Jeep engine help -- Paging Rick!

[Johnny_L.A]

A friend of mine has a Jeep Wrangler (I don’t know what year) with a 4.0l six. She says it’s running a little rough.

@friend

The Jeep went in for a tune up while I was off that week, still has this rough running feeling that no one can seem to pinpoint. [Mechanic] says it’s a vacuum leak somewhere but every time I take it to him to fix he can’t find it or just doesn’t know where to start looking for it. It’s been doing this for over a year now, it runs fine besides that and even better when the weather is warmer so I’m not taking it in until it turns into a problem.

<2nd email>

The Jeep is vibrating really bad, feels like it’s coming from underneath. Could be the throttle body. [Mechanic] did mention that, might be a hole in the exhaust pipe too. Yes, the manifold is cracked but it has been for a while, way before this rumbling occured. [NB: The exhaust manifold on Jeep 4.0l engines tend to develop cracks. – J L.A.] I don’t ever drive on the freeway anymore so I don’t know if it needs to be blown out so to speak. I do know that it runs a little smoother in warm weather and that you only feel it when you’re at a stand still with the engine running.

[Mechanic] said it could be the engine mounts too. I just had a transmission mount replaced, and older cars tend to loosen up with age and vibrate a lot more. Not really sure exaclty what’s causing it but it’s running fine other than this issue.

Rick and other automotive Dopers, any guesses as to the source of the problem?

[Fubaya]

No idea about the rough running feeling, but what did she replace the transmission mount with? I replaced my factory mount, which is pretty soft, with a heavy duty poly mount that vibrated like crazy.

Does she have factory tires/suspension? A little suspension lift can put the driveline in a bind and cause vibrations when stopped and/or just as it starts moving.

[Gary_T]

I gather the symptom is strictly a rough idle, with the engine running well otherwise.

Checking for obvious vacuum leaks is a good place to start. Next thing to check is carbon build-up in the throttle body. Not much point in going any further until those two areas are known to be in good shape.

[Johnny_L.A]

@Fubaya

Does she have factory tires/suspension? A little suspension lift can put the driveline in a bind and cause vibrations when stopped and/or just as it starts moving.

It’s been a while since I’ve seen her in person, but I think it might be raised a little. Maybe just the factory ‘up-country’ option. (She bought it used.) Wouldn’t vibrations caused by the suspension have cropped up earlier?

@Gary_T

I gather the symptom is strictly a rough idle, with the engine running well otherwise.

Checking for obvious vacuum leaks is a good place to start. Next thing to check is carbon build-up in the throttle body. Not much point in going any further until those two areas are known to be in good shape.

Her mechanic friend thinks it’s a vacuum leak, but couldn’t find one. I hadn’t thought about the throttle body.