Mercruiser 3.0 engine failure

emoe7749

Recruit
Joined
Jun 23, 2012
Messages
2
Hello everyone. I am a new member to this forum but have sought advice from Iboats for many years and this community doesn't disappoint. I have a perplexing issue on my hands and need advice from some pros. We have a Pontoon: 2006 25' Sun Tracker Regency with a Mercruiser 3.0 I/O. Two weeks ago it was running fine and then started missing for 10 min before it just stopped, dead in the water. On the wrong side of the lake. Through a series of diagnostics recommended by the Mercruiser manual, I determined that I have low compression, by doing a compression test, in all 4 cylinders: between 50 and 41lbs.

The consistency of the low compression makes me think that something might be wrong with the timing gears, as in they may have skipped a couple of teeth or come out of sink. Any other thoughts on what might be wrong before I tear into this motor? Is that something that would be considered common for the 3.0? Finally, is this motor an interference engine?
 

kd7isf

Petty Officer 2nd Class
Joined
Aug 20, 2004
Messages
162
Re: Mercruiser 3.0 engine failure

Hi,

Based on your description I'd have to agree with you EXCEPT for the fact that it's almost impossible for that engine to do that -- you would literally have had the equivilant of an explosion in the front of the engine if that happened -- there is no timing belt or chain -- just a big gear on the cam shaft and a smaller one on the crank, they mesh directly together and they are very thick metal.
No timing skipping here..

However, there are other possibilities.............

1st, how many hours are on this engine? Generally marine engines rot out before they wear out, but not always....
-If it has a lot of hours your rings could be worn, but that usually doesn't show up all of a sudden, the engine gets weaker and weaker and weaker until it just wont run anymore......

2nd, How old is the manifold? Let it sit for a few days and then try to run it on a hose -- you could have a rotted out manifold and that in turn could have allowed water to get into your exhaust valves, rusting them out.
Replacing the manifold and having a valve job done will fix that but you will have to remove the head and take it to a machine shop to get that done... Sometimes the mainfold is fine but the riser is rusted out... Same results but usually the rear cylinders are worse then the forward ones...

Are you sure you did your compression checks correctly and that your compression gauge is not broken?
If you rented one, did you make sure that the valve stem was in it? They are very commonly missing!!

I ask because your problem sounds ignition related to me.. I think your ignition module is on its way out. The problems above slowly kill an engine (except in the case of jumped timing), ignition modules instantly kill an engine (electronics either work or they don't)..
I would first check the ignition fuse and make sure its still good and in the hole tight. I would also check the connections to the distributor -- one loose one is all it takes...
I would also check the shift interupter -- easy check, unplug it and see if the engine runs. If yes, it's bad or your lower shift cable is out of alighnment. This is a black electronic thing on the shift bracket.. It has two wires with bullet conections -- just pull them apart to unplug them, push them together to plug them back in. Polarity doesn't matter, this thing is just a switch...

Another thing that could easily cause these symptoms is a failed fuel pump, a clogged fuel filter, or an empty fuel tank (it happens........:rolleyes:) Is there fuel in the yellow line from the pump to the carb? If there is, thats a sure sign your fuel pump is bad...

Hope this helps
 

emoe7749

Recruit
Joined
Jun 23, 2012
Messages
2
Re: Mercruiser 3.0 engine failure

Thanks for your extensive answer, you've thought of a few things I haven't. Here is my response:

Based on your description I'd have to agree with you EXCEPT for the fact that it's almost impossible for that engine to do that -- you would literally have had the equivilant of an explosion in the front of the engine if that happened -- there is no timing belt or chain -- just a big gear on the cam shaft and a smaller one on the crank, they mesh directly together and they are very thick metal.
No timing skipping here..


However, there are other possibilities.............

I agree with you on this, which is why I am reaching out, it just seems so unlikely to be the case.

2nd, How old is the manifold? Let it sit for a few days and then try to run it on a hose -- you could have a rotted out manifold and that in turn could have allowed water to get into your exhaust valves, rusting them out.
Replacing the manifold and having a valve job done will fix that but you will have to remove the head and take it to a machine shop to get that done... Sometimes the mainfold is fine but the riser is rusted out... Same results but usually the rear cylinders are worse then the forward ones...


The motor is an 06 so this seems unlikely as well. My friend has a scope camera, I could use that to see if the exhaust valves are rusted but I would think this is also unlikely but a possibility I hadn't thought of.

Are you sure you did your compression checks correctly and that your compression gauge is not broken?
If you rented one, did you make sure that the valve stem was in it? They are very commonly missing!!


I am sure I did it correctly, I used the screw in plug style. The tester is a cheap Chineseium tester so this could be the issue, though it has worked in the past.

I ask because your problem sounds ignition related to me.. I think your ignition module is on its way out. The problems above slowly kill an engine (except in the case of jumped timing), ignition modules instantly kill an engine (electronics either work or they don't)..
I would first check the ignition fuse and make sure its still good and in the hole tight. I would also check the connections to the distributor -- one loose one is all it takes...


my suspicions were initally ignition as well. My coil ohmed out incorrectly so I replaced it along with the following: cap, rotor, wires, plugs and I found a used distributor shaft and components (I was told the magnetic pickups in the distributor sometimes go bad and cause timing issues) I have a bright blue spark and have followed the procedures in the Merc manual for testing the ignition. I could try swapping modules to see if that fixes something.

I would also check the shift interupter -- easy check, unplug it and see if the engine runs. If yes, it's bad or your lower shift cable is out of alighnment. This is a black electronic thing on the shift bracket.. It has two wires with bullet conections -- just pull them apart to unplug them, push them together to plug them back in. Polarity doesn't matter, this thing is just a switch...

This is something I havent tried. I bypassed this because it seemed as though I had the ignition figured out. It would be worth a shot however.

Another thing that could easily cause these symptoms is a failed fuel pump, a clogged fuel filter, or an empty fuel tank (it happens........) Is there fuel in the yellow line from the pump to the carb? If there is, thats a sure sign your fuel pump is bad...


I tested the fuel pump and it seems to be working; there is fuel in the carb, filters are clear and have been changed recently. The clear line does not have fuel in it.


Thanks again for the long post. I appreciate the advice as I am 20+ hrs into it and dont want to pull the motor apart if I have missed something.
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,469
Re: Mercruiser 3.0 engine failure

Hello everyone. I am a new member to this forum but have sought advice from Iboats for many years and this community doesn't disappoint. I have a perplexing issue on my hands and need advice from some pros. We have a Pontoon: 2006 25' Sun Tracker Regency with a Mercruiser 3.0 I/O. Two weeks ago it was running fine and then started missing for 10 min before it just stopped, dead in the water. On the wrong side of the lake. Through a series of diagnostics recommended by the Mercruiser manual, I determined that I have low compression, by doing a compression test, in all 4 cylinders: between 50 and 41lbs.

The consistency of the low compression makes me think that something might be wrong with the timing gears, as in they may have skipped a couple of teeth or come out of sink. Any other thoughts on what might be wrong before I tear into this motor? Is that something that would be considered common for the 3.0? Finally, is this motor an interference engine?

Ayuh,... Welcome Aboard,.... Pull the dist. cap, 'n see if the rotor is turnin'....

If the fiber gear on the cam got tore up, it stops turnin'...
No way for it to run outa sync...
 

kd7isf

Petty Officer 2nd Class
Joined
Aug 20, 2004
Messages
162
Re: Mercruiser 3.0 engine failure

Ummm.. You "Bypassed" the shift interupter?
If you plugged the two wires that normally connect to the shift interupter together, YOU DISABLED THE IGNITION.

If you want to take the shift interupter out of the circuit, just unplug it...

Hope this helps!
 

kd7isf

Petty Officer 2nd Class
Joined
Aug 20, 2004
Messages
162
Re: Mercruiser 3.0 engine failure

So, did you get her running again?

Please post your solution if you did..

Thanks!
 

kd7isf

Petty Officer 2nd Class
Joined
Aug 20, 2004
Messages
162
Re: Mercruiser 3.0 engine failure

Ummm, are you still there?

Would like to know if you got it running and what the solution was...

Please don't be anothe runnaway!!

Thanks
 
Top