F150 compression test results

[TellMeWhen]

I'm looking to purchase a boat with a 2004 F150. It has just under 100 hrs. The mechanic did a full service and fuel system overhaul, and gave the all clear. The compression test came back at 195 across the board. Is this good?? I searched on here and the numbers looked much lower? Help please.

[boscoe99]

Numbers look on the high side to me but perhaps it is the gauge.

[robert graham]

They do seem high but as long as the readings are within 10% of each other then generally considered O.K.....but the only way I'd consider buying the rig would be with a good sea trial, checking starting, idle, mid-range and especially wide open throttle/RPM....should show you about 5500 to 6000 RPM at full throttle...

[Sequoiha]

I did a compression test on one yesterday, at the customers request, (personally I think a compression test on a 4 stroke is a waste of time,) A leak down test is more accurate, anyway, my readings were 205 across the board, make sure the throttle is wide open. and the batteries fully charged, the results will be way off if not..

[TownsendsFJR1300]

I did a compression test on one yesterday, at the customers request, (personally I think a compression test on a 4 stroke is a waste of time,) A leak down test is more accurate, anyway, my readings were 205 across the board, make sure the throttle is wide open. and the batteries fully charged, the results will be way off if not..

Agreed 100% ^^^.

A leakdown test will give a much better idea of the health of the internal parts of the engine.

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..

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.

(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.

[boscoe99]

Contrarian view here on the differential pressure (aka leakdown) test being the best and/or it providing the definitive answer. It is just one of several tools that is available to the mechanic to help him diagnose a problem with a motor. I look it as more of a test to find out what is wrong with a motor not if something is wrong with a motor. And neither is it a procedure that will guarantee that a motor is in good working condition.

Outside of aviation, a compression test or a differential pressure test is not part of any recurring maintenance procedure. These tests seem to be done in the marine world only as part of a buyer's inspection or at such time and the motor appears to not be running well. Doing just a differential pressure test can fool a mechanic.

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.

Take another example of excessive carbon build up in the combustion chamber but it is not affecting the valves or rings. The differential pressure test will indicate that all is well. The direct compression test will tell him that for some reason there is too much compression pressure being made. He needs to investigate further.

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.

I perform a dynamic compression check first. If it indicates all is well and if the motor is running well I stop there. If the dynamic compression check indicates something is amiss, and if the motor is not running well, I then move on to the differential pressure test. Depending upon what the differential pressure test indicates it might be time to inspect the cylinder via a borescope or even to pull a cylinder head.

[boscoe99]

Now lets talk cylinder pressure.

Ambient air pressure is nominally 14.7 psi. An absolute pressure gauge will indicate this value at sea level. A typical air pressure gauge of course will indicate 0 psi.

An F150 has a compression ratio of 9/1. So, using mathematics, we take an air pressure of 14.7 psi and multiply it by 9 to come up with a pressure of 132.2 psi. So, theoretically, performing a compression check with an absolute pressure gauge the result would be 132.2 psi. Doing the test with a gauge that reads gauge pressure at sea level (0 psi) the value would be 132.2 minus 14.7 or 117.6 psi.

Now we have got to figure in the temperature rise due to compression. When air is compressed it gets hot. When air gets hot the pressure rises. How hot will the air get if compressed by a factor of nine and how much will that temperature rise affect the pressure seen on the gauge?

I don't know. I should have paid more attention to physics back in school. But in my simple mind 195 psi of gauge pressure seems too high.