Engine Compression Range?

NIGHTHAWK4570

MEMBER
2001 FORD RANGER
3.0L
V6
2WD
AUTOMATIC
163,000 MILES

Regarding cylinder compression testing, while I know that a cylinder should never have compression of less than 100 psi and that all cylinders should read within 10% of the highest reading cylinder, what psi range would be considered good engine compression?

Also, how much difference in psi readings will there be, if any, between doing a compression test with the engine cold as opposed to an engine that has been ran to operating temperature and then allowed to cool enough to work on?

5 Replies

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

Hi,

For some crazy reason, Ford doesn't provide a minimum pressure. Instead, they indicate that the lowest and highest cylinders are not to be more than 25% different.

I attached the directions right from the manual below. Take a look and see if it is helpful. Also, let me know what is going on, the compression you are seeing, and if you did both a dry and wet reading.

Take care,

Joe

See pics below.

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Sunday, January 22nd, 2023 AT 4:27 PM

NIGHTHAWK4570

MEMBER
13 POSTS

I was actually asking because I'm searching for a used, replacement 3.0-liter engine and one of the used part dealers I've contacted provides compression readings for the engines they sell. For one engine they have available, they've given me the following readings for the cylinders, and it was a cold compression test:

1 - 180 psi
2 - 180 psi
3 - 180 psi
4 - 190 psi
5 - 180 psi
6 - 190 psi

Do these numbers seem high?

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Monday, January 23rd, 2023 AT 7:28 AM

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

Hi,

If those are the compression readings, that sounds really good. Do they guarantee the engine and their readings?

Let me know.

Joe

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Monday, January 23rd, 2023 AT 5:40 PM

NIGHTHAWK4570

MEMBER
13 POSTS

Not sure if they'll guarantee those compression readings, but I'll definitely ask.

I've heard that too high of compression readings can sometimes be an indicator of carbon buildup. With readings of 180-190 PSI, do you think there is any cause for concern about this?

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Monday, January 30th, 2023 AT 1:44 PM

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

Hi,

Certainly, anything that builds up on the top of a piston will increase compression. However, to have it that equal throughout is very unlikely. You could remove a few spark plugs and borrow a bore scope from a parts store and look down into the cylinders to see if anything seems excessive.

Let me know.

Joe