Cracked Exhaust Manifold?

Jmazza0

Jmazza0

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Joined
May 27, 2020
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8
Location
San Diego
Good evening everyone, I've recently noticed a drastic change in my Jeeps gas mileage from around 15mpg to now 9-10mpg. I'm pretty good at keeping up with the maintenance (no check engine lights), and I know my lift & 35" tires don't help but I've had those modifications on for the past 4 years I've owned my Jeep. I checked all my oil levels and they are just over full and still clean. I cleaned my air filter, and even tried adding some Lucas to my fuel to see if that'd help but doesn't seem to of had any effect.

From my recent research I'm seeing there is a chance it could be from a cracked Exhaust Manifold fooling the O2 sensor making it run rich. I've done some visual inspections and haven't found anything that looks like a crack, although it's hard because of how much old mud has been caked up on the pipes. it looks like I have aftermarket exhaust manifold from what I could see but I'm not sure when it was installed as my previous owner had to have done it.

Also my jeep does have a bit of a ticking sound while cold, not sure if that's also a sign of a crack?

As for my methods of testing fuel mileage, it's about 60 miles to the mountains from my house so I'll usually fill up before I go wheeling and reset the odometer once full. Then I will hit the last gas station before my turn off for the trails and do a quick calculation of the miles driven. I'll do the same thing for the ride back as most of the drive is on the interstate (I try to keep it under 65mph which is about 2400rpm).

What are your guys's thoughts for the sudden drop in mpg? Thanks for your time!
 
Bummer, it looks like you never got any replies to this one. I recently posted something similar here. If you're still on the Forum, can I ask what you ended up doing?
 
What motor do you have? If you have the 4.0, they all knock, especially cold. Change the oil every 3-5k and it'll be fine.

WRT mileage decrease, it could be anything. If you really want to check the exhaust manifold for leaks here's what I'd do:

With engine cold, soak the exhaust manifold in dish soap water. Heavy on the dish soap - you want it to stick. Run the engine and quickly look for any bubbles. Since the manifold gets hot it'll burn off fast. If you see bubbles, you have a leak.

Other things that could be impacting your mileage:
- Tire pressure
- Alignment
- Any additional weight
- Sticking brake caliper(s)
- Other engine loads (A/C, electrical, etc.)
- Drivetrain oil problems

Those are the first things I'd check.
 
Does your engine tick when you give it gas under load?

I just replaced my cracked exhaust manifold. One of the biggest changes I noticed was the ticking of the engine whenever I gave it gas was gone. Now, all of the exhaust noise goes out the back.
 
Windrac said:
What motor do you have? If you have the 4.0, they all knock, especially cold. Change the oil every 3-5k and it'll be fine.

WRT mileage decrease, it could be anything. If you really want to check the exhaust manifold for leaks here's what I'd do:

With engine cold, soak the exhaust manifold in dish soap water. Heavy on the dish soap - you want it to stick. Run the engine and quickly look for any bubbles. Since the manifold gets hot it'll burn off fast. If you see bubbles, you have a leak.

Other things that could be impacting your mileage:
- Tire pressure
- Alignment
- Any additional weight
- Sticking brake caliper(s)
- Other engine loads (A/C, electrical, etc.)
- Drivetrain oil problems

Those are the first things I'd check.
Click to expand...

Thanks for this. I have the 2.5L from 97, and I know it cracked once back in 2001, and was welded, not replaced (I think).

That's a good list, but I can eliminate all of them except for the drivetrain oil one. Can you say more about what the problem would be, and how I'd check for it?

In the meantime, I'll see if I can try your soap water test.
 
TJRick said:
Does your engine tick when you give it gas under load?

I just replaced my cracked exhaust manifold. One of the biggest changes I noticed was the ticking of the engine whenever I gave it gas was gone. Now, all of the exhaust noise goes out the back.
Click to expand...

Yes, I have a ticking sound. I'd describe it as a bit like if I had a leaf stuck somewhere, or like a piece of paper or playing card ticking against a fan blade or a bike spoke. It speeds up and slows down with my RPMs. I hear it most around 2800 RPMs, when I'm cruising gently enough to not be making a lot of engine noise.
 
Jmazza0 said:
Good evening everyone, I've recently noticed a drastic change in my Jeeps gas mileage from around 15mpg to now 9-10mpg. I'm pretty good at keeping up with the maintenance (no check engine lights), and I know my lift & 35" tires don't help but I've had those modifications on for the past 4 years I've owned my Jeep. I checked all my oil levels and they are just over full and still clean. I cleaned my air filter, and even tried adding some Lucas to my fuel to see if that'd help but doesn't seem to of had any effect.

From my recent research I'm seeing there is a chance it could be from a cracked Exhaust Manifold fooling the O2 sensor making it run rich. I've done some visual inspections and haven't found anything that looks like a crack, although it's hard because of how much old mud has been caked up on the pipes. it looks like I have aftermarket exhaust manifold from what I could see but I'm not sure when it was installed as my previous owner had to have done it.

Also my jeep does have a bit of a ticking sound while cold, not sure if that's also a sign of a crack?

As for my methods of testing fuel mileage, it's about 60 miles to the mountains from my house so I'll usually fill up before I go wheeling and reset the odometer once full. Then I will hit the last gas station before my turn off for the trails and do a quick calculation of the miles driven. I'll do the same thing for the ride back as most of the drive is on the interstate (I try to keep it under 65mph which is about 2400rpm).

What are your guys's thoughts for the sudden drop in mpg? Thanks for your time!
Click to expand...

Yes that's your issue a cracked manifold. I've replaced mine about to be for the 3rd time in 6 years. The good thing is that it has a lifetime warranty, but it is the point of doing it. I have a 99 Tj 4.0 Auto.
1st at 60k
2nd at 120k
Now at 149,653.
This is the only issue that I have ever had with my Jeep.
THE INFAMOUS TICK!!!
 
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