Help me fix grandma's mustang please - Mustang Evolution

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Old 06-22-2014, 12:29 AM   #1
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Unhappy Help me fix grandma's mustang please

1996 Mustang GT convertible. 76,000 miles on the clock. Rarely driven. Still has poly intake and will update that when I visit granny next as there is a slight drizzle by the temp sensor, but don't think it's related.

I show up to help with things around the house and started driving her Mustang to give it some exercise. Runs great. She said she had to get towed home a year ago because it didn't start, but started later on. It's been sitting, but she starts it up everyweek. She even puts in Lucas fuel injector cleaner at last fill up.

So I give it exercise, go through the gears, it squaks em hitting second. Fast, runs great. She drove it to her work and had to get towed home as it wouldn't start, or would barely start then stumble, then go 20 ft then stop. It's now at half tank.

Get's towed home, I get fuel pressure guage kit from autozone. Look for 30 Psi and 38 with regulator vacuum hose disconnected and plugged. Boom get 38-40 psi. Car runs perfect. I let it idle, get hot for hour. Drove it let it idle, everything was fine.

I take it to walmart that night, totally fine. Next day I run to home depot and it sputters a little. Check engine light comes on. I get the code P0175. Lean or rich mixture detected 02 sensor right bank switch off? I think. I clear the code.

Now after 180 miles various driving, no codes. Now here is the problem. At full throttle right at 4,000 RPMs there is a loud popping by the engine compartment. Maybe backfiring into the intake or afterfiring in the exhaust. It is only at 3/4 to full throttle at 4,000 RPM and higher. Pulls hard up till then. I drained tank, clear and slightly yellow fuel. No sediment. Replaced fuel filter. Same thing. Still pops. So after searching I read a guy filled his tank and problem went away indicating fuel pump. He replaced it and took care of problem.

I filled it up. Still popcorn pops above 4,000 RPM. Its like clockwork. Sounds like someone popping off a 22LR randomly. I checked injectors 15.6 ohms. I drove it with fuel pressure regulator vacuum off, Mass Airflow Sensor unplugged, everything. It acts the exact same. Light turns green, I punch it, it lays rubber pulls hard, and whamm starts popping at 4grand. I even hooked up the fuel pressure guage under the hood to the wiper, I drove at regular speed getting 30 PSI full throttle it goes up to 38psi and is at 38psi while sputtering.

I do not thing its a plug or wire issue. I think the fuel pressure regulator is doing fine, The fuel pump maintains pressure even at full throttle for a while. So I don't think it's that.

Last thing, if I drive in first gear and ease slowly on up to 5300 RPM or so no pops, but if I give it full throttle it pops right away. I was thinking catylatic converter, but that usually prevents RPM like a brick wall right?

I'm at a loss and I don't want my widowed granny to get took by a mechanic while I'm gone. Planning to try cam position sensor and o2 sensor and maybe hit cats with mallet to see if screen is loose.

Please help (sorry for long post, but wanted to give all info).
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Old 06-22-2014, 11:22 AM   #2
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You have done an excellent job diagnosing this problem.

I'd be suspicious of the 02 sensor.
If you search through cliffyk's posts, he links to a method for testing 02 sensors using a propane torch.
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Old 06-22-2014, 04:30 PM   #3
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My thought is fuel pump. On my 96 I had the exact same symptoms, I diagnosed very much in detail as you have and still can't believe that changing the fuel pump solved the issue. I did all of the Maintinance items first to no avail, then changed the fuel pressure regulator and then the fuel filter and still the popping at WOT, finally dropped the tank and up graded my fuel pump and no more popping and having the car fall on its face. If you do decided to change the pump make sure to also get a new filler tank grommet so you won't have to repeat the awkward exercise of dropping the tank again.
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Old 06-22-2014, 08:00 PM   #4
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And you still had all the pressure on the fuel guage?
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Old 06-22-2014, 08:09 PM   #5
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Would you be so kind as to go to RockAuto Parts Catalog and click through the options to the 4.6 SOHC and tell me which fuel pump is the right one? Can I just replace the pump independantly of the fuel gage sending unit? Or do I replace the whole thing. Autozone is $225 and i'm trying to be frugal with grandma's money.
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Old 06-23-2014, 02:16 AM   #6
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A stock performing fuel pump (loaded) can be replaced for around a hundred bucks, if your comfortable disassembling and wire splicing you can do just the pump for less. Rock auto has more choices than autozone but both have loaded and unloaded units. http://www.rockauto.com/catalog/more...483&cc=1134223
Is what I bought from autozone only because it came complete hanger assembly and pump all wired up, so I simply pulled old unit out and dropped the new unit in. Since then I have used my old hanger unit to build a dual 255 for my future upgrades. Make sure drain your tank before you drop it so it's more manageable and to replace any grommets or gaskets to ensure that you only have to do this exercise once, it's not that bad of a job and you should be able to do it under $150. And a couple of hours.
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Old 06-23-2014, 06:22 AM   #7
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If it's maintaining fuel pressure, then the pump is fine. Backfire (popping) under load (driving) can be ignition related. Have you done any troubleshooting in this area? Have you had the cat (or exhaust) checked for any backpressure issues?

You can toss parts at this for a long time; eventually you'll fix it, but at a large hole in your wallet.
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Old 06-23-2014, 06:33 AM   #8
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If it's maintaining fuel pressure, then the pump is fine. Backfire (popping) under load (driving) can be ignition related. Have you done any troubleshooting in this area? Have you had the cat (or exhaust) checked for any backpressure issues?

You can toss parts at this for a long time; eventually you'll fix it, but at a large hole in your wallet.

+1

A few years back my LT1 was doing the same thing. Ran fine unless I nailed it, then backfired like crazy. Took it to dealer and they threw a bunch of parts at it. After $800 it still did it. They wanted me to spend more but I was done. Brought it back home and parked it for the Winter. In the Spring had it running in the garage and noticed sparking by the main coil wire coming off my Optispark (distributor for LT1). I pulled the wire off and while holding it in my hand it literally just fell apart and landed on the floor. I picked up a new wire and replaced it, works fine since. I can't figure out how they did not discover this when they needed to unplug and replug this wire when replacing the Opti.
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Old 06-23-2014, 10:34 PM   #9
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My mechanic friend finally called me back. He's a troubleshooter too, not a parts replacer. He mentioned ignition too. I'm going to disconnect alternator tomorrow and drive it on battery around the block to see if that helps (you know, without the AC ripple). He said to check the plugs but to search first and see if these engines had spark plug breaking off problems. Will search bout that tomorrow.

Thanks for the info, I'll start troubleshooting ignition tomorrow. Thanks for steering me in the right direction.
Been building grandma a shed so no time for mustang yet.
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Old 06-28-2014, 11:29 PM   #10
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Update. So I disconnected the alternator and drove it. No change. I installed all new spark plugs. Then it ran super super great. Still some popping around 5500 RPM, but minor. Then next day I took it out cold and ran it. It ran flawlessly. When the engine warmed up it started the popping again around 4500.
I'm thinking it ran with the right air/fuel mixture while in closed loop mode (cold engine) then as the computer started receiving sensor input it leaned it out. I'm thinking 02 sensors now.

When grandma has intake changed to aluminum crossover, I'll suggest injector cleaning and replacing or at least testing o2 sensors.

Thanks for all your help.
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Old 06-28-2014, 11:41 PM   #11
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Originally Posted by GrannysMustang View Post
Update. So I disconnected the alternator and drove it. No change. I installed all new spark plugs. Then it ran super super great. Still some popping around 5500 RPM, but minor. Then next day I took it out cold and ran it. It ran flawlessly. When the engine warmed up it started the popping again around 4500.
I'm thinking it ran with the right air/fuel mixture while in closed loop mode (cold engine) then as the computer started receiving sensor input it leaned it out. I'm thinking 02 sensors now.

When grandma has intake changed to aluminum crossover, I'll suggest injector cleaning and replacing or at least testing o2 sensors.

Thanks for all your help.

Have you ruled out the plug wires yet? That was my problem. You were moving the wires around while you replaced the plugs, that might have been enough to make the problem come and go. Just a suggestion.
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Old 06-30-2014, 11:40 PM   #12
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they are newer 8mm wires. I ran it while it was pitch black and scarily ran my hand across each one. I didn't get zapped. Phew.... (My wife just asked how bad it would have been, I'm telling her it would have been worth it to figure out the problem...you all understand.)

Anyway, It ran perfect cold, and popped when hot. I'm thinking o2 sensor. I told her to keep the coolant full until she can do an intake swap (her neighbor or a mechanic) It needs it anyway. I also told her I'd go to rockauto and get her o2 sensors for her neighbor to install.

Am i on the right track with this diagnosis? REcap

I fired it up and romped on it cold. Ran perfect. once heated up it popped at the high RPM. I'm suspecting the o2 sensors causing it to lean out, which then gets the plugs hot causing preignition while the intake valve is still open which causes the backfiring. (backfiring is in intake, afterfiring is in exhaust just so our terms are understood.)

I think its the o2 sensors because in closed loop mode, the computer is injecting a generously rich mixture till the o2 sensors can judge the exhaust to make fine adjustments.

I believe this car has four sensors. A right bank, left bank, and two down the exhaust a ways. I was told the right and left are for mixture adjustments and the two down range are for emissions. Is this right?
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Old 07-01-2014, 12:12 AM   #13
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Correct
The two upstream sensors, in front of the cats, are what the computer uses for engine management.
The two downstream sensors, after the cats, are just there for emissions purposes to tell the computer if the catalytic converters are functioning properly.
You only need to replace the right and left side upstream 02 sensors.
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Old 07-01-2014, 12:14 AM   #14
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I like Motorcaft.
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Old 07-01-2014, 10:42 AM   #15
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Rockauto has motorcraft DY761. For right front. Are the two front the same? With a female connector 4 prong. I'm not at the car to check, left granny's.
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Old 07-01-2014, 06:08 PM   #16
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It looks like, on the 96's, both sides use the same 02 sensors, so yes, that is the correct one.
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