F250 compression #s

[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".

[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]

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]

[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?

[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

[99yam40]

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?

seems like most do not believe the test you did was a valid one

but if one cam is advanced farther than the other at cranking it seems like it could be seen with the eye

not sure how the oil pressure changes the timing of the cam.
I take it it pushes a device that advances or retards the cam.

someone want to enlighten us on that.

[boscoe99]

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?

Zeno and I are having a side discussion.

[boscoe99]

seems like most do not believe the test you did was a valid one

but if one cam is advanced farther than the other at cranking it seems like it could be seen with the eye

not sure how the oil pressure changes the timing of the cam.
I take it it pushes a device that advances or retards the cam.

someone want to enlighten us on that.

Consider yourself enlightened.

[zenoahphobic]

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?

Its all good what is being said, albeit repetitive or the same from different points of view and different world times in the order of posting.

The differential test points have been dealt with.

The 25psi remains a mystery.

And in all likelihood your VCT is stuffed, cannot be fixed, only replaced.

If replaced, then we can retest compression and wipe away that 25psi as being anything but an anomaly of the type of test, and has nothing to do with the cylinder compression ability in operation.

[rodbolt17]

if at idle one cam is showing about -2 and the other is showing +12.
either the cam was installed incorrectly,the VCT assy is sticking or the VCT filter is severly clogged and wont allow oil to return.
cam timing will affect compression reading.

and I gotta know, how did you hold the throttle valve OPEN when doing the compression test?

the cam angle on YDS should show both banks within about 4* all the time.

the ECU is not designed to move one due to any issues.

it is designed to move both based on RPM and throttle angle.

don't know how this got so crazy.
all you need to know is CAN it mechanically SEAL said cyl.
if it can your ****en there and just neeed to find out why the cams wont follow each other.
if it CANNOT then you need to find out WHAT is NOT sealing and fix that before proceding.
as for the 0%.
I was doing a leakdown on an F350, all were less than 7% and 2 showed 0%.
that I attributed to my gauge simply could not measure a very small leakage.

I have seen the 0% reading on a few others over the years. doesn't concern me as long as ALL do not read 0%, then I would suspect my test equipment.

kinda like testing a circuit with a standard digital meter yet the spec is in micro mo's.
standard meter wont see a resistance that small.

[TownsendsFJR1300]

I would think with what you posted above, the most likely would be a dirty filter (wherever those are located).

Especially since you haven't mentioned any failures of those cams.

As noted earlier, they cannot be serviced, but have you seen any/ many fail?

We don't know the PO's maintenance to the engines. How temperamental is the internals are for the variable cams?

The Op did check alignment and everything appeared dead nuts on.


Seems its getting narrowed down in any case...

[rodbolt17]

no
he did not check it correctly.
you have to line up the mark on the reluctor with the tab cast in the block THEN look at the cam sprocket marks.
the TDC marks on the flywheel are NOT used to set the cam sprockets.
it can be seen with the flywheel in place but its not easy.
the filters are under the OCV.
the plastic ones are about 70 bucks each.
the later paper style are like 8 bucks.
Yamaha recommends replaceing them about 400 hours.
the valve cleaarence is about 400 hours.
never ever gets actually done.

the maint actions are in the book.
the 10,50,100,200,400,500, and 1000 hour OR yearly, monthly specs are all in the maint guide.
10 100 hours services does NOT equal 1 1000 hr service.
is it my fault it takes 100 bucks woth of gaskets and 5 hours or so to check valve lash on an F350?
nope but it is recommended every 400 hours.

same as cam chain tensioner check on the F200/225 v6. no one ever does it till it fails.

[99yam40]

if at idle one cam is showing about -2 and the other is showing +12.
either the cam was installed incorrectly,the VCT assy is sticking or the VCT filter is severly clogged and wont allow oil to return.
cam timing will affect compression reading.

and I gotta know, how did you hold the throttle valve OPEN when doing the compression test?

the cam angle on YDS should show both banks within about 4* all the time.

the ECU is not designed to move one due to any issues.

it is designed to move both based on RPM and throttle angle.

just some thoughts this morning

Is it possible that there is a special way to hold the throttle plates open during a compression test?
seems like the compression difference is possible a result of cam timing being different.

the cam timing is different because the VCT is not retarding the cam on both motors on the port side
"+14 degrees at 2500 rpms" the cams even out with RPM so does that mean they are not stuck?
or is it that only the one filter on both motors is plugged up and not letting the oil return to pan.

kind of looks like the filters would be the thing to check on

and the proper alignment of cams

I have no clue how you would test to see if cams were stuck

[formula29]

mystery partially solved, I just swapped vct gears from port to starboard, now I have 195 psi starboard and 170 psi port