Please Help Me Fix Grandma's Mustang

Discussion in 'SN95 4.6L Mustang Tech' started by Granny's Mustang, Jun 22, 2014.

  1. 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).
  2. ALWAYS have the alternator checked for excessive AC ripple (bad diode) BEFORE replacing the cam sensor. Alternator problems will cause false cam DTC's.

    Almost all auto parts stores will do the test for free.

    I don't think your problem is cats. Why? Because it makes good power.
  3. What does DTC mean? Something Timing Control?
    I know a lot about cars, how they work and how they function, but when it comes to computer controlled you almost have to know the system and how the system specifically tells each thing what to do, was expecting it to be something I could unplug and isolate via closed loop mode, but to no avail.
    #3 Granny's Mustang, Jun 22, 2014
    Last edited: Jun 22, 2014
  4. Diagnostic Trouble Code. Onboard in the OBD 2, It is for safety of the engine and driver, stores safety codes.
  5. Almost sounds like you are disappointed it could be something so simple and boring as a bad alternator.

    A bad alternator diode will create excessive AC ripple that will mess up the cam sensor signal. The forums are full of ppl that got a cam related DTC and replaced the cam sensor only to have the same code re-appear.

    Bottom line. Today's cars simply will not run right without a strong battery and charging system. Every problem should start off with a review of the charging system. Cut corners here at your own risk.

    DTC=Diagnostic Trouble Code. The codes that the computer generates when things go wrong.
  6. okay. there is no code from the cam sensor but I'll check it tomorrow. My previous post was just saying that I was disappointed to not find out what it was last night after all the troubleshooting.

    Thank you very much

    Also, does the intake vacuum leak detection with carb spray really work?
  7. I don't like to spray carb/choke cleaner all over a modern car. The harsh chemicals may damage gaskets. This trades one problem for another. I have a propane based leak detector that does the same thing without damaging gaskets.

    Smoke is good.

    I ran across a post once where someone blew cigar smoke and used a a leak detector.
  8. 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.