Low Compression Problem

[mochosla]

Hi, I need some advice. I took my boat out and it has this rattle after 2200 RPMs (no rattle if not in gear or neutral). I thought it was timing. My manual told me to check the compression in all my cylinders before adjusting the timing. I did and found out that 3 out the 8 pistons have 90 PSI while the others have 130 to 150 PSI. I dropped some oil in the cylinders to see if the problem could be the heads but the compression jumped to about 120 which tells me its the rings (right??) Now I don't know what to do. Replace the engine, change the rings only. What to do??

HELP PLEASE!!!


HELP PLEASE!!!

 

[Don S]

Re: Low Compression Problem

Rings might bring up the compression, but they probably aren't causing the "Rattle"
Without hearing it, it's impossible to determine if that rattle was rod or main bearings, or lifters.
You have any other info such as what engine it is, year, model, make etc.?

 

[mochosla]

Re: Low Compression Problem

I'm sorry. The engine is a 1983 OMC 350.5.7. Now, the rattle sounds just like when a car's timing is off. I don't believe the problem is a rod or main bearings as there is no noise if I rev it up in neutral.

 

[Gary H NC]

Re: Low Compression Problem

Rattle as in spark knock? The timing being off a little,advanced to far will get you spark knock but the low compression is another thing.
Are the 3 low psi cylinders next to each other? Try a leak down test..

 

[mochosla]

Re: Low Compression Problem

It is a spark knock. 2 of the cylinders are together while the other one is on the other side of the motor. What do you mean by a leak down test?

Thanks.

 

[Reel Poor]

Re: Low Compression Problem

Click on the link: Leak Down Test

 

[Gary H NC]

Re: Low Compression Problem

Any marine or engine shop should be able to do the test for you.Usually takes 30 minutes or less.It will tell you if you have bad head gaskets,valves not seating ect.
Its a starting point instead of tearing the engine down and guessing or just throwing parts at it.

If it is a spark knock you won't hear it out of gear like you stated.Just like a car,most spark knock is when the engine is under load.

Also you can try retarding the timing a couple degrees and see if it helps.Or run some 93 octane gas.

 

[mochosla]

Re: Low Compression Problem

One last question, what's a spark knock? is that due to timing?

Thank you very much for your help.

 

[Reel Poor]

Re: Low Compression Problem

Here's another great tool for engine testing: Reading A Vacuum Guage

Detonation/Spark Knock Article

 

[180shabah]

Re: Low Compression Problem

Same website as the leakdown test, different topic, Detonation and Preignition.

http://www.moddedmustangs.com/forum...wo-things-that-can-kill-your-motor-vt518.html

 

[Gary H NC]

Re: Low Compression Problem

Yep,as the link above tells you,Spark knock can be from low octane fuel or timing issues.
When you did your tune up you mentioned did you set the points,dwell and timing?
I have had auto engines spark knock and just backing the timing off 2 degrees or so resolved the problem.
Thats why newer engines have a knock sensor that will tell the computer to make timing and mixture adjustments automatically..

 

[ziggy]

Re: Low Compression Problem

i don't know, but to me, the detonation is not as relevent as the low compression. if ya got low comperession, what diff. does it make if it detonates? maybe i'm wrong in that statement, straighten me out if i'm not hip please. i'm not sayin timeing it proper aint nesseassary by any means, do time it correctly useing the proper oem method for yer rig. but i'd be lookin at the compression issue first, then the detonation issue, if that's what it is....for locatein a rattle. i use a hearin hose. any ol hose held up to yer ear and then probe for the rattle with the other end. amplifies the sound for helpin determin where exactly it is rattlein at.....
instead of a leak down test ya could do just the air leak test too. bring yer piston to tdc (both valves closed) for whatever piston in question. hold piston in this posistion by whatever means and inject 20-30 psi of air into the sparkplug hole. if ya hear air out the carb. intake valve question, out the exhust, exhust valve question, out the oil fill hole, rings question..........happy new years......

 

[180shabah]

Re: Low Compression Problem

Ziggy, did you go to the link that RP gave in his first response above?

What you describe doing instead of a leak down test - IS a leakdown test. You just have no numbers to go with it.

 

[Reel Poor]

Re: Low Compression Problem

Ziggys post got me to thinkin.....Y'all hold on...LOL

It's very possible that your detonation problem has been occuring at an inaudiable level and has already had a chance to do damage to the pistons causing the low compression #'s.

I do agree with Ziggy that the first problem that you should be taking care of is the compression problem.

Doing a cylinder leak test (with the guages for comparison reference) is your best test for the problems your describing.

 

[ziggy]

Re: Low Compression Problem

well, i skim read it. a real leakdown test would probably revel much more than the air leak test i'm referin to. guess i'm just tryin to help him find the right direction to go w/o buyin a bunch of tools (ie leakdown tester) + learnin how to use them and interpert the results. or worse, off to the dealer for a rapein. (a note, when i'm up against it, i'll take my rapein anyday, tail between my legs, to find out the facts if ya got a good dealer to go to, that ya trust, if ya gots to) a little air injected into a cyl. and listenin to where it exits ain't to hard for even me to understand. as ya say, a leakdown test w/o numers. i got my info from mercruiser service bullitin 97-25, titled gasoline engine compression test. i'd provide it, but i've don't have a link to it. found it here on iboats but didn't bookmark the link, sorry......only got it on pdf and for whatever reason it don't seem to want to copy and paste......

 

[Don S]

Re: Low Compression Problem

Not sure what the big deal with the leakdown test or air is.
He put oil in the cylinders and the compression went up to what the other cylinders were. That means there is a problem with the rings.
So the rings are stuck, rusted, both, worn out, who knows. But any way you want to look at it, the heads need to come off and look.

But here is a link to that Service bulletin ziggy mentioned.

http://www.boatfix.com/merc/Bullet/97/97_25.pdf

 

[ziggy]

Re: Low Compression Problem

It's very possible that your detonation problem has been occuring at an inaudiable level and has already had a chance to do damage to the pistons causing the low compression #'s.

seems like good reasonin, but a real bummer idea in general. hope that aint it, but seems plasible i'd think. with 90 psi in a hole, maybe it aint blowed the top of the piston out.....yet...time for some diagnoisin maybe........lots of good ideas here. like useall.....

man it sure is good to be hangin on iboats on new years eve. lots of worse things to do..........

 

[ziggy]

Re: Low Compression Problem

thankyou dons. i been lookin for that link for a while.......even searched the www. let alone iboats for it specificaly....

He put oil in the cylinders and the compression went up to what the other cylinders were. That means there is a problem with the rings.

guess it's that plain and simple. addin oil to cyl. and havin compression come up indicates rings, ya just can't argue with that statement......perhaps this is a case of over diagnoisin? good luck with yer adventur m,,,,,

 

[Gary H NC]

Re: Low Compression Problem

Yep,the low numbers on 3 cylinders would make me want to pull the heads..

Anyway....HAPPY NEWYEAR!!!<

 

[mochosla]

Re: Low Compression Problem

WOW!!! I just can't believe how much help I got from you guys. This is an awsome website and you are all true comrades. I will do the pressure test and try the leak down test as well.

One last comment, the pressure did not jump all the way up to where the rest of the pistons are. It jumped up about 20 PSI.

Anyway, thank you again for all your help.

I will let you know what happens.