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3.0 Vulcan Compression Test Results


Old 05-12-2015
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Join Date: May 2015
Location: Lake Mary, fl
Posts: 2
3.0 Vulcan Compression Test Results

The idle on my 2002 3.0 is a little rough. Did plugs, IAC, cleaned throttle body, ran injector cleaner... Idle got a little better but it's still a bit rough. Seems fine while driving.

Miles = 280k and climbing :-)

I just did my compression test and the results were:


WOT, warm, all plugs removed, fuel relay disconnected.

1 and 2 are lower but is this still within range? I've heard within 15% is ok. Other posts I've seen say FORD says within 75% is good.

I did a second test cold and with a stronger battery and come up with these.


Obviously 2 is low. I put a little oil in the cylinder and the pressure shot up to 200.

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Old 05-13-2015
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Yes #1 and #2 are your problem.

Average is 155psi warm and 165psi cold, so the 137psi is low and the 125psi very low
An no, it isn't within any spec.

That is what is causing the partial "misfires" at idle
Compression is like a rubber band, the more you wind it up the more power is released when you let it go.
So lower compression cylinders don't add as much power to the crankshaft when they fire, you notice this more at lower RPMs, idle, it is just a lower power firing which you can feel or see as a vibration, not a complete misfire.

As for the cause of the lower compression, when Ford resigned the 3.0l heads in '99 they went from 8mm valves to 7mm valves as part of the change.
There was a TSB about '04-'06 3.0l Rangers and a recessed exhaust valve seat problem causing lower compression and eventually full misfires at idle.
I have read posts from Ford service techs that this issue started with the '99 head design and was just TSB'ed when it became an obvious issue.
So your '02 heads could have the start of this issue, or maybe not, just an observation.

Leak down test is more definitive to ID whether it is rings or valves that are leaking the pressure out.

The jump from 125psi in #2 to 200psi reads like you put in alot of oil, lol, generally speaking if you add oil to a low cylinder, and there is a ring problem, pressure will come up to the "average" or slight above, if it is a valve issue then pressure will go up but still be below average.

Rings can be faulty but I would expect it either in all cylinders or just 1 cylinder, 2 would be an oddity.

Head gaskets can leak between cylinders, pinging/knocking damages the softer metal rings on head gaskets which can allow them to start leaking.
Vacuum gauge on idling engine will usually drift from 6" to 17" if there is a leak between cylinders.
If you apply air pressure in one cylinder(like lead down test) then listen with a small hose in the adjacent cylinder you can hear the air escaping if gasket is bad.

Last edited by RonD; 05-13-2015 at 12:06 PM.
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Old 05-13-2015
Thread Starter
Join Date: May 2015
Location: Lake Mary, fl
Posts: 2
Thank you! Yes, I think I added wayyyy too much oil. I kept reading to add 2 tablespoons so I added one, another half, on my way up to 2, and realized it seemed to be way too much so I stopped.

So what would you recommend I do next? Deal with the idle or do a leak down to see if the valves are the issue?
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Old 05-13-2015
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Leak down test would get you info about rings or valves and also gasket check, so that would be my next test.

For those wondering what Leak Down test is, you apply a known amount of air pressure to a cylinder at TDC via the spark plug hole and measure the pressure it can hold, the difference in the known pressure and the "held" pressure is the Leak Down %.

Metal rings and against metal cylinders and metal valves against metal seats are not air tight, a brand new engine will leak pressure, generally you will see a 5%-10% leak down on good cylinder.

If you apply a known pressure of 100psi to a cylinder and pressure gauge on cylinder shows 95psi then you have a 5psi leak down or 5%, this would be a newer engine.

Say you apply 100psi and get 85psi on gauge, 15psi leak down, you then add teaspoon of oil, and turn the engine over a few times, then retest that cylinder.
Held pressure(85psi) will always go up, it is just by how much that you are testing, if it only goes up a few psi then valves are leaking, if it goes up above 90psi then rings were leaking.
Leak down is like compression test in that testing one cylinder won't tell you much, you need to test a few to get averages.
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