Troubleshooting Misfire

[Guess vs Diagnose]

My '94 land cruiser (366k miles) has developed a misfire that comes and goes. Its like a on/off switch is thrown between modes. The cruiser always starts quickly, and when not misfiring , idles and accelerates smoothly. When misfiring it runs rough at idle and has a significant loss of power and vibrates when accelerating. I used a laser temperature gauge on the exhaust manifold and found that the cylinder closest to the firewall is the culprit. When not misfiring, all 6 ports heat up evenly, but when misfiring that one cylinder doesn't heat up. I pulled the spark plug and it had some carbon build up, but not horrible. However, I found that the plug wire insulation was cracked and a piece of the rubber was broken loose at the end nearest where it connects with the plug. I replaced all the plugs and plug wires with new OEM parts. It ran fine. I drove about 50 miles (cruiser ran great) and thought problem solved. Nope, I stopped for a few minutes and upon restarting the missing returned.

I've read many threads already, but not sure what's next. I could guess and throw new parts at the problem, but I'm betting y'all will point me to diagnostic techniques that isolate actual source of failure.

Thanks

 

[mudgudgeon]

Check for vacuum leaks.
Any/all rubber vacuum hoses on the engine can be a culprit.
Check the rubber intake hose from air cleaner to engine.
Check oil cap oring, PCV grommet, dipstick tube orings.
Check all the small hoses under and around the intake manifold

Check wiring harness to the injectors, and around the EGR valve.
Clips on injectors get brittle and can break. Heat from EGR can melt insulation inside the wrapped wiring loom

Check timing.
Check distributor is in good condition, cap and rotor contacts, and not oiled up.

If you're sure this is isolated to pot 6, check injector wiring harness

 

[banjo1981]

I just stared a thread with your EXACT symptoms. I’ll link it and you can see if your cruiser fits the bill.

 

[banjo1981]

Vacuum Leak - Spark Plug tube seals?

So I’ve been trying to diagnose a rough running cruiser (1993 FZJ80 with 277k miles). So far I’ve checked, cleaned and replaced the following: Removed and cleaned the EGR valve and EGR vacuum solenoid. Both are in working order and flowing freely now. Replaced the PCV valve and grommet...

forum.ih8mud.com

For me it was a leaking HVAC control valve directly above the #6 injector plug. Coolant dripped down in the connector well and shorted the circuit. Thus cylinder #6 not firing and running like crap. It would come and go like yours and then finally shorted for good.

To find out it was a shorted injector, I had to remove the upper plenum half to access the injector connectors. I removed the injector harness connectors and ohmed each injector until I found the bad one reading waaaay too much resistance. Correct resistance is somewhere in this forum, but I think it was like 14 ohms +/- 0.3