91 XJ 4.0 Rough Idle at Start-Up, hard to re-start when warm

[EricT93]

Hello everybody!

So my 91 4.0 XJ ran fine when u bought it (a few months ago), then I went mudding, it started running horrible(hard start, sputtering, cutting out; virtually undrivable). I replaced cap, rotor, plugs, wires, tps, crank sensor, fuel pump and filter, and it was only after we wired the fuel pump to run constant with the key on, that it started running decent again. This is when I started seeing the symptoms I'm seeing. After sitting for 7+ hours it starts right up, but idles a little rough for a minute, but once its warm if I shut it off I have to pump or hold down the gas pedal to get it started. At first I thought it was leaky injectors, so I swapped to neon 703s and it started running perfect, for about a week, then it started with the symtoms again. Oddly, I replaced the PS pump a week ago, and it started running great again, then 2 days ago it started back with the symptoms. It seems like whenever I work on the driver side of the engine, it runs better for a short period of time, maybe something to do with the "50 first startup" learning or whatever. Idk, I'm stumped at this point.

[4.3LXJ]

Sounds like you might have low fuel pressure due to a bad check valve. Putting a fuel pressure gauge on it would tell you for sure on that. You can rent one I think at Autozone, or some other parts house. Also check to make sure you don't have a fuel line getting hot at the manifold or near exhaust causing it to get vapor in the line

[BlueXJ]

Also check for a vacuum leak. Rough idle is often caused by vacuum leaks.

[EricT93]

Also check for a vacuum leak. Rough idle is often caused by vacuum leaks.
Checked :/ Thanks though.


Sounds like you might have low fuel pressure due to a bad check valve. Putting a fuel pressure gauge on it would tell you for sure on that. You can rent one I think at Autozone, or some other parts house. Also check to make sure you don't have a fuel line getting hot at the manifold or near exhaust causing it to get vapor in the line

I'll check the lines. I tried priming the pump before starting which is supposed to bypass the check valve problem, but nothing changed, still took a while to start.

[EricT93]

So things got a little slow here at work so I went out and inspected the engine bay. I inspected the crank sensor and it had come in contact with the exhaust manifold, causing it to melt though the loom and partially expose one of the wires. I swapped it out (thank God for warranties) and it fired right up, and idled smooth. I'll keep everybody posted on if it reverts to it's old symptoms!