When doing a compression test on a 7.3 do you... - Diesel Forum - TheDieselStop.com
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'99 & up 7.3L Power Stroke Engine and Drivetrain Discussion of the 99 & up 7.3L Power Stroke diesel engine and drivetrain in the 1999-Up Super Duty trucks and Excursions. No gas engine discussion allowed except on transmissions and drivetrain that pertain to all models. Please confine discussion of topics in this forum to those items that are specific to the 7.3L Power Stroke engine.

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Old 08-11-2008, 02:21 AM   #1 (permalink)
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When doing a compression test on a 7.3 do you...

After reading some scary stories of Compression numbers, I realized I never tested mine. Had 122,000 mi when I got it, has 184,000 mi now. Runs fun but you never know.
Anyways, when testing on these motors, all GP's out, is it best to disconnect the main UVCH connector or each injector connector when doing the test. Is there anything else that needs to be DC'd for the test? Was stock compression 400psi or 450psi?
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Old 08-11-2008, 05:01 PM   #2 (permalink)
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Probably the best way to keep it from trying to fire is to pull the IDM power relay. With the valve covers off, you probably have the 42 pin connector over the driver's side disconnected. That will do the trick too.

I tested my compression at around 75k and all cyls were right around 400. A friends tested in the mid-high 400's, (don't know his mileage at the time). AFAIK, there's no initial compression specification. The book says that if the lowest cylinder is within 75% of the highest, the engine's OK. If they're all way down, you'll be seeing other evidence of that, such as hard starting.

You should crank with the starter for the same number of "hits" on each cylinder for the best comparison.

Hope that helps.
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