Iac madness!!!

[ponkotsu]

Okay, I've got my Aviator running pretty good but still getting an intermittent rough idle. And I've noticed that the RPMS's still seem high at times. Especially when revving the engine they seem to take much too long to come down. I noticed that I have to brake more than I should down hill. According to scan data the actual RPM is generally higher than desired.

So I did an experiment. I unplugged (Electrical plug) the IAC and went for a test drive. The R's are low at times but in general the truck runs way better with the IAC unplugged. And, I got 17mpg on a test loop I usually get 12mpg!!! Based on the MPG alone I have to conclude that something is F'd up.

With the IAC unplugged, I get some small flow through the bipass hose which I think is normal. If I unplug the IAC bipass hose from the air cleaner the engine will run rough which tells me the MAF sensor is doing it's job.

So clearly I have an IAC system problem. But is it the valve, PCM or something else? This is a brand new IAC valve. It's an Hitachi, not OEM but allegedly the Hitachi is OEM.

Any thoughts on this would be appreciated. This IAC valve is special order and costs almost $200 from the dealer so I don't want to change it out on speculation again...

 

[ponkotsu]

As much as I hate to believe, I think this IAC is junk. I plugged the PCV and Evap systems and it made absolutely no difference in how it runs. Disconnected and plugged off the IAC and the high idle roughness is gone completely. Actual idele RPM was always below the desired which would indicate no vacuum leaks. Looks like I'll be ordering another expensive part.

 

[swshawaii]

Faulty Ford or Motorcraft IAC? When it comes to sensors or anything electrical I refuse to use anything but OEM. Too many horror stories with aftermarket crap, and a few bad experiences myself. Interesting, RockAuto lists the Motorcraft CX1864 @ $99. Stealership pricing, or do you mean gouging?

http://www.rockauto.com/catalog/x,carcode,1440615,parttype,6072

 

[ponkotsu]

I can get it at Amazon for $75. I guess if it doesn't fix the problem I can return it.

 

[makuloco2000]

when the iac is unplugged the engine should die, either it is sticking open or there is a large vacuum leak. I am betting on the vac leak personally. No codes?

 

[ponkotsu]

when the iac is unplugged the engine should die, either it is sticking open or there is a large vacuum leak. I am betting on the vac leak personally. No codes?

Never had a code on this engine for anything. When it's fully warmed up it won't die. Hovers around 500RPM, but that's if I actually plug the IAC bypass hose. If it's cold and I unplug it will die. There's no vacuum leaks. I've been over it a dozen times.

Found something interesting though. This seems to be a common problem amongst the 4.6 Mustang crowd. Lots of them make a IAC restrictor gasket. Apparently Ford ramps down the RPM slowly on these engines to comply with emissions. That creates the annoying high and sometimes rough idle, along with the vehicle coasting rather than slowing down when you take your foot off the gas. I don't have a gasket, so I improvised. I filled the nipple on the air cleaner housing with hot glue then drilled out a 5/32" orifice. Damn thing is running beautiful now. Let's see if it idles okay when it's cold though.

 

[ponkotsu]

Did some more testing on the IAC today. Removed it from the throttle body and plugged off the holes. I ran the engine and watched the pintle valve. It barely moves at all. Even when revving the engine and the duty cycle is high I can barely see it move. So looks like for sure it's a bad valve or the PCM driver is fried. Hope it's not the PCM...

 

[ponkotsu]

So just for the heck of it I thought I would try closing down the throttle plate a bit to give the IAC valve more room to adjust the idle. So I put my vise grip on the factory crimped screw fully expecting it to be tight, and what do you know, it was LOOSE. Seems somebody had adjusted the idle speed up a bit. Not enough to be really obvious but just enough to mess up the computer. I turned it in about 1 turn or so to drop my base idle to about 450rpm.

Idle speed is behaving very differently now. When I decelerate the engine RPM follows the vehicle speed very closely. Engine RPM will hit 600rpm about the same time I come to a complete stop, whereas before it would come down to about 800-900 and then step it's way down to 600 over a few seconds. I guess the PCM algorithm isn't trained to compensate much for the IAC performance. Seems like it just comes down to the same value each time regardless of the throttle adjustment. Still getting some erratic behavior so I'm thinking the IAC is bad, but I'll give it a few days and see how it goes.

 

[ambrosiaB]

I got an IAC valve from amazon, fits beautifully but the gasket has round holes instead of the holes that are trapazoidal and match the openings on the iac is that a bad thing? Should i put the old gasket back on??

 

[ponkotsu]

I got an IAC valve from amazon, fits beautifully but the gasket has round holes instead of the holes that are trapazoidal and match the openings on the iac is that a bad thing? Should i put the old gasket back on??

I don't think it will matter as long as it doesn't leak. Air flow won't be an issue. I would be hesitant to use a non OEM IAC though. Heard too many problems.

 

[ambrosiaB]

Do you have any opinion of delphi?

 

[GopherBaroque]

Dear ambrosiaB, I just swapped out the original (200K miles) IAC with Delphi unit from Rock Auto, at half the cost of local. Delphi part number is CV101032 but the part that came out of the box is ABV0068, made in Japan. Hmm, that corresponds to the original Hitachi part and it seems to be a nice(ly made) unit that plugged right into my "AA" style connector.

PS also came with fiber base gasket, easy to plug-and-play.