I've been trying to get a 97 to pass emissions, but I keep having a misfire come and go that I (and my mechanic) can't figure out. I have replaced spark plugs, spark plug wires, distributor cap and rotor, and the fuel injector for cylinder 1 (the cylinder the code is popping up for). Mechanic checked compression, said it looks good and that multiple people in the shop drove it around with no hint of my misfire. I went to pick it up, because he said it would pass emissions as is with the miles they put on it, and the misfire was back (sums up my luck). I left it running and brought him back out. It is a slight misfire, not super obvious, but he said it was the most anyone had been able to get out of it so far. He said it shouldn't make a big difference driving it around, as long as the light would stay off long enough to get my emissions done (a service his shop and a lot of others in the area don't offer). If it will pass with it, great, but I do want to get it fixed. Where it has spark, fuel, and compression, neither of us know what to do with it. I'm hoping someone here has had a similar issue and has been able to figure it out.

97 Stock Cherokee SE with a 4L Inline 6 and 222,000 miles previously owned by someone with no regard for maintenance

Before talking crap on the mechanic, he is a long time family friend who I trust whole heartedly to be thorough and wouldn't lie about what he was experiencing with it. He only charged me $12 for some fuel system cleaner and taking up a pretty good part of his day trying to find the issue.

 

It sounds like you changed all the ignition system parts I think it could be a weak valve spring that would cause a lifter to pump up .

 

That's what I was thinking when I took it to the mechanic, but we agreed that it would be weird it doesn't happen all the time and that I have good compression. Wouldn't a weak spring ruin the compression because the valve can't fully seat?

 

No because the engine is moving slower . it doesn't enough oil pressure to pump up the lifter .

 

If you're trying to get away cheap shoot a can of Seafoam, do the burn some water trick and/or change to higher octane.

Do you have an OBDII reader? What specifically is the code-- 301? What specifically is the compression?

The thing about OBD Codes is that some need longer to complete testing, like driving 60-80 miles otherwise they show up at the emissions stations as Readiness Not Complete and they say say go drive around some more then the code pops up. Consequently, hoping to pass may never work.

I use an "entry-level" oscilloscope to zero in on those issues, images upon request.

I'd also yank the valve cover and inspect those springs.

 

The "water trick" was the only thing that fixed my buddy's 99.
There's carbon on the valve seat on cylinder number one.

 

Another vote for a good decarbonization.

Is the P0301 the only code?

When you "replaced" the FI, did you get a new one or did you swap with one of the other injectors?