[daniel g]

How so? Assume that a cylinder wall has a serious score in it from about the bottom of the ring travel to only close to the top of the ring travel. The engine is running poorly. Perform just a differential pressure test and it may indicate that the cylinder is in perfect condition. The valves are completely closed and the rings are sealing perfectly since they are impinging against the cylinder wall above the score point. The mechanic is perplexed as hell. The test is telling him that mechanically the motor is sound but it runs like crap. He rotates around his own axis trying and trying to figure out what is going on. Had he done a dynamic compression test he would see that the problem cylinder was way down on pressure. [links]

[FreddyFender]

Take another example of a sticky valve. The valve snaps itself closed but is slow in doing so. The differential pressure test may indicate that all is well. But again, the motor runs like crap. A direct compression test gets performed and low and behold, a cylinder is down on pressure.

[jescro]

(My F150 engine is slightly newer, low hours, flushed religiously and one seal, SS spring was indeed broke). Of course, both seals were replaced and the LU pressure tested. here

[doublec]

There is a set of seals under the water pump that normally DO wear out and need replacement. I would ask if those were replaced recently and just be aware of the cost. more

[derka]

Also agreed with a good sea trial, paying attention to slow speed idle (as well as what Robert posted), should be 700 +/- 50 RPM's..

[loadingpicker87]

Now you can take your compression tester and thread the hose into any cylinder that you want to start with.

[knight]

Something that a lot of people wonder about. There has to be a reason that causes an engine to have low compression, and there is. [links]

[hairtransplants789]

In my personal opinion, and experience of having performed possibly up into the thousands of compression tests. That it is most definitely better to do the compression test with a little warmth to the engine. more

[fflaguna]

You might be wondering this question, as I’ve had to answer it many times over the years to newer mechanics and boaters. Which is a great thing to me, because it shows a person’s willingness to learn! more

[becauseIfuckingCANbitchz]

did one on a master craft yesterday with an Ilmor 5.7.

[chmod755]

could be a lot of things,cam timing,cranking speed.
that's why I don't do compression tests.
I do leakdown tests.

[draaglom]

I can do a leakdown test in about 1/2 the time of a compression test and it way more accurate.

[tabshift]

BTW I'm sure you are aware that the F225 and F250 has/had corrosion problems so that should also be a concern.

[ShadowOfHarbringer]

I recently asked about an F150 that's been sitting in a Marine shop boxed in a crate for over a year and one main answer I got was to do a leak down test which is way better than a compression test. Just sharing.

[darrob]

make sure ecm data looks good and sea trial motor if possible. ive seen way too many tricks having the motor idle on the stand to be satisfied. fuel system issues which present themselves under load is most common and most costly.