Very high revs at start-up

Bingo

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This is a very rare problem that happens a couple time a year for me.

It has only happened after shutting off the car and letting it sit for up to an hour before starting it up again.

Essentially, the engine revs to 2000+ and when I press the throttle, it'll rev higher and maintain that RPM and not come down. The last time it happened (a couple days ago), RPMs kept rising to about 4000 before I turned it off. The only way it gets better is for me to initiate a stall or for it to stall out on its own.

Possible issues:

  • I have an e-Force supercharger with an SCT 3 tune.
  • I also had an issue with the smart junction box getting wet which caused my daytime running lights to activate.
  • Throttle position sensor?
  • Anything else?

It's such a sporadic thing, which makes me think it's not a dirty MAF or throttle body.

Anyway, if any of you have had this happen, or know what it might be, please chime in. I feel like if I start replacing stuff, I'll just be chasing ghosts and waiting months to see if it happens again.

Thanks.
 

boomer9999

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I have experienced this high rev start condition on my 07 GT with a Magnacharger. The car drove fine, but everytime you started it the rpms would go up to like 3k for several seconds and then come down. I figured it was just the supercharger tune but I was wrong.

So one day the temp sensor failed on my supercharger. I had to remove the SC to replace the sensor since on the Magnacharger it was located on the back next to the firewall.

Upon removal, I found one of the blue O-rings rolled over where the supercharger seals to the intake port. Apparently this caused enough of a vacuum leak to lead to this rev problem at startup.

I used a thin coat of motor oil on the cylinder head for reassembly so i could line up the bolts holding the SC in place without rolling those gaskets.

Three years later and never had the rev problem again.
 

Bingo

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I have experienced this high rev start condition on my 07 GT with a Magnacharger. The car drove fine, but everytime you started it the rpms would go up to like 3k for several seconds and then come down. I figured it was just the supercharger tune but I was wrong.

So one day the temp sensor failed on my supercharger. I had to remove the SC to replace the sensor since on the Magnacharger it was located on the back next to the firewall.

Upon removal, I found one of the blue O-rings rolled over where the supercharger seals to the intake port. Apparently this caused enough of a vacuum leak to lead to this rev problem at startup.

I used a thin coat of motor oil on the cylinder head for reassembly so i could line up the bolts holding the SC in place without rolling those gaskets.

Three years later and never had the rev problem again.

You would get that on every start-up?

The thing is this happens a couple times a year for me. I think it would be a regular occurrence, like you experienced, if it was a physical problem, but the randomness and irregularity of what I'm experiencing makes me think it's a computer gremlin.

Thanks.
 

boomer9999

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You would get that on every start-up?

The thing is this happens a couple times a year for me. I think it would be a regular occurrence, like you experienced, if it was a physical problem, but the randomness and irregularity of what I'm experiencing makes me think it's a computer gremlin.

Thanks.

yes, the high rpm varied in, well, height, but on every start.
 

brose37356

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This happens to mine also, high revs up to 300 rpms, installed a new gt 500 tb and a jlt series 3 cai, my tuner installed the required tune, drove it home it sat for 4 hours and it reved up again when I started it. Any ideas
 

06JET

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Mine was doing it after quite a few mod's all at once. Jon Lund was able to tone it down but unlike yours mine was consistent every time I cranked it. It was definitely in the tune. It would go from 0 to 3000 rpm and would hang for what seemed an eternity then slowly come back down..
 

boomer9999

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i'm still convinced it was related to my vacuum leak on the intake o-ring. I did not change the tune or clean anything when i removed the supercharger for the new IAT sensor.

I'm trying to imagine how these cars start. It used to be set the choke and high idle on the carb and it would step down as the engine warmed. What I guess happens is that the computer sets the idle air control motor to a certain position, reads all the relevant sensors and then you fire her up. The question is, what happens next. Obviously the IAC motor is opening up and letting more air in to get the rpms to go up. Why does this happen?
 
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46addict

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To my knowledge there is no external IAC on these cars. Throttle blade angle is controlled electronically by the PCM.

Any opening past the MAF is considered a vacuum leak so a bad o-ring on the intake manifold causing the RPM to rise makes sense. To confirm whether it's a vacuum leak look at the long term fuel trim log and if it deviates outside of .95-1.05 you know something is wrong. A LTFT value greater than 1.05 indicates a lean condition. Centri setups won't have the same exact issue because the install doesn't require a new manifold.

To rule out the basics first I would do the throttle body relearn procedure. This just involves flooring the pedal and letting go with the key on/engine off and repeating the process a few times in one sitting. Google it to get the exact steps.
 
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06JET

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Bingo
Have you checked out the gas peddle. It could be an issue with it. Look at the wire connection/plug and see if it is corroded. Could be internal to the peddle. Let us know what you find.
 

Bingo

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Well, it left me on the side of the road last night. Drove about 10 highway miles and it went into failsafe mode. P2104 and P2112. Forced idle and throttle actuator stuck closed.

It would fire up but not maintain idle and died basically immediately upon startup.

I ordered a new throttle body motor and it should be here Thursday. If that doesn't work, I'll look at the pedal electronics and then the TPS (which is a pain in the ass, fucking loctite on those screws). After that, who the hell knows? Wiring harness? PCM?

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redfirepearlgt

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^^^ Sounds like you are on it. You may wish to also make sure your connector pins are clean AND have not been recessed or pushed in which could cause poor connection between the PCM and the Throttle body Motor.

FWIW - since 2005 Ford Mustang GT's are drive by wire and are torque demand based. This has eliminated the need for an IAC. All idle control is done through tuning.
 

Bingo

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Issue resolved. Throttle body motor took a shit. Installed the new one and it fired right up. Thanks for the help, all.

Sent from my SM-G935V using Tapatalk
 

Bingo

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I started it up and the CEL was still on. I turned it off, pulled and cleared the codes, and fired it back up. Idled beautifully. Then I let it sit for about 20 minutes and when I went to drive it, it did the whole 4000 RPM idle thing again and finally settled down after a few minutes.

I'm going to try the idle re-learn and clean the MAF, but it makes me think the failed throttle body motor was just a symptom of a larger problem. Would bad/erratic voltage being sent to the motor cause premature burnout? Could the PCM or tune be the culprit? The wires/connectors going to the TB appear to be in good shape.
 

redfirepearlgt

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I started it up and the CEL was still on. I turned it off, pulled and cleared the codes, and fired it back up. Idled beautifully. Then I let it sit for about 20 minutes and when I went to drive it, it did the whole 4000 RPM idle thing again and finally settled down after a few minutes.

I'm going to try the idle re-learn and clean the MAF, but it makes me think the failed throttle body motor was just a symptom of a larger problem. Would bad/erratic voltage being sent to the motor cause premature burnout? Could the PCM or tune be the culprit? The wires/connectors going to the TB appear to be in good shape.

Doubtful. It would take a lot of AC ripple from a failing alternator to do damage to a DC stepper motor, and you'd likely be seeing other issues well before hand. But it is worth checking. Verify with the car running that AC ripple reads less than 0.5 VAC (500 mVAC).
 

Bingo

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So I recently emailed JimIII @ JDM about this issue and he told me to look at the throttle body; specifically the internal gears. I took the TB off and fiddled around with it, and it seemed to operate normally except for when I didn't let it slam close. If I eased it closed, it would get stuck an extra degree or two past "closed". I loosened the throttle body motor assembly bolts and tried it again, and it did not get stuck open. For the past few days, I haven't had any high-idle starts, so I'm thinking the issue was the motor assembly was just pushing too hard on the gear. I'm hoping it keeps behaving well.
 

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