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Old 12-01-2011, 09:42 AM
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Rough Idle, Vacuum Leak?

I have an 04 Overland.
It seems to have a slightly rough idle. I also notice what sounds like a vacuum leak but cant identify it. I can see all hoses and connections and they all look normal and attached to where they are supposed to be.

The roughness goes away once on the throttle. It almost feels as if I am missing when under load, but no codes.

I have uploaded a video on youtube to hopefully show the sound of the engine. You can also see how slightly the engine shakes.

Here is the link

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Old 12-01-2011, 10:25 AM
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Re: Rough Idle, Vacuum Leak?

Do you have an error code or check engine soon light? I had this last year and I, like you looked at all the hoses for the Emissions System up under the hood. I also did some reading about the error code I had which suggested I had a small leak.

Someone within the forum, maybe a mechanic, wrote back telling me to look at the vacuum hoses around the back axle on the drivers side. that is where the vacuum canister is located on the 2004 overland, back behind the rear bumper. Anyway, the fellow suggested i look real close at the hose that runs over the axle as it usually cracks. He was right on. If I remember right, that hose is 3/8 ID or at least 3/8 ID worked for me. I didn't have rought idle, but i did get the error code which also includes a loose gas cap or leaking gas cap.

Good Luck
Dave
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Old 12-01-2011, 11:02 AM
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Re: Rough Idle, Vacuum Leak?

Quote:
Originally Posted by Me Fix View Post
Do you have an error code or check engine soon light? I had this last year and I, like you looked at all the hoses for the Emissions System up under the hood. I also did some reading about the error code I had which suggested I had a small leak.

Someone within the forum, maybe a mechanic, wrote back telling me to look at the vacuum hoses around the back axle on the drivers side. that is where the vacuum canister is located on the 2004 overland, back behind the rear bumper. Anyway, the fellow suggested i look real close at the hose that runs over the axle as it usually cracks. He was right on. If I remember right, that hose is 3/8 ID or at least 3/8 ID worked for me. I didn't have rought idle, but i did get the error code which also includes a loose gas cap or leaking gas cap.

Good Luck
Dave
Thanks, I will check that out.
As for codes, I have NONE.
One thing a fellow Jeeper told me this morning while looking for this source of the rougher idle is he posed the question to the cams on the HO motor. He asked if they were more aggressive which would cause the slight shake.
Seems like a good question.
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Old 12-01-2011, 01:55 PM
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Re: Rough Idle, Vacuum Leak?

You mentioned in your origional post that "it sounds like a vacuum leak". That is why I chimed in with my experience. I have the HO motor as well. It had 125,000 on it when I bought it. When you buy used, you have to wonder about the maintenance habits of the previous owner. So far so good with my ride, but the HO motor could cause issues ay beyond a vacuum leak giving you rough idle.
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Old 12-01-2011, 02:23 PM
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Re: Rough Idle, Vacuum Leak?

Yeah that was me that suggested that rear evap hose over the axle, but these hoses are separated from the intake by the purge solenoid. A leak back there would cause an evap leak code not a vacuum leak persay.

A low idle shimmy *can* be decently normal, I have seen some pretty shakie 4.7l's not counting any misfires. I have seen it come and go too, my WK with 183k miles on the clock occationally gets a little shakie when I come up to a stop light and it idles down, no misfires count, and its up to date on all its services (new air filter about 4k ago, plugs about 18k ago, oil every 3k, new pcv valve 18k ago, completely removed and cleaned intake manifold and throttle body, all new intake and exhaust gaskets 18k ago)...

Rough idle can also be an indication of poor volumetric efficiency, the faster the engine spins the less a poor seating valve matters, obviously if its not seating properly at all or something is keeping it from closing you will have a dead miss, but if there is something like carbon buildup on the valve or seat (very common on the 3.7 and 4.7 engines) the cylinder can bleed off enough compression to cause incomplete combustion in that cylinder when its going slow, but when the engine speeds up it is going too fast to bleed off enough compression to effect combustion significantly.

Keep an eye on your coolant level, I have seen 4.7l engines etch a path from a water jacket to a cylinder before (electrolosis, why you want to drain and fill your coolant as suggested by chrysler). Something mechanical like this wont really throw a code unless it leads to a full on misfire.

The vacuum leak noise you complain of can be decently common with air passing through the IAC, the plastic intakes dont insulate the noise as well as metal intakes. You may want to remove your throttle body, unscrew the IAC from the throttle body and clean both the throttle body and the IAC well with some throttle body cleaner. This may help slightly.

That said there may be a vacuum leak, but without being there its hard to pinpoint (naturally). One thing you can do to help locate the noise is place your hand in the imaginary line between your ear and somewhere under the hood, move your hand around the engine and listen for you to "cover" the noise... then look in that area.

And the HO cams are not aggressive enough to cause a lope.
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Old 12-01-2011, 09:13 PM
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Re: Rough Idle, Vacuum Leak?

Thank you 2005JGC and Me Fix. Both have been very helpful.
How do you feel about Mystery Motor Oil, or whatever its called. Is that worth looking into to clean any carbon?
I took off the IAC and it wasnt as bad as some of the pictures I have seen. Cleaned it all out. The vacuum noise is coming from the air intake. As soon as I took off the plastic baffle over the throttle body, the noise was much louder. I am sure all I am hearing is the velocity of the air going thru the tube and into the TB.
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Old 12-03-2011, 11:22 PM
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Re: Rough Idle, Vacuum Leak?

I had the same noise and idle, I replaced the timing chains and tensioners and cured both problems. I searched for a vaccum leak like a mad man and tried different oils with no luck. After doing some research figured out it was the chains and sure enough when I tore into he eng I found a frozen tensioner and the plastic guides were disintegrated which caused the tick. It was the chain slapping. Might be something for you to consider.
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