Mercruiser 2.5 new tick has appeared - ex gasket or lifters? Something else?

[Scott76]

First time boat owner here. Have built many motorcycle engines and a couple cars so I'm mechanically inclined, but some of this boat stuff has me completely clueless. Motor is a 2.5 Merc, when I got it the wiring was a mess and would not run. Got that sorted, new plugs, oil and filter, needed a new alternator also, adjusted the carb and it was purring like a kitten and a seemed to run out great in my opinion. It always made a little 'puff' kinda sound that I could hear at idle, almost like a tiny steam engine. I just assumed it was a characteristic of this motor. Boat never gets over 140, and I have no leaks and the oil looks great.

I had it out in Lake Michigan the other day, and about halfway through the outing I noticed the little puff seemed to have turned into a louder tick. At idle, it sounds like it did when I first got it running. Under load past 2k, it's noticeably louder. My first thought was that I had hurt something, since the waves were more than I should have been out in (novice boater, little open bow 19 footer) and I also ignorantly had neglected to trim it down all the way. I never revved it past 3.5k however. After doing some research, I checked the bellows, thinking they could have torn or become dislodged, but everything looks great.

I have heard that some say using multi-weight oil can increase these noises? When I changed it I ran Rotella in it since I have a ton on hand, and was told that diesel oil is the preferred oil if straight 30 is not available. I'm not sure I buy this, as it sounded great for the first 5 minutes of being out.

My next step is to check valve lash, and I'll be well happy is that's all this is. I will say, this does not sound like a clattery rocker to me, it very much sounds like a Jeep with a bad exhaust manifold gasket. :lol:

What has me confused is how the risers and all the marine exhaust/cooling components function. Every 'Merc 2.5-3.0 exhaust leak' thread I find concerns overheating and water leakage. I have none of those issues. I'm a little hesitant to pull the manifold off, it's so foreign to me. I mean, the carb sits on top of the exhaust manifold? I assume the intake is integral passages? Is there water in the system I need to drain beforehand? I sprayed soapy water on the gasket between the block and the manifold and saw no bubbles, but I'm not even sure I'm looking/spraying at the right areas. I'm used to headers and drag pipes!

I do have a Merc stern drive service manual, but this area is quite confusing to me and I'm concerned I'm not understanding things correctly.

Could someone break this down for me, and help me diagnose what may be going on? Pics would be a big help, also. Thanks!

 

[alldodge]

Howdy

Noises are hard to determine over the net. If a boot was damaged it would cause water leaking in. Do you have any water which leaks in?

Your motor break down is here
http://www.mercruiserparts.com/2-5l-gm-153-i-l4-1987-1989-0b531854-th

The intake/exhaust comes in a 1 piece elbow
http://www.mercruiserparts.com/bam/subassembly/30903/963/160
Or a 2 piece elbow
http://www.mercruiserparts.com/bam/subassembly/30903/963/150

 

[Scott06]

I'd listen with a stethoscope or piece of tubing to the ear to see where it's coming from. You seem experinced enough to know an exhaust leak so probably is that. Look at the drawings provided and come back with where you hear the leak. Yes the manifold is integral intake and exhaust with a water jacket to keep it cool, unlike headers there's no airflow in an engine so the exhaust is jacketed and after the 180 elbow the water dump into the exhaust gases to go overboard. There's always a little water running through there to cool it, when thermostat opens this is where the hot water goes (unless you have closed cooling).

There is a manifold drain plug on under side a little aft of the carb, block drain is under it as well just above the oil pan rail . On the Rotella it's perfect to use, isn't gonna cause any issues that weren't already there

 

[Scott76]

Okay thanks. So basicallly, drain the manifold, pull the carb, detach the rubber riser boot and the manifold should pull off, exposing the gasket?

 

[kenny nunez]

One way to find an exhaust leak, [if you have one] is to pour some transmission oil down the carburetor to show the leak while giving it a lot of throttle while running on the flush out, also good for killing any mosquitos.

 

[Scott76]

Okay I'm not opposed to that, but I had been told that running off of the muffs at anything too much more than idle was bad for the outdrive?

 

[mickyryan]

well i have taken mine to 2k on short bursts on muffs, no problems i wouldn't run long at 2k because the drive itself isn't being cooled by surrounding water so it can heat up if you are hearing atick from a 4banger its usually either rocker number 1 the fuel pump slapping exaust flapper using a broom stick you can easily identify where the offending noise is coming from by holding to ear and walking down the engine, if you dont have a engine stethiscope , also piece of pipe works as well

 

[alldodge]

Okay I'm not opposed to that, but I had been told that running off of the muffs at anything too much more than idle was bad for the outdrive?

Its not so much bad for the drive, its bad for the impeller. Most city water supplies even at idle may not provide enough water thru a 3/4 house to keep the impeller needs. As rpm's increase the supply gets less. So anything above 1500 rpm can start to starve the impeller. If not enough water the blades start to burn

 

[Scott76]

well i have taken mine to 2k on short bursts on muffs, no problems i wouldn't run long at 2k because the drive itself isn't being cooled by surrounding water so it can heat up if you are hearing atick from a 4banger its usually either rocker number 1 the fuel pump slapping exaust flapper using a broom stick you can easily identify where the offending noise is coming from by holding to ear and walking down the engine, if you dont have a engine stethiscope , also piece of pipe works as well

Not the pump. Been converted to electric.

 

[Scott06]

Yes the way you mention is how you remove the manifold ( you could leave the carb on the manifold) , but as mentioned by others may want to spend some time makings sure it is the exhaust gasket, if you haven't already.

 

[Scott76]

Oh yeah, makes sense. Remove the fuel line and pull the whole thing off. I'll try the stethoscope/screwdriver/broom handle method tomorrow.

 

[Scott76]

Just updating, finally had a chance to swap in a new exhaust manifold gasket. Old one didn't look too bad at all, there was a decent sized chunk missing from the area where I was hearing the noise, but it wasn't very close to the port. Running it in the driveway on muffs, seems like the noise is gone. Then again, that's only taking it to 2k for short bursts so tough to tell for sure. Hoping to get it on the lake in the next couple days to know for sure.