Granny's Mustang

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).

Granny's Mustang

Nothing? Did I do something wrong? I searched for two days before posting here.

Granny's Mustang

deleting my membership here.

MaxxRPM

I think no one is responding because it can be anything......Throttle Position Sensor?..............
Bad plugs?.................
Bad wires?...............
Coils? Bad wire connections? Vacuum leak? Blocked airway in the intake or maf?..............

It sounds like the went through the fuel system pretty good. Now it might be time to check out something failing in the ignition system. Either bad sensors or bad connections aswell.

JC316

Check for vacuum leaks. It's leaning out too much and causing your backfiring issue.

Granny's Mustang

I sprayed cleaner on engine in every nook and cranny. No rpm rise or stumble. New plugs, seemed to help but now its back to the same thing. I think its leaning out and it toasted the old plugs.
Nothing wrong with fuel system. I drove it with the fuel guage ran under the hood to the wiper. 30psi while cruising and 38 while full throttle for a long time.

Did the pitch black test while running to look for arcs. Did the "tense run hands across each wire and hope not to get shocked" test. No shock. I unpluged the mass airflow sensor and drove it like that. Changed nothing. I removed alternator to double check that a bad diode didn't cause AC ripple throwing off the cam position sensor.

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?

Z28KLR

Yes, the two upstream sensors are for engine management (air/fuel ratio, etc), the downstream sensors verify catalyst efficiency only.

See if there's anything here you haven't thought of yet.... www.obd-codes.com/p0175

calistang05

Had a similar problem with my 05 wanting to stall out and sputter plus a bank 02 sensor CEL as well I changed my plugs and my 02 sensor all 4, and still no fix. Finally manned up and took it to the stealership and it was my throttle body sensor that went bad. I'd get that check out as well.

Granny's Mustang

One thing about the stealership is that they have the test parts. They plug it in and then drive it. If it didn't work then do something else. Besides their diagnostic equipment. It all depends if they have a non-parts-throw-at-it lead mechanic there. Might take it there for the $99 diagnostic. might be worth the time.

Granny's Mustang

That website also says rich condition, it is definitely lean. Which the little autozone printout said "rich or lean". anyway...
Thanks for all of your input