I have a 97 with the 4.6L
I've had issues trying to get my truck running right (misfire in cylinder 8- P0308) and have spent a small fortune doing so. I have now discovered that I have absolutely no compression in cylinder 8. We took the valve cover off and nothing jumps out as being wrong, but none of us have ever been into an engine this new either. What should we look for or test next?

 

check for broke valve springs on that cylinder. look closely they can be hard to see.

 

Next step is to pull the head and see what’s going on in there. If you could with the intake manifold removed see any of the valve look and see if there is a chunk missing. At the shop I work at we used to have a nice snake camera until it grew legs lol. If you have something like this just put down into the head to look around at the valve. But if nothing is noticeable your next step is to pull the head to see if any intake/exhaust valves are broken or there is piston damage.

 

If the Valve's are seating then check the crank case pressure. If it's breathing heavy then you have blow - By.

 

I'd check cylinder leak down next before taking anything else apart, gotta figure out where your compression is going, then you'll have a better idea of what you need to take apart and fix next.

 

Thanks for all the info so far. I forgot to add that I checked the vacuum and it was reading 15-16, which I believe is acceptable from what I've read.

 

15-16 in/hg is slightly low at idle. I like to see around 18-21 holding steady.

 

Can I remove the head without pulling the motor? If so, is there a special trick to it? It looks like it will be tight.

 

Sure, I've done heads in my truck with the engine in, but I would pull the engine next time.

 

[QUOTE=Quintin;3579862]I'd check cylinder leak down next before taking anything else apart, gotta figure out where your compression is going, then you'll have a better idea of what you need to take apart and fix next.[/QUOTE I agree with Quintin. Perform a leakdown or have one done. If the leakdown is low listen for the air leaking and determine if its going into the intake, crankcase or exhaust. This usually will lead you in the direction of the problem. Air into the exhaust leads to a exhaust valve concern (stuck open broken or bent valve or broken seat). Intake would be a intake valve issue and crankcase is usually a hole in the piston. If you get good leakdown then look for a follower off or broken valve spring as the air pressure will push the valve back into the seat and seal the chamber giving good leakdown. If the follower is off then the valve will be closed all the time showing good leakdown. Another possibility is something like a bonus/foreign nut, screw,or piece of flashing from the intake stuck in the intake valve keeping it open and allowing the air out. These bonus parts get in the intake plenum sometimes during assembly, bounce around until they land just right and get sucked into a valve. It happens sometimes even though the engine plants and VO plants take great care to prevent it. Remove the intake and check in the intake valve inlets looking for something stuck in the valve holding it open. I have found aluminum flashing from the truck intake casting that was not found in the intake after casting and get stuck in a valve. Probably not the reason but just some food for thought. A bent rod from a hydrolock will give you low compression but not zero. There are other possibilities for your dead cyl but these are just a few to give you an idea of what you are looking for. Good luck.

 

Yup. The general flow of diagnosis for anything is least intrusive to most intrusive. Tear down and inspection should be your last step, always start with the easy stuff and go from there.

 

UPDATE: We did the leakdown test and there was no leak. It held 130 PSI for a minute or two before we released the pressure. So we started playing around, inspecting springs etc. Upon doing so, on the second valve from the rear (part of #8) the camshaft roller follower fell off. We are assuming this means it is just a bad spring and we'll be replacing all springs on that side tomorrow. Anything else I should be looking at or checking on first?
Also, we checked compression in the other cylinders and they were 130, 129, 130, 130, 132, 130 & 133. Is that good?

 

I just wanted to bump this real quick because I'm getting ready to go get springs soon unless someone has anything else I need to purchase or check besides the springs.

 

The follower off is the reason for no compression. When an intake valve follower comes off you get no compression because no air can be pulled into the cyl to compress as the valve is stuck closed. First look at the cam lobe that operates the follower that fell off. Is it damaged/worn down? If not then look at the follower and check for a loose roller, it will wobble or flop around instead of having a good controlled rotation. Now look at the valve spring. If it is broken it will ususally rotate into itself and appear shorter than the others. You should also be able to move it with your fingers. The spring keepers may or may not both be in place as well. Now the next thing to check (and probably the problem) is the lash adjuster. It is the small hydrolic lifter looking thing in the cyl head pocket at the opposite end of where the follower would be. (opposite the valve stem) This is probably collapsed (broken). Grab the tip of it with needle nose pliers and pull it straight up and out of the pocket. You may have to rotate the cam so the lobe heel is over the lash adjuster to get clearance to pull it out. If the small middle portion of the adjuster moves in and out or is stuck in the down position then it is no good and has to be replaced. When these fail they collapse and allow sufficent clearance for the follower to fall off. Just get a new one and replace it in reverse of removal. Then slide the follower end over the valve stem and push the other end up to the tip ( as much on top of the lash adjuster as possible) of the lash adjuster. Using a long large screwdriver place the tip of the driver behind the small bubble or dome of the follower and while prying against the cam lobe(carefull) snap the follower up over the lash adjuster,make sure it is seated and that's it. Probably no need to replace all the springs, followers or lash adjusters unless you suspect they are bad as well. If this is all thats wrong than it's your lucky day sure beats removing the heads etc. Good luck.

 

I am heading to Florida in a couple hours. These guys on this site are excellent and I'm sure will help if you have any further questions. I personally wouldn't change all the springs and other parts unless they are defective. They should last the life of the engine and sometimes people damage other parts (like valves) during the process. Take care.