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Performance issues ONLY with the AC off?!

2K views 7 replies 3 participants last post by  Killjoiboi 
#1 ·
So I know that usually running the AC, especially on full blast, hinders performance to some point. In my case however I am running into the opposite issue?
When driving the car with the AC off, there seems to be some resistance and the second I press on the gas it shifts up and is really sluggish. On the other hand, when the AC is on and fan is running, usually with fan on 4 or higher, the car accelerates like a dream and feels as light as a feather. During both of these conditions the AF ratio and vacuum/boost are gauges are all reporting the same data. I am suspecting that this may be an issue with the alternator, AC compressor, or radiator for some reason, but I have not been able to find any post from my research on this type of issue or any possible causes to this. I am not usually social in any way but this issue has got me out of my comfort zone and asking for help on this forum because it has saved me in the past with other issues. I will list mods I have done which may be relevant.

Any help would be appreciated, anyone have any ideas of what could be causing this issue? Also, does this sound like it can be fixed with a tune since I have not gotten a custom tune?

2017 Mustang Ecoboost Premium Convertible
Mods:
JLT CAI
CP-E intercooler
MGK sparkplugs
JLT Oil separator<driver side>
Mishimoto off road downpipe
Muffler and resonator delete
Diablosport 93 octane tune
 
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#3 ·
I would start with your tuner, as it sounds like something in the ECM is being toggled (the wrong way) with the A/C. Your opening line sums-up what you would expect from a normally running car.

Just noticed your text says "not custom tuned" but the "Diablosport 93 octane tune" is listed. So, in that case, how hard is it to flash back to stock? That would at least tell you if it's tune-related.
 
#4 ·
Why didn't I think of that? Lol Thanks so much for you're help, I was just on my way out and made sure to grab my tuner to try that out. I wouldn't be surprised if that was the case since when I got the tuner it wouldn't work and they had to ship me out a new one. After that they still had to update it multiple times to get it to work, the whole ordeal took a couple of weeks.

I guess thats what I get for trying to save a few bucks and not getting the Cobb tuner.

Fingers crossed that this is the case because I was planning to get a custom tune pretty soon.
 
#5 ·
So took the car for a spin with the stock tune and then reverted back to the 93 octane tune to compare. Sadly I'm still getting the same results running with the AC on vs off.

I do have an oil cap breather coming in to avoid building up too much crank case pressure but I feel like that wouldn't help here since the engine only helps run the alternator/compressor for the AC. I am out if ideas as to what this could be.
 
#7 ·
That click is the AC compressor clutch engaging. Totally normal. I noticed my cylinder head temps drop & stabilize around 190F when my AC is ON. This is even in Phx when it's 115F ambient. Lower cylinder head temps surely help avoid pulled timing by the ECU, not increasing performance, but delaying the loss maybe? :unsure:
 
#8 ·
That sounds plausible. I noticed the cooling fans were set to not come on until the Temps are at 205°f which i thought was a little hight since my average cylinder head Temps are around 180-185°f. I set them to come on at 180°f to see what would happen and it looks like that helped but still monitoring performance with AC on and off to be sure.

Even though I've done a fair amount of mods and maintenance I still consider myself a novice at this so please let me know if I am wrong.
 
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