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post #1 of (permalink) Old 05-09-2008, 09:50 PM Thread Starter
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Cracked maifold symptoms

I have had some odd symptoms lately with my TJ. It acts like it is not getting enough fuel when you rev it up from an Idle. It is a bit sluggish when going from stopped to moving in first gear. I was cleaning the carb today. I have had a ticking when cold for a while, but it went from a tick to a woosh. So after investigating I find a crack right down the middle of my manifold. I have had it throw a few codes relating to o2 sensors but nothing that stayed after a battery disconnect.

So Would a cracked mainfold cause poor running? It leaks enough exaust that I got a bit swimmy headed while trying to diagnose it.

Ya only go around once, best to enjoy it the first trip.
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post #2 of (permalink) Old 05-09-2008, 10:36 PM
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It could, yes... it would throw off the 02 sensor readings.
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post #3 of (permalink) Old 05-09-2008, 11:22 PM
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Yes, definitely.

The exhaust actually pulsates, so it's drawing fresh air in as well as blowing exhaust out.
The fresh air gets in and causes the O2 sensors to read the wrong values, affecting everything.
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post #4 of (permalink) Old 05-10-2008, 05:12 PM Thread Starter
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New manifold will be in monday. Hopefully if the weather holds out I will get the jeep apart monday as well and maybee back together tue. It is not that difficult, just takes time.

Ya only go around once, best to enjoy it the first trip.
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post #5 of (permalink) Old 05-10-2008, 06:22 PM
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Quote:
I was cleaning the carb today.
So, when did TJ's have carbs?

I recall an issue that RRich had where there was an arch in the coil rail causing what sounds like similar problems you are having.

I really don't think a cracked manifold will cause that big of a problem....

Rich....can you add some input?

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post #6 of (permalink) Old 05-10-2008, 08:04 PM
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Anything's possible. But he said he first heard, then found a crack. I assume it's the exhaust manifold.

The exhaust pulses, especially that close to the head. Most of the time it's pressure in the manifold, but between pressure pulses it actually goes negative pressure. That's the principle used by Pulse Air Systems, even headers work on the pulsing principle.

We don't usually think about that negative pressure, but it's there. Even stock manifolds use that principle to scavenge the exhaust from cylinders - just like a header.

Sometimes we can see that negative pressure at the tip of the exhaust pipe too. Hold a piece of paper against the pipe's tip, you can see it flap, especially at a slow idle. But sometimes you can't see it, because the long pipe, cat, and muffler act as a big damper.

When it's during the negative pressure, it's pulling air in. The Oxygen in the "false air" gets on the O2 sensor(s), giving a wrong signal to the PCM, causing the engine to run funny. Not only mixture but timing can be affected by that extra O2 that wasn't from the engine.

Yup, no carbs on TJ's, but he's probably thinking of the throttle body.

Tip on finding an exhaust leak up front - dark night outside, strong pencil beam flashlight like a mini-mag lite, and carb cleaner.
Turn out the lights, pour a little carb cleaner down the throat - it'll smoke. Shine that pencil beam around the manifold, head pipe, O2 sensor etc - if it's leaking you'll see the smoke from it and the source.

Be careful not to breathe the fumes - they can be deadly.
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post #7 of (permalink) Old 05-10-2008, 08:37 PM
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Yes, the 'false air' can cause an issue with the o2 sensor....but....by the time the 'false air' gets down to the 02 sensor, it is so diluted that I just don't see how it can cause the problem Jeff is noteing.....

My vote is something else....in addition to the cracked exhaust manifold.

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post #8 of (permalink) Old 05-10-2008, 08:45 PM Thread Starter
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I don't need any help finding it, it is about 1 1/2 inches long and may well be cracked in two. There are actualy 2 cracks.

As for this issue I have had hesitation and timing issues for a year or so and the manifold has made noise for at least that long, but when it would warm up the noise would go away and the jeep would run right. Over the last few months the noise wasn't quieting, the engine timing seemed off and the engine ran like it was starved for fuel but smells ritch. Stumbles to a start and when you try to rev it up from an idle it stumbles. If you get past that say to 1200 rpm, runs fine, let of to shift and right back at it. The jeep actually sounds like it is sucking air straight down the intake as if the butterfly were wide open.

Ya only go around once, best to enjoy it the first trip.
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post #9 of (permalink) Old 05-10-2008, 10:35 PM
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Dawg - there could be other issues too, certainly can't rule that out. Yes, it's diluted, but it really doesn't take much extra O2 to screw things up. It's surprising how sensitive it is.

Some systems divert a little air from the air pump to "upstream" during warm-up to trick the ECM into thinking it's too lean so it richens up. It's their "choke" or cold enrichment. So we know it works.

At idle and low speeds the duration of negative pressure is longer, allowing even more O2 in through a leak. That makes it most pronounced at the low end.
Even a loose/leaking headpipe gasket has been known to cause problems.

I've never had much luck trying it, but some folks say you can blow air pressure where you think it may be leaking. Any change in RPM says it's leaking.

A smoke machine can also find one.

Us old guys have a tough time visualizing how and why closed loop is so touchy. Action - reaction - which changed your action - different reaction now, changing your action again.

Who'd ever thought a vacuum leak can make it run rich?" Or lean, or no change at all?
"Things is jus gettin too complicated!"

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post #10 of (permalink) Old 05-13-2008, 05:16 PM Thread Starter
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Well lets just say the manifold didn't get replaced today. I got it torn down to the point that I could remove the manifolds and thought I should loosen the hardes ton first so I went about putting together some extensions to get to the rear bolt only to find it was already gone. It had aparently broken off prior to this incident.

So knowing I din't have the time or patience to tackle it before the bash I will be arriving tomorow evening on time with a cracked manifold.

Ya only go around once, best to enjoy it the first trip.
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