Low compression tahoe

07 chevy tahoe 5.3 random misfire at low rpms. Codes are p0300 and some evap codes (I didnt pay much attention to those, I havnt gotten to the evap part of the book and honestly dont know that much about it all). Scanner should cyl 4 and 3 current misfires. Cylinder drop test showed cylinder 4 to be the only non contributor. Have spark from number 4 coil. Plug looked like crap from not firing so I swapped it and the plug wire with cylinder 2. No change. I did a relative compression test and found a low cylinder. Tried to sync it with a secondary ignition probe. Pretty sure number 4 is the low cylinder. I'm new to the scope and I know I had my relative compression test good but with my secondary ignition I kept getting a "varying" waveform. I'm honestly not sure if I have it set up correctly and could use so help there. From the still shot of my relative compression and ign sync it looks to me like the timing is off. What I'd like to do is watch a secondary waveform on the number 4 cyl and see if I have an injector problem? Causing a cylinder wash down and low compression. If that can be done. Ik paul has showed an ignition waveform that is from a no fuel misfire. I also did an injector balance test and they all were within a half pound of each other. I'm trying to rule out the injector before proceeding with a mechanical problem.

Other things to note the misfire seems to level out at about 1500 rpms. My upstream 02s both banks, are constantly running rich (.800mv or more) and I have no stft or ltft correction. However the freeze frame data showed such. My cam and crank rpms show about 20 to 30 revolutions different and get closer with higher rpm. Issue? I'm not sure. I dont think cyl 3 is actually misfire since it's the next cylinder after 4 in the firing order I think the scan tool is getting false info on that one. Cylinder 3 has a noticeable drop when doing drop test. Also had a code for overheating. Customer states they never saw it overheat and misfire issue started after a car wash. Vehicle has 224k miles. It does smoke on initial start up, white smoke.

What's the 8th digit of the VIN? It'd be good to know the exact engine code.

It sure sounds like you're dealing with a collapsed AFM lifter and/or cam lobe damage It'd account for your low RPM misfire, the low compression, and the spark plug looking like garbage (oil consumption). #4 is one of the AFM cylinders (1, 4, 6 and 7), and I agree that the counts on #3 are likely a result off being adjacent to #4 in the firing order.

I believe you've ruled out an injector problem with the balance test. I also don't think the CMP/CKP counts are an issue at this point.

Can you post your relative compression waveform? We may be able to help you get better results from that test. If not, then you're down to manual compression and leak down testing, IMO. Positively ID the low cylinder and find out where the compression is going.

The other option is to pull the valve covers and observe the rocker arms during cranking. They should all move the same amount. A rocker that doesn't move represents a collapsed lifter or wiped cam lobe.

i dont have a picture of the waveform at this time, i can get one friday. I figured the injector was good since the balance test showed good. I threw a compression gauge in number 4 cylinder and it took several revolutions (about 10) to get 90psi in the number 4. i should be able to add oil in the cylinder and recheck compression to rule out rings? correct? But how can i locate a lifter, or valve stem seal or valve seat? Also wouldnt a collasted lifter cause the valve not to open, low compression would be caused by the opposite from lifter/valve being open?

Also if it was a timing chain problem i would have issues in more than one cylinder?

You can definitely test for a cylinder wash/piston ring issue by adding oil to the cylinders. A leaking valve seat will show up on a leak down test. A valve stem seal? :huh: I've never seen one cause a compression issue.

A lifter that's collapsed won't necessarily show up on a leak down test, since it won't cause a sealing issue. :silly: At that point you'd have to observe rocker arm movement as I described earlier.

An intake lifter that's collapsed, or an intake lobe that's worn down, will cause low compression due to a low charge of air getting pulled into the cylinder on the intake stroke. Both a lifter or a camshaft problem amount to a lack of intake valve lift/duration. A similar exhaust valve problem will still result in good static compression (on a gauge) because there's still something to compress even if the exhaust valve is completely closed.

i can tell a worn cam lobe or lifter by pulling valve cover and putting a dial indicator on the bad cylinder vs a known good? Thanks so much for the info, youve really shed some light on the issue