O2 Sensor Cleaning For Fuel Economy?
#16
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Year: 1990
Model: Cherokee (XJ)
Engine: 4.0
#18
CF Veteran
I don't believe it is. A vacuum leak will add air to the air-fuel mix, making it more lean, which will cause the ECM to add fuel. The ECM is monitoring the TPS and the O2 sensor, adjusting the fuel accordingly. The vacuum leak messes up the relationship between the TPS position and the amount of air in the mix. It's like having a mis-calibrated TPS.
I've never been able to wrap my head around an exhaust system under pressure pulling outside air in......
Not to mention how many Jeeps I've had with cracked exhaust manifolds, and when observing the fuel trims, oxygen sensor movement, and MAP readings with the factory tester, were all fine......
Not to mention how many Jeeps I've had with cracked exhaust manifolds, and when observing the fuel trims, oxygen sensor movement, and MAP readings with the factory tester, were all fine......
#19
CF Veteran
I don't believe it is. A vacuum leak will add air to the air-fuel mix, making it more lean, which will cause the ECM to add fuel. The ECM is monitoring the TPS and the O2 sensor, adjusting the fuel accordingly. The vacuum leak messes up the relationship between the TPS position and the amount of air in the mix. It's like having a mis-calibrated TPS.
I've never been able to wrap my head around an exhaust system under pressure pulling outside air in......
Not to mention how many Jeeps I've had with cracked exhaust manifolds, and when observing the fuel trims, oxygen sensor movement, and MAP readings with the factory tester, were all fine......
Not to mention how many Jeeps I've had with cracked exhaust manifolds, and when observing the fuel trims, oxygen sensor movement, and MAP readings with the factory tester, were all fine......
#20
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Year: 1990
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Then why do the fuel trims and oxygen sensor readings not change?
Not a race engine?
Not a race engine?
#21
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Year: 1998 Classic (I'll get it running soon....) and 02 Grand
Model: Cherokee (XJ)
Engine: 4.0
You might intuitively think so, but consider the "Log" intake manifold on pre 99 4.0s. If a vacuum fitting on the log leaks, that air entering is distributed in the vacuum air that does not have any fuel yet. Most leaks of this nature are compensated for by the IAC with no net effect. If not, it idles too fast.
Not so fast. The IAC can only adjust so much. If the leak is beyond that range, it will lean the mixture. And if you are above idle, the TPS issue kicks in. The air leak essentially throws the calibration off, as there is more air entering the cylinders than the ECM thinks there should be. Again, lean mixture.
#22
CF Veteran
Not a race engine. Headers only really start increasing performance when the intake and exhaust openings overlap. It is called scavenging. With enough overlap, you get unburned fuel and air coming out the exhaust before the intake stroke really gets going. Hence the crappy idle
#23
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Year: 1990
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I still don't buy it based on the empirical evidence I've seen.
#25
Senior Member
Thread Starter
Found what could be the source of a sizeable vacuum leak. Mickey mouse repair on a rubber vacuum coupling on the valve cover. I disconnected the vacuum line at the manifold and plugged it. We’ll see how/if things change.
#26
CF Veteran
That should go directly to the air cleaner. Shouldn't be any vacuum there. In fact you will have positive crank case pressure on that line
#27
Senior Member
Thread Starter
There is one near the front of the valve cover that goes to the air cleaner. This one towards the rear of the valve cover goes to the intake manifold.
#29
Senior Member
Thread Starter