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V8 compression loss diagnosis (Read 348 times)

pct

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Baker City, OR
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V8 compression loss diagnosis
Feb 27^th, 2012 at 4:39pm

Hello!

First off i'm new to pico scope. I'm having a blast with this tool tho Grin

. My boss has a 1976 ford f150 with a 390. I've been playing with it in my spare time. we know it needs an engine so i decided to diagnose it with our new tool. i did a cranking compression and found cylinder #7 low (see attachment 1). i comfirmed with a mechanical gauge at 60 PSI. Now why is it low???? I checked everything!! i did a running compression (attachment 2) nothing really stuck out. Engine vacuum (attachment 3) no sign of a intake valve problem. exhuast pulse (attachment 4) nothing stood out. then i hooked up to the crank case through the dipstick tube and unhooked the pcv. Wink

you can see the change after #7 fires. i confirmed this with a cylinder leak down test and a wet compression test. next time i won't have to!

compression_001.psdata (558 KB | )

psi_compression.psdata (574 KB | )

vacuum_001.psdata (542 KB | )

exhaust_pulse.psdata (660 KB | )

crank_case.psdata (656 KB | )

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drdiesel1 Super Nerd Picogroup Offline Posts: 1,243 Nevada USA Nevada USA Gender:	Re: V8 compression loss diagnosis Reply #1 - Feb 27^th, 2012 at 6:40pm Slick ain't it It took me about 30 seconds to diag. a broken timing belt yesterday
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DaveR Senior Member Picogroup Offline Posts: 169 milwaukee Wisconsin USA Gender:	Re: V8 compression loss diagnosis Reply #2 - Feb 27^th, 2012 at 9:19pm drdiesel1 wrote on Feb 27^th, 2012 at 6:40pm: Slick ain't it It took me about 30 seconds to diag. a broken timing belt yesterday Dr. diesel how did you use pico to diagnose broken timing belt?
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drdiesel1

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Re: V8 compression loss diagnosis
Reply #3 - Feb 27^th, 2012 at 11:48pm

DaveR wrote on Feb 27^th, 2012 at 9:19pm:

drdiesel1 wrote on Feb 27^th, 2012 at 6:40pm:

Slick ain't it Smiley

It took me about 30 seconds to diag. a broken timing belt yesterday Smiley

Dr. diesel how did you use pico to diagnose broken timing belt?

Relative compression test Smiley

2.0L Dodge Neon with two low holes @ 40%. Popped the rubber plug on the belt cover and voila, no cam gear drive Smiley

I use this feature a lot after getting use to it on my IDS. The IDS works the same, but it ID's the cylinders for you. It's so quick and easy to test for mechanical issue's Smiley

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Mick LSD Guide Offline I learn every day Posts: 7,471 Toronto 400ft Ontario Canada Gender:	Re: V8 compression loss diagnosis Reply #4 - Feb 28^th, 2012 at 7:03pm drdiesel1 wrote on Feb 27^th, 2012 at 11:48pm: Relative compression test How about this one jeep 4.7
Back to top	relative_compression_003.gif (80 KB \| ) Mick IP Logged

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