duped
Petty Officer 1st Class
- Joined
- Aug 26, 2009
- Messages
- 307
Hey guys, having some issues I would like some input on.
Engine is a 1995 Mercruiser 5.7 I installed last year. I bought it as a long block to replace an '86 305 and I am using all of the 1986 peripherals (Thunderbolt IV ignition, Alpha one gen 1), so for all diagnostic purposes, consider it a 1986.
Anyways, engine ran great all last year and the start of this year. Put a 6 hour trip in Saturday with no issues. Sunday, on the same tank of gas... launched it, started up, let it run at 1000-1200 rpm for a few minutes to warm it up, and completely out of nowhere the idle dropped by a few hundred RPM and it began to apparently misfire or run rough. There was no noise of anything breaking and no valvetrain noise before or after. Adding throttle didn't clear it up and it runs rough and shakes at all RPM regardless of load or any other factor. The exhaust note is that of a tractor.
From last season, boat has NEW ignition sensor, cap, rotor, coil, plugs, Edelbrock 1409 carb and marine electric fuel pump wired to oil pressure switch. Also has new alternator and batteries. Not that it's totally relevant, but all exhaust manifolds were checked and gaskets replaced at time of engine install.
So far to find the problem....pulled the plugs. #8 looked a bit fouled. Put a brand new one in and tried a different spare plug wire on that cylinder....no difference in running even with a clean brand new plug....still bad.
Tried new coil from other project....still bad.
Disconnected tach for the hell of it...still bad.
Used spark tester and verified all plugs are firing as they should..still bad.
Checked timing, still at 8* BTDC as it has been since I put the engine in.
Thought maybe it had a stuck valve so I checked compression today following the merc service bulletin and everything is in spec. These numbers were obtained after just one or two strokes so thats even better i suppose...
"Dry" numbers
#1 160
#2 155
#3 160
#4 150
#5 160
#6 155
#7 160
#8 135
Added a squirt of oil to number 8 and got 160 right away, which points to rings. Not a huge concern of mine as the engine ran well, and I can put a hotter plug in to stop the fouling. However...the fouling was NOT what was making the engine run bad, as the clean plug did nadda to help it. SO I'm still left with no answers.
Have new plug wires on the way just because the originals are almost 30 years old. Haven't done much checking on the fuel system besides that the fuel pump runs. Plan on replacing the filter too.
I feel like I have the bases covered here, but can anyone give me something else to look at? I thought I had a handle on this thing after successfully swapping the engine, but I am getting to be stumped.
Engine is a 1995 Mercruiser 5.7 I installed last year. I bought it as a long block to replace an '86 305 and I am using all of the 1986 peripherals (Thunderbolt IV ignition, Alpha one gen 1), so for all diagnostic purposes, consider it a 1986.
Anyways, engine ran great all last year and the start of this year. Put a 6 hour trip in Saturday with no issues. Sunday, on the same tank of gas... launched it, started up, let it run at 1000-1200 rpm for a few minutes to warm it up, and completely out of nowhere the idle dropped by a few hundred RPM and it began to apparently misfire or run rough. There was no noise of anything breaking and no valvetrain noise before or after. Adding throttle didn't clear it up and it runs rough and shakes at all RPM regardless of load or any other factor. The exhaust note is that of a tractor.
From last season, boat has NEW ignition sensor, cap, rotor, coil, plugs, Edelbrock 1409 carb and marine electric fuel pump wired to oil pressure switch. Also has new alternator and batteries. Not that it's totally relevant, but all exhaust manifolds were checked and gaskets replaced at time of engine install.
So far to find the problem....pulled the plugs. #8 looked a bit fouled. Put a brand new one in and tried a different spare plug wire on that cylinder....no difference in running even with a clean brand new plug....still bad.
Tried new coil from other project....still bad.
Disconnected tach for the hell of it...still bad.
Used spark tester and verified all plugs are firing as they should..still bad.
Checked timing, still at 8* BTDC as it has been since I put the engine in.
Thought maybe it had a stuck valve so I checked compression today following the merc service bulletin and everything is in spec. These numbers were obtained after just one or two strokes so thats even better i suppose...
"Dry" numbers
#1 160
#2 155
#3 160
#4 150
#5 160
#6 155
#7 160
#8 135
Added a squirt of oil to number 8 and got 160 right away, which points to rings. Not a huge concern of mine as the engine ran well, and I can put a hotter plug in to stop the fouling. However...the fouling was NOT what was making the engine run bad, as the clean plug did nadda to help it. SO I'm still left with no answers.
Have new plug wires on the way just because the originals are almost 30 years old. Haven't done much checking on the fuel system besides that the fuel pump runs. Plan on replacing the filter too.
I feel like I have the bases covered here, but can anyone give me something else to look at? I thought I had a handle on this thing after successfully swapping the engine, but I am getting to be stumped.
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