Hi, i made this account specifically to ask about this because i have no idea what to do. My car, especially when i stop but sometimes when i’m driving too have an issue. RPMs jumps and the whole car is shaking. Firstly it was small, rpms jumped slightly there was a small shake and that was it but over the course of the last 8 months the problem is getting bigger. RPMs can jump for 30 seconds or for the whole time i wait at the red light, the whole car is shaking and sound like the engine is suffocating but it doesn’t. Yesterday the issue grew even bigger because my engine stopped 3 times in 15 minutes. It is complete unpredictable as one time i can drive for several hours and it works right but another time as soon as i turn on the engine the whole car is shaking.
I changed fuel lines, ignition system cap; cables; splitter finger; spark plugs, engine starter and ignition coil, nothing helped.
I have LPG but the problem appears no matter if i drive on lpg or gasoline.
I heard that it may be LPGs fault. That the bigger engines don’t work well with it and the person that told me this had similar problem with his 4.6l engine with lpg. He said that the problem disappeared when he took off his LPG installation. I take it into consideration but almost everyone in my country have lpg in theirs xj’s and i didn’t find anyone complaining about similar problem on my local forums.
Do any of you had similar issue and know what to do about it or have idea what i can do about it?

 

Make sure your intake manifold bolts are tight.

How long since new plugs, wires, cap and rotor?

 

I will check it. I changed them 8 months

 

ALL those parts?

Don't forget the intake bolts................

 

What year are we talking about?

And out of interest: what is the 'splitter finger' you mentioned you replaced?

 

yes, i didn’t changed them all at once but i did it over the course of one month.

 

And the intake manifold bolts?

Do you know how to check for vacuum leaks?

 

They are pretty tight but also in pretty bad condition so i will replace them. I also have a plan to take out the intake manifold to check its condition because judging from the outside and the time it might also be bad, but i have to wait a couple days.
TBH i don’t know much about it(checking for vacuum leaks) but i’m doing research about it now.
Answering someone else’s question it’s 1998

 

Intake manifolds don't go bad.
Neither do the bolts.

Have you checked them all? You can just tighten them up.

Do you know how to check for vacuum leaks?

Ever cleaned your throttle body and IAC?

Any engine codes?