1994 FZJ80 random misfire

[MontyAndCruiser]

Couple weeks ago our Land Cruiser started to misfire occasionally at idle, but would still run fine under load. Then it progressed to misfiring almost all the time at idle, and still usually fine under load, and then a few days later, misfiring all the time.

I knew the distributor o-ring was leaking, so i pulled off the cap to make sure the distributor wasn't filled with oil or something stupid. Found a VERY worn cap and rotor while i was in there, and some kinda suspect plug wires.

SO, i ordered plugs (NGK Vs), wires (NGK), cap, rotor, and distributor o-ring, since i couldn't get anything but the spark plugs locally.

Parts trickled in, i ended up doing the plugs/wires/cap/rotor before the o-ring came in. All seemed well for a couple days.

Wife has reported twice in the last 4 days that the truck has briefly misfired at idle only during her commute, both times midway. So, i'm not convinced that replacing all the badly worn ignition stuff has actually fixed anything, though it was probably a good idea anyways.

Things i've read across the internet (on this forum, specifically):

1) It's definitely my fuel pump (highly doubt due to random nature and only dropping 1-2 cylinders at worst)
2) It's definitely my fuel pump relay (see above, and it's not struggling to start at all.)
3) It's definitely a cooked engine harness due to EGR (Is this harness on intake or exhaust side?)
4) It's definitely my coil (Maybe?)
5) It's definitely a cracked intake tube (While not brand new soft and pliable, i saw no damage to the pipe when i did the ignition tune up, but maybe i should throw a new one at it anyways?)


Anything i'm missing here? I DID notice that the spark plug tube seals are leaking pretty good on 2 of the cylinders, but the random nature of it leads me to believe that yes i should fix this, but no it's not likely to be my issue.

It did throw a code a couple times prior to the ignition refresh, but has not throw a code either time it misfired post-refresh. The Code was 26, which appears to be "rich condition." This is what i'd expect with an ignition misfire.

 

[Malleus]

The misfire and rich air/fuel ratio are not necessarily related, although they can influence each other. For a little background, have you checked this:

 

[Malleus]

My first guess, beyond your initial work (why didn't you use OEM parts?) would be the O2 sensor.

Rather than accept random advice from "the internet", try using the FSM. Troubleshooting has to be accomplished using a deliberate, planned, verifiable method, or it's a waste of time. Unless you're good at picking lottery numbers.

 

[MontyAndCruiser]

Thanks for the diagram on the o2 sensor, i'll check the resistance and see what i find. But, the CEL ONLY came on after the truck had been misfiring for some time, and would go away when cruising smoothly.

I used the NGKs because well... they're copper like oem, i could get them quickly, and i've used them on every Japanese car i've owned since 2001. The plug wires was mainly habit, from putting them on every Japanese car i've owned since 2001, but if that was a horrible idea, i'm all ears. I was planning on going back and making damn sure that they were all seated properly over the plug, since that's really the only "intermittent" scenario i can imagine beside the toasty harness around the EGR scenario. EGR was replaced shortly before i got the truck, so i doubt it's malfunctioning currently, but that doesn't answer whether or not it toasted the harness some in any of the 208k miles prior.

Appreciate the help. I don't know that i can do the test drive procedures, i'd have to drive a bit farther outside of the city to be afforded that kind of space and lack of traffic than i trust the truck to go at this point.

 

[Malleus]

The MIL doesn't always coincide with the problem. Sometimes the problem exists without the lamp. Sometimes you don't notice it. Can't say, I'm not there.

Test drive it on jackstands. If you've on got two, pull the front driveshaft. I wouldn't do this, but driving it is the only way to troubleshoot it.

The engine harness you're referring to runs along the rear of the intake side. It should have a wrap on it.

I'd strongly suggest a copy of the FSM.

 

[Malleus]

I replace plug wires on every 80 I buy. I keep them, because even though they're nearly 20 years old, they're still in spec and will work, albeit not as well as new. No harm done, it just costs more money. My dealerships can get parts out of Jacksonville in a day. I use them only when I have to have something tomorrow. I can't imagine UPS can't get to Indiana from Florida in a day.

 

[inkpot]

So when it does misfire, is it a constant miss, or is it a very random miss? Is it the same cylinder that misses, or is it all of them at random?

 

[MontyAndCruiser]

So when it does misfire, is it a constant miss, or is it a very random miss? Is it the same cylinder that misses, or is it all of them at random?

Constant miss, can watch the tach bounce in perfect rhythm. Feels like one cylinder, at its worst prior to the ignition refresh it would sometimes feel like it was dropping two.

The MIL doesn't always coincide with the problem. Sometimes the problem exists without the lamp. Sometimes you don't notice it. Can't say, I'm not there.

Test drive it on jackstands. If you've on got two, pull the front driveshaft. I wouldn't do this, but driving it is the only way to troubleshoot it.

The engine harness you're referring to runs along the rear of the intake side. It should have a wrap on it.

I'd strongly suggest a copy of the FSM.

The MIL coincides with the problem. If the problem persists for a long enough time (multiple minutes), the light comes on. If it doesn't, it doesn't (like now, when the issue is only happening briefly and sporadically). The light has never once come on while the truck was not misfiring.

I have a copy of the FSM, thank you for the reminder. I'm asking here to 1) get help from people with hands on experience and 2) document for the next person that might have the problem. Most of the threads i found talking about this didn't have actual closure.

I will say this, though: The o2 sensor, even if it was bad, would not cause a misfire. It could be not present at all, and it would not cause a misfire. This is an OBD1 vehicle, a bad/missing o2 sensor at worst would cause a rich condition, but not enough to cause a misfire.

 

[Malleus]

The EGR probably didn't cook the wires just because it's new. Look at where the loom is located. It should be off the manifold, away from the EGR piping.

A good visual once-over is never a bad idea. Everything should look like it needs to be where it is and is in visibly good shape. Like this (maybe a little cleaner, maybe not):

Oh, and your antenna should be plugged in. This was before I fixed mine.

 

[Malleus]

Misfires are hard to diagnose, especially over the phone, which is why I've posted what I have.

If you're sure it's a misfire (which means the plug or plugs aren't firing when they are supposed to), go after it. That's a simple ignition problem. Based on your description, I doubt it. You have replaced all the (typical) ignition components which fail. The coil and igniter either work, or they don't (in my experience).

Problem is, this can also be a fuel problem masquerading as an ignition problem. Or an ECU problem. This is why the engine control troubleshooting flowcharts are so long.

If you're not, assume the engine isn't running at optimum and proceed to troubleshoot from there.

 

[Apolo]

Sorry for digging up an old thread but I’m having the exact same issue. Did you ever get yours sorted out? What was the end result?

 

[NGA60]

Any conclusion on the miss issue? I am having the same issue, on a new to me 94 FJZ80

 

[inkpot]

Have either of you checked the condition of your main wiring harness. Many rigs have lost the plastic brackets that hold it in place. Especially the one that keeps it away from the very hot EGR tube, just below the heater valve?

 

[NGA60]

Have either of you checked the condition of your main wiring harness. Many rigs have lost the plastic brackets that hold it in place. Especially the one that keeps it away from the very hot EGR tube, just below the heater valve

Can you send a picture of the location. This is my first 80, always ad 60 series

 

[Kernal]

Near EGR valve and EGR pipe , the large bundle of wires that runs along the firewall then turns forward to run along the left side of the valve cover. If that harness bundle droops too low onto the hot EGR pipe the wires inside can get melted (insulation) causing shorts.

Read through this thread and the links in the thread:

Do yourself a favor and WRAP YOUR ENGINE HARNESS!!!

Years I’ve been here drinking deep from the well of knowledge and experience, and I don’t know how the heck I failed to learn about this common issue, but after an unfortunate PITA experience that I realize now could have been easily avoided, I’m on a crusade to encourage my fellow eightians to...

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