Engine compression vs leak down test expert needed

[KNOT-RIGHT]

Originally Posted by Rage

Thanks stevesxm,

Bob Lloyd of Full Throttle Marine also suggested that poor ring sealing below TDC could be the cause of my lower compression but good Leak Down results. Yes to 100 psi air source but I did not track the source of the leakage ie intake / exhaust or crankcase though I definately heard leakage for all cylinders. That will be done next as well as a heavier oil in the low psi cylinders to see it that improves ring sealing. I tried a light oil (Merc Storage Seal oil) squirted into the low cylinders but that made no difference in the compression reading.

Bill

I would repeat the leakdown and try to target the source
of the leak by listening to it. Its going to be eithier
the crankase, Exhaust, Or intake.
I normaly pull the valve cover on the head and take the rockers off. If you find that it is comming from a valve
then lightly tap the valve with a brass hammer to try
to reseat it. Most of the time its a hanging guide or carbon on the exhaust seat.
A cold engine will not give accurate readings on the rings
the bigger the bore the worse it is.

Your on to it now just keep going.
Good Luck!

[Rage]

Yes, with all the good information and insight everyone has provided herein and elsewhere I now know how best to proceed to sort out this issue.

I will post my findings which unfortunately will be at least a month away since that is the earliest that I will be back to the lake.

Many thanks!

[Rage]

Originally Posted by stevesxm

i have a milwaukee that uses the same camera technology and it can see anything. also, any of the basic non camera types on ebay ( i looked and there are a 200 of them...) will do the job as well. the only thing you have to make certain of is if it has its own light source. for a couple hundred bucks its a tool that you almost can't be without .

The BS I showed above was one of them.

[GO4BROKE]

Originally Posted by stevesxm

if this motor had rods bent bad enough to affect cranking compression, he wouldn't have to be wondering whether he had to take it apart. his post would have read " gee, does anyone know how to fix my back seat ? i started the boat this morning and a connecting rod went through it"

I've seen this too. slightly bent rod from water ingestion. NO noises, only a miss at idle, picked up the cylinder off idle. Perfect leak down, slightly low compression.

[stevesxm]

Originally Posted by GO4BROKE

I've seen this too. slightly bent rod from water ingestion. NO noises, only a miss at idle, picked up the cylinder off idle. Perfect leak down, slightly low compression.

this is an argument i can't win. but ignoring that for a minute because i'm bored today...

so you are saying that you can bend a rod with water ingestion, such that the rings will still seal at leak down but the .002 shorter the effective rod length becomes will make a lower cranking compression number difference that you can see on an analog gage ? really ? you mean that ?

and going slightly further... youre saying that you could bend a rod sufficiently thru ingestion badly enough to actually affect a cranking compression number and yet not do so much other damage like pin the rings in the lands or bend a valve or two , that you would still be scratching your head as to what else might be wrong ?.... and STILL maintain good leakdown ?

well... i suppose anything is possible. but i think the realistic chances of either of those scenarios being possible is probably a 10 decimal place number away from zero.

[HotPursuit]

Originally Posted by Rage

#1 is 125 psi, #3 is 135 psi, the rest are ~150 psi. Engine rebuilt 36 engine hours ago and all cylinders were ~150 psi after a 3 hour engine breakin. The 36 hours mostly cruising 2500 rpm and wake boarding 2200 rpms.

Not bent rods.Its side by side cylinders sounds like you hurt a head gasket.When your leaking down #1 check plug hole #3 to see if air is leaking from that plug hole.I have burnt 1 layer on a Cometic gasket or Fel Pro SS layered and had your problem.Engine ran fine.10# crossing to the next cylinder is hard to notice. Hope this helps.

[Rage]

Originally Posted by HotPursuit

Not bent rods.Its side by side cylinders sounds like you hurt a head gasket.When your leaking down #1 check plug hole #3 to see if air is leaking from that plug hole.I have burnt 1 layer on a Cometic gasket or Fel Pro SS layered and had your problem.Engine ran fine.10# crossing to the next cylinder is hard to notice. Hope this helps.

Good suggestion. I am running Cometic's. Hard to imagine that I hurt a Cometic with the HP I am running but will check everything that makes sense and that makes sense. Thanks.

[Lmarth]

I too, noticed that the cylinders were adjacent. There's not a lot of material between cylinders. In a prior life, I blew a head gasket during a race at the Glen (BBC with aluminum heads/iron block). It torched a channel in the head between two adjacent cylinders! I used to look for relative differences in the sound level between cylinders. Typically,you'd hear almost nothing from the intakes if you had the intake off and the scope down in the port. The exhausts would be noisier measured with the headers off and the scope in the port. The big 'whoosh' would be leakage past the rings measured by dropping the probe into the crankcase. Having the intake off allows you to drope the probe down past the cam. The best stethoscope I found was one borrowed from an airline many years ago on a business trip that they now charge you $5 for to use for a few hours. Nice and flexible. Again,it's relative differences I'd look for between all the exhausts,etc. Good luck!

[Rage]

Just an FYI that was mentioned in the earlier thread, Engine compression vs leak down test expert needed, Leak Down Test results were good for all cylinders.

[jeffswav]

Do you have a flat tappet cam or roller. I had a flat tappet cam go down before and caused lower readings. If you have a roller it would be unlikely. I have been their before. My engine has been out 3 times. I pull my own engines and do my own assembly work. Machine work is still expensive and not having a running boat sucks. l would keep investigating and would not wait until spring when you could be boating. you can learn a lot just by pulling the intake and valve covers. Good luck!!