4.6L engine issue

stangman16 · Apr 1, 2005

On my '99 Mustang GT 4.6L I experience a problem when I come to a stop, almost a little before I completely stop. My car idles at 400 then back to 700, and slightly down to 500 before it goes back to regular idle at 750 or so. I've replaced the IAC motor, and cleaned my TPS sensor. The car has about 68,500 miles, and I've used fuel injector cleaning solution in the gas tank to see if it helps. The car is all stock, and my parents bought the car new and sold it to me at 48K miles. So, I'd say it's had a good life thus far. It's been well maintained with oil, filters, (maybe not fuel filter) and for overall daily driving. This problem started at about 55K miles, and I've yet to get it resolved, any help would be greatful!

ProCharger GT · Apr 1, 2005

vaccuum leak?

bdcardinal · Apr 1, 2005

take off the plenum and clean out all of the passageways. i had this problem on my mustang, and cleaning the plenum fixed it. make sure you especially clean out the passageway for the IAC. use carb spray and wrap a rag around a screwdriver.

stangman16 · Apr 1, 2005

ProCharger GT said:
vaccuum leak?

How do i check for that?

I'll give the plenum a good cleaning, I'm hoping after work tonight, if not then tomorrow. Thanks for the advice!

Gunfighter · Apr 1, 2005

sounds like a vacuum or TPS issue

JonJon · Apr 1, 2005

i had that problem in the winter when I used my AC. i never resolved it.

i have aftermarket tb/plenum/maf

Jinx · Apr 1, 2005

Important Note

If you decide to use carb cleaner to remove carbon buildup around sensor areas (TPS, MAF, etc.) be sure to buy the kind that says "sensor safe" on the label.

stangman16 · Apr 1, 2005

JonJon said:
i had that problem in the winter when I used my AC. i never resolved it.

i have aftermarket tb/plenum/maf

I remember it had originally started doing this back way when, and it was only during use of the A/C. Now it happens almost anytime I stop, and more so when I have the A/C on. I'll look for the vacuum leak, and also clean the parts with that sensor-safe cleaner I have. I'll let you know if it works, but I'm at work right now, I prolly won't get to it 'till Sunday.

hognutz · Apr 1, 2005

IAC or vaccum leak would be my thought.

white-04 · Apr 1, 2005

Sounds similar to a problem I've had with a different car. Am I guessing right that you have an automatic? If not, then I'm probably wrong. I had an old Grand Am that would lose rpms under breaking. Ended up the solenoid needed replacing. Hope that helps.

stangman16 · Apr 1, 2005

white-04 said:
Sounds similar to a problem I've had with a different car. Am I guessing right that you have an automatic? If not, then I'm probably wrong. I had an old Grand Am that would lose rpms under breaking. Ended up the solenoid needed replacing. Hope that helps.

Nope, I'm a manual transmission driver only. Can't stand automatics, just my personal preference - I'm not dogging them, I just don't feel as in control with the car as I do with a manual tranny.

Kilgore Trout · Apr 1, 2005

I dunno, wasn't there a thread about low freon causing compressor to cycle on and off and cause idle problems...? Could that be it?

bdcardinal · Apr 1, 2005

Kilgore Trout said:
I dunno, wasn't there a thread about low freon causing compressor to cycle on and off and cause idle problems...? Could that be it?

there was a thread about that. have you ever serviced your A/C? you could try having it evacuated and refilled with fresh R-134a.

stangman16 · Apr 1, 2005

bdcardinal said:
there was a thread about that. have you ever serviced your A/C? you could try having it evacuated and refilled with fresh R-134a.

Nope, I can surely say since my parent's bought the car back in '99, it's never had an A/C recharge or anything... i suppose it's well worth a shot. How much do those run, and could I run down to a local Advance auto parts store to do it?

stangman16 · Apr 1, 2005

Kilgore Trout said:
I dunno, wasn't there a thread about low freon causing compressor to cycle on and off and cause idle problems...? Could that be it?

would it cause this even when I don't have the A/C on? I hardly use my A/C, it's been rather good weather, and the extra HP i get from it feels good.. hehe, I like to conserve them ponies.

bdcardinal · Apr 1, 2005

you have to take it to a shop to dispose of the R-134 in there. its illegal to let it vent(thats a federal thing, but a CA nazi thing). you can buy a self recharge kit for probably $20 or so from any parts house. i forget how much R-134 mustangs take. if you ask the people at the parts counter, they should be able to look it up for you.

white-04 · Apr 1, 2005

R-134A recharge is pretty easy. Bought one from Advanced auto for like $30, had a really good DVD instructional video in it, nice pressure guage and 2 or 3 bottles of R-134A. Re-charged one car and was able to check the pressure in my other vehicle with it. It was worth the $30.

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