I am at my wits end. 95 blazer spider and poppet style fuel system. About a year ago it started a miss in cylinder 3. New plugs, wires, spider assembly, nothing would cure it. Compression was about 15lbs less than the rest. Finally bit the bullet and pulled the head. Found a broken valve spring on the intake. Valve looked good so I put a valve spring in and put it back together. Ran good for a couple weeks and then started the same P303 code. Put delco cap and wires on it since you guys said that is all that works on these. Still code P303. Compression is still at 145lbs. The rest are about 160lbs. Switched the poppets between 3 and 5, no difference. No fuel leaks in intake. Fuel pressure good. Checked valves, still set correctly. I am ready to burn this thing!!! Some days it is barely noticeable. Some days it is a completely dead hole. The exhaust smells rich. What am I missing?????????????

 

When you did all of the work with the spider, did you replace the internal lines running from the spider to the back of the lower intake plenum? If not, you may have a leak there. A fuel pressure test will tell you if there is a leak.

Also, did you lap that valve in prior to putting the new spring on? A leak down test will tell you whether or not the valves and/or rings are leaking and potentially leading to the misfire in the cylinder.

You could also run through the valve lift on that cylinder compared with the rest to see if the cam lobe is wiped away.

 

It is dry inside the intake. No leaks there. I did not lap the valve. I tested it with parts washing fluid and it was dry. My engine builder was there helping me and that is what he recommended. Lift appears to be the same. I did not put a dial indicator on it.