Misfire Rabbit Hole

[GeoRoss]

I did a long laundry list of maintenance on my 1994 Land Cruiser only to end up with a misfire. I also have a new CEL, #25, that is probably related. I was pretty bummed.

I left it with my mechanic for him to use his fancy oscilloscope to test the ignition while the family and I went to play in the snow. He told me that in the secondary ignition there is incomplete combustion in cyclinder #4. He did mention off hand some of the other cylinders looked "weak". He told me the next step was to test injector flow. He wants to monitor fuel pressure while firing individual injectors to look for relative differences. I don't have much experience on this kind of diagnostics so I am reading up. I am also cheap and didn't want to spend $300 on something I'm not sure is the problem.

Part of my maintenance was to solve an EGR code 71. I let my mechanic put a generic VSV to bypass the stock one. It didn't run right unless I pinched the main vacuum hose coming out of the air intake. I admit this was a dumb move. The Cruiser was parked for about 10 months until I could get to it. The modulator and valve tested out, so my plan was to replace the VSV and all vacuum hoses under the intake. The original VSV did test bad.

While the intake was off I took the opportunity to have the injectors cleaned. I did this locally and they are matched within ~2% according to them. I had to replace the injector wiring housing for injector 1 and 6. The fuel filter only has about 50K miles on it so wasn't changed.

Here is my plan,
1. Vacuum- I'm first going to look for vacuum leaks because it is sort of easy. I'll use propane. I guess it is possible for a vacuum leak to isolate to a single cylinder, but it would be unlikely as far as I know.

2. Wiring- Then I'll look at the wiring and grounds. My understanding of the oscilloscope tells me that the injector is firing correctly so wiring problems are unlikely but possible. It is fairly easy to test. I did a tune up with OEM plugs, wires, cap and rotor. The scope indicates the ignition system is good and working.

3. EGR- I'll go through the EGR system, but will an EGR fault isolate a misfire to a single cylinder? He did say other cylinders looked "weak".

4. Fuel- Read up on measuring fuel pressure. Is it possible for a weak pump or fuel pressure regulator to give a single cylinder misfire? The mechanic did say that some of the other cylinders looked "weak". I do have a stock pump and sock I can throw in easily. The tank was mostly empty for a year, so bad gas could be a problem. I did limp to a gas station a few blocks away to put 5 gallons of 91 octane. I later filled it up with more 91 octane.

5. Injectors- Pull the injectors and retest them.

I'm wanting to do the easy stuff first even if they are unlikely, then move to the more invasive tests. I guess I could have a stuck valve. When I pulled the injectors I can't be 100% sure some gunk didn't fall into the injector holes. I did shop vac the crap out of each one before installing the clean injectors.

Any advice, corrections or offers to fix it for me will be appreciated.

 

[baldilocks]

What did your mechanic mean by "weak cylinders"? Did he test compression? How was it running prior to your tune up maintenance? Have you re-checked plug gaps? Have you replaced the spark plug tube grommets/seals? The FSM lists leaking tube seals as the first priority when diagnosing a misfire.
Answer to questions in #1,3 and 4 is no.

 

[Tools R Us]

A failed O2 sensor can cause fuel mixture problems, how old are they? On the early rigs, the O2 sensor wiring isn't routed the best, can rub in the driveshaft, short out. The injectors, just being done, should be good? Compression test, if so what numbers?

 

[GeoRoss]

What did your mechanic mean by "weak cylinders"? Did he test compression? How was it running prior to your tune up maintenance? Have you re-checked plug gaps? Have you replaced the spark plug tube grommets/seals? The FSM lists leaking tube seals as the first priority when diagnosing a misfire.
Answer to questions in #1,3 and 4 is no.

The sparkplug grommets were replaced 30K miles ago. I did just replace the valve cover gasket so I could reseal the leaking cam half moons. Wish I had done that when I did the spark plug grommets 30K miles ago. I'll pull that spark plug. The mechanic's diagnosis is only from the scope pattern on the secondary ignition pattern, the incomplete combustion and weak combustion. No compression test.

The truck was running fine even with the EGR code 71. It ran like crap after trying the aftermarket VSV relocation antics. It smoothed out when I pinched off the main vacuum hose coming from the intake.

The plugs are OEM, but I didn't check the gap.

 

[GeoRoss]

A failed O2 sensor can cause fuel mixture problems, how old are they? On the early rigs, the O2 sensor wiring isn't routed the best, can rub in the driveshaft, short out. The injectors, just being done, should be good? Compression test, if so what numbers?

The O2 sensors only have about 40K miles on them. I can check them out.

I hope the injectors are fine. I have to trust the local guys and they will be my last thing to look at. They may have done something.

I've never done a compression test, so I'll read up on that.

 

[Tools R Us]

Check the harness to the O2, often rubs on something causing issues, or has connection problems. On the early rigs, we find this more often than bad sensors.

 

[baldilocks]

The wire harness to inboard O2 sensor travel down from the main engine harness above the starter and across the top of the transmission. Sort of a pita.

 

[BlueCruiser84]

So is the misfire in cylinder 4? or is this a random misfire? I don't know what code #25 is.

Single cylinder should be pretty easy. Random misfire is trickier.

 

[GeoRoss]

The O2 sensors only have about 40K miles on them. I can check them out.

I hope the injectors are fine. I have to trust the local guys and they will be my last thing to look at. They may have done something.

I've never done a compression test, so I'll read up on that.

No visible chaffing on the underside harness. I'll look up where to check continuity of the whole circuit. In listing out the similarities I noticed the PAIR valve can cause a code 25. When I was replacing vacuum lines I removed two of the top screws of the reed valve that hold the hard vacuum lines. In my searches I see that this may not have been a good idea.

 

[GeoRoss]

The wire harness to inboard O2 sensor travel down from the main engine harness above the starter and across the top of the transmission. Sort of a pita.

According to the oscilloscope it is a misfire on #4. The mechanic told me the pattern looks like incomplete combustion. He the told me some of the other cylinders show "weak" combustion patterns on the scope. I'll quiz him more on this. I have no clue how to interpret those scope patterns.

 

[GeoRoss]

Well, here we go. I'm loosely going through the order of things to check in the FSM for the code 25 and for a misfire. There is some overlap.

Code 25:
Grounds to body, intake and head all cleaned and checked. Not surprised it didn't help.
Checked the E1 circuit. It is ok.
Checked the injector wiring at the ECU. It is ok.
Checked VAF wiring at the ECU. It is ok.
Checked the O2 sensor circuit at the ECU. It is ok.

-I don't know what the fuel line pressure is. It looks a wee bit like a PITA. Getting to that firewall banjo sucks. I do have a NIB fuel pump and sock. Fuel filter only has ~50K miles, regulator is original.
-I don't know if the injector #4 is ok yet. I'll have to pull stuff off to do that, but they were cleaned and checked locally during my maintenance flurry.
-I haven't checked the PAIR system. I did loosen some of the housing screws that hole the vacuum hard lines during my flurry of maintenance. I hope I didn't fubar it.
-I need to check the O2 sensors with the multimeter and while the truck is running at the diagnostic port.
-I haven't checked the ignition system, but according to the mechanic the oscilloscope indicates that system, primary and secondary is functioning. I can go through it later.
-ECU is a black box.

Misfire according to the FSM.
-Oil leak, I should have that covered. The plugs I replaced were pretty oily. Based on the intake I hope it was from the PCV. I did replace the PCV and plugs. They are OEM and I didn't check the gap. I should have. I'll check their gap when I do a compression test.
-Fuel quality, I'm sure was bad. Nearly empty tank of year old gas, but later filled up with premium.
-EGR system, one of the reasons I was pulling things apart. I know the VSV is good. The modulator was clean. I'll check its function with my vacuum gauge later. I resealed the obviously leaking "fuel filter #1" on the intake that I replaced. I need to wait a couple of hours for the rtv to cure before starting. My brake booster hose sounded like it is leaking too.
-Sparkplug, ignition circuit and distributor should be ok according to the oscilloscope. I can go through it later.
-I checked the ECT circuit at the ECU and it is ok.
-Compression needs to be check, but I'll need to run the engine first.
-VAF circuit at the ECU checks out.
-Injector wiring from the ECU checks out.
-ECU is a black box.

I may have time today to pull the #4 spark plug and see what it looks like and test the EGR function according to the FSM. I need to quiz the mechanic some more on his diagnostics. It looks like it takes abit of experience to interpret the pattern. I need to test for vacuum leaks on the intake as that could be the problem. I'm not sure if overall fuel delivery problems could localize a misfire to a single cylinder. If this "weak" combustion pattern he told me is real, then my pea brain sees how that could be it. Compression testing will help me rule out a sticky/stuck valve I guess.

 

[ajax1]

Make sure fuel injectors are plugged in correctly and that they alternate plug color every other one, code 25 is rich, I was getting 26 and 25 which were rich and lean, replaced o2s deleted pair and that seems good so far, temp sensor can cause 25 also, and wouldn't be first time I've seen injectors sent out that said were good and came back s***ty, u could put screw driver onto that injector and listen to it should have rhythmic sound to it

 

[GeoRoss]

In my hubris that the local shop did a good job on the injectors I put off listening to them. While I was letting the engine warm up to test the EGR system I pulled out the stethoscope. 1,2,3,5 and 6 all have a nice sharp click-click-click using the stethoscope. 4 sounds dull which I assume is just resonance from the others.

Looks like I'll be pulling the intake. I'll be calling the shop that cleaned them tomorrow. I'm now wondering if I should send them out to someone else as I don't think I can trust the local shop now. I know #4 gas voltage, so it's not an electrical problem.

My EGR failed the warm engine test and putting exhaust gas directly into the intake. I don't have a way to apply vacuum directly to the EGR valve. Booooo!

 

[GeoRoss]

A quick electrical question. At the ECU I get +12V at all the injectors when I test according to the FSM. My reading of the EWD tells me that this tests for continuity of the entire injector circuit. Clearly the positive is good, but is the ground side good too?

 

[ajax1]

If 4 sounds different and number 4 cylinder is the misfire I'd think number 4 injector is causing this have seen it before , all 6 of mine sound exactly the same and have 300k on them

 

[george_tlc]

The injector system has +12V going common to all 6 injectors when ignition switch is at run/start.

The ECU then fires a driver circuit to provide a momentary ground path to each of the individual injector (6 injector drivers) - this provides the current to activate the solenoid in the injector to allow fuel to come out the nozzle. So, having +12V to each injector is only step ONE. You need to verify that the ECU is providing that ground path and that the connector on the injector has not been damaged etc. You can pop the injector connector off #4 and measure the resistance. Should be in the 13 ohm range iirc (the FSM tells you all of this........).

Oh, and NEVER trust the work you have farmed out to someone else. Check everything.

cheers,
george.

 

[GeoRoss]

The injector system has +12V going common to all 6 injectors when ignition switch is at run/start.

The ECU then fires a driver circuit to provide a momentary ground path to each of the individual injector (6 injector drivers) - this provides the current to activate the solenoid in the injector to allow fuel to come out the nozzle. So, having +12V to each injector is only step ONE. You need to verify that the ECU is providing that ground path and that the connector on the injector has not been damaged etc. You can pop the injector connector off #4 and measure the resistance. Should be in the 13 ohm range iirc (the FSM tells you all of this........).

Oh, and NEVER trust the work you have farmed out to someone else. Check everything.

cheers,
george.

So with the ignition on, I was testing the entire circuit to the ECU when I would either ground to the ECU ground pin or a body ground. I guess I could have a faulty ECU.

 

[BlueCruiser84]

Double check the harness connectors at the number 4 injector. Half of mine broke when I pulled them off. Also, the harness can rub on the firewall under the glovebox. I had couple chafed fuel injector wires when I was chasing this same problem (didn't fix the issue, but needed attention anyway.

A faulty ECU is very unlikely on a 94, but *could* be the issue.

 

[baldilocks]

Have you compared the resistance through #4 injector to the others? I would accomplish a compression test before tearing into and replacing anything unless you know someone local who would allow you borrow an ecu to slave in for trouble shooting purposes. That would be a quick, easy way to rule out your ecu.

If you end up sending your injectors out, RC ENGINEERING In L.A. has been used by many on Mud including myself. IMO their price was reasonable and turn around time was fast.

In post #2 I asked, how was it running before you did the tune up? When, exactly, did the misfire start happening?

 

[GeoRoss]

Double check the harness connectors at the number 4 injector. Half of mine broke when I pulled them off. Also, the harness can rub on the firewall under the glovebox. I had couple chafed fuel injector wires when I was chasing this same problem (didn't fix the issue, but needed attention anyway.

A faulty ECU is very unlikely on a 94, but *could* be the issue.

I did break two injector housings and replaced them. I did check the incoming wire harness at the ecu and it was fine. I wish I had checked resistance of the injector wiring harness from the ecu. The voltages were the same across the board so I assume the connections and the circuits are good from the ignition switch/battery to the ecu. I could be wrong. Right now it looks like either a bad injector or ECU. I'll ask a friend if I can borrow his ecu from his 94. I agree it is a low probability failure point but is easier to rule out than to pull the injector. My pie grabbers will have a hard time accessing anything related to injector #4 without at least pulling the throttle body.