4.0 Running Rich

[Bruce20]

Jeep had a check engine light for upstream o2 sensor when I got it and I replaced it with a Ngk sensor and it is running really rich. I can smell it and the exhaust is sooty. I’m getting a horrible 9 mpg. It definitely has a misfire because the rpms jump around at idle. Will new plugs fix both issues?


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[Jerry Bransford]

The O2 sensor replaced was located just below the exhaust header? How long since it got the replacement NTK O2 sensor?

 

[Bruce20]

1998 4.0 with the 2 o2 sensors and one cat. I changed it 200 miles ago and I thought it just took a little bit. Check engine light went out and it was still acting up so I thought I would post it up.


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[Jerry Bransford]

Does it run any differently or better when it's cold?

 

[Bruce20]

It runs the same when cold.


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[Bruce20]

It also smells really bad of gas when I shut it off or when I give it the beans, it smells like burnt fuel.


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[Jerry Bransford]

Engines usually run a little rich when cold but running rich when warmed up could be from any of several possibilities

-could be a bad IAT/temperature sender fooling the computer into thinking the engine is cold.

-bad coolant temperature sender.

-missing or incorrect thermostat, it must be a 195 degree thermostat.

-clogged air filter

-bad upstream O2 sensor wiring or connector.

What temperature is the gauge showing?

Just a few initial ideas.

 

[Bruce20]

If it was a bad thermostat, wouldn’t it not get up to the proper operating temp of between the 2 and 1 in 210 because it gets up to temp 6 minutes. I’ll also add that I’m losing antifreeze and I can smell it. That is another problem I’m searching.


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[Bruce20]

Air filter was clean, sensor is new and it cleared the check engine light by itself.


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[ohiopatriot]

Pull the vacuum hose at the fuel pressure regulator and check for fuel. With that low of fuel mileage and soot and smell at the tail pipe that's a high probability.

 

[59 wagon man]

I think we just chased a similar problem of running rich in my 59 chevy. it has a 383 stroker with fuel injection. the fuel pressure regulator blew an internal diaphragm and it was dumping gas into the vacuum line attached to the regulator

 

[Bruce20]

Pull the vacuum hose at the fuel pressure regulator and check for fuel. With that low of fuel mileage and soot and smell at the tail pipe that's a high probability.

So what do you think the problem is?


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[Jerry Bransford]

The TJ's fuel pressure regulator doesn't have a vacuum line connected to it, plus it's really just a fuel pressure limiter.

OK thermostat is ok. Was the IAT sender replaced?

 

[ohiopatriot]

Regulator diaphram ruptured.

Located on fuel rail. There is a rubber diaphram inside.

 

[Bruce20]

Alright, so what am I doing? How do I pull a vacuum line if there isn’t one?


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[Bruce20]

The TJ's fuel pressure regulator doesn't have a vacuum line connected to it, plus it's really just a fuel pressure limiter.

OK thermostat is ok. Was the IAT sender replaced?

I don’t know what the IAT sender is, so I did not replace that.


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[Jerry Bransford]

Regulator diaphram ruptured.
View attachment 60951

Located on fuel rail. There is a rubber diaphram inside.

The TJ's 4.0 doesn't have that. Its fuel pressure regulator sits on top of the fuel pump assembly on top of the gas tank.

The 2.5L engine has a pressure damper on the fuel rail, that's what that Omix Ada device is. That is covered at
http://m.4wdmechanix.com/?url=https://www.4wdmechanix.com/Jeep-Fuel-Pressure-Requirements?r=1&utm_referrer=#2810

"1997-2002 four-cylinder 2.5L MPI engines use a device similar to the pressure regulator. This is a damper, however, and not the fuel pressure regulator. These models use a filter/regulator at the fuel tank like the 4.0L chassis. The damper is strictly to offset electric fuel pump pulsing and smooth out pressure at the fuel supply rail."

This is the TJ's fuel pressure regulator which is used with all TJ engines ...

 

[Bruce20]

Can I test the IAT sensor?


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[Jerry Bransford]

I don’t know what the IAT sender is, so I did not replace that.

That's the intake air temp sender and it's screwed into the intake manifold. Its location varies but this is typical...

 

[Bruce20]

That's the intake air temp sender and it's screwed into the intake manifold. Its location varies but this is typical...

View attachment 60955

Can I somehow test it before I buy a new one?


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