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Old 01-28-2014, 09:45 AM   #1
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Need Help! Jeep TJ Won't Start Unless IAC is removed

Hey folks,

Just need a second opinion on this. 99 TJ 4.0 Manual one morning decided to crank, but not start.

Cranks strong but won't turn over.

All fuses and relays good. Fuel pump seems to be priming. I put in new battery, new Mopar crank position sensor. Nothing changed. Only got it to barely start if I pushed in gas pedal all the way down, and then barely ran.

Decided it might be the IAC valve. Removed it this morning, and left the hole open. Tried to start Jeep, and it roared to life! Although it ram up to 3000 rpm's right away and I shut it down of course. Put IAC back in after cleaning it a bit and Jeep turned over but ran like crap and wanted to stall right away.

So I am sure now that my cheapo Autozone IAC is bad. New Mopar unit in one the way in the mail.

My question: Do you guys think the IAC is the only problem based on it starting with the IAC removed. Why would it only start when I removed the IAC completely. Seems odd.

I also ordered a new Throttle Position Sensor and Coolant Temp Sensor to be safe because I read that these can also play a role but don't want to be throwing too much money away on sensors.

Any advice or past experiences would be great. I have searched and read every forum post there is on "cranks but no start" problems. Never heard of IAC preventing a start.

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Old 01-28-2014, 10:02 AM   #2
Knows a couple things...

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The IAC is the engine's sole source of air when you're not stepping down any on the accelerator pedal. So if the IAC doesn't open enough to let air into the engine, the engine will not start.

Does the engine start normally when you step down on the accelerator pedal just a tad? If so, I'd suspect the IAC too. Stepping down on the accelerator pedal opens the butterfly valve which lets air into the engine.

And for sensors, I'd get out of the habit of using anything from the auto parts stores. They normally only stock cheap sensors made in China.

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Old 01-28-2014, 10:16 AM   #3
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Originally Posted by Jerry Bransford View Post
The IAC is the engine's sole source of air when you're not stepping down any on the accelerator pedal. So if the IAC doesn't open enough to let air into the engine, the engine will not start.

Does the engine start normally when you step down on the accelerator pedal just a tad? If so, I'd suspect the IAC too. Stepping down on the accelerator pedal opens the butterfly valve which lets air into the engine.

And for sensors, I'd get out of the habit of using anything from the auto parts stores. They normally only stock cheap sensors made in China.

Thanks Jerry, and yes I have learned my lesson on the aftermarket sensors!

Yes, the engine starts barely if I hold down the pedal with the IAC in, but runs poorly, like it is not getting enough gas or air. But with the IAC removed and the hole left open it starts right up without the pedal involved, but the rpms go through the roof. I did read that the IAC can fail, or at least overcompensate if the TPS sensor is bad, so that is why I ordered that one as well.
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Old 01-30-2014, 12:00 PM   #4
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Hey Mfarr75, I'm experiencing the very same issue. Removed IAC cleaned it, that seemed to help a little. Next morning back to the same problem. Once I get it started, idle it up to warm motor. The Jeep will idle on it's own. Did the new IAC fix the problem or were there other issues?

Thanks,
Tyler
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Old 01-30-2014, 12:11 PM   #5
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Originally Posted by woodward4x4 View Post
Hey Mfarr75, I'm experiencing the very same issue. Removed IAC cleaned it, that seemed to help a little. Next morning back to the same problem. Once I get it started, idle it up to warm motor. The Jeep will idle on it's own. Did the new IAC fix the problem or were there other issues?

Thanks,
Tyler

Actually, I am waiting on UPS to deliver my new Mopar IAC today some time. I am really hoping that it the case. I will post an update to let you know if that did the trick.

I can tell you that when I took the IAC out of the throttle body (while still connected) to the harness to check its operation, it DID NOT move at all when I turned the key on. I understand it is supposed to retract when the key is on, but before cranking so to let enough air in the intake to start. Mine stayed in full boner mode no matter what, so I think the IAC motor is shot (or hopefully not an electrical/PCM problem) and the IAC simply closes off the intake air.
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Old 01-31-2014, 11:46 AM   #6
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This might help. I learned this from a GM mechanic. Un Hook your battery terminals for 20-30mins hook back up. This puts your PCM into learning mode for the next 3000 miles. I have you had your battery checked? Mine was bad and previous owner had a group 58 instead of 34. I was being shorted on CCA. My problem seems to be fixed.
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Old 01-31-2014, 07:07 PM   #7
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Hey everyone, promised I would update in case others needed to know the outcome. UPS was a day late in delivering my Mopar IAC valve. Anyway put it in and THAT WAS IT. Jeep started right up. I didn't even need to disconnect the battery or anything. Acted just as it should.

Lesson: DO NOT buy an IAC from Autozone, etc. Just use the Mopar unit. My Autozone IAC failed after just 4 months. The plunger was stuck in the fully extended mode and would not respond to the computer. Therefore the IAC was not allowing any air into the engine for start-up purposes.

Also, there was no gunk in the intake and the IAC was clean as a whistle, so I can't blame gunk on causing the IAC to fail. This was pure Chinese engineering at work!

Woodward, glad you got sorted out. Your IAC might be on its way out though. If if happens again, IAC is probably your issue.

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