Thread: IAC on the fritz?

So I just got done replacing the LIM gaskets on my car. Intentionally took my time with it to triple check torque on fasteners and make sure any vacuum lines were adressed if they tore or broke (only lost the "T" at the snout!). Put the Fuel pump relay back in, filled the coolant, connected the batt, primed the fuel lines (a few times for good measure), and went to start it up. It hesitated to start, as if there was not enough fuel in the line yet, so I let off it, and tried to start again. It started, but idles terribly, and when I can get it to keep idling long enough, it's showing a misfire on all 6! No codes have triggered yet, and my LT & STFT's are significantly better than before I did the gaskets, so I'm inclined to think there is not a vacuum leak. I have already spent another day going over the stuff that I have already checked a few times again, and every wire is connected that was disconnected, spark plug wires are in the right place, plugs are a little dirty, but not fouled out, The fuel pump can be heard priming every time I turn the key to the on position, and there is fuel at the schrader valve! If I give it a little gas I can make it start every time, but if I don't, it's a bit of a crap shoot if it will start right up or not. I unplugged the maf while running, and it ran worse. I also left the IAC unplugged, and it seemed to at least idle better, but went apesh!t when I put it in gear. I'm thinking/ hoping this is all from the IAC, but it's just odd that it would die at this particular time. I have read many a post that went down a similar rabbit hole without much resolution, so hopefully this will be enough detail for someone to see something that I'm not. Also, does anyone know what pins to measure resistance at on the IAC?

Sorry for the novel-

thanks for the input on the maf. I still have not been able to go out & get a GM replacement one since mine went out a month or two ago, but prior to the lim gaskets, the aftermarket seemed to be doing ok. as far as the wires on the ICM, I never disconnected anything there or moved it other than spark plug wires, but I'm going to take all of that off sometime today to see if maybe something got nicked anyway. I was able to keep it running long enough for a little data logging, and now I'm curious.....what should the fuel flow rate be at idle? I'm seeing .016xxx gpm, which translates to just under 4lph, so I'm wondering if there's something up there...then again, maybe that's normal, IDK.

Step 1 in diagnosis is ask if anything was recently done to the vehicle. You said you did the gaskets, therefore it's about a 99% chance it's something that was done when you did the work. Most sensors will not break when removed and reinstalled etc.

Sounds a lot like a big vacuum leak to me (aka unmetered air)
How well did you clean your gasket surfaces?
Did you reconnect the EGR tube under the TB at the LIM?
Did you reconnect the two mafs on the back of the motor?

A test you can do is get some carb/brake clean and get it running. Have a buddy spray all around and if the engine changes.. look in the area that was sprayed at the moment.

Hope you get it fixed up asap

Thanks for the input, and yeah, I would absolutely think something broke/ tore/ wasn't reconnected/ etc. when I was putting it all back together, but it doesn't seem to be a vacuum leak. I've gone all around the top 2/3 rds of that motor a few times over now, checking hoses, places where I put gaskets, etc, and no fluctuations. Egr was reconnected, I didn't notice any cracking on the tube, but it's easy enough to take it back off and check...the evap sensor and map sensor are both reconnected (hoses and electrical). after taking off the carb and cleaning it, and replacing the IAC (old one fell apart when I pulled it out the last time), I'm down to only 4 cylinders missing at idle now. I have checked electrical connections at the Maf and IAC, I never disconnected the ground at the ICM, but made sure it still looked good just to be sure. It just seems like at anywhere near idle (less than 1k rpm's), it acts like it's about to choke out. If I hold the throttle at or above 1k, it sounds fine. I'm gonna go old school with it tonight after work and see if I can hear anything with a length of tube to my ear.

I'd second a big vac leak. Typically a messed up sensor would get worse with more throttle. Lower the rpm the more vac it's pulling, the crappier it will run with a leak.

If the tube cracked, you likely can not see it or get it with tb cleaner in order to change idle. Pull the tube, pull back the piece that holds it to the intake and look from the inside of the tube to the outside while holding up to the light. Might not be your issue, but it sure seems like you have a vac leak.

Silly question.. brake booster hose is reconnected?

Originally Posted by BillBoost37

If the tube cracked, you likely can not see it or get it with tb cleaner in order to change idle. Pull the tube, pull back the piece that holds it to the intake and look from the inside of the tube to the outside while holding up to the light. Might not be your issue, but it sure seems like you have a vac leak.

Silly question.. brake booster hose is reconnected?

I did not end up pulling the EGR tube back off, I did however, replace every vacuum hose other than the booster (which was connected and in good shape, but anything is possible). Also, as time allowed over the weekend, I pulled the fuel rail back off, replaced the injector o-rings with the ones that came with the LIM gasket kit (really didn't think I needed to originally, as the ones on there now are about a month old), and re-seated the fuel rails with fresh grease on the o-rings. Took care of a couple of other, unrelated repairs while I was at it (water pump, VC gaskets), but everything back together, cleaning everything I took off before putting it back on, and BAM! issue resolved! I'm guessing it was the FI o- rings that were the culprit, so it seems it's a good idea to replace them every time I take the rail off, regardless of how old they are.

-Also, thanks for the input, guys!

Or .. and I've seen it.. the rail didn't seat properly. Glad you got it. guess that 99% thing is still accurate.