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Anyone get an occasional rough idle when the car is hot and on e85?

Andy13186

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Occasionally my RPM will drop to like 450 a couple times and come close to stalling, usually when the car is warm then it goes back to normal idle. Happens on e85 but also happened once on the stock tune on 93 octane. Any ideas on what could cause that?
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I get something similar, but normally when motor is cold or cooler. Backing out of stall at store. If I give it slight throttle, let clutch out, push clutch back in while still rolling, the rpms will drop to almost a stall. I’ve only had it stall maybe twice so far.
 
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Andy13186

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I get something similar, but normally when motor is cold or cooler. Backing out of stall at store. If I give it slight throttle, let clutch out, push clutch back in while still rolling, the rpms will drop to almost a stall. I’ve only had it stall maybe twice so far.
Mine is an auto and it happens when stopped in first, hasnt stalled yet for me but I am slightly concerned since it dipped pretty low about 5 different times over about 2 months. I wonder if its the fans kicking on but idk, I wouldnt think that would happen on the stock tune.
 

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Occasionally my RPM will drop to like 450 a couple times and come close to stalling, usually when the car is warm then it goes back to normal idle. Happens on e85 but also happened once on the stock tune on 93 octane. Any ideas on what could cause that?
I've had it happen once or twice. The one time I can recall it was after a long road trip. Automatic, it happened it 1st gear.
 

michail71

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Occasionally my RPM will drop to like 450 a couple times and come close to stalling, usually when the car is warm then it goes back to normal idle. Happens on e85 but also happened once on the stock tune on 93 octane. Any ideas on what could cause that?
I had a Mercedes that would do that on occasion but was told by the dealer it was normal. I've never seen my Mustang do that yet.
 

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Andy13186

Andy13186

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Hmm it actually stalled today at a redlight. I wonder what could cause this issue. The car is fine 99% of the time but every once in a while when warm the idle will get rough and this time it actually stalled.
 

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Hmm it actually stalled today at a redlight. I wonder what could cause this issue. The car is fine 99% of the time but every once in a while when warm the idle will get rough and this time it actually stalled.
Never had any warm idle issues. A really cold start is quite rough with E85, as is a warm restart. The only time my idle chugged, is when I started it after it had been sitting for 5 days, and the overnight low was 32F. Starting at that temp was bad. It would catch, fire a big, chug, then die. The second attempt, I gave it some throttle after it started firing, and all was well.
 

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I've had my E85R tune now go into a weird startup idle where is cycles high and low RPMs 730/1100. It's very rhythmical (about 1/2 second low and 1/2 second high) so it has me thinking it's something ECU/Software related with warm starts.

I checked with my tuner but he didn't seem aware of any such issues. I'm trying to catch it in a startup log but so far it has been elusive.
 

Zinc03svt

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Hmm it actually stalled today at a redlight. I wonder what could cause this issue. The car is fine 99% of the time but every once in a while when warm the idle will get rough and this time it actually stalled.
Hey, did you ever sort out the idle stall issue? Mine has stalled now 2x in gear at a stop. WTF? Bad e85?
 

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I had mine stall twice on me recently on a cold night. Fired right back up without issues. Next day it didn't stall once and then I put in 10 gallons of 93 and the car behaved much better. I'm thinking it was the cold weather and no IMRCs on my tune.
 

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Zinc03svt

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I had mine stall twice on me recently on a cold night. Fired right back up without issues. Next day it didn't stall once and then I put in 10 gallons of 93 and the car behaved much better. I'm thinking it was the cold weather and no IMRCs on my tune.
Yeah, I’m going to burn through this tank and switch back to pump on stock tune and see if issue persists. ??
 

Bluemustang

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Yeah, I’m going to burn through this tank and switch back to pump on stock tune and see if issue persists. ??
What was the weather like when this happened?
 

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Mine is an auto and it happens when stopped in first, hasnt stalled yet for me but I am slightly concerned since it dipped pretty low about 5 different times over about 2 months. I wonder if its the fans kicking on but idk, I wouldnt think that would happen on the stock tune.
It could be the fans. I watch mine do it and it's always when the CHT is at 198*
 

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50-55 degrees. It would every once in a while dip at idle. Now it is doing every time I drive it. Died at stop light.
It could be the weather and E85. In colder temps I've found an E30-E40 mix makes the engine behave better. Are you on flex fuel tune or is it an E85 race tune? If on flex fuel I would add some 93 octane to it and see what happens.
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