandy

 

Removing either battery cable will clear the codes as well as the Keep Alive Memory (KAM) in the computer. Now the computer will have to relearn the characteristics of the sensors. You may notice a few small driveability issues before the relearn process is completed.

 

Since he disconnected the positive cable and no change in codes, they must not be in memory, correct? Sandy

 

Or they come right back?

 

He ran a KOER test and I don't think KAM is active unless it is from a KOEO test. Sandy

 

I would stop chasing your tail on the EEC and the sensors. If the truck runs well and keeps reporting the same code I would assume you actually have a lean condition and the EEC is enriching the mixture to compensate. A vacuum leak or leaks is the most likely culprit. This is supported by the idle not changing when the IAC was disconnected. (It wouldn't hurt to check the O2 sensor harness for chafing or other physical damage, just in case though.)

Just bite the bullet and replace all of the under hood vacuum lines and such. It is a pain, but it will save you problems down the road. This will include all the vacuum hoses run off the intake vacuum tree, the PCV system, brake booster vacuum line, and the fuel vapor lines from the vapor canister to the throttle body. Buy the best quality hoses and such as you can afford to avoid future problems. I like Gates for hoses.

You can test individual vacuum circuits and parts with a hand vacuum pump and only replace what is broken too. Be aware that the EGR vacuum solenoid deliberately vents when not on, so that circuit will not hold vacuum without bypassing that solenoid.

If the above procedure does not reveal any leaks then you can move on to suspecting intake gaskets, which you will have to remove and replace to check.

FYI there is a white vacuum line that feeds the vacuum motor for the blend door (passenger side, near the firewall) that is notorious for failing and causing lean codes or the air conditioning coming out of the defrost vents. Also the "coffee can" vacuum reservoir will develop pin holes from rust.

 

Good points above. The coffee can usually will rust under the bracket where it is mounted. Since you can't see under it, you should go under the fender well and remove the two bolts/screws to remove the can and check that area. Sandy

 

Spot on. I actually noticed the vacuum line for the recirculate door was rotted completely. I fixed it last night and ac is working like it should again but I haven't checked codes or tried iac since I fixed ac vacuum line. I will check coffee can when girlfriend gets back with truck