Every once in a while, if I let my 92 4.0 sit for a weekend without cranking it, it will misfire and run really rough. Mostly it doesn't like sitting (whether its in park or drive holding brake), as once I hit the gas it pretty much drives normal until I have to sit at a light or park for some reason, then the roughness continues. One time is even died a few months back and wouldn't start for a while (although sometimes it would and would just sputter and die again). Thankfully it eventually started though.

Any ideas as to what could be going on?

 

Bump

 

Have you done a tune up recently? spark plug gaping would be the first thing I looked at. then a full tune up will new spark plug wires, rotor, rotor cap, and new copper plugs that were properly gaped. 75-100$. after that i would look at the IAC (idle control valve) 45$ and throttle position sensor 70$... i would do those things is that order... all easy as far as technical skills go.

 

I was having less sever problems and maybe i didnt need to perform all of these but it really smoothed out my start up and idle. im going for the 784 injector swap this weekend to get even better performance.

 

I think it just needs a little servicing. Engine oil change, clean air filters... Basic stuff. Should be ok..

 

Sounds very similar to what my jeep is currently doing. If you let it get worse, it may throw a CEL for the IAC. I pulled it out and its all gunked up with carbon. Once I cleaned it she ran great. But I. ant figure out why the carbon built up so fast. This has happened twice in the past 3 months. My thoughts are a vacuum leak which hasn't been found quite yet

 

I recently cleaned the whole TB and it has fairly new oil and new plugs gapped to spec. It only does it if it sits for a few days and after 5-10 minutes of driving it runs normal again. This ha only happened 3 times since I got it in April.

 

It also has reman Bosch 4 hole injectors I recently put in.

 

Have you tried seafoam? Both the deep clean stuff and regular in the vacuum lines? Or mass air flow sensor?

 

I used seafoam in the intake when I got it in April. I haven't tried any deep clean seafoam. I've also recently started using MMO at every fill up for about a month and a half.

 

Does it crank hard and blow smoke out the tailpipe once it starts? Could be an injector leaking down if that's what's happening.

 

That time is did hard start, but on a regular basis it does not

 

Oil smell gassy after it sits? If so, I'd pull the plugs and look for the wet one. Refurbed injectors can be sticky if they sat too long between the cleaning and running them on an engine.