AC Bogging Issue - Mustang Evolution

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Old 07-22-2014, 10:17 PM   #1
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AC Bogging Issue

I have a 2003 Mustang GT with 139,000 miles. When I start the car with it cold and turn the ac on the car tends to bog down with the rpms going under 500 rpm and the car shakes and wants to die before going back up to around 850 rpm. It only does this once the whole time the car is running and only when the car is cold. I did the battery trick by unplugging the negative side of the terminal for 15 minutes to reset the memory on the car's computer which actually helped the car for about a week before going back to it's old habits. The tune on the car is good so I can only imagine I have a bad sensor or a clogged TB. I'll check the TB but I just wanted some opinions on what you guys think it might be. Thanks in advance for reading and posting.
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Old 07-22-2014, 10:38 PM   #2
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Could be a weak iac valve.


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Old 07-22-2014, 10:59 PM   #3
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About 2 years ago I replaced it for another idle issue which turned out to be my 02's haha. Theirs a chance I guess that it could be bad again but I really don't think so. The car has Longtubes, and off-road H, flowmaster catback, JLT First Gen Ram Air Intake, MSD Coil on packs, and a custom tune from wichita dyno using an SCT tuner for performance mods. I recently changed the Fuel injectors but still the stock poundage for the injectors cause I had one that was going out and replaced all of them because of the high mileage on the car.
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Old 07-23-2014, 12:29 AM   #4
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Change the PVC valve and check for other vacuum leaks. And the ac may need some oil. I don't know which kind it is but I remember when I changed the ac on my truck I had to put in some ac oil with the r134a.


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Old 07-23-2014, 01:29 AM   #5
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My GT does this!.. When I turn on the AC rpms will drop. I always figured its just the car starting up the AC hah I don't know.. AC takes power so figured that's why the revs drop a bit
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Old 07-23-2014, 07:10 AM   #6
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Most ECU have an A/C predict signal that lets the ECU know you turned on the A/C and thus bumps the idle up a bit to account for the extra load. I would replace the IAC and like the others said check for vacuum leaks.

Does the compressor sound normal?

If it's making a rattling sound, like marbles on glass, it could either be low on oil or be over/under charged. Of course it could also be mechanical damage.
Look for dirt near the A/C fittings which would signal a leak.
Does the clutch cycle normally?

If it sounds normal and cools alright it's probably fine.

Seeing as disconnecting the battery makes it work for a while, I would lean towards the IAC.

They are high failure items in our cars and are cheap enough to just replace.
Make sure the TB is clean and like others have said PCV hose/valve is clean as well.
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Old 07-23-2014, 01:57 PM   #7
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Simple solution, turn the knob to vent or off when you start the car. Give itself a second or two to get going and then kick on the AC. Does it still do it? You could very well have a shot AC that is fine when its free wheeling but sticky/worn out when engaged. This would account for your problem.
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Old 07-23-2014, 04:07 PM   #8
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I was able to buy the iac which I will put in the car tomorrow and see if thats the issue. If not i assume i have a bad ac compressor. Today it did the exact opposite haha. It just had a hanging rpm of about 1300 so i assume its the iac going bad. Its just hard to believe after a little bit over 2 years that its gone bad again.

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Old 07-24-2014, 05:09 PM   #9
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Ok so here is an update as to whats going on. I did find out that I had a sluggish IAC valve. I bought a new one and was on the car for a little bit but when my car was last custom tuned by wichita dyno they had used the malfunctioning IAC and my car's idle was hanging at 2000 rpm once I got the engine speed up by driving it. I put the old IAC back on and the car ran like it had been running before the switch. So what I have to do now, with it being my daily driver and I have school loans coming due next month so I'm saving money, is to have bama tunes send me an email tune and when I get the tune I will switch to the new IAC. Its been a process today haha.
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Old 07-24-2014, 05:28 PM   #10
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Try disconnecting your battery for 5 minutes or so to clear the computers memory. With the new IAC installed, of course.
Unless the tune had your idle set at 2000 RPM's, I don't see how that could be the problem...
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Old 07-24-2014, 05:30 PM   #11
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I meant 15 minutes or so.^
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Old 07-24-2014, 05:32 PM   #12
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I let the battery sit for a bout 15 minutes unplugged. Still had the same reaction. The IAC comes with a lifetime warranty so i tired 2 new iac's with the car thinking one got stuck open or closed and still had the same affect. With the old iac the car ran just like it did before the switch so its probably the tune.

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Old 07-24-2014, 05:35 PM   #13
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Has anyone ever "adjusted" your throttle body to try to compensate for the low idle?
Is it the stock TB?
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Old 07-24-2014, 05:38 PM   #14
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The throttle body is stock. Its probably never been off of the car. I know its not clogged because i checked it today.

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Old 07-24-2014, 05:49 PM   #15
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You should clean it if it's never been off the car.
What I'm asking is, to your knowledge, has anyone ever adjusted the little set screw that's on top of the throttle body.
If it's the stock throttle body, it should never need adjusting.
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Old 07-24-2014, 06:12 PM   #16
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Not to my knowledge. Everything idle wise was adjusted with the computer. I've never touched the set screw myself.

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Old 07-25-2014, 07:50 AM   #17
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Assuming nobody touched the screw, you shouldn't have to play with it unless something is broken. Straybullitt's advice is good. Check for binds as well and make sure throttle cable is connected correctly and the plastic grommets are ok etc.
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Old 07-28-2014, 12:35 PM   #18
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Well I put on the new IAC again and a new tune from bama and the car still has the high hanging idle. I put the old one back on again with the new bama tune and I think I'm just gonna rock it like that for a bit and see what happens. Ill keep everyone updated. This is just really frustrating.
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Old 07-29-2014, 02:20 PM   #19
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So the car is finally fixed. What ended up happening is that the IAC's I bought from autozone would make my car's idle hang at 2000 rpm. I ended up going to the ford dealership here in wichita and bought one of their IAC's just to make sure it wasn't a bad part. Put the new IAC on and unplugged the battery. Let the car sit for about 15 minutes and fired the car up.... perfect idle. My hanging idle was gone and the sluggish response from the old IAC was gone. Test drove the car just to make sure and all was well. I learned my lesson that Ford OEM products are what you should go with on your car as far as maintenance goes. Car drives like a dream now. Thanks for everyone's input. It helped me out a lot.
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Old 07-29-2014, 03:37 PM   #20
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I always try to use Motorcraft when it comes to sensors and things like that. They fail just like everything else, because odds are that you're replacing a Motorcraft part, but I feel that if there is ever a problem with a bad "batch" of parts, it will be identified and resolved fairly quickly as there are thousands of Ford dealers who, strictly, use Motorcraft parts.
This thing with the AutoZone IAC's seems like it has been going on for months now. The "big box" auto parts stores can get away with selling an inferior product because of their "lifetime guarantee" and the knowledge that, on average, people don't keep their cars very long after things start wearing out and breaking, especially, if they are replacing broken parts with seemingly new, but defective parts. The frustration grows and they are soon looking for a new vehicle.

Glad you got it figured out!
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Old 07-29-2014, 03:58 PM   #21
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Thanks man I guess I just wish it wasn't so expensive haha. $182 with tax for that sensor.
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Old 07-29-2014, 04:07 PM   #22
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Yikes!!!

I think they are around a $100 at Rockauto...
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Old 04-25-2016, 07:06 PM   #23
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I know this post is really really old haha but turns out I had a bad alternator. Replaced the alternator and cam sensor and I didn't have the problem anymore.

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Old 04-25-2016, 10:44 PM   #24
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Quote:
Originally Posted by robartel89 View Post
I know this post is really really old haha but turns out I had a bad alternator. Replaced the alternator and cam sensor and I didn't have the problem anymore.

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Battling the same issue right now, thanks for the heads up.. At least I know not to buy an autozone iac lol


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Old 04-25-2016, 11:04 PM   #25
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Haha not a problem. The issue came back when it was cold a little bit but its probably just my battery. I've had the battery for about 5 years.

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