F250 compression #s

[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???

[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.

[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.

[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?

[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...

[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.

[boscoe99]

I am going to stick my skinny neck out and guess it is a piston ring/cylinder leakage issue. Do the differential pressure test and find out for sure.

Now if you think the crank shaft is not phased correctly with the cam shafts then a quick visual inspection will let you know for sure. Rotate the crank shaft until the alignment mark on the drive gear aligns with the alignment mark on the cylinder block. Verify that the alignment marks on the driven gears for each intake cam shaft aligns with the corresponding alignment marks on the exhaust cam shaft. If at first they don't line up rotate the crank shaft another 360 degrees. See illustration below.

You can always remove the valve covers and verify there is clearance on both intake valves and both exhaust valves for each cylinder when the piston for that cylinder is at top dead center on the compression stroke.

[TownsendsFJR1300]

The more information gathered, especially with both engines doing the same thing, sounds like when the engines were put together again, one cam is a tooth off (on both engines-same mechanic, same mistake).

[formula29]

By the way, both of you guys helped me big time a couple years ago when I bought a used f300xca and the thrust bearings went out, it was way out of warranty and one of you guys recommended asking for Yamaha goodwill which I would have never thought possible and they came through with a new crank case assembly, that was the only other 4 stroke I have owned and I actually assembled the motor myself and it ran great afterward, I have quite a bit of mechanical knowledge but some of these things are much easier to diagnose when you are familiar with the model and work on them everyday.

[formula29]

How do the motors run by the way? Good, bad or ugly? If badly running, what are the symptoms?

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.

[rodbolt17]

the valves wont care if it is hot nor cold.
they seal regaurdless.
if I do a cold motor and note leakage past the rings I look for are all the same leakage?
in this case where I lost almost 30 PSI on the compression test on ONE bank, I would suspect someone or something is holding a valve open slightly or the wrong cyl head is installed.

run a motor much that cannot seal,valve open,head gasket leaking or the piston ring gaps lined up and failure is swift and sure.

in this case IF an intake valve were leaking,remember we no longer use intake vacuum to move fuel, with a 170PSI compression its can fire on the power stroke.
now this combustion pressure is not only going to push on the piston it will JET a HOT gas flame past that small leakage.
this leads to severe metal erosion on whatever is leaking.
valve faces burn off,seats melt lines get etched in blocks and heads,pistons get melted spots.
don't trust the air and pulling up to TDC much.
the above referenced 5.7 I posted 4 cylinders while injecting 100 PSI(standard for my tester) I could pulled each cyl right on by the compression stroke with one hand and a 1/2 drive ratchet on the crank bolt.

had that cyl been sealed I would have snapped off the crank bolt long before I pulled past it at 100 PSI.

you already know this F250 has an issue,now ya gotta find out what it is.

if nothing else bring each to TDC compression stroke,add about 100 PSI and simply use your ears.

if an intake is leaking(hot or cold) it will blow air back into the intake and past the throttle plate.
if an EX valve is leaking (hot or cold) it will blow air into the ex system.
if the rings are leaking(hot or cold) it blows air into the crankcase,this can be heard and felt by simply removing the crankcase vent line at the air box.

on a COLD engine I am not real concerned with 20-25% past the rings,anything more may mean an issue.
but valves seal hot or cold.

if it cannot seal cold it will not seal hot.

its how the doing vacuum tests on the intake and ex valves before and after refacing and seat cutting work.
we know it seals before it boxed and shipped back.

so even with your broken tester you can test for valve leakage.

[boscoe99]

You don't need to do anything at all with respect to trying to get the crank shaft to line up with any marks.

Remove all spark plugs. Rotate the motor until the number one cylinder is coming up on the compression stroke. Put your finger over the spark plug hole (or over the tool that was inserted into the spark plug hole, to which the air hose will be attached) and you can feel the pressure being developed as the piston comes up to top dead center.

Then attach the air hose to the tool. Adjust the regulator so that there is about 20 psi being applied to the cylinder. Rotate the crank shaft so that the piston comes up to top dead center. You will feel top dead center when the resistance to the rotation of the crank shaft diminishes. Have someone hold a wrench on the crank shaft nut just in case air pressure tries to turn the crank shaft. Which it won't if the piston is on top dead center.

Helps to have the lower unit removed although not necessary.

Adjust the air pressure regulator so there is 80 psi being applied into the cylinder, as noted on the primary gauge. Note and record the air pressure on the secondary gauge. If you want to know where any leakages is coming from you will need to use your ears. Listen into the oil fill cap opening for crank case leakage. Listen to the exhaust for exhaust valve leakage. Listen into the intake manifold for an intake valve leak.

Repeat the process for each cylinder. Let us have the results. Oh, the motor needs to have been recently run so that it is hot when the differential pressure test is performed.

[rodbolt17]

dude.
this aint that hard.
the 4.2L and all the v8 motors have NO index marks.

simply bring #1 to TDC,you can use a slim screwdriver to find it.
does not have to be PERFECT like a two stroke.
you can miss a few degrees either side.

now simply make your own timing mark that lines up with the flywheel index mark.

you can fasten a bent paper clip or use a magic marker.

remember in a complete cycle(720*) #1 is going to be at TDC twice.

once you find TDC #1 compression stroke simply test it.
if the ex valves are open(ex stroke) you will hear air blowing past the ex into and out of the ex system.
simply rotate it 360,now you will be on compression.
now listen for air past the throttle plate and the ex system.
any air requires investigation as to why that valve is leaking.

if you have an engine with no flywheel index mark you simply find TDC of the easiest accesable cyl
divide the number of cylinders into 720(4 stroke) that will tell you how many degrees of flywheel rotation is needed to fire the next cyl in the fireing order.

in the case of a V8 you will have 4 marks at 90*,V6 3 marks at 120* inline 4 will be two marks at 180*.

its actually simple.

[boscoe99]

Oil pressure was aprox 80psi ad idle, the problem with HF gauge was the second indicator gauge is broken, the needle doesn't move at all, I am more obsessed with figuring out where the problem lies than anything, things like this drive me nuts until I can figure it out, I actually enjoy repairing engines, just not when I pay for a good running engine.

How do the motors run by the way? Good, bad or ugly? If badly running, what are the symptoms?

[99yam40]

Just in case you do not know it yet,
Rodbolt is a Yamaha master tech

Pay attention to his posts
I am just someone that likes to read and learn from the posts on here