2006 Chaparral with Carbureted 5.0 Mercruiser having alarm and limited power.

[crepko]

I have a 2006 Chaparral with a 5.0 Mercruiser (carbureted). We rebuilt the top end of the engine last fall because the previous owner blew a head gasket and cracked the port side head when it hydro-locked. We replaced the port side head, gaskets and rebuilt the sterndrive replaced impeller and all exhaust gaskets and flappers. We set the base timing to 10 degrees per the service manual using a ground to the white wire for baseline timing setting. Engine runs fine and sounds great on ear muffs. Oil pressure and temperature are perfect on the gauges. When we took the boat out on the water for the first time a few days ago we were getting a long audible alarm when we were trying to get the boat up to speed and on plane. The rpms would not go over 3000 and the engine felt weak. Engine was not missing or running rough at all at idle. The alarm seems only to happen when the engine is under load. Stops when we get on plane but it takes a long time to get there. I looked at taking the boat in for service but everyone here in the north DFW area is saying 6-8 weeks to get to it (half the summer). Is there anybody out there who can give me some ideas on what to check to resolve this issue. I am pretty handy with a wrench. Also we checked timing advance and it seems to only be advancing to 20 or so total degrees at 3200 rpm, seems a bit low by car standards.

Also we replaced spark plugs, wires, cap and rotor during rebuild.

 

[crepko]

Also we did have a single backfire on our 1 hour outing. Couldn't tell if it was through the carb or exhaust though.

 

[Bondo]

Ayuh,..... Welcome Aboard,...... That alarm only sounds for low oil pressure, hi water temp, or low drive oil,.....

Check the Contents of the fuel filter for anything but clean fresh gasoline,....
If it's lean detonation could destroy it in minutes,....

 

[crepko]

Ayuh,..... Welcome Aboard,...... That alarm only sounds for low oil pressure, hi water temp, or low drive oil,.....

Check the Contents of the fuel filter for anything but clean fresh gasoline,....
If it's lean detonation could destroy it in minutes,....

Hello,

Thanks for your input. We did replace the fuel filter after the rebuild. Is there a fuel pressure sensor for these carburated setups? Am I looking for water in the fuel? We did fill it up with fresh fuel but it still had half a tank of fuel in it when we filled it up. Also I changed the oil 4 times after the rebuild to get all the water out. Maybe still some water in the oil? Oil pressure on the guage looked good the whole time (over 50 psi). I did not replace the sending unit at the back side of the intake manifold. Also the oil level was slightly over the full position.

 

[tpenfield]

When you rebuilt the top-end of the engine, did you do any sort of inspection (or work) on the bottom end of the engine, such as the rotating assembly? Usually, when you hydro-lock and engine you run the risk of bending some of the connecting rods (pistons) and possibly damaging the crankshaft. The fact that the hydro-lock cracked the cylinder head, would indicate that something else may have sustained damage as well.

 

[crepko]

When you rebuilt the top-end of the engine, did you do any sort of inspection (or work) on the bottom end of the engine, such as the rotating assembly? Usually, when you hydro-lock and engine you run the risk of bending some of the connecting rods (pistons) and possibly damaging the crankshaft. The fact that the hydro-lock cracked the cylinder head, would indicate that something else may have sustained damage as well.

I talked to the machinist who magnafluxed the heads for me and he said that these chevy small block heads fail easily when hydrolocked and there is a good chance the lower end is not damaged if all the pistons are coming all the way up the bore and there is no visible cracks in the pistons etc. Also the engine sounds good after rebuild of top end ,no vibrations, knocking or anything like that, it just seems low on power. Like a car would if there wasnt enough timing advance. So on the fuel issue, are you thinking a lean condition caused the backfire? Should I reseat the filter maybe? is there a connection for checking fuel pressure?

 

[nola mike]

No fuel pressure connection, you'll have to cobble something into the existing lines. Did you *check* the fuel filter contents? Looking for water and crud. Water can for sure cause these symptoms. Double/triple check your plug wiring. Compression test? Overfull oil level is worrisome. Don't know what ignition you have, but if a TB V with a knock sensor, that might explain the timing as well.

 

[kenny nunez]

Try the 6 gallon outboard tank test which will either point out or rule out the fuel supply from the tank. Quite often there is a restriction in the tank suction tube or even a fuel hose that has collapsed internally as well as the anti siphon valve.

 

[crepko]

No fuel pressure connection, you'll have to cobble something into the existing lines. Did you *check* the fuel filter contents? Looking for water and crud. Water can for sure cause these symptoms. Double/triple check your plug wiring. Compression test? Overfull oil level is worrisome. Don't know what ignition you have, but if a TB V with a knock sensor, that might explain the timing as well.

I did not check the fuel filter contents when I replaced it. Unfortunately its long gone now. Maybe a temporary inline fuel pressure gauge would be a good idea? I will change the oil and filter again and put the synthetic in this time at the proper level. Its only slightly overfull because I put too much in.

 

[nola mike]

Replace the filter again and check its contents this time, and/or run off an external tank like Kenny suggested. The oil level was concerning if it was rising (suggesting water getting into the crank case). Synthetic vs dino oil isn't your problem.

 

[crepko]

I looked up the ignition timing details in the service manual for the Mercruiser 5.0 Thunderbolt Carburated engine. It says that the Mean Best Timing spark advance will be disabled if an audible alarm is presnet. This seems exactly like what is happening in my boat (low power but no bad noises). Opinions? (Not sure if this is a light load scenario though.)

MEAN-BEST-TIMING (MBT) SPARK ADVANCE

During light load cruising, the Ignition Module maintains optimal ignition timing by making small spark advance adjustments. At a given rpm, the module will add a small amount of advance and wait to see if there is an rpm change. If rpm increases, it will increase timing more. The module will continue to advance timing until it no longer gets an increase in rpm.
Conversely, if it senses an rpm drop, it will start to retard some of the spark timing. Between 1200-4000 rpm the Ignition Module can add approximately 10-15 degrees of spark advance to the base spark timing curve.
NOTE: The Audio Warning System is also connected into the Ignition Module circuit. If the audio warning system becomes activated by the closing of one of the audio warning system switches, the MBT feature is deactivated.