Compression Test Questions
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Compression Test Questions
Hey all,
As of right now I'm in the process of a semi-teardown of my 300 in my 85 F250. The engine isn't original, and I'm led to believe its from an 80-82 based on the oil dipstick location on the pan. I plan on doing a valve job on the head as I don't know the exact mileage on the motor, but I would like to avoid a bottom end rebuild if the compression is decent.
As of right now I have no manifolds or carb on the truck, so will that affect anything when I check compression in each cylinder? Also, I know to compensate about 10% lower pressure testing a cold engine (not ideal, I know), but what are some average compression readings on these engines? I've seen everything from 80 to 160 psi but can't find a good thread, and my manual says the lowest pressure cylinder should be no less than 75% the pressure of the highest, so I'd like to get a solid average. Any help or extra experience here would be great.
Thanks!
As of right now I'm in the process of a semi-teardown of my 300 in my 85 F250. The engine isn't original, and I'm led to believe its from an 80-82 based on the oil dipstick location on the pan. I plan on doing a valve job on the head as I don't know the exact mileage on the motor, but I would like to avoid a bottom end rebuild if the compression is decent.
As of right now I have no manifolds or carb on the truck, so will that affect anything when I check compression in each cylinder? Also, I know to compensate about 10% lower pressure testing a cold engine (not ideal, I know), but what are some average compression readings on these engines? I've seen everything from 80 to 160 psi but can't find a good thread, and my manual says the lowest pressure cylinder should be no less than 75% the pressure of the highest, so I'd like to get a solid average. Any help or extra experience here would be great.
Thanks!
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Joined: Nov 2015
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From: Angier, NC
When doing a compression test the throttle should be open so with the manifold(s) off is not a big deal.
What might be a better test is a leak down test. It can tell you if the valves are bad and what ones, intake or exh., easy with manifolds off or if the rings are bad.
As for not wanting to pull the bottom end apart I can under stand that but a bad or good compression test is not why I would not pull it apart.
If you had the motor running what was the oil psi when up to temp? Low oil psi is a sign of warn bearings be it rod, main or cam bearings and/or crank warn.
So not knowing the oil psi the only other way I know of checking bearings & crank is to pull it apart.
It has been 10 months since I did a compression test on my 300, truck was new to me and motor had a miss. Don't remember the readings number wise as I was checking to make sure it did not have a dead hole. But do remember the readings were not bad and pretty close to each other. Replacing the plugs that did not look too bad, fixed the miss.
Sorry I cant help more and good luck with your project.
Dave ----
What might be a better test is a leak down test. It can tell you if the valves are bad and what ones, intake or exh., easy with manifolds off or if the rings are bad.
As for not wanting to pull the bottom end apart I can under stand that but a bad or good compression test is not why I would not pull it apart.
If you had the motor running what was the oil psi when up to temp? Low oil psi is a sign of warn bearings be it rod, main or cam bearings and/or crank warn.
So not knowing the oil psi the only other way I know of checking bearings & crank is to pull it apart.
It has been 10 months since I did a compression test on my 300, truck was new to me and motor had a miss. Don't remember the readings number wise as I was checking to make sure it did not have a dead hole. But do remember the readings were not bad and pretty close to each other. Replacing the plugs that did not look too bad, fixed the miss.
Sorry I cant help more and good luck with your project.
Dave ----
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I'll definitely have to grab a leak down tester--I've heard of that before but it never even crossed my mind. Thank you for that tip!
Also, oil pressure has always been good. I've never had any strange noises or any smoke while it's running. A lot of this is a "might as well" situation since a gasket set is all of 50 bucks or so, and access to the head will be simple while I have everything torn down. I figure at least a valve job and head gasket will be nice peace of mind when it's -5 this winter and trying to move a bit of snow.
Thank you again for the info!
Also, oil pressure has always been good. I've never had any strange noises or any smoke while it's running. A lot of this is a "might as well" situation since a gasket set is all of 50 bucks or so, and access to the head will be simple while I have everything torn down. I figure at least a valve job and head gasket will be nice peace of mind when it's -5 this winter and trying to move a bit of snow.
Thank you again for the info!
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