Unsteady idle and rev hang, bad IAC?

tsundave

Member
Dec 1, 2020
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America's Finest City
Hey guys, just wanna get some feedback before I shotgun parts at this 95 Cobra.

Last month I picked up a used 75mm BBK TB that came with a TPS and an IAC. Got a decent deal on it, but I never seemed like I could get it to idle at stock rpm or get it to come back to idle without hanging, especially after some time on the road, where it'd rev hang above 2k rpm. I poked around here and a few other forums and it seemed like it wasn't an uncommon issue with BBK TBs. I cleaned up the old TB, put it back on and still had the same issues. I figured I fat fingered something getting it on and caused a vaccuum leak; sprayed probably half a can of carb cleaner around the engine and couldn't find squat. I read through the Surging Idle Checklist (probably should've done this first), pulled codes and came up with a 412, Idle Speed to Low. Disconnecting the IAC bogs the engine at idle, but plugged in it won't snap back to idle if you blip the throttle and whenever the fan or A/C compressor kicks in the idle speed bogs before it smooths out. I have two IACs and two TPSs (one is new), and I get the same condition on either TB with either part (which wasn't present before I put the BBK TB in). Have I just not found the vaccuum leak or are both my IACs tits up?

Thanks in advance.
 
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McFly2

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Nov 21, 2020
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I presume you did take negative battery lead off for 20 minutes to reset PCM after TB install? Some times the only way to find a vacuum leak is with a smoke machine. Midas found my vac leak with smoke machine at EGR valve (smoke poured out) - $89 diagnostic test with scanner diagnostics included. I had already replaced the EGR Valve a few weeks before. I introduced worse problem with new EGR. I checked my original EGR valve after the fact. It was leaking some (not to bad). With EGR valve off [blowing into the assembly - should not allow air through; so smoke should not escape thru EGR while installed on the car during smoke test. Smoke escaping at IAC is normal. Bad IAC are known to work intermittently (hard to diagnose even with the best scanner) if bad, causing erratic scan read, and idle.
 

tsundave

Member
Dec 1, 2020
9
5
13
America's Finest City
I presume you did take negative battery lead off for 20 minutes to reset PCM after TB install? Some times the only way to find a vacuum leak is with a smoke machine. Midas found my vac leak with smoke machine at EGR valve (smoke poured out) - $89 diagnostic test with scanner diagnostics included. I had already replaced the EGR Valve a few weeks before. I introduced worse problem with new EGR. I checked my original EGR valve after the fact. It was leaking some (not to bad). With EGR valve off [blowing into the assembly - should not allow air through; so smoke should not escape thru EGR while installed on the car during smoke test. Smoke escaping at IAC is normal. Bad IAC are known to work intermittently (hard to diagnose even with the best scanner) if bad, causing erratic scan read, and idle.
Yeah, sort of. I pulled the 20A Maxi fuse to the ECU. I've been asking around sone of my local mechanic friends for a smoke machine.

Also, OBD codes don't apply to the 5.0 SN95s. It's still EEC-IV.
 
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7991LXnSHO

wanna catch the space herp
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Sep 1, 2010
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If you are lucky, the BBK or other aftermarket TB will whistle when the IAC is working. That’s not supposed to happen, but I have not fixed that F# because it is helpful.
It sounds like you need to reset the base idle, and actually use the surging idle checklist. Also clean and dielectric grease the IAC and TPS connections.
 
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tsundave

Member
Dec 1, 2020
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America's Finest City
If you are lucky, the BBK or other aftermarket TB will whistle when the IAC is working. That’s not supposed to happen, but I have not fixed that F# because it is helpful.
It sounds like you need to reset the base idle, and actually use the surging idle checklist. Also clean and dielectric grease the IAC and TPS connections.
Yeah I read that they whistle, mine definitely did. Connections were cleaned and greased, I do it religiously. I'll read through the list again.
 

tsundave

Member
Dec 1, 2020
9
5
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America's Finest City
Fixed the issue. Smoke test didn't really reveal anything on the EGR or IAC side, but the cabin looked like I was hotboxing, smoke was coming in through the brake booster. I had access to my buddy's AllData account, too, which lead me through a diagnostic tree to check IAC resistance. Both the IACs I had were out of spec at ~17 ohms. Oreilly's had one, I checked theirs and it read ~11 ohms. Put it in, went through the reset procedure for base idle, and no more rev hang.

Now on to the brake booster.....:rolleyes:
Screenshot_2021-05-01 KE4 - Measure IAC Solenoid Resistance (Computers and Control Systems) - ...png
 
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