4.6L Gt has no power - Ford Mustang Forum
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post #1 of 33 (permalink) Old 3 Weeks Ago Thread Starter
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4.6L Gt has no power

Hello! Trying to pin point what is going on with my 2003 Ford Mustang GT sohc, automatic.

Unsure if I’m able to post a video of what it does here. Sometimes when I start it, it’ll start fine, I put it in drive with my foot still on the brake and the rpm will shoot up to 1000rpm and drop to 400rpm and then level out at about 800rpm moments afterwards. Sometimes it works fine but sometimes if I put my foot down it’ll accelerate to 40km and that’s all it got with no power at all, if I gradually press the pedal down I’ll get to about 80km but the car is suffering. But it don’t happen all the time (and when I bring it to a garage it’s when everything seems fine)

After reading different posts I have changed the all the ignition coils, spark plugs, egr valve. Cleaned the MAF sensor, idle control valve and throttle body. Check the vacuum system for a leak. Replaced the fuel filter.

Managed to get a computer hooked up to it and took it for a ride to monitor fuel pressure and at idle it’ll be around 40psi +, when I step on it, the pressure will drop and if I hold the pedal down with the car suffering it’ll get as low as 9-10PSI.

I’m not getting a engine light so I have nothing to go by

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post #2 of 33 (permalink) Old 3 Weeks Ago
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It sounds like you have a partially clogged fuel filter on the pickup tube in the tank. Or a fuel pump that's going away.

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post #3 of 33 (permalink) Old 3 Weeks Ago
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This is a double post for same issue. Can a mod combine the threads?https://www.allfordmustangs.com/foru...fuel-pump.html

Thanks!

Looks like advice on this thread is same as other saying fuel filter so at least same advice on both. haha

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post #4 of 33 (permalink) Old 3 Weeks Ago
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Quote:
Originally Posted by MustangLife View Post
This is a double post for same issue. Can a mod combine the threads?https://www.allfordmustangs.com/foru...fuel-pump.html

Thanks!

Looks like advice on this thread is same as other saying fuel filter so at least same advice on both. haha
It hardly ever turns out to be the filter.
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post #5 of 33 (permalink) Old 3 Weeks Ago
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Quote:
Originally Posted by 90lxwhite View Post
It hardly ever turns out to be the filter.
It may very well not be the filter, but it seems to be the common suggested next move on this thread and the other thread. If it is an old filter with say more than 30,000-40,000 miles on it than it won't hurt to replace a needed item anyway... Right?

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Past Mustangs in order and color of text.
(1.)2000, (2.)2004, (3.)2006GT/Auto traded for (4.)2006GT/Manual, (5.)1996, (6.)2011GT, (7.)2003GT, (8.)2006GT, (9)2013 Boss 302, and (10.) 2002GT
(I have owned a few non Mustang cars over the years as well, but always try to have a Mustang with it or come quickly back to a Mustang!)
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post #6 of 33 (permalink) Old 3 Weeks Ago
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Sounds like bad fuel pump. It shouldn't get that low in the psi. Thought I would share a little but more information below.
Most people often forget about the importance of having oxygen sensors that are working properly. Oxygen sensors provide feedback to the pcm to add or subtract fuel. When one of the banks(cylinders 1-4 for example)is running too rich the sensors tell the pcm to subtract fuel. When this happens you can get a bucking or hesitation under throttle(load). When a lean condition is detected the oxygen sensors tells the pcm to add fuel. I'd suggest dataloging the car to figure out exactly what's going on. The elm adapter for ford on amazon, downloaded with the forscan software will make easy work when troubleshooting issues. Very easy to learn and use. One bad oxygen sensor and you can have a bucking or hesitation issue with your vehicle. These do need replaced every 75,000 miles, due to electrical tolerances that wear out over time. Everytime you turn your key forward power is being provided to these sensors. On a v8 motor you have two upstream and 2 downstream. They all 4 work in synchrony to provide the air to fuel ratios to the pcm to make adjustments as you drive. Without these and your car runs like crap, loss horsepower, loss gas mileage, or exactly the opposite depending on each scenario. Either way bad oxygen sensors can wreak havoc on your motor internals and significantly reduce the life expectancy of your motor.
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post #7 of 33 (permalink) Old 3 Weeks Ago
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Usually a fuel pump would just quit, I would definitely check the fuel filter first as it is less expensive than a fuel pump that is pumping fuel. I would also run Chevron Techron in the fuel to clean out the injectors as well


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post #8 of 33 (permalink) Old 3 Weeks Ago
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Quote:
Originally Posted by Onesick99gt View Post
Sounds like bad fuel pump. It shouldn't get that low in the psi. Thought I would share a little but more information below.
Most people often forget about the importance of having oxygen sensors that are working properly. Oxygen sensors provide feedback to the pcm to add or subtract fuel. When one of the banks(cylinders 1-4 for example)is running too rich the sensors tell the pcm to subtract fuel. When this happens you can get a bucking or hesitation under throttle(load). When a lean condition is detected the oxygen sensors tells the pcm to add fuel. I'd suggest dataloging the car to figure out exactly what's going on. The elm adapter for ford on amazon, downloaded with the forscan software will make easy work when troubleshooting issues. Very easy to learn and use. One bad oxygen sensor and you can have a bucking or hesitation issue with your vehicle. These do need replaced every 75,000 miles, due to electrical tolerances that wear out over time. Everytime you turn your key forward power is being provided to these sensors. On a v8 motor you have two upstream and 2 downstream. They all 4 work in synchrony to provide the air to fuel ratios to the pcm to make adjustments as you drive. Without these and your car runs like crap, loss horsepower, loss gas mileage, or exactly the opposite depending on each scenario. Either way bad oxygen sensors can wreak havoc on your motor internals and significantly reduce the life expectancy of your motor.
I could be wrong, but I don’t believe the O2 sensors affect fuel pressure. When they say “add or subtract fuel” they’re talking about the length of time the injector sprays fuel. When the motor is cold it’s not using the 02’s. If you were to do a pressure test at idle with the motor cold and at operating temp it should read the same pressure.
Can this guy put a fuel test gauge on the rail and test the pressure? Does it work the same as the 302’s, when you test at the rail with the vacuum line connected and disconnected from the fuel pressure regulator? Disconnecting the vacuum line on the fuel regulator on the 302 simulates wide open throttle. So by disconnecting it with a gauge hooked up it’ll show what the fuel pressure will be full throttle.
The rear two O2 sensors don’t make any adjustments to the fuel trims.
*Edit*
No vacuum hose on the fuel pressure regulator it seems. This thread talks a little about it.
https://mustangforums.com/forum/4-6l...ml#post6596636

How about the fuel pressure sensor?
https://www.stangnet.com/mustang-for...essure.520927/

Last edited by 90lxwhite; 3 Weeks Ago at 05:59 PM.
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post #9 of 33 (permalink) Old 3 Weeks Ago
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Quote:
Originally Posted by MustangLife View Post
Quote:
Originally Posted by 90lxwhite View Post
It hardly ever turns out to be the filter.
It may very well not be the filter, but it seems to be the common suggested next move on this thread and the other thread. If it is an old filter with say more than 30,000-40,000 miles on it than it won't hurt to replace a needed item anyway... Right?
Looks like he already changed it. He mentioned it in post one. I guess we missed that.
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post #10 of 33 (permalink) Old 3 Weeks Ago
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"could" be a clogged cat.

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post #11 of 33 (permalink) Old 3 Weeks Ago
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Quote:
Originally Posted by kenv View Post
"could" be a clogged cat.
I don’t believe that would cause low fuel pressure.
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post #12 of 33 (permalink) Old 3 Weeks Ago Thread Starter
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Thanks for all the feed back guys! I’m out of town and away from my car for the past week. I have ordered a fuel pump and fuel filter. I’ll be changing it when I get back and will post with the results. If the 02 sensors were bad would I get a could or not necessarily?

Thanks again!
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post #13 of 33 (permalink) Old 3 Weeks Ago
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Quote:
Originally Posted by Benmichelin View Post
Thanks for all the feed back guys! I’m out of town and away from my car for the past week. I have ordered a fuel pump and fuel filter. I’ll be changing it when I get back and will post with the results. If the 02 sensors were bad would I get a could or not necessarily?

Thanks again!
Probably yes a code. Focus on the fuel pressure problem because we know that you have one. O2’s aren’t going to be “it.”
If I was “throwing parts at it” I think I’d try the fuel pressure sensor before the fuel pump. It’s a cheaper part and no need to drop the fuel tank.
https://www.partsgeek.com/gbproducts...SABEgIIoPD_BwE

Maybe someone on here knows how to test the fuel pump before taking it out?
Have you scanned for codes and didn’t get any, or you just don’t have a check engine light?
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post #14 of 33 (permalink) Old 2 Weeks Ago
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Quote:
Originally Posted by 90lxwhite View Post
I donít believe that would cause low fuel pressure.
Yeah, probably not, but it could cause the flooring it with no power thing.
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post #15 of 33 (permalink) Old 2 Weeks Ago
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Quote:
Originally Posted by kenv View Post
Quote:
Originally Posted by 90lxwhite View Post
I don’t believe that would cause low fuel pressure.
Yeah, probably not, but it could cause the flooring it with no power thing.
No. When he floors it the fuel pressure drops to 10 psi. 10 psi = no power.
O2’s aren’t used in open loop which is what it is in when at wide open throttle.

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