F250 compression #s

[99yam40]

I just got off the phone with a friend that has the same motor, he went out and scanned his motor and it was 1 to 2 degrees at idle on both banks so that confirms what rodbolt said about it being way off at +12 degrees at idle,

this motor could be a chance to do a compression test on a different F250.
it should show what would be a proper PSI using your tester.
195 or 170

[formula29]

why are we still harping on the differential test? The intake cam being +12 degrees advanced seems to be a more likely problem unless we want to dwell on weather I had the YDS hooked up properly? while it has been pointed out the advance timing is not right I have not herd any insight as to why it could be advanced like it is?

[boscoe99]

[boscoe99]

900 hrs on the motor total, I am guessing half that on the new crank case based upon when it was replaced, just a guess, obviously the problem I am chasing is the 25# difference between cylinder banks, its not right, I know Yamaha doesn't produce a given # for what the compression should be but what do they generally have when new? 195? or 170?

Minimum compression pressure is stated as being 107.3 psi at 68 degrees F with the throttle wide open and all spark plugs being removed when the test is performed. There is no mention of an atmospheric pressure standard.

This is just a reference value and not an absolute value.

[boscoe99]

Pull the valve cover and simply check to make sure that all valves are closed when the piston is at TDC on the compression stroke. Closure will be indicated by a slight amount of clearance between the cam shaft lobe and the bucket shim.

This will tell you if the cam drive mechanism is phased correctly.

[boscoe99]

This is the issue when one does not fully understand one's instruments.

An "inappropriate" leakdown tester can easily show "0%" loss of air on quite a wornout engine!
I use the word inappropriate because each case must have the correct tester. If we assume the OP tester is operating correctly, but there is a chorus out there that says you must have some leakage, a scenario could be:
He has a large compressor that puts out lots of air easily at 100 psi. The compressor can output 10,000 psi at 10,000 cubic meters per minute. Does not matter as long as the gauge is maintaining 100 psi.
His regulator responds readily to maintain that pressure.
His orifice (not getting personal) between the two gauges is also large allowing air to readily flow. Yes, an over large orifice can skew the results seen on the second unregulated gauge.
His leak is small or even a bit more (all engines leak).
His second gauge reads 100psi.
We are told this means there is no leak.
Nonsense, all it means is that the compressors air in the entire system can be maintained at 100psi!.
Air could be hissing out everywhere! But his tester doesn't pickup the leak because his orifice (not getting personal) is too large and/or the gauges orifices are too small acting with too much dampening restricting their reactions. Yes, an over large orifice can skew the results seen on the second unregulated gauge.
The only useful thing he has, if the above is true, are his ears. Listen for the leak(s).
Just a word of caution.

If I were doing a differential pressure check and saw zero loss I would suspect something is amiss. As amiss as seeing two engines both of which have the exact same out of kilter compression readings and which are not making sense.

Someone mentioned an orifice with a .125" diameter. Seems strange to me. I have never seen an orifice size published by Yamaha but the FAA specifies an orifice ID of .040" when smaller size cylinders are being tested. In their case a smaller size piston is one with a diameter less than 5". I can't think of any airplane piston motor that has a piston diameter as small as a Yamaha F250 piston with an OD of 3.7". .040" is a long way from .125".

[zenoahphobic]

I have checked all of the timing marks, they are kind of idiot proof, I think either something is wrong with the cvt or it may be the wrong one maybe from an 05, I only say that because the port and starboard share the same part# but they look a little different, they have different markings on them. I need to go look at my friends motor tomorrow,

The variable cam timing on these engines was raised some time back. You may want to search past threads. Off the top of my head I think it was concluded there is no "serviceable" parts to it.

[zenoahphobic]

I thought the leak down test was the cats meow. The be all and end all to solving problems.

For having 900 hours, a leak down test showing 0% loss of air is quite remarkable.

What is the objective? Is there a problem with the motor?

This is the issue when one does not fully understand one's instruments.

An "inappropriate" leakdown tester can easily show "0%" loss of air on quite a wornout engine!
I use the word inappropriate because each case must have the correct tester. If we assume the OP tester is operating correctly, but there is a chorus out there that says you must have some leakage, a scenario could be:
He has a large compressor that puts out lots of air easily at 100 psi.
His regulator responds readily to maintain that pressure.
His orifice (not getting personal) between the two gauges is also large allowing air to readily flow.
His leak is small or even a bit more (all engines leak).
His second gauge reads 100psi.
We are told this means there is no leak.
Nonsense, all it means is that the compressors air in the entire system can be maintained at 100psi!.
Air could be hissing out everywhere! But his tester doesn't pickup the leak because his orifice (not getting personal) is too large and/or the gauges orifices are too small acting with too much dampening restricting their reactions.
The only useful thing he has, if the above is true, are his ears. Listen for the leak(s).
Just a word of caution.

[formula29]

The ecm sees the cam timing through a cam position sensor, there is one on each cam, evidently the cam is advanced at cranking speed, the vct doesn't do anything until oil pressure builds up, so either the wrong vct pulley is installed or mine in broken and not letting the intake cam retard to its resting position, I don't know, I have never really had to mess with one of these so I will figure it out, there is in no way a need for another leak down test or new gauge, that will not change the timing,

[99yam40]

Did you not say that it all comes back into matching degrees at a higher RPM?
I am not sure how the ECU/scan sees the cam timing
Something is a problem at cranking or idle, but not higher RPM?

Seems strange that if the cam timing is off then the sealing of the cylinders would show a problem

I would try to find a different tester to try just to make sure

[TownsendsFJR1300]

Well, sounds like your narrowing it down...

Perhaps the dealer would replace both cams with new, correct ones...

[formula29]

I have checked all of the timing marks, they are kind of idiot proof, I think either something is wrong with the cvt or it may be the wrong one maybe from an 05, I only say that because the port and starboard share the same part# but they look a little different, they have different markings on them. I need to go look at my friends motor tomorrow,

[TownsendsFJR1300]

So does that mean the cams are simply not installed correctly?

I worked on a small Yammy 40 HP maybe where the one cam was OFF TWO belt cogs and still ran good. We popped the belt off and the cam, on its own, jumped into the correct position.. (non-interference engine)

[formula29]

I just got off the phone with a friend that has the same motor, he went out and scanned his motor and it was 1 to 2 degrees at idle on both banks so that confirms what rodbolt said about it being way off at +12 degrees at idle,

[TownsendsFJR1300]

The purpose of the Schrader valve in a compression tester is so the multiple compression strokes needed for the psi measurement can accumulate in the gauge. Got nothing to do with reading at your leisure or letting air out of the gauge.

A shrader valve (as in your cars wheel) strictly holds air in (and more air as pressure allows) and allows you to release it.

And yes, when you push the valve in, it releases the air stored inside.

I guess we're playing with words today...

Sorry OP for the hyjack, but I'm on the same page as you with both engines, same banks, with the advanced timing are all the one thing in common..

Does the dealership have a variable cam engine (used, on a boat being fixed, etc) that you could do a compression test (for the high and low #'s) on EACH BANK?? Curious if another motor with that set up does the same.

One way or the other, you can make a more informed decision on what to do next...