I was told that I must have a broken valve spring...

[Just00Jake]

I started getting a random misfire code (P300) on my 2013 Tahoe LS 5.3. It then turned into a misfire on cylinder #2 (P302). I changed all the plugs and wires figuring it was probably due for a tune-up. Of coarse I bought OEM Delco Irridium plugs and good wires. No change, so I paid a "mechanic" to run diagnostics on it. I was told that I needed a new coil pack for #2, so I replaced them all, no change. I had it checked for vacuum leaks and none were appearant. So I asked what it could be? I was told that it must be a broken valve spring, without compression I'd get a misfire code. I picked up a compression tester and actually got compression on my #2 cylinder. I am not sure how many times it cranked but the gauge read 90psi. My question is if I had a broken spring causing a misfire, wouldn't that mean that one of my valves would always be open? Wouldn't that prevent me from registering any compression or am I totally nuts? I hope I am nuts because if the spring isn't broken then it's gonna drive me nuts trying to figure out what to try next. This all started with an intermittent rough idle 2 months ago. I appreciate any help. Thank you

 

[OR VietVet]

What is the compression on the rest of the cylinders, with all plugs out of all cylinders? You are correct, if the spring(s) are broken, the valve could be hanging open unless it is stuck in the seat for some reason. The spring may be ok but the rocker has a problem. You need to at least pull the valve cover and LOOK AT IT! You may have to remove head to get an even better look. The 90 psi is not a good compression reading. Are you hearing any strange noises? What did the old plug look like when removed and compared to other plugs?

 

[Just00Jake]

The plugs all seemed fine. They were all the same. I did start with changing the O2 sensors. I'm going to pull the valve cover tomorrow and will let you know. I've seen videos where people are doing compression tests and they say how many cranks they do to get a certain psi. I'm not mechanically illiterate but this is kicking my butt. I'll post my findings asap

 

[Geotrash]

The plugs all seemed fine. They were all the same. I did start with changing the O2 sensors. I'm going to pull the valve cover tomorrow and will let you know. I've seen videos where people are doing compression tests and they say how many cranks they do to get a certain psi. I'm not mechanically illiterate but this is kicking my butt. I'll post my findings asap

Is the engine making any noises? Is it running rough? A broken valve spring usually results in a dropped valve on these engines. And you'd know it.

Check your fuel injector connectors and make sure they're all snug.

 

[Foggy]

90 psi cranking compression sucks...............
You have a piston issue or a lifter issue is my guess
A healthy gen IV engine should have at least 150 psi on all cylinders

 

[swathdiver]

Time to pull the valve cover for a looksee.

 

[Dustin Jackson]

Yup open it up and see what is broken. Engine is probably in good condition just need to fix whatever broke.