Misfire and code 25/26 - back again! (1 Viewer)

Feb 23, 2011
First off, let me apologize for the long post – I’ve tried to limit this to only relevant info to the issue. My truck is a bone stock US ’94 FJZ80. I am going crazy trying to troubleshoot a misfire and loss of power, accompanied by code 25/26. This started back in 2019, I later discovered the truck was consuming coolant, didn’t have the money to the do the HG at the time, and parked it for several years.

Back then, I deleted the PAIR system, installed new Denso O2 sensors in the PAIR holes, replaced the air filter, and replaced the plugs, wires, cap and rotor, and PCV valve to no avail.

Last fall, I finally had my HG money and a buddy offering to lend a hand, so I started doing a baseline on everything under the hood. During my initial run through, I did the following relevant things (plus a lot of other stuff):

  • Pulled the head, sent out to the machine shop to get it cut, pressure tested, and the valves done.
  • Deleted the EGR and had the machine shop make me some nice…. Keychains? Installed those using the gaskets from the OEM head gasket set and shorted the EGR temp sensor wires.
  • Got rid of the EVAP system (bad canister, so just decided to delete it). Vent from the tank vents into the fender now.
  • Every possible vacuum hose that could be replaced was replaced. My only remaining vacuum lines are the two PCV hoses to the valve cover, the lines to the fuel pressure VSV and FPR, the short one to the dashpot, and the one to the brake booster. These are all brand new OEM. The ports for everything else have been capped using OEM caps (except the big one that used to go to the EVAP - that has a Chinese-y cap on it).
  • New OEM plugs (gapped to spec), reused the new-in-2019 plug wires, cap, and rotor
  • Head was reinstalled following the procedure in the FSM with new OEM head bolts and the OEM gasket set. Set the ignition timing with a timing light per the FSM with the jumper in the diagnostic port.
  • Sent the fuel injectors to Witch Hunter to have them cleaned and tested. #1-5 came back fine. #6 was bad. Replaced #6 with new OEM. The #5 connector was in rough shape and one of the wires broke off when I disconnected it. This was replaced with a new connector pigtail from Napa and soldered/heatshrank.
  • Replaced the fuel filter with new OEM.
  • Harness was mildly melted around the EGR. I pulled off all the original harness covering in that area and inspected the wires and didn’t notice any damage jumping out at me, so I wrapped them in new harness tape and put them back.
  • Installed a new OEM fusible link set.
  • Replaced all the various other gaskets that came in the OEM gasket set.
After I finished the head job, the truck fired right back up. Full power, no misfires, no CEL. It ran exactly as I remembered it running years back (except some birf clicking… but that’s a problem for another day). I assumed my bad #6 injector and/or HG was the source of my misfire woes and figured life was good.

It ran great for about 100 miles until one day I fired it up from a cold start and my misfire was back. Drove it around the block and Code 25/26 reappeared as well. It was so down on power, I had trouble getting up the hill to make it home.

Things I’ve done for troubleshooting:

  • Fuel pressure test (using a banjo bolt adapter on the inlet end of the fuel rail) – tester shows around 44 psi with the jumper in the diagnostic port and the engine off.
  • Disconnected the ECU and measured the resistance of each fuel injector circuit through the entire wiring harness path from the glovebox area to the injector. All the resistances were in spec.
  • Did the same thing with the heater circuits on the O2 sensors – from the ECU all the way back to the sensors. Resistance was also in spec.
I’m trying to decide where to focus my efforts next and hoping you all can give me some advice, because I’m really starting to get discouraged. After all the time and money I’ve poured into this, I just want to be able to drive and enjoy the dang thing.

My gut is telling me this is an electrical issue – what kind of problem goes away when you take the whole engine apart and put it back together, only to return at random later? Maybe I missed something in that harness by the EGR? If I did, what’s the best way to figure it out? Just visual inspection? My other thought is that a vacuum leak opened up somewhere and I just haven’t found it.
Feb 23, 2011
Finally fixed this madness… I shook various sections of my wiring harness until I heard the engine speed change while shaking it around the #1 injector. The connector was making enough contact for it to ohm out correctly on a meter, but not enough contact to actually run the injector. I cut the plug off and spliced on a new one and the engine is running great. I initially wrote the injectors off as fine because I had good fuel pressure and a good meter reading… lesson learned!

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