Help I fear for the worst

I have a 2007 Tundra with the 5.7.

I've had a steady misfire on cylinder 8.

Here is what I've done:
Replaced plugs,
Swapped coils,
Swapped fuel injectors,
Replaced pcm,
Cold compression test was just under 90 for cylinder 8

I've pulled the valve cover and can see no broken springs or anything out of the ordinary.

Oil is clean and coolant is clean.

Wiring to coil and injector looks good as well.

 

Did you save or take pictures of the old plugs?

 

They were all dirty, cylinder 8 had a good bit of oil on it so I did the valve cover gasket as well.

Here is the only picture I have and can't tell you which cylinder it was

 

Reset the computer? (Pull a battery cable for 20mins)

 

Also clean your throttle body, I can see it’s dirty from here, and your MAF, with proper cleaners

 

What is compression in the adjacent cylinder? Head gaskets often leak between two cylinders since that is where the gasket is often smallest and where heat builds up. Hopefully that’s not it, but low compression on two adjacent cylinders can be a clue.

 

Other cylinders were over 120 each cold

Computer was reset yesterday when I did the pcm.

Also I'll give the throttle body some brake clean treatment but would that really ONLY cause a cylinder 8 misfire? If it were random I'd understand.

I've also checked for vaccine l vacuum leaks and found nothing

 

90 does sound a little low. 120-150 is usually pretty healthy. Does squiring a little oil in cylinder 8 immediately bump up the compression?

 

I've got an endoscope on order, I'll try the oil trick in the cylinder tomorrow to see if it is any better.

Here were some shots from today and I see nothing jumping out at me

 

I was going to suggest an borescope to take a peek in that cylinder but it looks like you’ve got that covered. Definitely take a look in a few other cylinders to compare.

Sometimes a leaking fuel injector will wash a cylinder and cause premature wear on the rings or cylinder bore, dropping compression without completing killing the cylinder. Eventually it will over time, but it can show up as lower than normal compression. What does your oil look/smell like, especially at oil change time? A washed cylinder can dilute the oil.

 

Miles on engine?

 

160,000.

Did oil last week and it looks and smells like oil right now

 

I don't think your missfire is caused by the lower compression rather the other way around.

 

Were the gaps and electrodes ok on the old plugs? When we look at the cylinder walls in 8 do you see any hot spots? Do you burn oil regularly?

 

Cylinder 8 looked like the others but with a bit of oil on the ceramic.

Noticed no oil loss/burning

 

You said you did the valve cover gaskets, did you do the tube seals as well? Could just be that seal leaking oil and fouling the plug on cylinder 8. And did you look super closely at the valve springs? Could possibly have one look intact but be cracked.

 

Do you have any codes? Cylinder #8 has a pretty specific code PO308 for a misfire. Also if I remember correctly, #8 is also vulnerable to a head gasket leak for some reason.

https://www.700r4transmissionhq.com/toyota-tundra/

 

I did do the tube seals and yes it is a p0308

I couldn't find and evidence of a head gasket leak

 

If I remember correctly it’s either 10% or 20% difference between cylinders. Since you have 120+ on all your other cylinders then the problem is head gasket or mechanical issue on 8. Not really much you can do about except a tear down.

 

Look the cam lobes over on # 8 and make sure it's not short stroking or as stated earlier a spring issue.

 

You could also do a leak down test. If it fails a leak down test then start with the head, head gasket and valves.

 

I agree with this. You can't see the spring in the first picture, but the lobe wear looks inconsistent. In the second picture the spring next to the green dot looks smashed/ out of seat on top.

 

It does look funky. I see this on die knockout springs all the time. Usually when those first break it will be the first 1/2 to full coil breaking off of one end. The flush-ground end of the spring is supported a good ways around the diameter against whatever it's pushing on, but the free coils beyond it are moving as a spring moves, so that turns into a high stress area of the spring.

 

Two different angles for that back spring

 

Did the rocker arm contact the retainer? Is that the bright spot?

 

Some photos from the inside

 

How does this compare to other cylinders?

 

This is about the average, cylinder 1 is dirtier. 4 is cleaner.

Edit:

I have run Lucas fuel injector cleaner in every tank and I have sea foamed the truck in the past.

I bought the truck with 145k and am around 160 or so now.

I feel like the 15k I've owned it this truck has seen more maintenance than the first 140k

 

Do a leak down test.

 

Can you look at the cylinder bore, especially the downhill side of the cylinder (where fuel or coolant would collect if left to sit for a while)? Also, what were the results of the compression test after a squirt of oil in the cylinder?