Engine Now the car's stalling when I brake for a light or stop sign...HELP

99 FLA PONY GT · Mar 12, 2013

My car is a 1999 GT coupe 4.6L V8 SOHC MOTOR.
I've been having this problem since January but living in Brooklyn, NY,
I only drive my car on my days off. On my roadtrip down to Silver Springs, Florida
for the 19th annual Ford & Mustang roundup down in Florida, my troubles began in Georgia.
I have a chip in my car that's as old as the car installed by Steeda in S.Florida.
They told me from day one, drive using premium which I always have.
During my trip down to Florida, I used mid grade gas & the car ran the same and good until I filled up
again using midgrade gas in Georgia. As soon as I pulled out of that gas station, the car ran rough
at idle when at a light. The revs would drop from 400-900 and the car would rock
back & forth as if I was playing with the brakes and rocking the car
to the beat of a good song. I thought at first it was bad gas but I added techron,
then dry gas & now 10 tankfulls of premium gas...so I think I've ruled
out the bad gas theory.
Today I changed the IAC valve, platinum motorcraft spark plugs,
Pcv valve, & fuel filter. Now my car is running even worse as it's now stalling out
constantly when I hit the brakes at a light to stop the car. I have to wait for the light to just about to turn green then I put the car in neutral, rev the motor & then pop to car into drive as I rev the car up.

What could my problem be? I've checked the intake for a leak with
Water & it appeared that's not the problem. Do anyone thinks that it could be the TPS-Throttle Position
Sensor?
Any help much obliged.

From_A_to_B · Mar 12, 2013

Mine use to do the same thing. Mine was a cut hose on my intake. since you checked that, i'd suggest checking the fuel pump.

Mustang5L5 · Mar 13, 2013

99 FLA PONY GT said:
They told me from day one, drive using premium which I always have.

Unless you have mods requiring higher octane fuel (which i don't see browsing your sig), you don't need premium gas. In fact, the name "premium" fuel is purely a marketing term as it's no "better" than 87 or 89 octane. The 4.6L SOHC is a low compression engine meaning it can make maximum power on 87 octane. So as long as you go to a quality gas station, the fuel coming out of the 87 octane nozzle is just as good as the fuel coming out of the 91+ octane nozzle. I ran nothing but 87 octane in my '03 GT, ran a best of 14.0 @ 99MPH in bone stock trim and averaged 25-27MPG highway.

Of course, that all means nothing if the gas used was "Joe Blow's discount gas" and full of crud and water.

I'm not sure i would blame the gas though. The amount of fuel in the lines at any time can run the vehicle for a minute or two before mixing with freshly pumped gas. Unless you were on E, and filled your tank and idled for a good 5 mins or so before pulling out, you may have still had old gas in the lines leading to the engine that hadn't yet mixed with the new gas.

Anyway, with all that out of the way, first thing i usually ask is if the check engine light has come on? I assume not and after running 10 tanks of gas through it with no improvement and not seeing the CEL pop on, that makes me think it's a mechanical issue and not electrical. I wouldn't then start replacing random sensors on the car as if any were truely at fault, you'd get a code and the CEL would come on.

I am however leaning fuel pump...however that's a guess because I would think you'd have a CEL code indiciating lean if it was a pump issue

COramprat · Mar 14, 2013

Mine did the exact same but I got a check engine code for fuel rail pressure sensor and fuel pump.

Jhussey1280 · Mar 18, 2013

When my fuel pump went out this is what happened as well. I did not get a CEL when my pump went out, we found out by checking the pressure at the fuel rail.

The basic difference between fuel octanes is burn vs explodes. It's hard to type in a short paragraph, but basically it's based on compression ratio of the engine, something like 9.3:1 or less and 87 octane is fine, higher compression they recommend higher octane. The name "Premium" and "Super" only refers to the fact that the fuel has undergone more processing and thus is more expensive to make. I always run what is recommended. I have a DiabloSports 93 octane tune, so I run 93 octane, but running a higher octane fuel without a need doesn't give you any benefits.

JJ Glo · Mar 27, 2013

Get yourself some MAF cleaner and clean the sensor.

My car was doing this for the past 3 days and I cleaned the MAF today and took it for a drive and it drives fine now.

Mattstang04 · Mar 28, 2013

+1 on cleaning the MAF. That just benefits, even if it doesnt work. Past that, change the fuel filter and check the fuel pressure. My 2 cents, good luck.

madspeed · Mar 28, 2013

pull your codes. Many do not trigger the check engine light

Engine Now the car's stalling when I brake for a light or stop sign...HELP

99 FLA PONY GT

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From_A_to_B

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Mustang5L5

That is…until I whipped out my Bissell

COramprat

...I can take it. I think.

Jhussey1280

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JJ Glo

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Mattstang04

madspeed

Colonel Mustard

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