I'm dealing with a random misfire that I've exhausted every option that I could think of and followed the tech service manual flow chart. There is one part in the diagnostics of the service manual that doesn't seem to be lining up and was hoping I could get some input. I'm checking the voltage of the ECM terminals for the injectors following the attached picture and none of the pins give me 9v-14v. I'm between 500-700mv on #10-#40, #60 and 5.5v on #50 (confirmed ground is good through another 12v output). My question: is this reading the output of the ECM or input?
I'm starting to wonder if my ECM is bad with all that I've already done.

My compression is good, fuel pressure is good, timing is good
Swapped coil packs
Spark plugs
Plug wires
Tried 2 sets of injectors, with the current set on there tested and confirmed good
New mass air flow sensor

I'm pulling DTCs
P0300
P0301
P0303
P0306

Thanks

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What year is your truck?

Also, do you have and OBD2 reader that can display live engine data? If so, what do your fuel trims look like? You installed a new MAF sensor; is it an OEM one or the auto parts store special? I would get some MAF cleaner and try cleaning the old one and see if that changes anything. Does it misfire immediately on cold start, or start misfiring after it's warmed up a bit?

At the ECU terminal, it is 'both' - The ~12V you should be getting at each of the #10-#60 terminals comes 'through' the fuel injectors while the ECU is not driving them open. When the ECU is driving the fuel injectors on, the voltage at those pins with pulse to near ground.

The voltage checks should be done with key-on, engine-off for this test, I believe. Make sure your battery is on a charger while doing this to avoid draining it...

-Charlie

98 4Runner 3.4L

I do have an OBD2 reader but here is the issue. My Fuelsys stays on OL and doesn't go into CL as it should once the engine temp reaches the temp that it should switch so my fuel trim stays blank. I'm thinking this has something to do with whatever is causing my misfire.

It misfires on start up and is not driveable. When I put it in gear it kills. The misfire is still there even after warm up.
No white smoke from the tail pipe

I performed the test with the key on, engine off. 500-700mv on all pins besides 1 which was only a little over 5v. I used a good battery, fully charged.

Do you have any suggestions on what my next step should be?

When did this misfire start?? Any back history on this project? As you went through all the coils, timing, injectors. I am wondering if there is a massive vacuum leak here.. As if its bad enough engines will not idle. Or a massive intake leak? all of these things can cause misfires too. As the misfires look like they are all on one side. So am thinking, the issue is on that side. Might be wrong but just thinking Good grounds on that side of the engine?? Maybe the injectors are loosing gnd and thus causing the misfire?

A couple years ago, I burnt a valve so I had the heads reworked and replaced, along with the timing belt.
The misfire started after this. I've had all the work double checked by a buddy who is a full time mechanic and everything checked out according to him. Took the intake off, used gasgacinch on all the gaskets for the lower and upper intake manifold, even took the timing belt off and redone for good measure. I also checked the valve clearance. He could only offer so much help due to distance and schedule. Even though I've tried multiple sets and also had the current ones installed bench tested by a reputable company, He left saying he still believes it's something with the injectors.

Even though every plug and hose was labeled on disassembly, I feel like I have to be missing something. I've done spray test for vacuum leaks with no luck but maybe a vacuum line was accidentally put on a wrong port. I just moved and a neighbor down the street has a 3rd Gen that I'm going to see if he'll let me compare everything side by side. I also see ground to both sides but I haven't checked continuity from the injector plug to the ground, I'll do that.

All this brought me to following the tech service manual diagnostics. Everything has checked out until I got to checking the pins at the ECM for the injectors. None of them read the specified 9-14v in the tech service manual, with the key on, engine off. This might confirm my mechanics hunch on something with the injectors, just might not be the injectors themselves but something with the wiring.

If everything was fine until the heads were replaced....This really should rule out the engine ecu.. (extremely rare on 3rd gens ). I would verify the grounds are good. I would also verify the wires to the injectors wires to. As wires can get damaged.. More rare though..

Why isnt 4 misfiring? 5 and 2 are good as no indication of errors there.

So if you swapped the coil with cylinder 5 with 3 does the misfire change?

Do you have the spark plug wires on the correct corresponding cylinders?

As if the order is wrong that would be causing issues as well..

What do your fuel trims look like? Not going into closed loop is a problem. Is your engine temp registering on the OBD scanner. Not the temp gauge on the dash, but on the OBD scanner itself?

Nothing changes when moving the coil.

The spark plug wires are in the right order, verified many times.

I did replace all the injector pig tails due to them being brittle, soldered and heat shrinked.

Fuel trims aren't registering because It's not going into closed loop. The temp is registering on the OBD scanner.

Wait a min.. It wont go into closed loop man you really do have some nasty misfiring going on if thats the case.. Will the cel start flashing??

So the mechanical side is good... Leaves fuel, air and Spark.. Moving the coil not changing anything. Tells me it probably not spark. Leaving injectors and air.

Are the misfiring plugs wet when you pull them out or dry? If wet.. No spark. If dry you have no fuel in that cylinder. :-)

You checked everything thoroughly. Checked for vacuum leaks.. So the intake is looking like it sealed. You checked and changed MAF.

I am hoping that you are using oem or oem mfg parts from reliable sources As counterfeit plugs wires, plugs and maf can cause all of this. Ebay and Amazon have some very good look a likes parts that are complete garbage.

As junk injector sets can cause this too if not from good rebuilt sources too.

May not be your case, I am just trying to think what else it could be.

Is there 12V at the injectors? (black-red or black-orange wire at each injector, key-on, engine-off again) Compare with the same ground you are using to check voltages at the ECU. Do this with all injectors unplugged and with all injectors plugged in except the one you are testing. (yes, I know this is a pain since the intake is in the way)

I really suspect a wiring issue (broken wires, grounds, etc.).

I'm assuming the battery was disconnected while the injector wiring was worked on?

-Charlie

Misfire is nasty enough that it kills when put into gear. I do get a flashing cel...

Yeah, I know better than to skimp on parts.
I am mostly confident it's not a parts issue. I've had the vehicle for almost 20 years, doing all the maintenance and mechanical work myself so I've learned the vehicle pretty well. It's been my favorite vehicle owned and set that giving up on it is not an option.

In the image the plug on the left is from a cylinder that doesn't have a misfire the one on the right is from a misfiring cylinder. Looks like the cylinder that isn't misfiring has a little more soot on it than the other.

They were brand new when the misfires started. 🫤

Edit: after pulling the plugs and seeing how bad they look I tried cleaning with a wire brush and put the plugs back into the opposite cylinders of what they came from. Now it suddenly won't even start. Pulled the same plugs and they are both wet with fuel, obviously not giving spark.

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