Sanity Check: Cylinder 4 Misfire (OHV)

[ratdude747]

Trying to find the cause of a cylinder 4 misfire... and making sure I'm on the right track. 1995 4.0 OHV.

For a long time (several years) I've had a warm start misfire that wouldn't resolve until the O2 sensors warmed up and the EEC transitioned to closed loop (at which point, the fuel trim on bank 1 would skyrocket for a bit). I tried replacing upper intake and fuel rail gaskets (along with fuel injector gaskets) a year or two ago to no avail. I also checked the lower intake bolts and they were tight.

For various reasons (mainly worsening rust), I haven't been driving it a lot the last year or so... but 6 months ago it developed a startup misfire that's become a constant misfire. EEC says it's cylinder, which a power balance test confirmed. My gut feeling says it's either the lower intake or the fuel injector; here's what diagnostics I've been able to do to:

• Confirmed spark
• Spark plug was dry but "glossy"... so hard to say if it's getting fuel or not Plug is not fouled
• Spraying carb cleaner into the air box had no effect
• Spraying carb cleaner around cylinder 4 intake had no effect
• Attempted a compression test only to determine the threads aren't sharp enough... couldn't get it to start in the hole.
  • But, I did confirm no galloping when dry cranking (fuel pump relay pulled)... so likely not a major compression loss
• Tried a spare EEC (different catch code, but same hardware and strategy codes), no change.

One other odd thing that happened today (having to drive it due to the rest of my fleet being out of commission at the moment): for a while, it was giving SRS and ABS lights when running (KOER), but not KOEO. OBD2 indicating system voltage correct... and when I tried using a couple of multimeters (analog and digital), I didn't find excessive noise when running (although at that point this issue ceased). Probably not related, but I figured I'd mention it.

So, plan on doing a lower intake and fuel injector job?

 

[franklin2]

I would keep trying to do a compression check.

 

[SenorNoob]

I had to buy a "spark plug non-fouler" to get my HF compression tester to reach the threads in the head.