Cylinder #4 issue

[Dougless21]

1993 pathfinder 3.0 running rough under load and idle is off. Replaced cap,rotor,plugs,and wires. Codes in computer show nothing and o2 test says it’s reading. Unplugged coolant sensor and drove it didn’t make a difference and also unplugged o2 and ran the same. Started unplugging wires at cap and #4 didn’t change with it off. Checked and had spark on both ends pulled plug and it isn’t wet. Disconnect injector plug had power and ground KOEO and had pulse reading while running. Ohm out the injector and was around 13.4. Plugged everything back in and could hear the injector clicking. Also checked compression on #4 and was around 155 psi. Any ideas ? 

[level9]

Some things to check for off the top of my head (that don't require tearing into things) -

 

could be a localized vacuum leak (ie. intake manifold gasket) - can be checked by spraying carb cleaner/starter fluid around the area while running

 

could also be a sticky valve - the last time I looked at my valves there was quite a lot of carbon buildup, concerning enough that I blocked off my EGR and put a catch can on the PCV line. Usually seafoam ingested via the brake booster vacuum line can be used to clean these off - just go slow to prevent hydrolock

 

 

[Slartibartfast]

Good testing so far!

 

You've tested everything about the injector except whether it's actually spraying fuel. It's electrically OK, and it's trying to work, but I suspect the actual spray nozzle at the tip is clogged up. If it was spraying, I would expect a wet plug (and exhaust that smelled like it was running pig rich, and possibly fuel in the oil as well). Unfortunately I can't think of a way to confirm this that doesn't involve taking the intake apart (or isn't stupid/more work than doing it properly).

 

Might be worth checking for a vacuum leak before tearing it down. I think someone here had a leak from the gasket between the injector and the intake manifold after they'd had the intake apart. It wouldn't hurt to shoot some carb cleaner around there to check, but if you haven't been in there recently, I would be surprised if it had failed on its own. Did the old #4 spark plug look any different from the rest?

 

Passing the compression test tells me the valves are closing. I guess if a valve wasn't opening, or if a port was blocked up with something, it could make compression but not run. But it would take a hell of a chunk of carbon to block up the intake port (unless the engine ate a rag or something), and from what I've read, the cam lobes in these are made from coffin nails and woodpecker lips. And, again, if it was spraying fuel, but not burning it, I would expect a wet plug.

 

I would remove the injectors and check/clean them on the bench. Do your valve cover gaskets at the same time if you haven't--this will also give you a chance to confirm that the valvetrain is OK, though again I doubt that'll be the issue.