Intermittent Rough Idle


Tman5293

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My 04 B2300 has a slightly rough idle that shakes the truck at stop lights. Sometimes it's worse than others and sometimes it's barely present. I have no DTCs and the spark plugs have been replaced in the last 6 months. The rough idle was present before I replaced the plugs. The engine seems to be idling at the correct RPM. There just seems to be a lot of vibration transfered through the chassis. This is a 2.3L DOHC. The only thing I can really think of is that it might be engine mounts going bad. The engine seems to run fine besides the shaking. When looking at the engine running under the hood you can visibly see the vibration.

Any ideas guys? I want to make sure I properly diagnose this before I kill myself trying to put engine mounts in it.
 


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Bgunner

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Start it up... have someone put it in drive while holding the brake and torque it up a bit... with you standing off to the side to watch the engine. If you have bad mounts... you will see the engine roll.
 

Tman5293

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If you have a vacuum gauge handy try hooking it up and seeing what it does. Here is a link to the vacuum diagnostics in the How to tech section for quick reference. If the engine is misfiring you will be able to tell easily with a vacuum gauge.

https://www.therangerstation.com/how-to/engines-fuel-systems/dealing-with-engine-vacuum/
If it was misfiring wouldn't I have a DTC for it? The truck seems to drive fine with no loss of power. I would assume that I would be down on power with a misfire.
 

Bgunner

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If the power is smooth then no misfire. You would feel it especially pulling hills. Now a vacuum gauge can tell more than a misfire to so check out the link if for nothing else just info.

Does your check engine light come on when you turn the key from off to run? This is just to be sure the light bulb is still working.

Be very careful when checking the motor mounts the way Uncle Gump suggested, while it is the best way to tell, you must not be in front of the truck when checking them. I can't stress how dangerous it is to be near a vehicle while in gear with someone pressing the gas and your head is under the hood, well not under the hood exactly.
 

Tman5293

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If the power is smooth then no misfire. You would feel it especially pulling hills. Now a vacuum gauge can tell more than a misfire to so check out the link if for nothing else just info.

Does your check engine light come on when you turn the key from off to run? This is just to be sure the light bulb is still working.

Be very careful when checking the motor mounts the way Uncle Gump suggested, while it is the best way to tell, you must not be in front of the truck when checking them. I can't stress how dangerous it is to be near a vehicle while in gear with someone pressing the gas and your head is under the hood, well not under the hood exactly.
Yes the check engine light works. All the lights come on when I start the truck up.
 

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Just spit balling on this one here till you check the mounts but a vacuum leak can make the engine idle rough also. Vacuum gauge would help tell that also. It would show up as low vacuum.
 

Tman5293

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Just spit balling on this one here till you check the mounts but a vacuum leak can make the engine idle rough also. Vacuum gauge would help tell that also. It would show up as low vacuum.
If I pick up a vacuum gauge, where would I hook it up?
 

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There should be a vacuum tree coming off the intake manifold usually positioned near the firewall side of the manifold. Multiple vacuum hoses would be connected to it and should have a few with just rubber caps plugging the unused orifices. This is the best place to tap into the vacuum system.
 

Tman5293

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There should be a vacuum tree coming off the intake manifold usually positioned near the firewall side of the manifold. Multiple vacuum hoses would be connected to it and should have a few with just rubber caps plugging the unused orifices. This is the best place to tap into the vacuum system.
As far as I could tell when I pulled the manifold the other day to do the valve cover gasket, there's only two vacuum hoses connected to it. One that runs to the vaccum ball and one for the brake booster.
 

wcurtin1962

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Check your motor mounts. When mine failed the engine was resting directly on the frame.
 

Bgunner

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As far as I could tell when I pulled the manifold the other day to do the valve cover gasket, there's only two vacuum hoses connected to it. One that runs to the vaccum ball and one for the brake booster.
You can use a T and put in between the manifold and hose to the canister since your motor doesn't seem to have the vacuum tree. My gauge came with a T adapter when I bought it.
 

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