My car started running rough around last fall. It would be an on/off problem. Recently I decided to clean up engine bay to install CAI I purchased awhile back. Cleaned MAF, air temp sensor and IAC. Car runs great now. Since weather temp dropped I started using the heater and the car idles and runs rough. Sometimes it even dies. When I turn it off, it runs fine. I've never really noticed but is this normal? Thanks in advance.
You can try cleaning the iac again, and look for a vacuum leak, especially behind the a/c controls, does it act up with the a/c on as well, or just the heater?
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That hunk of junk? Yeah, well I've made a few special modifications to that yellow 99 Gt, it makes the Kessel run in 12 parsecs and eats turbo japs and poops chevy's
I did clean the IAC the other day, still no change. I didn't try turning a/c on and never thought of checking for vac leak behind control. I'll check out both procedures tomorrow. Thanks.
No problem, let us know what you find and we'll go from there.
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That hunk of junk? Yeah, well I've made a few special modifications to that yellow 99 Gt, it makes the Kessel run in 12 parsecs and eats turbo japs and poops chevy's
Well, I drove around for a while with a/c on and same thing. Idle eratic between 400-1000 rpm. Wouldn't the idle be a little steady high when a/c is on? Checked for leak behind controller and there were none.
Might be a solenoid problem, when the compressor kicks on it the solenoid bumps up the idle. Try checking the plugs and looking elsewhere for vacuum leaks, but it may also be a problem with the control box. How did you determine no vacuum leaks, sometimes you can't see them, you may have to run the car and use a little carb cleaner, but you have to be very careful in the interior, you don't want to mess anything up. Also see if it does it when you just run the vent, it may be a vacuum leak by the duct door.
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That hunk of junk? Yeah, well I've made a few special modifications to that yellow 99 Gt, it makes the Kessel run in 12 parsecs and eats turbo japs and poops chevy's
One other way to check this is do a WOT pull with say AC on,usually if u have a vacuum leak on the HVAC system the air distribution will change to defrost. This is the default postion for Ford products.
Usually a vacuum leak on HVAC will not cause it to stall because it normally is not that big of a vacuum leak.Usually if the leak is bad enough to cause a stall u normally will hear it.
Are u running underrun pullies? If so this could be causing voltage to drop to low with accessories running at idle.
well i hear all mustangs idle a lil rough with the heaters or a/c on mine fluctuates about 200 rpms but it shouldnt be dieing
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BLACK 1999 Mustang GT 5-speed
Smoked healights, tailights, and foglights,Mach 1 Chin spoiler,American Racing Rebels with Toyo Proxes 4,BBK Cold air,
BBK X-pipe with Flowmaster mufflers,
MGW short throw shifter,
Spec Stage 1 Clutch with the Steeda Ajuster kit
Strange 10-way Shocks/Struts, Prothane iso's, H&R SS spings, M&M CC Plates
Hello all, I made sure connections were proper and checked for leaks by hand while heat was on. I'll try the carb cleaner to make sure.
The are no UDP's on the car, It's all stock. I'll also try out the WOT test before I use the carb cleaner.
And belive me, there are times I'm on the highway ready to get off,I'll throw it in neutral(5spd) and let her caost. As soon as I do the revs drop and car dies.
Well, now that the weather is getting colder I've usin gthe heater more often. I noticed something wierd though. When the I run the mixed or defog selection, the idle acts up. But when in the vent or floor selection, Idle is fine.
I've taken your suggestions and did everything I could to find the problem, unless I missed something? I apologize for bringing this up again but I just want to make sure the car is o.k.
racin366, I know you mentioned early on about a sensor, do you think that that may be the problem? Where is it located and is there a way to test it? Thanks again.
2000 Mustang runs rough when HVAC in use 
