P0300 UPDATE

P0300 rant..
01 V6 293k
So I’ve been chasing this code for around 2 months now. Some of you may have seen me mention it in other threads but I’ve been waiting for the right moment to make a thread on it cus I wanted to exhaust all other options first. I’m only making this thread now because I may have solved it by accident and I’m curious as to what others think about it and how it may have been causing the misfire.

Scroll to “here’s the good part” if you wanna just get to the point.

P0300 random cylinder misfire (two or more are randomly misfiring)

I only noticed the misfire upon starting (always on cold starts, here and there on hot starts) other than that the truck drove fine. I’ve done a LOT of reading on here and just about replaced everything that’s been suggested to replace.

Stuff that’s been replaced recentlyish
-2 years ago: fuel filter, valve cover gaskets, pcv valve, all hoses, spark plugs, wires, coils. All OEM. (Followed Timmy’s video on the valve cover gaskets to a T. Cool dude).
-This year: IAC valve, rebuilt TB (MaxBore), fuel pump.
*Fuel pump was the first thing I replaced since the misfire started. Yes this is all going in order*
-Past few weeks: cleaned MAF, all 6 Injectors (from motor west), fuel pressure regulator, upstream air/fuel sensor (denso).

All parts were replaced with either OEM, Denso, or Standard.

Once I realized nothing was working I feared my HG might be starting to go. No oil in coolant and no coolant in oil. No milkshakes yet that I could see. No excessive smoke. No overheating. I noticed a little coolant loss so I got scared for a minute but then I remembered that I’ve been taking my TB off quite a few times this year and every time I do a little coolant spills out. To me this made sense since I haven’t topped it off for awhile. It’s now topped off along with the resivor. Just in case I also did a compression test finally and all my numbers are 180-190 dry. With these numbers I shouldn’t need a leak down test right?

It took me a few days to properly do the compression test cus of my lack of tools and time (wrench extensions, shitty spark plug socket, bad threading, laziness). So, I had my spark plugs out a few times for a few days. Finally got the tools eventually.

HERE’S THE GOOD PART
Before I put my spark plugs back in the last time after a successful compression test (8/10/22) I put on some dielectric grease on the metal ends of the plugs and a little inside each coil pack. My startup misfire has been gone ever since! No p0300. No shaky starts. I wasn’t expecting this. Usually when I cleared my CEL it would come back later that day or the next morning. Sometimes it would intermittently clear itself and then come back later or the next day. Almost 4 days now and only the p0420 has come back (which I don’t care about right now cus that’s been on and off for a few years). Unless they’re related? What do you think?

1. Did the dielectric grease really fix my misfire? My plugs, coil packs, and wires are all still relatively new I would say. They all looked good to me, no cracks or shorts that I could see. Should the spring in the coil packs be pushing the pack off the valve cover a little (contact with the spark plug) or should the coil pack be sitting flat on the valve cover?

2. Did my new upstream O2 sensor just need some time to adjust? The gasket on that was absolutely toast. I’ve read that these need some time to adjust to the computer before you start seeing mpg difference. What about misfire difference?

Side thoughts:
1. After I did the injectors and FPR 10 days ago, it didn’t misfire for the rest of the day when I was running around town. Started up easily. I was convinced I fixed it until the next day when the p0300 came back.
2. During the compression test of cylinders 1 & 3 there was a loud squeak you would hear as the engine cranked. I didn’t think much of it..

I’m sure I’m forgetting to mention a few other things but that’s the gist of it. So I guess there’s no real problem anymore (knock on wood) but I’m curious to hear your thoughts on all of it. I’ll upload some pictures of the journey.

 

Few

 

I suggest only on the inside of the coil boot for the dielectric grease. A tiny bit on the metal tip of the plug is okay but mostly keep it to inside the boot. Could’ve helped, I’ve been using it on my plug jobs since I got the truck. Been using it at work in underwater LED applications for years. It works when used appropriately. Great at keeping moisture and corrosion away, and to prevent rogue arcing.

 

Perhaps it was rogue arching causing the misfire

 

Just possible the dielectric grease allowed you to push the spark plug boot on fully making a better electrical connection.

I have been using it since the seventies to just allow easy spark plug boot removal it also helps to push them on easier.

 

I forget where I saw it, FSM or a note in the actual plug set bag, but it says "do not remove grease from the wire plug ends". This is a different location (where plug wire plugs in to coil) and they mention it for this location because you can see it but this is indication that the grease is good.

 

Do you recall which wire kit you went with? OEM? Or those blue denso ones everyone on TW seems to like and use? Or perhaps another brand?

 

UPDATE
So the CEL came back and after some more reading and YouTube I think I put the grease on incorrectly. I put grease on the plug ends that meet the coil (wrong). And I put grease inside each coil without spreading around in there (also wrong). Nonetheless, the p0300 went away for 4 days so I’m convinced that I did something partially right.
I took everything back out yesterday. Cleaned the plug ends (not the spark end) from grease and instead spread a layer of it over the white ceramic portion. Then I put some grease on a cue tip and spread it around on the inside of each coil boot. Hoping this helps.

 

Oem wires, coils, and plugs.

 

2nd the oem because it seems this engine is more sensitive than most to wire issues. Look at FSM how they specify routing at every support and even that 45 degree twist on the last cylinder.

I don't know if they are really this sensitive but for a few bucks I don't want to find out. Also you get all new clips and often the old ones break.

I have only replaced mine once since bought truck new so they do last.

 

Denso cables are black......

 

NGK's are blue.....

 

With most of us using the iridium plugs, these cables have become a non wear item.
Snug metal to metal grip at both ends is what your after.....just need a premium set...

Id be more worried about.....is having a 'HIGH QUALITY ENGINE GROUND'.
These are body to battery ground......and then body to block.....

As these trucks get older everything electrical needs a good ground.....
A lot of times this is a source of injector problems......especially random ones....
and with plugs firing you can overwhelm your iffy ground....
may even back feed amps thru other systems that are depending on that same weak ass ground.....
ouch.....poor ecm.....

Some have suggested running a direct engine to post ground, as I did...
Slid some pvc tubing over some braided cable strap.

Had a boss who had his house's breaker box lose its common.....over a week it destroyed all major appliances....washer, dryer, frig, microwave.....
110 was coming in....but no where for it go except for it to reverse into the other appliances commons.....fried everything......
LG 47in lcd survived tho....crazy...

 

I agree. It isn't a bad idea to always assume you have a loose or bad ground. Go over and check them all, why not? The problem lies where the power dies..

In the past I have recommended to my field service customers numerous times over the years to have their utility company come out and check specifically the neutral wire from the street to the house. This is a free check. Those neutral wires have a habit of loosening up over time and then you get wonky flickering lights and just weird stuff in general all over the property. I still have a voicemail from a customer that swears I saved his house form a giant electrical fire. Connections need to be tight.

 

They sell probes that start around $60......
......but that may not tell u whole story without everything firing on all cylinders at speed under a highway load......

 

UPDATE:
Noticed my coolant resivor might have dropped a little since I filled it last week. Radiator still full. Also noticed white smoke coming out the tail pipe during my cold start today while it was in the 70s so not like it was cold outside. This kinda re-ignited my HG suspicion.
Then I learned about the block tester kit which is suppose to confirm a HG leak or head crack. Blue=good Yellow=bad (leaky HG or cracked head). Instructions say to test for about 2 minutes, I probably tested for almost 10. Stayed blue the whole time! I also capped the hose to the overflow. Also no bubbles in the coolant itself and looks clean.

SO. After doing this test and my compression test both being successful, is it safe to say that I don’t have a head gasket leak or cracked head yet? Or is still possible? Where else could I be losing coolant? I seem to get consistent exhaust condensation. Could some of that be coolant? I read somewhere that coolant can evaporate from the radiator cap?

 

I’ve also checked all grounds that I could find. They’re all solid from what I can tell.

 

I had a Supra with bad HG, it was obvious exhaust went into the coolant because temp gauge all over the place and coolant bubbling.

But on my 3.4 it seemed like coolant only went from coolant passage into cylinder so maybe exhaust test wont work. I think the acid test is coolant in the cylinder. This was obvious for me and many others - early symptom is a miss for a few seconds on startup which progresses to white smoke on startup.

It might be worthwhile to do a coolant system pressure check and possibly pull spark plug cold and check for coolant in cyl, hi tech way is with a camera probe (never did this) but you can probably come up with a low tech way using something absorbent.

Finally check your oil filler cap because a milky haze will show up there well before oil milkshake.

 

Ya my next test is probably going to be the camera probe just to triple check for coolant. And I think my oil looked good.

 

So I just made the drive from San Diego to Sacramento a few days ago. But before I left I put in Blue Devil head gasket sealer. Seemed to start better for a little but next day the misfire came back. Anyways, the truck drove just fine all the way. I was constantly monitoring coolant temps with my FIXD scanner, staying around 184. Got as high as 195 going though the grapevine and as low as 175.
I popped the hood at one of the gas stations and noticed coolant leaking somewhere behind one of the pulleys. My resivor was still relatively full. I made sure to bring plenty of coolant just in case.
Anyways, once I got home I took it to my buddy’s shop to have them scan it over and give me their opinion. I personally know these guys very well as I grew up with a few of them (they’re 3 brothers), and their dad owns the whole shop. As well as their mechanics have years of experience and they’ve worked on my truck in the past many times. So, they believe I have two problems and not just one like I’ve been wanting to believe. Basically my coolant is leaking from the water pump so it just needs to be replaced. I figured as much, but that still leaves the misfire. They believe my cat is clogged up and is causing that. Since I’ve had the p0420 for years and never dealt with it they think that it’s just been getting worse and clogged until now it’s causing to misfire. Thoughts?

I just bought an Aisin water pump, thermostat, and Toyota red coolant. Timing belt should still have some decent life left on it but I may or may not replace it anyways. I also don’t have the crank shaft pulley bolt which I read is a one time use. My old man has offered to go to the dealership for me and getting the crank shaft pulley bolt while I’m tearing the water pump out. I might just have him grab a timing belt as well.