Ok i got a big problem here

[deftsound]

So my car has a really wierd idling problem.

Its like ill be coming up to a light going 60mph, i take it out of gear put it in neutral and just coast and the rpm's drop to like 400 and then back to 800 and it bounces and sometimes goes below 400 and the car dies which is really unsafe.

The thing is when im just sitting at a stoplight it idles fine, it only does it when i put in neutral and coast like when im slowing down.

Ok I know i have a misfire i need to fix, i replaced the IAC so i know its not that, I really dont think its because lack of backpressure, what the hell could this be??

 

[DTNODYA]

Maybe a bad IAC?

Loose vaccum hose somewhere/leaking ...

I guess, dirty MAF or IAT or TB, but I bet it's a vaccum leak somewhere since you have a new IAC. Check ya hoses.

Have you changed your fuel filter in a while?

Also check your throttle body stop screw.

 

[deftsound]

yea i replaced the iac so i doubt its that, as far as a vacuum leak i would think if it was that then it would be idling badly all the time not just when i was coasting in neutral,

yea i changed fuel filter about 8k miles ago, i guess i should clean the maf and tb, replace pcv valve too, what is the IAT?

 

[DTNODYA]

Intake Air Temp. sensor. It's the sensor that is between the MAF and your throttle body (couple inches up from the MAF). I don't exactly know what it does, I think it reads air temp and adjusts the computer .... Regardless, one time I had one crap out in my truck and it made it idle like complete crap. I think it would throw a code though ....

For your PCV, just take it out and shake it. If it doesn't rattle then you can get a new one, or you can just spray some cleaner up in there until it rattles (ghetto fix but I've done it).

If you do clean your MAF, use electrical cleaner spray and be very careful with the little filament. MAF's are not cheap!

Also, check around your air filter to make sure everything is connected good. One time, I had my K&N come a little loose and it did exactly what you are describing with the fluctuating rpm's in neutral.

 

[Matt's 03 GT]

It could be a vacuum leak. Check all of the lines to see that they have not fallen off and are in good shape.

 

[Kilgore Trout]

Low freon...

 

[mrvax]

Low freon...

WTF???

 

[Jsun3701]

If it was freon how would that ecceft the idle??????????? Check some of the other threads, take a little break cleaner and spray around your vaccum while the car is running.If you have a leak it will inhale the cleaner and cause the idle to go up. Like i said make sure to read up on the problem. I used this as a solution to finding a V. leak in my car. It worked just watch wear you spray the clenaer if the car is hot.

 

[Rickey_Bobby]

SAME DAMN PROBLEM

I just put up a post about this. SAME EXACT PROBLEM. I had my car misfiring for a week because coolant was leaking out of the intake manifold gasket down into the plug holes.
The leak and misfire is fixed now, but I have this idle issue just like you. So far it has cost me a bumper and a lowe's shopping cart (engine died, as did the power brakes, thus leading the the violent death of a shopping cart)

I'm pretty sure i dont have a vac leak. and my supercharger piping is all fine.
and when I rev it up sitting still it idles fine.

let me ask you this though...
have u noticed a differance in fuel milage or a decrease in the Vac # you pull when your cruising on the interstate?

the reason i ask is when this stalling happened I noticed that at 70mph, I normally pull 12 in/Hg of vacuum. Now I only pull 10 in/Hg. This obviously affects gas milage as I normally average 17mpg, and I'm down to under 15mpg.

It may have no relation, but both of my problems are intake air related, so it makes me wonder....

2 heads are better than 1

 

[deftsound]

no my gas mileage is fine, i get over 22mpg on the highway, but i dont have a s/c of course

 

[deftsound]

im gonna check the air intake i suspect that coudl have come loose, also ill check the pcv and if all else fails ill screw with the maf, ill give an update tomorrow thanks guys

 

[CobraRed_96_GT]

drive you car under water and check for bubbles so see where the vacuum leak is but seriouslt check ur IAC again

 

[Kilgore Trout]

Low freon will make the idle go up and down because the a/c compressor cycles on and off rapidly. Does it do it when the a/c controls are set to the off position?

 

[deftsound]

yea i never use my a/c so thats probably not it but i do know my a/c compressor is leaking

 

[Kilgore Trout]

The a/c compressor operates in all positions except straight up and the settings immediately to the left and right of straight up. It is on in the other four positions, defrost and a/c.

 

[Rickey_Bobby]

The a/c thing is interresting... I just went out and tried it. drove around the neighborhood on different setting speeding up and coming to a stop. Almost killed someone's cat. It definately still stalls with the a/c switch straight up and down. and the a/c blows really cold.

switched out IAC... no fix
switched out MAS... no fix
Tried unplugging IAT... no change
switched IAT... no change
checked hoses AGAIN! nothin.

I do have one question as for as a source for a leak.

The breather hose that goes from the intake to the valve cover.
That is inline AFTER the MAS... what is to keep is from sucking too much air in after the sensor? what if the engine has abnormal blow-by thus forcing (a small but noticable quantity) of EXTRA air into the intake unmetered causing a lean condition.
Why would this only happen when moving? (i'm stretching it here) possibly the higher RPM's result in more blowby from the motor, givin a few seconds at low RPM the oil has a chance to better coat the cylinder walls and bring blow by back down
-or-
the natural tendancy for the computer to keep the RPM's up while moving, coupled with the extra blow by air, creat the lean condition. the leak alone isn't enough to cause the problem, but when the IAC opens on top of it the problem pops up.


Thoughts anyone?

 

[the98stang]

use the diablo man. You can get readings from the TPS, MAF, load, spark, o2 sensor, fuel trim, etc. Then you can go from there.

I have the SAME exact problem. Put in a new IAC and TPS, no luck. New plugs, fuel filter, PCV too. Saturday I'm going to use the programmer to get some numbers..

 

[deftsound]

oh man too bad i sold my diablo, im gonna check the pcv today and check the intake but i think it was doing it before i put my intake on

 

[the98stang]

oh man too bad i sold my diablo, im gonna check the pcv today and check the intake but i think it was doing it before i put my intake on

Crap that could have helped you out ALOT.

Try carefully cleaning the MAF. I didn't see your mod list, but if you have a K&N FIPK, the oil from the filter can cause the MAF to get really dirty....

 

[blacksheep-1]

good point 98 stang, that crap that K&N uses has killed a lot of MAFs.