Background: 99 Mustang GT, 5 speed, aftermarket CAI, newer IAC, new TPS. Rest is stock.

So just recently, my engine has been acting weird. When I shift into neutral, the RPMs hang at around 1500-2000, where before it would go down to about 800-1000. They hang so bad, that when I shift I don't even need to hit the gas with the clutch because there is enough RPM to handle it. But when I come to a stop light, it sounds terrible and I am sure my fuel mileage is being killed.

My CAI was installed last year and I completely cleaned out the TB. My car uses to want to stall when I would start it, and replacing the IAC fixed that and all had been good for the past year or so. A friend suggested the TPS, and despite replacing that it is still having this issue.

Tomorrow I plan to remove the CAI and TB and clean it out again, but is there anything else someone can suggest?

Thanks!

 

I'm have the same issue while going into neutral. But when I come to a full stop it goes to normal. Mine I know is bad because at idle it makes a fluttering sound. It won't stay partially open to allow 1k rpms while moving but it will allow 700 while stopped. Did you buy a ford replacement?

 

No both were from Autozone/Advanced Auto.

Mine will hang at idle too sometimes, but does usually come down.

 

I don't trust autozone and advanced parts anymore like this. I used to but I'd rather stick with ford. I plan on getting one next week

 

I agree..The Duralast stuff is crap.
Ive had so many tension pullys replaced that where DL and they pretty much gave me one free every year (1 year warranty) that all they lasted.
got the Motor Craft one and it been great for last year so far.

OEM on most stuff really is the only way to go,like they say you get what you pay for

Just my 2

 

So do you think the IAC or TS is bad? I will call Ford tomorrow I guess.

 

iac i would say

 

Thanks. So I went to Autozone and did a warranty exchange on the IAC (free, so why not). That seems to have helped on the short trip back.

However...

There is a strange "drone" coming from the front end of the car when at about 1500-2000 RPM. I heard it when I had the idling issue cause it would remain at that RPM, but with the idle now responding correctly, I can replicate the sound while standing at the engine. Almost sounds like its coming from the intake or radiator. My air filter is on tight and I double checked all vacuum lines. If its not one thing, its another!

 

A worn IAC can cause that honking/humming noise--read more here; also many people have reported this happening with new IACs, with Autozone being mentioned in many of those reports.

If you have NAPA store nearby get their Echlin brand part # CRB 229573 for your '99. The Echlin brand products are always top quality

 

I guess it must be that piece of crap IAC then... I was just at Autozone and they said the only other time they've heard a noise like that, was when someone wasn't using an air filter..

Thanks for the Napa tip, I just called and they have them in stock so off I go. I thought today was Good Friday?!?