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Advice needed please 80 psi compression test on all 4 cylinders

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555simonsimon555

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"Hi all,
As title,I have

"Hi all,
As title,I have compression tested my mercruiser 3 ltr Lx and all the readings are very similar at 75/80 psi, i have squirted a few mls of 30 weight oil down each plug hole and readings have not changed at all.
There is alot of pressure in rocker cover, to the point if i connect it up whilst idling then engine will stall, oil despite being changed was slurry almost immeadiately.
Any ideas? I am pulling the engine this week to rebuild, but i'm still at a loss as to where exactly the problem lies...."
 
"greyed oil, means water is ge

"greyed oil, means water is getting in. 75/80 psi could be a bad head gasket, but could mean worse.
If your lucky could be a cracked head, but slim across all cylinders. I'd like to know what you find."
 
"Hi Brian,
I recently

"Hi Brian,
I recently purchased the boat, it had not been winterised at all....

I had the head off,bores were spot on, no lip and honing marks still present.

I fitted new valves, pushrods and gaskets, the machine shop said the head was fine and did not need skimming, and he knows his job..the head was assembled and torqued correctly.......i did find some water in the manifold when i removed it, and there are no cracks in the block under the manifold where i believe they usually are... thanks for the input
"
 
"yea, typical for the port sid

"yea, typical for the port side to crack behind the manifold. If the block and head is Ok then the low compression could be incorrectly adjusted rocker arms, bad piston/bore clearance or ?? hm..
If the int/exh manifold has internal cracks that will let the engine ingest water (common problem with us inline enignes). I had a new one go bad after 2 runs."
 
Cracked manifold will let wate

Cracked manifold will let water into the top half and hydro lock it if enough gets in while turning over.

"oil despite being changed was slurry almost immeadiately"

Water in the oil usually comes from a cracked block.
 
"thinking more, BTW - by the w

"thinking more, BTW - by the way, if you have a lot of air coming from the valve cover then you have either to much piston to cylinder clearance, bad/incorrectly installed rings, stuck rings or heck missing rings. If the engine was rebuilt and honed to much and old pistons installed then the piston to cylinder clearance is to high causing blow by, causing a lot of air coming out of the oil fill in the valve cover and could cause foaming of the oil.

It will run with 80psi across the board but it wont have the power it should."
 
"it sounds to me if you have h

"it sounds to me if you have hissing noise on the top end them maybe the crank and cam didnt get timed together correctly ie not letting out the exhaust when its calling for. quite a mystery there as far as the 80 across the board id try another comp tester just to see what it says

my
2_cents.gif
"
 
"Thankyou chaps for the input

"Thankyou chaps for the input !
Brian, as far as i can tell the engine has never before been taken apart, of that i am pretty sure, it has 328 hours so not too many.
Surely if it was ring/s not all 4 would be shot, and as when i put oil into plug holes the compression got no better at all then that would confirm that at least the compression rings were doing their job?.
As for the rocker arms i followed the book, which stated 3/4 turn from zero lash, which i can only presume is the correct amount? And.... would it not read zero if they were staying open?

A thought..... if they were not opening enough it would take longer to get the correct compression into the chambers? ie not the 3 compression strokes which is considered the norm?

Guy, i ran the engine with new oil and filter for perhaps 3 minutes and the slurry was present, would a cracked manifold allow THAT much water in so quickly?

Thankyou for your help again chaps.... it has me stumped!"
 
""would a cracked manifold

""would a cracked manifold allow THAT much water in so quickly?"

If it is a cracked ex. manifold I'm surprised that it didn't hydraulically lock up the engine while it was running. I'm thinking more towards an internal block crack. Maybe not? Pull the spark plugs to see if they have been steam cleaned. If yes, then it may be the exh. man. has a crack."
 
"the trick with the oil in the

"the trick with the oil in the cylinders will only give you a better compression number if the rigngs and bores are reasonably serviceable. the real indicator here is the amount of blow by you have. i think if you have the pistons out , you are going to find the rings stuck in the pistons. it is very common if water has been present. do yourself a favor. before you have the thing apart piece meal 5 times, pull it out now, hone the bores and do a fresh set of rings and bearings and the valve job you are already working on and make your life easy the first time. the parts are cheap and at least you'll know where you stand."
 
"Hi all, thankyou all very muc

"Hi all, thankyou all very much for the opinions, I have pulled engine now and stripped it down..... I can see no problems at all with the block, i have tested the inlet manifold and it is indeed cracked, and the low compression was down to the piston rings all being stuck in the pistons,so steve....spot on diagnosis ;) All the bearings are all showing signs water ingress in the oil, so all will be changed as well.

Once again many thanks to all...."
 
"when you go to hone the bores

"when you go to hone the bores, i strongly suggest that you use a brush hone to do it. they come from a company called brush research in california and they are just terrific. a 240 grit will refinsh those bores to brand new without increasing the clearences and leave you with a surface that will seat the rings almost instantly with very very little break in time and almost no contamination. they will deal with you direct and the products are inexpensive for what they do. 100 tmes better than a stone hone."
 
"as a suggestion, have the cam

"as a suggestion, have the cam bearings removed and have the block hot tanked to get it cleaned out and get some new cam bearings too.."
 
"it is hard to argue against d

"it is hard to argue against doing things thoroughly and properly as brian suggests. having said that though, what he is suggesting changes the project an order of magnitude. if the motor looks structurally good and ran acceptably well except for the lack of bore sealing, you can have , essentially, a 100% sucessful " rebuild" by doing a good hone, through soap and water clean and hi pressure air of the various passages , new rings bearings and gaskets and a valve job. followed by a concientious assembly. and you can do that in your basement for probably less than 500 bucks. ( assuming you have the skills)

if you send it off to get tanked, you are now looking at some machine shop doing the cam bearings and any other babbited bushings, all the freeze plugs and whatever else they rope you into... and they'll forget to put the oil galley plugs back in etc etc. and it will cost you another two or 300 bucks. i am cynical about machine shops. of the many i have used to do hundreds of motors and components, i would say the percentage of time they have gotten everything correct and to spec is probably less than 30 per cent. i end up checking absolutely everything they touch and often find important errors. so for me, the machine shop route is only for those occassions when it is essential... which in your case it may very well be... only you can know that after measuring and inspecting everything. but for casual cleaning, for my money, i would do it myself and in 30 years i don't think i have seen two sets of cam bearings that actually NEEDED to be replaced... people do it because they feel guilty if they don't or because they damaged them taking the cam out of the block."
 
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