1997 5.7 Thunderbolt power loss in drive after warm

[Boaterguy65]

I have a 1997 mercruiser 5.7 thunderbolt V ign. It just happened yesterday, never had a problem with it at 180hrs. tune up and manifolds replaced less than 100 hrs/2yrs ago.
After 15 minutes running @ 35mph 3500RPM maintaining a cool 140 degrees temp I start losing RPM and power, I hear popping, bogging when I force the throttle up while in drive. If I take it out of drive, throttle only, it seems fine engine wise revs even and smooth. In drive it loses RPM slowly untill my max RPM is 2000 and im limping along. Once she cools off its back in the game power restored and im "off like a prom dress". I made it from Honeymoon Is. to Indian Rocks Beach FL when it started to act up.

Compression is good, choke seems ok, all cylinders firing, cleaned and inspected cap and rotor, seems ok. Checked lower unit its cool and spins loose w no noise. It only pops and boggs under a load
The coil looks old but I dont have a voltage meter here.

Any suggestions? I am a certified ASE technician on cars and I hold an A&P license for aircraft so I hate shotgun maintenance, or guessing. I am far from my rollaway but I brought a tool bag, Should I just replace the coil and dump some B12 in the tank? HELP!! :%

 

[Don S]

Re: 1997 5.7 Thunderbolt power loss in drive after warm

Sounds like you have developed a plugged vent, or have an air leak in the fuel system, or even a fuel pump going bad.
Have you checked the fuel pump pressure and volume?
Also check the boats fuel system. Here is a thread on how to test it.

http://www.iboats.com/bbboard/bbBoard.cgi?a=viewthread;fid=24;gtid=1107009

 

[Bondo]

Re: 1997 5.7 Thunderbolt power loss in drive after warm

It only pops and boggs under a load

Ayuh,........

Sounds like Fuel Starvation........

 

[linkhorn]

Re: 1997 5.7 Thunderbolt power loss in drive after warm

The problem is in the fuel line.
Restricted flow allows smooth running at low rpm and starves it at high rpm
Remove the anti siphon device at the top of the fuel tank. If your fuel line has a screen at the end of the gas hose remove it also.
Your fuel filter will catch any disbris. Change the filter often.

I have a 1997 mercruiser 5.7 thunderbolt V ign. It just happened yesterday, never had a problem with it at 180hrs. tune up and manifolds replaced less than 100 hrs/2yrs ago.
After 15 minutes running @ 35mph 3500RPM maintaining a cool 140 degrees temp I start losing RPM and power, I hear popping, bogging when I force the throttle up while in drive. If I take it out of drive, throttle only, it seems fine engine wise revs even and smooth. In drive it loses RPM slowly untill my max RPM is 2000 and im limping along. Once she cools off its back in the game power restored and im "off like a prom dress". I made it from Honeymoon Is. to Indian Rocks Beach FL when it started to act up.

Compression is good, choke seems ok, all cylinders firing, cleaned and inspected cap and rotor, seems ok. Checked lower unit its cool and spins loose w no noise. It only pops and boggs under a load
The coil looks old but I dont have a voltage meter here.

Any suggestions? I am a certified ASE technician on cars and I hold an A&P license for aircraft so I hate shotgun maintenance, or guessing. I am far from my rollaway but I brought a tool bag, Should I just replace the coil and dump some B12 in the tank? HELP!! :%