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Very Violent/Rough Idle

Old 05-11-2019, 10:05 PM
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Exclamation Very Violent/Rough Idle

Hi! I just replaced my Exhaust Manifold on my 1987 Jeep Cherokee XJ 4.0L 6cyl. When I went to start of the jeep the engine started roaring like I was putting the pedal to the metal. I Immediately shut off the truck, and looked under the hood. There was white smoke coming out from the front of the engine and smelt very bad, like burnt plastic. I than did some research and read that it could be because of a faulty fuel injector. Is this the case? Should I replace the fuel injectors? The fuel injector pigtails are all on the right injector... Please help!

Thanks!
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Old 05-11-2019, 10:56 PM
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What kind of injectors do you have? Stock?

Also it is common for white smoke to be present right after doing intake/exhaust gasket or new mainfold. Not the crazy high pedal to the floor board though....
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Old 05-11-2019, 10:57 PM
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No. A fuel injector will not do that. You can't make the motor run faster (higher RPMs) just by having more fuel added. Your motor runs faster (roars) because it is getting more air. Air is likely getting into the combustion chamber from some source other than the throttle body.

When you put on your new exhaust manifold and re-installed the intake manifold, it probably did not sit flush against the mating surface. So air is getting sucked in around the intake manifold. Usually, this is because the rear portion of the intake manifold is hung up on the rear most locating dowel. It is not easily spotted from the front and/or above. But 9 times out of 10, this is the cause.
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Old 05-11-2019, 11:04 PM
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^ What he said but still if you have stock injectors I would change them to 746 Bosch injectors from a volvo. Othere will work also bit that's what I used.

"......but my stock injectore are fine......"

Until they ain't. When your 3 hours from home and they start leaking at the seams. It isvery easy to change them over. Then if they start leaking all you need is an O ring. Always keep a pack of fuel injector O rings in your jeep, in case they start leaking when your way away from a store.
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Old 05-11-2019, 11:06 PM
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Originally Posted by Chick-N-Picker View Post
^ What he said but still if you have stock injectors I would change them to 746 Bosch injectors from a volvo. Othere will work also bit that's what I used.

"......but my stock injectore are fine......"

Until they ain't. When your 3 hours from home and they start leaking at the seams. It isvery easy to change them over. Then if they start leaking all you need is an O ring. Always keep a pack of fuel injector O rings in your jeep, in case they start leaking when your way away from a store.
That's a good tip. I currently do not carry any. I should remedy that.
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Old 05-12-2019, 06:22 AM
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If you cover the throttle body and the engine doesnt die you have a large vac leak. Covering the TB will also make the leak more pronounced. Dont cover the TB with anything that is dirty or can break apart. I use a 1/2 plate of aluminum that I keep specifically for this/runaway diesel.
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Old 05-12-2019, 06:33 AM
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Originally Posted by Chick-N-Picker View Post
^ What he said but still if you have stock injectors I would change them to 746 Bosch injectors from a volvo.

He shouldn't be changing anything until he gets this massive problem solved.
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Old 05-12-2019, 10:00 AM
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Originally Posted by jordan96xj View Post
No. A fuel injector will not do that. You can't make the motor run faster (higher RPMs) just by having more fuel added. Your motor runs faster (roars) because it is getting more air. Air is likely getting into the combustion chamber from some source other than the throttle body.

When you put on your new exhaust manifold and re-installed the intake manifold, it probably did not sit flush against the mating surface. So air is getting sucked in around the intake manifold. Usually, this is because the rear portion of the intake manifold is hung up on the rear most locating dowel. It is not easily spotted from the front and/or above. But 9 times out of 10, this is the cause.
I just checked it and it sits on the dowels and looks to be flush. I might need to tighten it better since I only used a wrench. I might really need to crank it down
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Old 05-12-2019, 10:03 AM
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I also have the upgraded fuel injectors on the way. I read online that it could be cause of that.
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Old 05-12-2019, 10:04 AM
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Originally Posted by Cummins93 View Post
If you cover the throttle body and the engine doesnt die you have a large vac leak. Covering the TB will also make the leak more pronounced. Dont cover the TB with anything that is dirty or can break apart. I use a 1/2 plate of aluminum that I keep specifically for this/runaway diesel.
How do I cover the throttle body?
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Old 05-12-2019, 10:13 AM
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Originally Posted by TrayXj View Post
I just checked it and it sits on the dowels and looks to be flush. I might need to tighten it better since I only used a wrench. I might really need to crank it down
You might need to torque it down properly.
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Old 05-12-2019, 10:34 AM
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Originally Posted by TrayXj View Post
How do I cover the throttle body?

Remove the air duct from the throttle body. that's the plastic pipe that goes from the air box (where teh air filter lives) to the TB. You'll see the TB intake. It's..... (going from memory) in the neighborhood of 3" diameter. Maybe a bit less. It looks like this:












Now this part is complicated: Stick the palm of your hand over the intake. No 1/2" aluminum plate required. Trust me, it won't suck you in.

Heh. I once got to watch as someone tried to shut down a runaway engine from an M-88 tank recovery vehicle that was sitting on the ground being worked on. We're talking about a twelve hundred cubic inch, 1500 horsepower air-cooled gasser that powers a massive, tank-like vehicle that can TOW a couple of tanks at the same time. The air intake was about 8" across. I know the measurement because what finally worked was a standard clip board, and it barely covered the intake.

That engine ate a t-shirt before someone tried the clip board.
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Old 05-12-2019, 11:24 AM
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Originally Posted by BlueRidgeMark View Post
He shouldn't be changing anything until he gets this massive problem solved.
I agree. He mentioned fuel injectors. Was just giving him some advice.....& if the intake wasn't sitting flush and he ended up having to take it back apart, might as well change them while it was back apart......but he said it's flush so he can get to them after he solves this.

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Old 05-12-2019, 12:45 PM
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Anything clean flat and sturdy. It's nice to have both hands free.
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Old 05-13-2019, 12:03 PM
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Originally Posted by BlueRidgeMark View Post
Remove the air duct from the throttle body. that's the plastic pipe that goes from the air box (where teh air filter lives) to the TB. You'll see the TB intake. It's..... (going from memory) in the neighborhood of 3" diameter. Maybe a bit less. It looks like this:












Now this part is complicated: Stick the palm of your hand over the intake. No 1/2" aluminum plate required. Trust me, it won't suck you in.

Heh. I once got to watch as someone tried to shut down a runaway engine from an M-88 tank recovery vehicle that was sitting on the ground being worked on. We're talking about a twelve hundred cubic inch, 1500 horsepower air-cooled gasser that powers a massive, tank-like vehicle that can TOW a couple of tanks at the same time. The air intake was about 8" across. I know the measurement because what finally worked was a standard clip board, and it barely covered the intake.

That engine ate a t-shirt before someone tried the clip board.
Awesome thank you! I will try that, I'll torque down the bolts first than do that second.
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