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Intermittent Rich Mixture And CEL.

AlphaWhiskey

NAXJA Forum User
Location
SE PA
I have a 1996 Jeep XJ Classic 4.0 Auto.

I am at my wits end with this thing. So ever since I first bought it 2 years ago its had bad fuel mileage. As in at best 150 miles per tank average is about 120.More recently I have had and intermittent Rich smell and Low power followed by a CEL for the Downstream O2 sensor heater. It also takes forever to warm up and usually wets the ground with fuel under the exhaust.

Things I have Replaced/tested

upstream O2
downstream O2
MAP sensor
Coolant sensor.
IAT sensor
TPS sensor
Tune up (Cap,rotor,plugs Fuel filter) This actually made the mileage worse.

I do have a small crack on the exhaust manifold, I am unsure if this is enough to make this much of an impact.

Anyone have any ideas.

If need be I can post Graphs of the o2 output.
 
Did you install Bosch or NTK O2's ? Did you test the ignition coil ? Is the thermostat functioning correctly ?

A cracked exhaust manifold will let fresh air into the exhaust stream and trick the ECM into thinking the mixture is too lean. This should be fixed.
 
If it's not warming up quickly it won't go into closed loop. If it has one of those fail safe thermostats or other nonsense, replace it with a proper Mopar or equivalent 195F thermostat.

NTK O2 are said to work the best, and as far as the crack in the manifold, lots of people have cracked manifolds and no all report horrible fuel economy, so it probably depends how bad yours is cracked.
 
Check out this thread on testing the heater, relay and power to the heater on the O2 sensor.

http://www.naxja.org/forum/showthread.php?t=1012701

Most really bad fuel economy issues are related to the O2 sensor, but as others have said anything to do with spark quality and timing and fuel feed and compression can be issues.

For sure it sound like the excess fuel has damaged the Cat converters and is now partly blocking your exhaust. They melt from the heat of handling excess fuel and close off or close up the gas path ways in the honeycombs.
 
Leaky or lazy fuel injector? Do a fuel pressure leak-down test. Then also test the fuel pressure while at idle.

Awesome suggestion, fuel pressure leak down test. I need to do that before I accuse my O2 sensor system of taking a vacation the last month or two. I overlook that one all the time.
 
Last edited:
Its has about 50-55 Psi of fuel pressure constant.

I have just recently replaced the thermostat, Once it comes to temp it holds fine but it takes forever.

I replaced the o2's with Bosch o2s and it really did not seem to make a differance.

Have not done a pressure leak down test yet.


Another oddity. when I was testing for vacuum leaks, I introduced a vacuum leak ( disconnecting the booster) The idle cleans up and it runs smooth.

Also Here is a graph of the upstream o2 voltage at idle.
8ttUjXb.png
 
This sounds like a stuck open T-stat for one

"It also takes forever to warm up"

Check the large wire to the rear O2 sensor after the Cat converter to see if it got too close to something and got damaged.
 
This sounds like a stuck open T-stat for one

"It also takes forever to warm up"

Check the large wire to the rear O2 sensor after the Cat converter to see if it got too close to something and got damaged.

Will do after work today. But the fueling problem has persisted way before the o2 CEL and has been slow to warm up even after a brand new 195deg t-stat.
 
Verify temperature with an IR thermometer.
Verify cooling system is working as designed (no air, circulation, maybe try a different brand of thermostat).
With a failed open thermostat you can get very bad fuel economy and low heat.
 
It is well known that Jeep 4.0L and Bosch O2 sensors do not play well together. Use NTK.

x2 on the thermostat, even below freezing it shouldn't take long to warm up.
 
Brand new parts including T-stats can be bad right out of the box. I test mine with boiling water before installing them. Focus on the T-Stat, CTS and complete O2 sensor and O2 wire and O2 heater relay system.

Does the OBDII show it in closed loop?
 
Bad or sluggish coolant sensor. Check the engine temp with an IR thermometer. I know
that you changed the coolant temp sensor - but did you buy a name brand sensor
(MOPAR, Standard, etc.)? A bad or sluggish temp sensor will send the wrong signals to
the PCM, and then the PCM will think that the engine is cold and tell the fuel injectors
to shoot more fuel into the engine than it actually needs. Before buying another sensor,
check the wire connector. give it a "wriggle test" (...wriggle the wires there and see whether
if the temp gauge is affected).
 
Its has about 50-55 Psi of fuel pressure constant.
[/IMG]

When you say constant, do you mean it stays in that range over the full rpm range?

Not familiar with the HO setup, but on Renix, the vacuum is used to keep the pressure constant relative to the intake manifold pressure.
 
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