F250 compression #s

[TownsendsFJR1300]

Correct, I connect the gauge with 0 psi, increase pressure up to 100 psi and both needles rise together and hold at 100 psi, I have also checked the gauge without it connected to the engine without andy backpressure and the secondary only reads about 70 psi

With the line not connected to the engine, there shouldn't be ANY PSI to peak of, much less 70.

Here's my unit, NOT hooked to an engine, free flowing air. 90 PSI set at the gauge, 10 psi showing (I'm assuming it slight back pressure in the hose, etc). There's something not correct with your gauges..

[formula29]

The leak down gauge is nothing more than a manifold with a regulator, as long as you have 2 functioning meters on it that's all you need, I have about beat this horse to death here, if its not a matter of a head gasket or wrong head I just don't know, time to take it back to where I bought them.

[99yam40]

But that should have lead to the different banks on the motors being different not the same side on both motors

At least he has been able to eliminate some things as the cause.

it would be nice to find markings on the heads that would lead to what has happened

[exeter27]

Are you aware that the Harbor Freight is initially built to work at 15 psi? That might be why your secondary gauge failed. In theory it's built to accept a full 100 p.s.i at the inlet - and with your new gauge in theory you could now read secondary pressure up to 100 p.s.i. But, that's a lot of 'theory' in a single sentence. I find nothing worse than diagnosing problem with my diagnostic gear - because they have led my on wild goose chases.

I like Harbor Freight for certain (i.e. non-precise) types of tools. I'd suggest purchase of a good leakdown tester. Better yet, build one very easily. At 100 psi, your ears have a much better chance of hearing a leak if there is one.

Speaking of that, I've not done a leakdown where I haven't seen some leakage. Good engines seal very well, but they still leak minimally. I'm suspect of your leakdown tester.

Keep at it - you will figure it out. And you should figure it out - as the guys said, if it's off by that much in compression tests - the engine(s) will fail at some point.

p.s. one thought. Is it possible that both engines were disassembled at the same time? If so, perhaps one bank of heads were swapped between engines (see 99yam40s post - with a twist!). That leads to two engines - each with a thick and a thin head - with similar oddities with compression values . But, this theory is a bit of a reach!

[formula29]

I don't see where I was comparing any tests? as stated from the beginning I am trying to figure out why one bank has 25 psi more compression than the other, for two engines to have the same values does not make sense to me, I did the leak down test as advised to eliminate valves not seating or blow by, so what is left boscoe? you are saying cylinder wear in the middle or bottom of the bore? just on one side of the motor? I guess if one side of the motor runs quite a bit hotter than the other it would be plausible, from what I know anything more than 15% between cylinders in not really acceptable,

[boscoe99]

he started this thread to find out if it was normal for one bank to be lower compression.

Well now he is trying to compare a compression test to a differential pressure test. The apples to oranges thing again.

Either test can have it idiosyncrasies. Either one is not the be all end all. Despite Mr. Rodnuts opinions to the contrary.

Think about where the piston rings are when the differential test is being done. At the top of the cylinder. Where the wear will generally be the least. It is possible for there to be nil wear at the top of the cylinder and considerable wear lower down in the cylinder where the piston ring speed is the fastest.

It is possible to have 0% loss of air during a differential pressure test but for the engine to have considerable piston ring/cylinder wear that will affect the outcome of a compression test and the performance of the motor.

Now why two separate motors would have less compression on the same side and at the same values is the mystery. I don't see a cylinder head as causing the problem. Piston ring and cylinder wear, possibly. But still strange for both to have the exact same situation.

[formula29]

I'm pretty sure the only thing left at this point is either it has a 225 head or head gasket for some reason.

[formula29]

Correct, I connect the gauge with 0 psi, increase pressure up to 100 psi and both needles rise together and hold at 100 psi, I have also checked the gauge without it connected to the engine without andy backpressure and the secondary only reads about 70 psi

[boscoe99]

he said before that the output gauge did not work, that is why I asked if he was still using the bad gauge.

Bosco do you know where the # s on heads are located?

Part numbers are not on 99% of Yam parts. There might be the engine prefix on the head (6P2) and then best that can be found is a head variation number such as 00, 01, 02, 03.

[TownsendsFJR1300]

To clarify, you have 100 PSI going in (gauge on the left) and ALL cylinders are showing 100PSI on the gauge next to it(showing the sealing of all internal parts of the cylinder)? IE, absolutely no leak down at all??

[formula29]

he said before that the output gauge did not work, that is why I asked if he was still using the bad gauge.


Bosco do you know where the # s on heads are located?

I replaced the secondary gauge with another off of a air regulator that I had, sometimes we have to make do with what we have laying around on the farm, anyway it works fine now,

[99yam40]

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?

he started this thread to find out if it was normal for one bank to be lower compression.

[formula29]

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?

[99yam40]

With 100 psi on the output gauge there would be no leakage. Nothing to hear.

he said before that the output gauge did not work, that is why I asked if he was still using the bad gauge.


Bosco do you know where the # s on heads are located?

[boscoe99]

I guess you were still using the HF gauge,
did you listen for any air leaking at the different places?

With 100 psi on the output gauge there would be no leakage. Nothing to hear.