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Unread 07-05-2013, 10:22 AM   #1
JayRay08
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Exhaust leak question (o2 sensor)?!

If i have an exhaust leak high up can it screw with the o2 sensor creating a rough idle? and if so will it throw a check engine light on? I have a 96 jeep grand 4x4 5.2L and almost everything else sensor wise has been replaced (with the exception of the o2 sensor & crankshaft position sensor). just trying to pinpoint this sparatic rough idle. Thanks

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Unread 07-05-2013, 11:00 AM   #2
Uniblurb
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So do you know you have an exhaust leak and where is the "high up" location? Yes, it can screw with your o2 sensor and heat up the whole engine compartment if it's around the exhaust manifold/header.

For a rough idle the 1st thing you should do is remove the throttle body and clean it along with the IAC. Just don't move/twist the pintel and here's a real good thread for cleaning the IAC.

http://www.jeepforum.com/forum/f13/h...4/#post7047001

Here's a couple tests for the crank sensor and o2 sensors. When testing the crank sensor make sure you have the black probe on the multimeter set to the lowest ohm setting on the middle B pin (ground cavity) and the red probe on the far right C pin with the locking tab on the connector pointing up. Any resistance at all and it's bad. Only go with Mopar sensors or your looking for problems.

On the o2 test it's just checking your element for resistance and not the heater on the o2 sensor itself. Good luck!
crankshaft-positon-sensor-test-p.-8d-8-fsm.jpg

o2-sensor-element-test-fsm-p.14-49.jpg

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Unread 07-05-2013, 11:12 AM   #3
dellis
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Quote:
Originally Posted by Uniblurb View Post
Yes, it can screw with your o2 sensor and heat up the whole engine compartment if it's around the exhaust manifold/header
Really? I don't understand how an exhaust leak in the exhaust manifold/header would cause an erroneous O2 sensor reading, since this would only affect the volume of exhaust gasses, and not the composition of exhaust gasses. Even with a hotter engine compartment, I don't understand how this would affect the O2 sensor reading. I'm open to education.

But OP, as Uniblurb was suggesting, if you have a rough idle you should be looking for a cause other than an exhaust leak. Vacuum leaks (including one at your intake manifold) and IAC problems are more likely. Have you checked those?
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Unread 07-05-2013, 12:00 PM   #4
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Quote:
Originally Posted by dellis View Post
Really? I don't understand how an exhaust leak in the exhaust manifold/header would cause an erroneous O2 sensor reading, since this would only affect the volume of exhaust gasses, and not the composition of exhaust gasses. Even with a hotter engine compartment, I don't understand how this would affect the O2 sensor reading. I'm open to education.
I dont have a lesson plan, but I'll give you the backyard mechanic or tribal knowledge version. Leaky exhaust before the o2 sensor not only lets exhaust out, it allows air in. The result of extra air in the pipe is a rich fuel mixture.
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Unread 07-05-2013, 01:32 PM   #5
dellis
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Originally Posted by ZeeJay1997 View Post
I dont have a lesson plan, but I'll give you the backyard mechanic or tribal knowledge version. Leaky exhaust before the o2 sensor not only lets exhaust out, it allows air in. The result of extra air in the pipe is a rich fuel mixture.
Um, but at no point would there be a vacuum in that area, so as to draw in air. It is under pressure greater than the outside air, and there should always be an outflow of exhaust gasses ... are you thinking of a huge hole?
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Unread 07-05-2013, 02:20 PM   #6
JayRay08
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hey guys, thanks for the great discussion. my IAC is brand new and i pulled the TB and scrubbed it clean with TB cleaner and a toothbrush. i know i have an exhaust leak because i can hear it (pretty loud). i was going to makeshift a fog machine i have to smoke the vacuum lines and the exhaust to pinpoint any vacuum leaks but im pretty sure the exhaust leak is high up because i sea foamed it and smoke was coming from high up but ill hopefully get this fog machine thing to work tomorrow.
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Unread 07-05-2013, 02:32 PM   #7
ZeeJay1997
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Quote:
Originally Posted by dellis View Post
Um, but at no point would there be a vacuum in that area, so as to draw in air. It is under pressure greater than the outside air, and there should always be an outflow of exhaust gasses ... are you thinking of a huge hole?
are you familiar with an air powered vacuum cleaner? same concept. of course it will be entirely dependent on the characteristics of the crack, but it can happen.
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Unread 07-05-2013, 03:40 PM   #8
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And if you look at some of the cracks in the pics on this forum, you'd swear pigeons could change the 02 readings,cause they are big enough for a nest. But here are some other links on 02 introduction.

http://www.ehow.com/list_5746543_eff...-manifold.html

http://honda-tech.com/showthread.php?p=47234733

http://forums.off-road.com/jeep-shor...-symptoms.html
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Unread 07-05-2013, 08:16 PM   #9
dellis
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So, after some reading, I think I may have this straight.

As an exhaust valve opens, exhaust gas is pushed out toward the catalytic convertor, creating positive pressure that pushes gas out the tailpipe. At this point, there is a momentum of air that is flowing out the exhaust. When this exhaust valve closes, the air flowing in the tailpipe will continue to flow based on its momentum and a siphon effect is created. This siphon effect creates a very short duration vacuum very close to the exhaust head. The pressure is lowest at the exhaust valve and increases toward the catalytic converter.

So, if a leak exists at the exhaust head gasket, then fresh air could be pulled into the exhaust at that point because there is a vacuum, and thus affecting the O2 sensor reading. But, the further away from the valve and the head gasket one goes, the less likely that this vacuum will have any effect on drawing in fresh air.

So, my assumption is that exhaust leaks at the exhaust manifold gasket or very close to the exhaust manifold gasket could influence O2 readings, but cracks further along the exhaust course (say, at the flange) would not influence O2 readings.
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Unread 07-05-2013, 08:41 PM   #10
ZeeJay1997
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A leak near the manifold will also burn the exhaust valve because the extra oxygen causes hotter combustion.

By virtue of the way exhaust systems are constructed, a leak at a joint will suck air down the pipe, most notably at the collector.
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Unread 07-05-2013, 08:49 PM   #11
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Dont forget that the other exhaust valves are opening right after the one being discussed has closed, causing a near continuous flow out the pipe....which could....rather...it WILL.. suck in outside air from the leak which will be detected by the sensor and the pcm will act accordingly by enriching the mixture, I assume.
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Unread 07-05-2013, 09:10 PM   #12
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Quote:
Originally Posted by dellis View Post
Really? I don't understand how an exhaust leak in the exhaust manifold/header would cause an erroneous O2 sensor reading, since this would only affect the volume of exhaust gasses, and not the composition of exhaust gasses. Even with a hotter engine compartment, I don't understand how this would affect the O2 sensor reading. I'm open to education.

But OP, as Uniblurb was suggesting, if you have a rough idle you should be looking for a cause other than an exhaust leak. Vacuum leaks (including one at your intake manifold) and IAC problems are more likely. Have you checked those?
A leak or crack in the manifold/header will cause "erroneous" o2 sensor readings. Actually they aren't erroneous/false because the o2 sensor is doing exactly what it's supposed to; determine how much oxygen is in the fuel mix and adjust the voltage accordingly while communicating with the PCM. And with a crack/leak it will let more air in hence more oxygen.

While it may seem like there's always exhaust pressure to prevent air from entering it's actually an exhaust pulse and not continuous. Also when you let off the accelerator air is pulled into the exhaust leak and have you ever heard an exhaust/engine pop/cough as cold air is drawn in? Particularly if there's little back pressure?

I had a cracked manifold/header where the tubes are welded together which is typical for a 4.0. I rode around for days with my scanner/reader attached logging data trying to figure out why my o2 sensor readings were jumping all over, particularly the upsteam sensor. This sensor is about 2' down from where the header was cracked. My 96 4.0 was also running real rich and couldn't figure out why. Tested the o2 sensor element, checked out ok, but bought a new OE/NTK one anyhow. Then I replaced my exhaust manifold and the upstream sensor settled right down with a normal fuel mixture. Still have the new NTK o2 sensor but see no reason at this time to install it.

So guess I do have some first-hand knowledge a cracked or leaking manifold/header will affect the way an o2 sensor functions.
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Unread 07-05-2013, 10:58 PM   #13
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Very good discussion. you all rock. Thanks
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Unread 07-06-2013, 04:13 AM   #14
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I will need to get a whole new exaust for my 5.2L in a little while. What are some good choices in a mid price range?
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Unread 07-06-2013, 07:05 AM   #15
JayRay08
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Correct me if im wrong but i heard if you want headers there 700 bucks. If you dont want headers youll need to replace that crush section on the y pipe from the headers. Then youll want 3inch pipe all the way thru with a new magnaflow cat and a flowmaster muffler. im looking to get a new exhaust myself and if i read correctly this is the way to go. again please correct me if i said something wrong
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