Opinion on compression numbers

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Nov 16, 2013 | 02:02 PM

mrberg89

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Opinion on compression numbers

Well folks after owning my truck for a year I decided to finally test the compression. Here's what I found

Drivers side front to back:
390
445
425
445

Passenger front to back:
435
435
410
435

Seems good to me, but the 390 kind of bothers me. Anyone think it's a problem? I tested because I'm planning on sticking a ton of money into her pretty soon (injectors, pump, radiator, suspension) and I wanted a good base to start with.

What y'all think?

Nov 16, 2013 | 03:02 PM

hairyboxnoogle

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I would question the lowish cylinder as well, but realistically those are pretty good numbers. I assume the test had been done after the engine was warmed up?

Nov 17, 2013 | 12:08 AM

ifrythings

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I believe ford lists it as 20 or 25% from the highest to lowest number, so by that your good to go. Haynes says 260-440psi for compression.

If you want to go deeper into it, do a leak down test to see what mite be up with that cylinder.

Nov 17, 2013 | 12:32 AM

mrberg89

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I seem to remember reading somewhere that ford says 25% difference or under.

For having at least 230k miles I think she's doing pretty good.

Nov 17, 2013 | 05:17 AM

BruteFord

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From: Over There

Was the cylinder that read 390 the first one you did, and how long after it had last run?

Nov 17, 2013 | 09:38 AM

Dieselamour

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From: Virginia

Typically, 10% variation is the rule of thumb, unless you have the manufacturer specify 20 to 25%- so you are good to go.

I am assuming this is a dry test. Edit: Do not perform a WET compression test on a diesel engine- end edit.

Last edited by Dieselamour; Nov 17, 2013 at 01:22 PM. Reason: corrected iformation