jereman

Hi all. First post. Been reading around for a while; decided to post because I couldn't find an answer. I'm having some problems and not exactly sure where to go next. I replaced an Idle Air Controller on the advice of the SMOG tech (I'm in CA) and as soon as I replaced it I started getting a CEL. Checked the codes, got 114 (IAC related.) Put my old one back in, got a different code 113 (also IAC related). Switched back and forth a few times and got the alternating codes consistently. I decided to try a new (third) AIC but I'm still showing a code 113. I've gone through the KOEO, stored codes and KOER codes and am still getting a consistent 113 with the new IAC.

My next guess is to try to replace the computer. I've checked most every sensor, checked (and fixed) vacuum lines, checked wiring, etc. I checked the signal voltage from the computer to the IAC and found that it was out of range by a couple volts and wondered if that was indicative of a malfunctioning computer. My book (Haynes) says the signal voltage should be "about 10.5" but I'm getting about 12.5. Normally I would think "hey, it's getting plenty of volts, probably not a problem" but because of my codes, and the symptoms (idles high; computer can't control idle during self test) I'm inclined to wonder if 12.5 volts is too high and doing something goofy to my IAC. Anyone know if that would be out of range? Could that be a different problem than the computer?

Thanks for any help. Also let me know if I should include any other info.

1991 F150, 4.9l, Manual 4speed w/odrive, ~250K miles (no speedo)

fltdriver

Test your TPS, should read .9-.97v closed throttle, 5v WOT, smooth increase as you open it up.

jereman

Cool. I'll check the TPS as soon as I can and report back.

jereman

Checked the TPS. The signal voltage looks right from the computer, the resistance looks right on the sensor - open and closed, and the voltage looks right coming back from the sensor - open and closed. It's hard to see a nice "sweep" with a digital meter but the voltage looks right open and closed and the basic movement is there.

After all that I did a real test--I unplugged it and drove a couple miles. Truck ran even worse; CEL on solid, idles really high and won't come down. Symptoms went away when I plugged it back in.

The symptoms are not much different than what I'm experiencing actually, but not quite as bad. Doesn't seem like the TPS to me.

Chris_1

iTrader: (1)

Maybe the smog tech didn't know what he was talking about. The IAC simply allows air to bypass the throttle plates to control idle speed.
Generally it works or it doesn't - sometimes just cleaning it helps.
I think the test on that one is to measure the ohms of resistance through the iac itself. Fltdriver can help you out with what that should be.
It could be you've got a wonky signal from your computer, but remember that every time you change a computer related item you have to give the computer time to adjust.

Chris_1

iTrader: (1)

Also just bumped up "testing the tps". Main thing you'll want to double check on that is the signal wire test.

Chris_1

iTrader: (1)

Might also be related to your temp sensor - that's what tells the computer to tell the iac to switch between high and low idle. sensor, not sender.

fltdriver

ECT sensor is what chris is referring to, I'm sure there is a way to test it but for around $10 I would just replace it.

Ohm test the IAC, should be 7-13ohms (set to ohms and use the two spades on the IAC's connector, test works regardless of current flow direction). If you get something else then the solenoid is shot.

This is an excellent write up with many pictures on how to properly diagnose, test, clean, and replace the IAC.
http://www.fullsizebronco.com/forum/...d.php?t=206960

jereman

Checking in; more info.

First, I blew it on one thing here. I was reading my book wrong and was going after the IAC instead of ACT (codes 113 and 114 in my system.) It is strange that the ACT codes would change when I changed the IAC, but I'm learning a little about how these computers work. They're smart, but they're not *that* smart.

ACT sensor: A few days ago I took it out and tested it by sticking it in the freezer for a bit and measuring the resistance, then putting it in boiling water and checking the difference. It seemed to change somewhat, but part of the insulator crumbled away so I got a new one anyway. Fast forward a couple days: I'm still getting 113, so I take some measurements and pull it out. The little sensor on the end that looks like a tiny capacitor or something was gone. Maybe it was just a junk one, so I put a new one in last night. Today I was still getting a CEL and sputtering/hesitation, so I disconnected the battery to erase all the stored codes. Ran a KOEO test: okay, Stored Codes: nothing, KOER: Pass. I drove it a few miles and started getting CEL so I ran the codes and am getting 113 again. (That's my only code now.)

Maybe I got unlucky with a couple bad sensors, but could there be a reason the system is frying that sensor? I checked the reference voltage to the sensor and get about 4.5 volts.

Good news is that I've ended up with lots of new sensors, vacuum lines, etc. in this thing and seem to have narrowed it down to something ACT-related.

Also, checked the TPS yesterday. Signal voltage seems fine, Seems to change resistance correctly when you open up the throttle, and I backprobed it and it looks like the voltage is right when it's open and closed too.

jereman

Above I mentioned that I replaced the Air Charge Temperature (ACT) sensor or also called" Intake Air Temperature" sensor a couple times. I was getting code 113 on a KOEO test, also on the KOER test (and stored.) The first new ACT I put in: the little sensor thing on the end disappeared (burned up?) right away. The second one I put in failed too, but I didn't want to believe it so I looked at everything else I could think of. I tested the wires for continuity all the way back to the computer, got a new (junkyard) connector to the ACT, checked all the other sensors I could, re-tested the ACT with two different meters, etc. When I convinced myself that it was in fact bad, I got another one. This time I got the lifetime warranty instead of the one-year one, I also checked it with my meter before putting it in. This time it worked! I've put about fifty miles on it and am not getting any CEL. I guess I just got a couple bad ones in a row. For what it's worth the ones with the brass body failed and the one with the black plastic body is good.

I still have some questions about my truck but I wanted to post this for anybody else chasing the same thing down.