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4.6L Gt has no power

5K views 32 replies 12 participants last post by  wmburns 
#1 ·
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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#3 ·
#6 ·
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.
 
#8 · (Edited)
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/4-6...570283-testing-fuel-pressure.html#post6596636

How about the fuel pressure sensor?
https://www.stangnet.com/mustang-forums/threads/fuel-pressure.520927/
 
#10 ·
"could" be a clogged cat.
 
#12 ·
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!
 
#13 ·
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/gbproduct...MIns36soTC5QIVd__jBx0EUgIlEAQYASABEgIIoPD_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?
 
#16 ·
99-04 has what’s called a return less fuel system. That sensor on the fuel rail is called a “fuel rail pressure sensor”. The sensor reads pressure and will tell the pump to shut off or on. Depending on if it has the required pressure or demanding pressure. Since your fuel system has no return line this is how it operates. A lot of guys ditch this system once they start making serious power because they just can’t keep up with the fuel demands. I agree with the ones saying focus on the pump or FRPS. The O2’s work by reading the fuel trims like a AFR gauge “Air Fuel Ratio” when it reads lean it tells the fuel system little more fuel. When they read rich they say little less fuel. WOT is off the MAF not the O2’s.
 
#17 ·
Z

Something you might want to check. I don’t know if this will affect the fuel pressure but I had a vehicle that had a blocked catalytic converter and it was a dog and acting weird. To find it , the mechanic drilled and tapped the exhaust pipe and measured the pressure. Not sure what it should be but I’m sure the info is on the web. Good luck
 
#18 ·
You all missed the part that I said just providing more information. I said the fuel pump is likely the culprit. Dont forget to check your charging system. A broken terminal on the battery will cause this. Happened to me. And I had 3 of 4 of my oxygen sensors go bad without a check engine light. I only brought up the oxygen sensors as a reminder to people how important they are to your motor. It was completely unrelated to the discussion as I said in the very beginning just providing some necessary information that people often forget about. The elm adapter with the forscan software can tell you alot about your car. It can read all data your car is providing to the pcm. Voltages, pressures, sensors, air flows, ect. I've diagnosed so many issues using it that I've probably saved over several thousand dollars using one. It's a must have for all mechanics or people learning to troubleshoot issues on their own.
 
#20 ·
, 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)


I was only responding to the symptoms not everyone's concern of a failed fuel pump. I was saying it can be more than a fuel pump that is failing. These symptoms can be a bad battery, I've had this exact issue before, blown head gasket, intake gasket leak, failed injectors, failed spark plugs, failed oxygen sensors. Here recently my camshaft sensor failed causing this exact issue. All of these i have experienced. But back to my original post. How do you know what the issues are unless you use a scan tool. I suggest using the elm adapter 30 dollars on amazon with the forscan software. Cheap and easy to pin point exact issues.
 
#21 ·
OP I know you said you are not getting a CEL. However when you turn the key to ignition before cranking does the CEL actually illuminate? If it does the bulb is good. If it does not then the bulb is bad. Have you still scanned for DTC’s even though you said you are not getting a CEL? A lot of vehicles can store codes in memory after so many key cycles and not illuminate the CEL. Always always always scan for DTC’s. Don’t just assume you don’t have any because the CEL isn’t illuminated. You can test a FRPS but you will need a multimeter and research how to test it. Or you could continue throwing parts at it.

I’m not going to argue over the whole voltage and O2 sensor ordeal. I’m well aware of their functions and what they have to do with the fuel trims. Their is a big difference between open and closed loop. I personally use SCT to datalog my car when I need to do so. Yes even a weak battery can play tricks and cause little issues with electrical parts. I will say that 99% of the time if there is an actual O2 issue it will trigger a CEL. Whether it be a voltage issue or they are reading lean or rich conditions. By no means does the down stream O2’s do anything other than emission BS. You can delete them and be perfectly fine.
 
#24 ·
And to answer your question bad wiring, bad connections can cause low fuel pressure and intermittent issues. Along with a bad battery connection.Its rare but I've seen it.
Yes, bad wiring can cause intermittent issues of all sorts. He says his will idle (sometimes) but it cuts out when he “steps in it.” That would eliminate an o2 problem wouldn’t it since the o2’s aren’t used at WOT? If he is experiencing this problem when the motor is cold then it’s not the o2’s because they’re not used when the motor is cold either.
Apples:eek:ranges kind of since it’s OBDI, but I’ve had the o2’s disconnected with the motor running and nothing out of the ordinary happened. The a:f might have been off but it didn’t stumble or anything.
 
#25 ·
He says his will idle (sometimes) but it cuts out when he “steps in it.” That would eliminate an o2 problem wouldn’t it since the o2’s aren’t used at WOT? If he is experiencing this problem when the motor is cold then it’s not the o2’s because they’re not used when the motor is cold either.

It depends. If he steps on the gas and gets too much fuel the oxygen sensors will read this and cut fuel which would have bucking issues or lost power. I experienced this on my mustang because I had several injectors that were stuck open. That was due to having my car sit for extended periods of time which is real bad on your cars. But otherwise I would agree with you. The sensors aren't used when the motor is cold or when at WOT. There is a seperate table in the tune just for WOT that provides values of airflow and fuel mixture. We all can guess on what the issue is but the only way of knowing is to use a scan tool to monitor data. The elm adapter for mustangs, Ford and mercury's is only 30 dollars on amazon. The software you can download and use is free and called forscan. Anyone trying to diagnose issues or working on their own car should have both of these. That's the only way to know what is going on with the car. Hopefully replacing the fuel pump fixes your issue as most of us agree this is the culprit.
 
#26 ·
Correct closed loop is only achieved once O2 sensors have heated to correct temperature. So at initial startup “cold” even at idle and cruising you are in open loop until those sensors reach temperature. The sensors do have heaters in them to help speed up this process. I would also advise only Motorcraft O2 sensors in Fords. Actually any electrical part aside from batteries I only use Motorcraft. Once you go WOT or get under a load it will go into open loop. The fuel trim readings from the O2 sensors are not used for open loop. The sensors might as well not even be there in open loop, because the ECU is not using it to modify its timing and fuel. There are times when you want the motor to run a set amount of timing and fuel, no matter what the conditions. Closed loop at say partial throttle is to achieve the best possible fuel mileage. Open loop at say WOT to allow the engine to run the safest it can, and that the factory fuel map is not modified in any way. This is a great example of why you need a tune altered for say a stock 4.6 2v vs a 4.6 2v with head work, cams, turbos, bigger MAF, fuel system changes, and so forth. You are going to need to change those fuel/air maps, and timing to allow this motor to run as safe as possible.

In no way am I trying to say that electrical issues can not cause symptoms/issues. It could be something like a bad ground that’s not always getting good connection. A weak battery, or anything else electrical related. His options are to take the car to a reputable shop and have a diagnostic done, or seek advice here and help educate himself. I’d personally select the last one of the two. It’s always going to be more beneficial to learn something vs paying someone to just fix your issue. We just got to try and refrain from arguing over what we think it is and give the best sound advice we can and allow him to troubleshoot.
 
#28 ·
I would “guess” it’s fuel system related. However I do not have the car in front of me to diagnose. The best I can do is go off the information the OP gave and give my opinion. No way or form am I trying to claim to “know” exactly what is wrong here. I’m curious if there really is or is not any DTC’s and if the OP actually scanned for any or just assumed there wasn’t any because the CEL isn’t on.
 
#31 ·
Once again thanks for all the input. Managed to get some time to work on the car. Changed the fuel pump, fuel filter, and filler neck. Seems to have fixed the problem 🙌🏼. I do have a P0460 cel for the fuel lever circuit malfunction so the gas gauge isn’t working but I’ll take it over the random bucking and not getting above 40km/hr.
 
#33 ·
P0460 - Fuel Level Sensor Circuit Malfunction

The PCM monitors the fuel level input (FLI) circuit for electrical failure. The test fails when the signal moves outside the minimum or maximum allowable calibrated parameters for a specified fuel fill percentage in the fuel tank.

  • Empty fuel tank
  • Fuel pump (FP) module stuck open
  • Incorrectly installed fuel gauge
  • Damaged instrument cluster
  • CASE GND circuit open
  • FLI shorted to VPWR
  • Damaged PCM
  • Overfilled fuel tank
  • Fuel pump (FP) module stuck closed
  • Damaged fuel gauge
  • FLI circuit open
  • FLI circuit shorted to CASE GND or PWR GND
  • CSE GND shorted to VPWR

Monitor FLI PID and FLI V PID in key ON engine RUNNING. FLI PID at 25% fill (with non matching fuel gauge) and FLI V PID less than 0.90 volts [for FLI PID at 75% fill (with non matching fuel gauge) and FLI V PID greater than 2.45 volts] indicates a hard fault.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/
 
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