LC guru's needed, misfire at idle. pulling out hair!

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Mar 9, 2013
This is a shout out to the LC gurus out here on MUD. I have been dealing with a misfire on cylinder #4 and I am in need of some advice. So before I begin I just want to disclose that I work at a Toyota Store in the service dept and my situation is currently stumping my best technicians. I have owned my 80 Series since May of this year.

So the misfire began after the head gasket was replaced and the cylinder head was overhauled by the machine shop we use.
The cylinder head gasket was replaced due to an external oil leak. Prior to the repair there was no misfire issue or engine run-ability concerns, no over heating ect. The check engine light was on due to a faulty EGR valve but there were no other issues. The EGR valve has not been replaced at this point.

The engine was using a bit of oil so when the head was off I had asked the technician to replace the valve seals at the same time. Once the head came off the valves were severely carboned up and the valve guides and seats were worn too. We decided to send the head out to be rebuilt. The machine shop reconditioned it top to bottom. At this point, to prevent any further oil consumption, I also decided to have the piston rings replaced as well. So new piston rings, rebuilt head and a over haul gasket kit.. so far so good.

Once the head came back from the shop we reinstalled the camshafts, measured the valve clearance and ordered the correct shims to get the valves into spec. All the bits and pieces went back together.

We did run into a few issues along the way, we managed to crush the #2 fuel injector connector and due to age, 2 of the other injector connectors broke. I had all 6 injector connectors replaced and miraculously at the end of it all the engine ran......... just not well.

What I am currently dealing with is a misfire at idle on cylinder #4 only. The ONLY fault code being thrown is P0304 (cylinder #4 misfire) When we look at the live data list we can see that Cylinder #4 is misfiring 80-90 times while idling but once the RPM's are brought up the misfire goes away.

So to try and isolate the issue we checked all the basic things, plugs, wires, timing, electrical connections ect. We rechecked the
valve clearance and found that cylinder #4 intake valves were too tight. We adjusted the clearance with a different shim but that did not help. Installed new plugs, now the plug on this cylinder seems to be getting some white ash on the tip already. Running to lean?
I will come back to this point later.

Compression is good - 162 psi
Leak down test - 0 psi loss
Swapped Fuel injectors - misfire stayed on cylinder #4
Checked pulse of the fuel injector wiring - pulses good
Checked to make sure there was no air getting past the injector o-rings - good
Stethoscope on the Fuel injector confirms its working
New plugs
Swapped Wires - no change
Inspected the inside of the Dist Cap. No issues seen
Ignition Timing is adjusted
Valve Timing - Good
Removed Intake manifold a 2nd time, make sure that all the gaskets are sealing - good
Blocked off EGR (EGR bypass) - no change
Swapped ECU's - No change
Inserted Borescope used to inspect valves to see if any visual issues noticed - None
Rechecked the valve clearance a third time - all in spec, none are too tight.

So at this point we are stumped... The only thing I can think of at this point is to take the head back off, send it back to the machine shop and have them recheck their work.

Now back to the valve being to tight on the intake side, if the clearance was too tight could that have caused damage to the valve (burnt the valve) If so, should that not cause a loss of
compression on the cylinder? I doubt that it is a fuel issue as we swapped injectors the misfire didn't change cylinders and the fact the misfire stops at higher RPMS tells me that perhaps its a valve issue.

Has anyone ran into this issue before?

Any help would be great!

Thanks in advance.

Did you check the engine wiring harness where it runs right next to the EGR? A lot of trucks have melted insulation on the wires near that valve. While the engine is idling reach around the back of the cylinder head, grab that harness where it turns and begins to run through the manifold towards the injectors and wiggle it around.
I have tried wiggling the wire harness but it didn't change the idle. The injector seems to be firing properly. The wire harness sheath seems to be in good condition. I'm not sure if it would be worth it to unwrap the wiring and inspect it further.
"Swapped Fuel injectors - misfire stayed on cylinder #4"
"Checked pulse of the fuel injector wiring - pulses good
"Swapped Fuel injectors - misfire stayed on cylinder #4"
"Checked pulse of the fuel injector wiring - pulses good"

Hey Kernal, am I missing something with the fuel injectors?
I'd pull the insulation off of the harness and check visually. A mechanic buddy of mine told me about an 80 series truck that came into his shop. The guy had "ecu" problems, had gone through 2 or 3 different ecu's and all sorts of other things that the previous mechanics thought might be the problem. Spent almost $2000 chasing a problem that nobody could find. When my friend got the truck he grabbed the harness and fiddled around with it enough that the miss went away. He said the outer insulation looked fine but when he took it off a few of the wires had their insulation melted away and fused together. He separated them, taped them off and put extra insulation around the harness near the EGR.
It sounds like you have been extremely thorough in your troubleshooting so far, kudos.

If your techs are like other Toyota store techs, I know they hate to work on these trucks, hope you're paying them well :)

So I think I got everything you've done, it almost sounds to me like something stupid simple, like the intake tube from the intake manifold to the air flow meter has a crack and it's registering as a misfire, simple to check.

The wire harness thing is a hassle to check and just wiggling it isn't a diagnosis. I've run into this twice, both times I didn't find it until I split it apart. Different burned wires cause different troubles. Again, a bugger to check, maybe put that off til you've eliminated other, hopefully more simple stuff. Also, in my experience, that registers as a misfire under moderate load rather than at idle.

I don't think a valve too tight would cause this, but someone else with more valve adjustment experience might check in.

Have you double checked the cam timing? I'm not sure if that would show up as a single-cylinder misfire though, just a thought.

I'll ask at work to see if anyone has ideas.

g'luck. Dan.
My first idea is a valve issue, very slight loss of compression , makes a bit of an issue at idle then at higher revs there is no noticeable impact, had this with water getting in by weeping head gasket or very slightly bent valve, this one took a long time to find ( comp was good only way we found it was to pull the head again remove all valve and test them with a dial indicator i wanted to kill the dude that rebuilt that head).
Thanks for the input guys its much appreciated.....

We are going to check the lobe of the camshaft this morning just to make sure that there is no rough spots or double bumps on there. If it looks good I am going to send the head back to the machine shop to have it reinspected.

Keep you all posted!
I would watch the valves on #4 while it is misfiring at idle with a timing strobe light trigered off #4 to see if any of the valves are sticking open intermittently while it is misfiring. If so, it might be as simple as running it until the tight spot wears down a little and stops sticking.
Distributor cap was checked and was visually good.

Can't help but wonder if there might be an issue with #4 there somehow that's not visually apparent? You swapped around various things to see if it affected #4 to do so, but it's kinda hard to do that with the dizzy cap. Might be worth swapping on a known good one to see what happens.
I second unwrapping the wire harness insulation near the EGR valve and inspecting the individual wires. Once you pull the intake and EGR valve, you'll have lots of room to work with. There's another place the wire harness might have rubbed through behind the glovebox. It seems unlikely since the engine ran fine before and this area of the wire harness would not have been monkeyed with during the HG replacement, but it's easy to take a look
Distributor cap was checked and was visually good.

Can't help but wonder if there might be an issue with #4 there somehow that's not visually apparent? You swapped around various things to see if it affected #4 to do so, but it's kinda hard to do that with the dizzy cap. Might be worth swapping on a known good one to see what happens.

I did a basic tune up in May when I purchased the vehicle. I replaced the plugs, wires, cap and rotor. I doubt that it is the cause but stranger things have happen. I did order a replacement this morning just in case.

The camshaft lobes looked good so the head is coming back off as we speak.
So it looks like the culprit is one of the intake valves, It's not seating properly. We can see where there is a uneven carbon tract around the one valve and on the seat of the head itself. I've sent the head back to the machine shop for a re-inspection. Hopefully they can get that fixed up and we can get the head back on tomorrow.

Keep you posted....

If one of the valves wasn't seating, it would show up as low compression unless it was sticking intermittently.

So the head came back from the machine shop yesterday.... The shop foreman reinspected the head and said that they found nothing wrong with it..... except it took them 3 days to reinspect it and it has 2 new valves installed, yah right!

So back it goes on today and fingers cross the misfire will be fixed!

Well that sounds promising. Best of luck! Hopefully that was it.
Best of luck!
I'm curious, how does the ECU detect a Misfire? When I had a misfire on #6, it was due to a head gasket leaking coolant.
It detects misfires as a momentary lag in the rotational speed of the crank by the CPS and it knows what cylinder was expected to fire at the time.

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