merc runs strange after full throttle running

[r2d2a]

Mercury 225 1998 OG625239 (fuel inj.)

Engine runs great even at full throttle but when I take it back from full throttle and try to idle it will do all sorts of popping and shaking. It won't stall though. So far others have looked at it and all the filters were changed. I took a look at it and found two things so far but don't know if they will cause this problem.

I found the engine isn't getting up to temp I don't think. One head (above t/stat) reads about 95f with temp gun while the other side reads about 84f. What should this engine be at normally and would this cause a problem like I am seeing?

The second thing I found was the lowest port side cylinder (5) had less compression than the others. It was at about 100 where the others were at about 115. This cylinder also had little to no reaction when I would take the spark plug wire off and reconnect it. All the other cylinders acted like they should. I changed plugs and know that it is not because of spark or the plug (I did a spark check). Would this cause a problem like I am seeing?

Thanks in advance.

 

[jerryjerry05]

Re: merc runs strange after full throttle running

Where are you located? Fill out the profile.You might be close enough that someone can help.
It should make a difference when you pull the plug.
The o-ring or head gasket could be causing this problem.Squirt some oil in the cylinder and see if it changes the compression results.
No change then remove the head or get a bore camera to check for damage.
AS far as tem goes if your taking readings on the hose then you might not have enough water to the motor.
Take off the thermostat housings and see if the thermo's are working right.
Lots of salt or corrosion?Post pics and come back with your results.J

 

[CharlieB]

Re: merc runs strange after full throttle running

Have you ever sent the injectors in for cleaning?

 

[jeffersondodson561]

Re: merc runs strange after full throttle running

Lowest cyliner on the portside is #6 and 15lbs difference between cylinder is getting into the grey area of powerhead damage. I would decarbon (heavy) cylinders and see if compression/performance changes.

 

[r2d2a]

Re: merc runs strange after full throttle running

Quick update, I didn't see all the replies till now though. I changed the t/stats just because I don't know when they were done last and they are a quick easy cheap thing to do so I figured might as well. It didn't change the temp though, it is still running at about 90f which I think is pretty cold.
I put some marvel mystery oil in the cylinder that was low and the compression did come up to about 125. I'm guessing this would be pointing towards rings being done.
I've never sent the injectors out for cleaning but I don't think that would help with the compression problem, I do see fuel in the water so I'm thinking this cylinder is getting fuel but not really burning it off like it should.
I've never decarbed a cylinder before but might as well give it a try, how would I go about doing that? And you are right about it being #6 cylinder.
Any other ideas? I'm starting to lean towards it being the rings and therefore it needing a lot of work or new powerhead/engine.

 

[jerryjerry05]

Re: merc runs strange after full throttle running

Remove the head and check the o-rings.It's cheap to fix that.J

 

[r2d2a]

Re: merc runs strange after full throttle running

here are a few pictures from that cylinder, it is a little hard to see in the picture but it looks like that cylinder is scored so that will probably be the problem. I will go ahead and try the head gasket and o-rings anyway since I have them and I have the head off. Anyone know the torque spec for the cylinder head on this engine? I know the pattern but not the torque.
Thanks