Can cracked intake manifold ruin gas mileage?

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Old 08-30-2010, 10:31 AM   #1
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Default Can cracked intake manifold ruin gas mileage?

My 98 XJ is all stock (other than 3" lift). Just over 80K on odometer. I am getting 10 mpg in city driving , just over 200 miles per full tank. The tire pressure is within specs, brake calipers about one year old and not seized. The oil looks OK and the engine is not burning any oil. I don't smell gas and there are no signs it is leaking fuel somewhere. The intake manifold has crack and and I had CEL which disappeared on its own. There is no error code in stored that I can scan. This jeep was getting 23 mpg on highway when I drove it from NJ first time, with half deflated tires, wheel alignment out of whack and seized calipers! Can this cracked manifold causing the jeep running rich? The engine running smooth, does not misfire or overheat. Please tell what you think, I just can't afford this any more

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Old 08-30-2010, 10:33 AM   #2
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Have you done a tune-up lately?

What Are the Signs of a Leaking Intake Manifold?

A leaking intake manifold can severely disrupt a vehicle engine's performance and cause a variety of abnormal engine signs and symptoms. Leaky intake manifolds prevent the adequate flow of air and fuel into a vehicle's engine and combustion chambers, a situation that has far-reaching impacts on engine performance, functionality and reliability.

  1. Engine Miss
  2. Many times, a vehicle with a leaking intake manifold experiences an engine miss, or an irregular engine cycle. A leaky intake manifold allows intake gases (air and fuel) to leak out into the outside air, disrupting normal engine combustion, which leads to an engine miss.Engine Noise
  3. A leaking intake manifold allows engine intake gases to escape during engine operation.These escaping gases, which are traveling at very high speeds, can be heard as loud hissing sounds as they leak out of a leaky intake manifold.Engine Hesitation
  4. By interfering with the amount of air/fuel mixture that reaches a vehicle's engine, a leaking intake manifold negatively impacts engine combustion and power. This can cause a vehicle to hesitate or stumble, especially upon acceleration from a stop.Low Engine Power
  5. It's common for a vehicle with a leaky intake manifold to experience reduced engine power. This is due to the amount of air/fuel mixture that escapes from the leaky section of intake manifold. The wasted air/fuel mixture gases impedes engine combustion, which in turn causes a power loss.Hard Starting Condition
  6. A leaky intake manifold can make a car hard to start. By allowing a large portion of the air/fuel mixture to escape rather than enter the engine, a leaky intake manifold prevents normal engine combustion from occurring, which in turn affects vehicle starting, especially a vehicle that has been sitting for long periods of time.
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Last edited by muddeprived; 08-30-2010 at 10:36 AM.
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Old 08-30-2010, 03:04 PM   #3
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Yes, it can reduce fuel mileage greatly and like Mudd posted, all of that is definetely true too. Go find yourself one from a JY and put a new intake/exhaust manifold gasket on there too......while you're at it, you may as well look to see if your exhaust manifold is cracked down by the collector because this will reduce fuel mileage too.
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Old 08-30-2010, 05:13 PM   #4
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A leaking intake manifold will mean that unmetered air is entering the cylinders from the outside due to the sucking nature of the downward motion of the piston, this means that the engine will run lean on mixture and will cause loss of performance and petrol mileage per gallon. if it was the exh manifold then you will indeed hear a blowing noise as the air is pushed out of the cylinder due to the piston coming up and pushing gasses out. this can also decrease the performance of the engine.
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Old 08-30-2010, 07:14 PM   #5
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Originally Posted by ffcgary1 View Post
A leaking intake manifold will mean that unmetered air.
I'm curious how that air is 'unmetered' considering it utilizes a manifold absolute pressure sensor as opposed to a mass air flow meter/sensor...

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Last edited by 89xjgrey; 08-31-2010 at 08:16 AM.
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