F250 compression #s

[formula29]

I will have it figured out on Monday, I didn't get much done today, I am going to go pick up a good leak down gauge, I was thinking about pulling valve covers to be able to see the lifter buckets are not too tight, if all is good there I am pulling the flywheel to verify the timing marks and maybe through on a new timing belt, have you guys ever seen a belt stretch enough to mess with the compression? I have seen chains do it, these belts look to be in good shape. I have noticed in all the service manual pics the part number and writing on the belt is right side up looking at it, the belts on my motors has the writing upside down, that should not matter if the setup marks on the belt are the same both directions, lots of what if's, however my hpdi 300s are still screaming and I'm taking that boat out tomorrow, go figure, I have really no problems with the time bombs and every 4 stroke I move into gives me a fit, thanks again for everyones imput and I will post up Mondays results.

[TownsendsFJR1300]

It'd be nice to know what maintenance (valve clearance checks/adjustments, or any repairs) was done to the engine by the dealer and before when the dealer got them..

My moneys on either the cams off, or someone adjusted one cam (same cam on both engines), when the cams were in an incorrect position..

Good luck in any case...

[zenoahphobic]

The motors run good and strong, they fire right up and idle perfect, and run on up to 6000 without issue, if I didn't know the numbers were off you wouldn't know by the performance of them running.

A bit of a contradiction, are the measured tests reliable?

[boscoe99]

Which would be preferable?

A motor that has perfect compression and zero percent loss of air but which runs like crap, or a motor that starts, idles and runs perfectly but which has much less than perfect compression and lots of loss of air?

If considering the purchase of a motor and I had the choice between two that started, idled and ran well, but one has low compression and the other has great compression, I would choose the latter. That would simply be a tie breaker.

But if I own a motor, and it starts, idles and runs well, seems to make rated power, does not use an abnormal amount of oil, and which has abnormal compression or leak down values, I am going to be happy as a clam with it.

My Honda mower knocks like hell when I start it. It runs just fine. Does not use oil. Sips gasoline. Cuts like a dream. The knock is of no concern to me whatsoever. If and when it goes, it goes.

[formula29]

A bit of a contradiction, are the measured tests reliable?

where is the contradiction? I have two engines that perform as they should, however on both engines the starboard banks show 25 psi less on compression test, there is an underlying problem, weather its worn rings, valves or a cam not indexed properly. when I sell this boat I'm sure the potential buyer will have a problem with uneven compression, so if needs to be corrected.

[zenoahphobic]

where is the contradiction? I have two engines that perform as they should, however on both engines the starboard banks show 25 psi less on compression test, there is an underlying problem, weather its worn rings, valves or a cam not indexed properly. when I sell this boat I'm sure the potential buyer will have a problem with uneven compression, so if needs to be corrected.

The difficulty we have with this is that we are not there to hear/feel the engine running. So when it is said the engines perform as they should nothing could be critically wrong. Prima face such a drop in compression on 4 times two cylinders contradicts that the engines run well.
Maybe I was suggesting the way you might be testing is introducing some error. I know when screwing in a compression guage I may have to use my other hand at times, my weaker left hand does not have the same tightening strength.

Not really suggesting you are doing something wrong. We just read the threads and come up with scenarios; because we just assume there is something wrong (because OP's say so).
Your engines may well be wearing differently from one bank to the other, eg greater ring wear, or not exactly the same amount of oil on the bores. Although Rodbolt says valves leak the same hot or cold, they seat differently depending on temperature, and "the same hot or cold" is not true about every other thing. That is, expansion and a nice warm oil (to correct viscosity) is not there on a cold engine. All these tests should be performed on a correctly warmed up engine to have proper meaning.

Theoretically, incorrect valve timing may alter the amount of air getting into or out of the cylinders, but this tends to be very small in a 4stroke. The ********g amount of oil in each bank creates ********g friction, which can also affect the cranking speed. Slower cranking speed invariably leads to lower pressure readings.

Just adding some thought.

I wasn't swearing, the form of the word different has been censored???

[99yam40]

where is the contradiction? I have two engines that perform as they should, however on both engines the starboard banks show 25 psi less on compression test, there is an underlying problem, weather its worn rings, valves or a cam not indexed properly. when I sell this boat I'm sure the potential buyer will have a problem with uneven compression, so if needs to be corrected.

one thing I noticed that Rodbolt posted was possibly wrong head installed.
I guess there are heads that create high compression and some with lower
But that would have to be true on both motors

[zenoahphobic]

If the compression was excessively high, which it is not, would suggest heads have been machined. This would also cam timing due to slight effective change in lenght of the cam belt.
But we have consistent drop in pressure, so if we entertain wrong heads then they must have come from a model with lower ratio, or the pistons might have been changed to lower compression ratio?
I can understand this in the car world, but I don't imagine such variation in Yamaha motors.
Worth another thought.
Over grinding of valves would also make compression lower.
Shorter spark plugs also.
And of course bearing regrinding could cause this also.
The mind wonders. Again in the automotive world, but in marine?

[rodbolt17]

the OP says compression on bank 1 is 185 even,bank 2 is 170 even.
compression is rarely even that way.
you cannot install a shorter or longer plug to change it.
you cannot over or undergrind a valve to change it,as long as the seat and face still seal.

its a freaking valve and it seals.
as long as the temp is inside its operating limits it seal hot or cold and on or off the cylinder block.
it is how we vacuum test them on the bench to see if the valve seals after a repair.

mechanical sealing on a valve is NOT dependant on temperature as long as the temp is inside the design limits.

piston to wall clearance is less than .0015.
never seen a mechanically sound engine fail a cold leakdown test.

I am 55 and have been doing leakdown and compression tests since I was 15.

we failed more engines with a leakdown than with compression.

typically if you surface a head it makes compression rise as you made the combustion chambers smaller.

so we know the engine has been repaired,most likely a block replaced.
we don't know if the heads were replaced ,surfaced or a junkyard set of mismatched heads were installed.
however now we DO KNOW the cam belt was installed incorrectly.
we also KNOW there is an issue on an intake cam cause at idle they both should read about the same and fluctuate between about -2 and +2 degrees.

what we DO NOT know is are the valves with the misposition cam being held slightly open?

how we can find the VALVE issue IF one exists is by a LEAKDOWN test and a set of EARS.

this I could find WITHOUT a leakdown tester.
I would simply dial back my air pressure to about 100 PSI,bring the suspect cyl to TDC compression stroke, apply air and simply listen.

the ABSOLUTE first thing you do in a trouble shoot is VERIFY can we MECHANICALLY SEAL this theoretically SEALED cyl.
industry standard allows about 15% leakage at an input of 100 PSI.
theory is good but it does not take into account the ring end gaps.
however I do see 0% leakage occassionally.

I got involved with an F115 making oil a few years back.
local mobile tech backyardigan with a tool box and a biddness card had been working on it.
the compression numbers the tech had wriiten on the cam cover was 140-145.
lookin good ya might think.
he had replaced all 4 injectors,the VST needle valve,rebuilt the fuel pump and replaced the T-stat and the CTS AND it still made oil.
ran perfect made 5900 RPM just made oil, a lot of oil.

when it came to me.
first thing I did was a leakdown test.
within 15 min I found #3 cyl had 85% leakage and you could use the crankcase breather tube to blow up ballons.
funny thing that motor was still under warrenty.
a call to Yamaha and I was pulling a power head for replacement.
I pulled #3 piston(after installing a Yamaha supplied block assy) and found the ring gaps perfectly aligned on #3.
I later made a few coins on that block assy .
but even with that much combustion gas(and raw fuel) leaking past the ring gaps the compression test FAILED to catch it.

what I am getting at is CAN that cylinder mechanically seal?

if it cannot you can toss all the parts you wish at it and do all the laptop hunting and guess wht?
you will still end up with a dead rabbit.

mechanical issue wont heal themselves.

in this case the compression test has already shown a POTENTIAL problem.

6 little motor shareing a common crank and a common throttle body yet 3 of 6 are 25PSI lower.
now I gotta know. WHY?
that is where a leakdown test or simply air pressure at TDC compression stoke comes in handy.
ya got a piston in a cyl with gap type rings can it seal?
ya got 4 valves per cyl,can all 4 seal?
ya got a head gasket around each cyl,can it seal?

a compression test simply wont pick up a small leak.
a small leak WILL be a bigger leak soon.

[zenoahphobic]

The OP stated 195 not 185.

Try grinding valves in 5 valve Yamaha motorcycle heads to their absolute limit. Guessing 3mm off each covering 75% of the head area. What does that do to the volume? Substantial, usually compensated for by shaving the head some 1mmm or more (if the valves were needed to be done to that extent, the head surface would have also suffered so machining probably necessary anyway).

Not trying to be critical, but the only obvious difference between a leakdown test and a compression test is that the former introduces the pressure at chosen levels, and the later internally by varying rotational speeds with a limit of the pressure produced. Leakdown testers are more sophisticated than compression testers. compression testers can be made to do more than just taking absolute readings. It would be far more appropriate that pressures are tested with the engine rotating.

You say that all pressure readings cannot be the same. What would be the reason for that seemingly illogical statement. Surely engines are built with exact tolerances and are "blue printed" to be made to great precision. Why would a dial pressure guage have better precision? they don't. Try more precise pressure transducers coupled with data recording and then compare the two means of seeing what is going on.

Just by rotating an engine by hand and a very good set of ears (maybe electronic) you will get as much information that you need to find a fault location. However, you will still need to teardown to "see" what the fault is.

I appreciate the value as a quick diagnostic tool, however what is "read" is subject to interpretation, that may not always be right.

[TownsendsFJR1300]

Got my pop corn!

[formula29]

I took a day off and cruised the inter coastal and had a great day on the water, I promise I will get to the bottom of this in the morning, I will do a full leak down test, if that comes back in spec I am pulling the flywheel to verify everything is lined up where it should be, while it is entertaining reading some of the directions this thread is going, well, we should all gain some sort of knowledge from it. One thing I have learned over the years of trouble shooting is that sometimes a fresh perspective from another brain can have positive results, one of the above posts questioned my ability to do the compression test, which while not knowing me could be a valid point, however I have built many various types of engines in the past and have a good working mechanical knowledge, I have checked the compression on the motors at least 3 times with the same results, the day I brought them home it was the first thing I did, both motors has the same results, I contacted the dealer and he assured me they just needed to be run, that they had performed a decarb on them in the yard, I said ok, I can buy that I guess, after I rigged the engines on the boat I ran them at home up to temp and rechecked, same numbers, I then put the boat in the water and ran it for 45 minutes anywhere from idle to wide open, I came home and pulled the plugs and rechecked the compression, same results both engines, I am still leaning toward either a stretched timing belt or incorrect assembly of something because it just doesn't make sense to me that two motors are having the exact same issue, keep up the good work guys.

[formula29]

ok, I have one more scenario for the night, since I have found that the port bank's which have 195 psi are the bank that have +12 degrees of intake cam timing at idle and should be 2 degrees, maybe the vct is broke and the + 12 degrees of intake timing during cranking is increasing the compression on the port bank higher than it should be and the 170 psi on the starboard bank is the normal compression?

[zenoahphobic]

I took a day off and cruised the inter coastal and had a great day on the water, I promise I will get to the bottom of this in the morning, I will do a full leak down test, if that comes back in spec I am pulling the flywheel to verify everything is lined up where it should be, while it is entertaining reading some of the directions this thread is going, well, we should all gain some sort of knowledge from it. One thing I have learned over the years of trouble shooting is that sometimes a fresh perspective from another brain can have positive results, one of the above posts questioned my ability to do the compression test, which while not knowing me could be a valid point, however I have built many various types of engines in the past and have a good working mechanical knowledge, I have checked the compression on the motors at least 3 times with the same results, the day I brought them home it was the first thing I did, both motors has the same results, I contacted the dealer and he assured me they just needed to be run, that they had performed a decarb on them in the yard, I said ok, I can buy that I guess, after I rigged the engines on the boat I ran them at home up to temp and rechecked, same numbers, I then put the boat in the water and ran it for 45 minutes anywhere from idle to wide open, I came home and pulled the plugs and rechecked the compression, same results both engines, I am still leaning toward either a stretched timing belt or incorrect assembly of something because it just doesn't make sense to me that two motors are having the exact same issue, keep up the good work guys.

Just another thought, tests should be done WOT. If not closed throttles can restrict the air available to compress. So that, and also differences in idle sync can also have an affect. Worth checking port and starboard linkages and the amount of opening at idle? (not familiar how this engine is setup).
Another thing, gauges can be poor. Sometimes the needles stick. If you watch the needle every stroke, it rises and goes past the read maximum and goes back a bit. The gauge has a spring action and some form of dampening. The maximum should also increase at every stroke. The rate at which this happens is also vital information. I.E the quicker the rise, and the quicker the reach of the maximum usually suggests a healthy engine.
However a poor gauge can be knocked about by the pulses of air. Depending upon the nature of these pulses, they can also determine where the needle rests on a cheap gauge. All is not lost here, if this is the case, no matter how bad the gauge is, it still tells you there is some differences between these cylinders and the other bank of cylinders!

[formula29]

Leak down test done, 100 psi imput and holding 100 psi, all cylinders, I pulled the flywheel and all the pointers are lined up, I am scratching my head, my next thought is finding the head casting numbers to see if they are correct, I want to give you guys a Monday morning laugh though! while performing a leakdown test you really should not have a 24" breaker bar still attached to the flywheel bolt! it will swing around LOL! someone did mention that earlier on, some things you have to learn the hard way.