I just pulled my valve covers off due to a valve train knock. How can you tell if a hydraulic lifter has failed? I have a 1995 merc 502 EFI.

With solid lifters, it is fairly easy because you will see an increase in lash. Hydraulic lifters will take up the slack. You may have to narrow it down with a stethoscope and then pull the noisy lifter to check it. If it is sticking or bleeding down a visual inspection may not tell you a lot, but at least you can ensure that the wheel is not failing.

If you are using a mechanical pump mounted on the block, the fuel pump pushrod can make noise like that.

Good call on the pump, I didn't even think of that. I put my hand on the intake plenum and I can feel the the tapping through it, that is why I am looking at the lifters.

Well if you pull the valve covers after running they should be pumped up. If so have someone start it and with valve cover off you could hear where its coming from and put pressure on the lifter end of the rocker and noise will change. Also a simple comp test will let you know where to start.

Theirs no fuel pump rod on a gen 6 engine,,the fuel pump is mounted to the sea pump.... If you have a bad lifter if you push on the side the push rod is on and if it were bad you would be able to compress it about 3/8''..they get real spoungie..

As 1969 said compression test should give you a hole that you should be looking at.

comp checked ok. Could low fuel pressure create a knock or tap? pulled the fuel line off and barely anything came out and there wasn't any pressure behind it.