Logo
FULL SITE MENU

Power loss after 15 minutes 7.4L Bravo

3

3rd Times a Charm

New member
This is bizarre. Push the power to 4500 to get on plane, then roll back to 3500 to cruise. 15 minutes later it will lose power down to 3000 regardless of throttle position. Fuel? Timing? Spark? It’s an old school 7.4 (92) that I can wrench on without a computer. I’m guessing the fuel pump isn’t keeping up with the acceleration, but I found evidence of arcing under the distributor cap. Really confused.
Help me Obi Wan
 
First thing given the symptoms start is check for a clogged fuel tank vent. When this occurs open the gas tank fill cap see if there is a vacuum in the tank. Alternatively you could rig up a 5 gal outboard tank to test.


You mention arcing. Have you replaced cap rotor wires and plugs ? Would test with spark gap tester when this occurs to see if you are loosing spark
 
3rd Times a Charm said:
Tank vent is clean and operable. Cap/rotor/plugs/wires and spark advance module all replaced. Just ordered a new coil…
Literally throwing parts at the problem
Click to expand...
Stop throwing parts at it and diagnose it.

In the service manual there are Ohm specs for the coil, ohm it out before replacing. Modules are expensive there is a test procedure for that as well.

You need to figure out if you are loosing spark or fuel. The spark gap tester can be used for verifying spark strength when it acts up. Take flame arrestor off and see if it squirts gas out accelerator pump when this happens. What does your fuel filter contents look like? Run it on a remote tank to see if the issue goes away
 
Agree with the recommendation above ..... proper diagnosis will improve your understanding and (very likely) save you a small fortune...

the pickup coil inside the distributor is another possibility...

it could also be the timing advance is not consistent...at 3500 RPM, the advance should be at max...if there is an intermittent input to the module, it max not provide that full advance...
 
Dieter said:
Stop throwing parts at it and diagnose it.

In the service manual there are Ohm specs for the coil, ohm it out before replacing. Modules are expensive there is a test procedure for that as well.

You need to figure out if you are loosing spark or fuel. The spark gap tester can be used for verifying spark strength when it acts up. Take flame arrestor off and see if it squirts gas out accelerator pump when this happens. What does your fuel filter contents look like? Run it on a remote tank to see if the issue goes away
Click to expand...
TBH? Throwing parts is pretty much necessary at this point. The Vessel is over 32 years old🤷🏽
I pulled the coil and tried to reference the part number…. The part number only had 4 digits, which spoke volumes about age. So these are things that definitely needed replacing regardless of my current troubleshooting.

Now I am leaning towards fuel flow. Since it runs up to 4500 with no problem, then runs for 15 minutes around 3500, before rolling back.
I’m thinking that I am opening the aft 2 barrels to get on plane , then the fuel pump is coming up short on pressure. I’m going to pull the Inline filter and report back. Thanks for the responses
Chris
 
Yout coil is simply a Internal resister coil, can be purchased at any auto parts store.

Napa sells them as I have purchased them there, INTERNAL RESISTOR COIL FOR GM.
costs approx $35-50.
 
3rd Times a Charm said:
TBH? Throwing parts is pretty much necessary at this point. The Vessel is over 32 years old🤷🏽
I pulled the coil and tried to reference the part number…. The part number only had 4 digits, which spoke volumes about age. So these are things that definitely needed replacing regardless of my current troubleshooting.

Now I am leaning towards fuel flow. Since it runs up to 4500 with no problem, then runs for 15 minutes around 3500, before rolling back.
I’m thinking that I am opening the aft 2 barrels to get on plane , then the fuel pump is coming up short on pressure. I’m going to pull the Inline filter and report back. Thanks for the responses
Chris
Click to expand...
You can rig up a fuel pressure gauge to see if that is what is happening. there may be a filter or sintered stone in carb inlet fitting as well as canister filter down by the pump.
 
On the fuel side, if the tank vent is open, its very possible you have a bunch of crud collecting on the dip tube in the tank...and it will drive you nuts because you can never "see it" directly...
 
Problem solved!!!!! Yes, no ECM happening here.
Was it a new coil? Perhaps yes or perhaps no….
As I was removing existing coil I back tracked the wiring harness wires leading to coil. Those wires were very inconveniently routed right over the plate on the Intake manifold that sent ‘heat’ to the Choke mechanism for the carb back in 92…. The carb choke is now electric so there was nothing to stop the wires from contacting the heated part of the intake manifold. The wires crossing the plate all had heat degraded insulation. I reinsulated the wires and suspended them above the plate with zip ties. I suspect this was the problem as I would have a great running engine for 20 minutes….once it warmed up, the insulation got soft and arcing to the intake manifold starting robbing ignition spark. Do I regret buying a new coil? Nope. New cap and rotor? Nope. Plugs, wires, spark advance module? Nope. This is a 454 in a 92’ Sea Ray 270.

Hullwise, the boat will exceed my lifetime. Anything motor related bears changing parts on an ongoing basis. Aircraft are a prime example of this same principle. Do you think a 737-700 Aircraft with 25’000 hours on it, is still tooling around on its original engines? Nope. That’s a good thing!

Chris
 
Congrats on making progress....I'd suggest a bit of caution until you can get the engine running, under load, at 3500 RPM for 15 minutes...you never stated the engine ran on plan for over 15 minutes after the 'wiring improvements'...
 
Back
Top