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Old 04-23-2016
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Location: Shelbyville, Indiana
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Rich fuel and chugging at idle

Before I take this to a mechanic I was seein if anyone would know what's causing this problem, my 94 4.0 runs and drives fine, but when you stop or let it idle, it starts chugging and eventually dies unless you lay into the gas, it's running really rich, you can smell it and it burns your eyes, and I'm pretty sure that's why it's chuggin, I checked for vacuum leaks and didn't find anything, I replaced all my plugs and wires and had my coil pack tested, I checked my IAC valve and my mass airflow sensor and they're fine, and I was told it could be O2 sensors, throttle position sensor, and exhaust leak and a bunch of other stuff, and I don't just want to replace a bunch of stuff until I eventually find it, the check engine light is on but I tried to put a scanner on it and it kicks the scanner off when I turn on the ignition and won't read it, if anybody knows what I could do to test it to figure it out or had the same problem and knows what to fix please help
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Old 04-23-2016
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No on the O2 sensors, if they haven't been replaced before then they need to be for best MPG, but they wouldn't cause your problem.
They last for 150k miles, then MPG starts to drop slowly, not much, but new ones will pay for themselves over the next 150k miles.

First thing to check would be the Fuel Pressure Regulator(FPR).
Passenger side top front of engine, it has the Return Fuel line connected AND a vacuum line.
You need to check if this vacuum hose has fuel in it.
FPR diaphragm can leak and fuel is sucked into the intake, causing what you describe.
Highest vacuum at idle so highest amount of fuel sucked in, then when driving lower vacuum so less fuel and you are using more fuel anyway, higher RPMs.

Replace FPR if fuel is found in Vacuum line.

Next is Sticking fuel injector, when you changed the spark plugs did one look much darker that the others?
Sign of injector not closing all the way and leaking fuel
You can do a fuel pressure test, '94 system should have 30-40psi when running and then hold pressure above 25psi after it is shut off, and it will hold that pressure for a few months, not minutes, lol.
And a system that doesn't hold pressure could have a bad FPR or leaking injector, even bad check valve on fuel pump but that wouldn't cause current issue.

Exhaust leak at the Exhaust Manifold can cause Richer running, only because the O2 sensor on that side "sees" extra oxygen which means Lean burn to the computer.
An exhaust leak also sucks IN air it doesn't just blow exhaust out.
So a pipe or muffler leak wouldn't cause that issue.

O2 sensors only "see" oxygen levels not fuel, too little oxygen means Rich, too much oxygen means Lean
If an exhaust leak is after the O2 then it would have no effect on lean/rich

1994 was last year of OBD1 protocol, ODB2 was used in 1995 and up vehicles
You can try reading codes via meter or CEL(MIL) read here:

But from your description the codes will be "engine running rich", lol, and you already know that.

Other sensors or controls(like IAC) can't really cause what you describe, almost has to be a mechanical failure, i.e. leaky FPR or injector

Last edited by RonD; 04-23-2016 at 03:28 PM.
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