98 gmc p0303 code

I have 1998 gmc with a 5.0 vortec motor with 108,000 miles on the motor. I keep getting a p0303 code. I changed the plugs, wires, disributor cap, rotor cap, fuel pressure regulator switch, switched fuel injectors, egr valve, ignition module. Coil was fine. Haven't replaced any o2 sensors. Tired of spending money and not fixing the problem. Buddy suggested replacing the "PCM/ECM".

Check your compression of that cylinder .
What plugs did you use.

compression was 75# on #1 cylinder & 125# on 3,5 &7. I forget the brand. I think either a/c delco or split fires on all of them

Compression pretty low on #1 and low on the others.
175 psi should be on all cylinders with no more than 5-10 % difference between each other.
Q, Did you take all plugs out and open the throttle fully, when doing the compression test.
Change # 3 injector for #1 ,and see if the misfire moves to cylinder #1 (P0301)

Did the compression test wrong. Forgot to open the throttle body. What a big difference. Compression is as follows Cylinder #1-180, #2-195, #3-190, #4-200, #5-190, #6-200, #7-190, #8-195. Someone suggested doing a leakdown test - need to get a tool for that though. Using Bosch platinum plugs. Checked the .060 gap while the plugs were out
I've never replaced an o2 sensor since I've had the truck

I hate those plugs! I've also watched alot of people chase misfire issues on those Vortecs because of aftermarket distributor caps causing flash over from cylinder to cylinder as well as poor quality ignition cables. I'd start with plugs and a distributor cap from A/c delco

I agree completely get them plugs out sure thats your problem, as Jeff said "I've also watched alot of people chase misfire" just because of Bosch plugs GM's don't like them .

I haven't yet met any vehicle that likes Bosch Platinum plugs. Get the proper AC plugs before you do anything else!!

[b]FYI: Finally fixed the problem. A buddy of mine checked everything I did and then did a runingncompression check on #3 cylinder. He felt the compression wasn't building up the way it should have. With the truck hot, we slowly poured water down the intake to shock any carbon off the valves. I removed the valve cover and keepers on both valves and found that the exhuast valve spring was broken. I replaced both springs and haven't had any problems since. After spending over $300 in other parts all I needed was to spend another $16.

I have a 98 sierra after replacing the distributor, cap, plugs, wires, cat, fuel pump, fuel filter etc(over the past three years)... I'm still having the same problem as you mind you after the distributor was done problem disapeared for the last six months. However it came back. I've noticed that the missfire only happens @ 3000 rpm for an extended period af time (for example going up long hills maitaining speed).I will try your fix of the valve exhaust spring.also I tried the bosch plugs as well, problem code was more frequent.