So I decided to replace my ‘88 GTA L98 valve stem seals as it would smoke when starting. I found no exhaust seals so went back with Howard’s Cams viton seals on all. While doing this I thought I would replace the plugs (AC Delco Rapidfire #2). Also thought I would replace the distributer cap and rotor.

I started the car and it sounded like it was not running on all cylinders. I decided to check the distributer cap and rotor. Then put the old back on. The car ran better with the old one so not sure why the new one didn’t run. Anyway, it sounded like a valve needed adjusted on the passenger side so pulled the cover to check and indeed one needed tightened. I started the car again and it idled great and let it warm up a bit. I went to give it more throttle and it started to knock and buck. Yanked the valve cover off and found nothing out of the ordinary(all rods in the lifter pockets and tight). Put it back together and started it. Same thing idled good then started to knock.

I am not sure if it’s a bottom end issue or what at this point? Any ideas?

 

We would just be guessing to. You'll have to do some investigating.
all valves adjusted correctly?
plugs the right length? (i would have stayed with the cheapie factory type delco plugs)
sure you have the correct firing order on the wires? A couple are easy to swap and itll run but badly. Sure it isn't a miss/backfire?
drop anything in a cylinder?

 

The plugs it has were AC Delco Rapidfire #2s so same same. I did not drop anything in the engine. I have adjusted the valves about 5 times to make sure they weren’t too tight. The good news is I have getting the valve covers off in pit crew timing. Anyway going to do more investigation and a buddy has an endoscope and inspection camera.

 

Double an triple check the firing order real quick

 

Yeah I pulled the wires off and strung them out to ensure they were right twice but can always check a third time.

 

So slept on it and it dawned on me, the knock only happens at higher rpm. I noticed a couple valves were what I will call soft (lifters). Meaning I could easily push the push rod down and I wonder if I have a couple failing listers. Anyone with experience?

 

The lifters are probably fine, but it sounds like they may be adjusted too tight. After adjusting them, if you then rotate the motor around, the hydraulic lifters will bleed down and seem like they are adjusted too loose. Then, if you adjust them again, they are too tight and will hold the valves open when the oil pressure pumps the lifters back up.

When this happens, I’ve found that adjusting them with the motor running works pretty good. Get the clips to put on the rocker arms to keep the oil from spilling everywhere.

Check the plug wires for the right firing order too. Follow each wire to the spark plug.

 

So played with adjustment more but did not adjust live yet. The engine idles great. I let it warm up then gave it some throttle and once again knocking.

Some suggested something in the bottom end but it doesn’t make noice all the time and I would figure it would if in the bottom end. I am going to try to inspect the cylinders with a camera/endoscope but not sure what I will be able to see through a spark plug hole.

 

I'm wondering if the timing somehow got advanced and when you rev up the engine the advance is causing pre-ignition, spark knock?

 

So a little more to the saga, the oil looks good so I pulled the spark plugs. They look ok nothing unusual. I decided to take an endoscope and look to see if there is anything in the cylinders where the noise is coming from. I saw nothing unusual there.

I disassemble the plenum and runners to take a look in the intake runners down to the valves to see if I see anything there.

I will attempt to adjust the valves with the engine running this weekend but I fear the random knocking may still exist. It’s sounds like a rock that randomly hits.

 

Has anyone ever experienced a lifter failure that only exhibits itself at higher rpm? Let me describe the noise. The car idles fine no issue or noise but then when I hit the throttle it sounds like a rock in the valve train and to date I have found no issues in the cylinders (checked via endoscope camera). I am going to take off the runner and plenum to use an endoscope to see if there is anything odd in the intake ports then I will adjust the valves once again while the car is idling.