Proper Engine Compression - 1985 Mercruiser 140

[incrediblened]

I'm having trouble with the search function on the website... so I'll just post my question.

I just checked the compression on my 140 mercruiser. It's got 125-130-130-125-100 psi from front to back. I realize that the 4th cylinder is low. I shot some lube in the cylinder and it was the same after testing. . Any way to improve compression on the cylinder without head-work. (The engine hasn't been running for months. Could this cause compression issues?)

Thanks,
Ryan

 

[mkast]

Re: Proper Engine Compression - 1985 Mercruiser 140

Usually the test is on an engine at operating temp.
These aren't the greatest numbers.
Truth be told, time to get some work done.
Cylinder head has to come off.

 

[Howard Sterndrive]

Re: Proper Engine Compression - 1985 Mercruiser 140

Not sure why you have 5 cylinders there, but 125 is just fine - you only have 1 low and 100 isn`t the end of the world.

Any way to improve compression on the cylinder without head-work.

always try adjusting the valves before condemning anything

comp gage on number 4 - pop the valve cover off and while cranking the starter, wind the rocker nut in until compression drops off towards zero, then back the nut off until compression comes up, keep going until the rocker starts rattling, and tighten it back up watch the comp gage.
I bet you`ll gain a few psi.
If you get it to 110 - I wouldn`t worry about much.

Remember - comp test with all spark plugs out, strong battery, throttle full wide open, choke off, blower running, fire extinguisher handy, and ignition disabled.

 

[Bondo]

Re: Proper Engine Compression - 1985 Mercruiser 140

It's got 125-130-130-125-100 psi from front to back.

WoW,... A 5 cylinder 3.0l,.... Got any Pictures,..??

I'm with Howard,... She's alittle Tired, but probably still has some Life left in her...
Although if it's the 5th cylinder at 100psi,... There might be mechanical work needed...

 

[futz]

Re: Proper Engine Compression - 1985 Mercruiser 140

pull the center plug on the dizzy so it's hanging and the motor can't start, crank the starter with the valve cover off, till the valve, is fully closed on the piston that has 100 psi reading.

Dribble gas on the valve, it leak out? Leave it there for a hour or 2. No leak down then it's not that valve, try the other one for the same piston. make sure it's fully closed. if there is no leakdown then it's the piston ring.

If it does leak then pull that valve and look at the seat it pitted, you can try polishing it with the head still connected. i use a shop vac and suck as i'm polishing the seat.

Then polish the valve, use a old piece of hose, inset the valve stem into a drill holding the hose, the valve will spin free inside the hose, and you polish the edge of the valve.

Re-install, torque the rocker, and then do another compression test.

 

[searay3]

Re: Proper Engine Compression - 1985 Mercruiser 140

Need a little help here...how do you inspect a seat and pull a valve with the head still attached?? I believe you would have to pull the entire head and disassemble that valve/spring/keeper to inspect or lap the seat/valve.

 

[incrediblened]

Re: Proper Engine Compression - 1985 Mercruiser 140

WoW,... A 5 cylinder 3.0l,.... Got any Pictures,..??

Yeah... I remodeled my engine a few years ago and put on a new addition. No pictures though... top secret.

(ok, it actually reads 125-130-130-100).

More information... I did the compression test cold (how much of a differience does this make)?

I just re-checked it. As I cranked it over (a little longer this time) compression crept up to about 108. Leak down settled at about 102 @ 5 minutes.

As far as the motor being worn out... I pulled this motor from a 1985 Wellcraft. The boat was so clean you could eat off of the bilge. It looked like the original owner had hardly ever used it, but it set outside in the sun, seats were shredded, floors were soft, engine looks brand new though, no hour meter available however. I don't know if all that means much though.

 

[Bondo]

Re: Proper Engine Compression - 1985 Mercruiser 140

but it set outside in the sun, seats were shredded, floors were soft, engine looks brand new though,

Ayuh,... Being #4 cylinder, if there was Ever any Water,... It laid in that cylinder...
That's My Guess as to why it's Low....

I'd Run it,... Then maybe have another Look when you Winterize it...

 

[incrediblened]

Re: Proper Engine Compression - 1985 Mercruiser 140

Leakdown is still holding at 100psi. (just for y'all's info).

 

[mkast]

Re: Proper Engine Compression - 1985 Mercruiser 140

Leakdown is still holding at 100psi. (just for y'all's info).

Define "Leak Down" and it's purpose please.

 

[incrediblened]

Re: Proper Engine Compression - 1985 Mercruiser 140

Define "Leak Down" and it's purpose please.

100lbs of pressure remained in the cylinder over the period of 30 minutes. Technically, leak down is measured by introducing compressed air into an engine to see if/where it is leaking out of the engine.

I just left the compression guage on the cylinder to see if the air pressure would remain in the cylidner.