1999 XJ, 4.0. just changed the exhaust manifold and now there's a bit of a rough idle. the rpm goes up and down at idle about 50-100 rpm. i think there could be a small exhaust leak where the manifold connects to the exhaust pipe. could it be the lil donut gasket thing? would that even cause a rough idle?

also i did not find any vacuum leaks and just did a tune-up (plugs, wires, rotor, and cap). just cleaned iac and replaced tps sensor and that did not improve the idle. no check engine lights either.

any ideas? thanks in advance

 

Does it get better when you increase the rpm's somewhat?


As a general guidance with any new symptoms after any work done, you'd first need to suspect the work that was done.
I haven't got experience with replacing manifolds but leaks are easy to occur.

 

since the intake and exh bolt together through the same place and the same gasket, either one could be causing leak. loosen all bolts try and reseat the manifolds and try again. if you have an obd2 reader see if your manifold press also fluctuates but that could be the result of the varying idle...also disconnect both your o2 sensors, if it goes away bad aft o2 sensor or the exhaust leak is causing your problem

 

thanks for the advice! just disconnected both o2 sensors and went for a drive - problem is still there.

it does get better when rpm's are increased. even the slight increase in rpm when the a/c is on levels it out.

 

Ok. Now, see if adjusting your throttle cable solves this.
Just do a search for that, it's been in a few threads
(Here or via Google)

 

adjusted throttle cable - still no improvement. thanks for the suggestion.

 

Well, that was only one possible cause, a few more to go.

​​​​​​ Any difference between in Neutral or Drive?

 

no difference between drive, neutral, or park.

 

It goes away when you step on the pedal is because the extra airflow through the intake is more than the extra air the leak induces and it covers it up..you will have to unbolt or mostly loosen and reseat both the intake and exhaust manifolds...somewhere there is a gap..most probably on the intake

 

thanks for the advice bluejeep2001. i loosened all the bolts holding the exhaust & intake manifold on and re-torqued them following the torque pattern. also the gasket is brand new. after doing that there was unfortunately no change to the idle.

 

Disconnect the t-stat sensor connector. See if it stables out...do you have an obd2 reader...see if any parameter it monitors fluctuates..running out of ideas

 

Have you already confirmed your airbox intake where it attaches to the TB doesn't have a small leak there? Just spitballing every component from A to B.

 

Spitballing or eyeballing? 🤣

 

is the the t-stat sensor the one on the thermostat housing or near the back of the engine bay next to the valve cover?

i will try using the obd2 reader as well.

 

that's a good suggestion, haven't checked but definitely will. thanks.