Update the computer or change cops and plugs

1955moose said:

Pull the valve cover for that 0 cylinder, wouldn't be surprised if you've got a snapped off valve spring, with the valve just free floating. Your motor should be the same 260 hp as mine, and I need all of them to pull a grade. Having tried 3 different gauges is smart, you must be an engineer in your trade. I would have stopped at 2 gauges. Let's hope for something easy for your fix.

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Well I figured there were 3 available and it only takes a sec to thread them on so why not. Lol.

If I had a busted valve spring wouldn't it make a noise loud enough that I'd here it without an oscilloscope?
Although I guess if the valve spring was broken right at one of the ends it probably wouldn't make a lot of noise but therefore would allow the valve to travel down and get smacked by the piston causing it to be bent..

You never know till you pull that cover. I had a good friends girlfriends Ford Fairmont with an in line 6. It was back firing through the carb, compression was 140 all 6. I pulled the VC cover and #3 spring was laying there, snapped off. Best guess the valve was bent closed holding compression. Weirdest diagnosis untill I yanked that cover. It was amazing the piston wasn't smacking the valve head, but it didn't. The other scenario is your piston rings are stuck on that bank, and partially stuck on that low compression cylinder. Do a leakdown test on those 2 banks, if it turns out to be rings, try a bottle of marvel mystery oil in your oil. You don't have the cam phasers so you should be ok. Anyway curious what you find.

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Moose, what do you think of Marvel Mystery Oil in our later cam-phaser motors?

for those of you still playing the home game with me...

turns out I wasn't testing compression properly. after being shown the correct way to do it all cylinders showed well within tolerable ranges.

so actual diagnosis...
After putting a smoke machine to it, turned out I had several failures in the upper and lower intake as well as the EGR, IAC, and a cracked brake booster grommet. so we took it for a spin and checked out the fuel trims.... they were maxed out. ..after tearing into that, saw some not so wonderful things on the valve covers, so I went ahead and replaced those also. as of yesterday (10/8) all seems well.

will potentially be taking it for another ride tomorrow to see if the trims have improved for sure. I do have a couple of questions about vacuum lines that we weren't able to figure out, But ill start another thread for that topic.

Big thanks for all the help/info!

Thank you so much for the follow up man! Glad it's going well