Power loss at cruising speed on 2002 8.1 port engine

Engines are '08', is a repower. (fuel injected, no chokes) Pickup tube was blown clean with compressed air. No resistance to low pressure airflow through the tube into the tank. No water/dirt in the Racor filter. No water/dirt in the FCC filter. New fuel line from tank to engine. Tank vent lines and vents clear and clean. Crusader tech states the issue is intermitent failure of the low pressure fuel pump (not common, but there have been a few failures). Pumps are not computer controlled (basically on or off). Plan to replace both low and high pressure fuel pumps 1st week of April, not difficult and not very costly. Won't know if this is the fix until relaunch and sea trial. Last failure was 3 hours into a 5 hour run (not good in 8-10 ft seas) and ran perfect the next day.

Sounds like crud in the tank is still viable. It justs stirred up and as fuel is consumed, it collects on the pickup tube's screen. If enough accumulates, you get starved for fuel. Removing the demand from the fuel pump "releases" the crud back into the tank...that's what makes 'finding' this cause so much fun.

One way to verify this is to insert a vacuum gauge into the suction line. if the vacuum increases as the engine is run, it indicates a restriction.

Using a fuel pressure gauge on the rail would also help in isolating the cause (between pump malfunction & inlet restriction). If the pressure gauge drops when the power falls off and the vacuum gauge was holding steady, you could be pretty confident that a pump issue is the cause. I'd still rule out an electrical intermittent before buying the pump though.

Mako: conceptually this is nice idea, however it's not real practical. In oredr to do this, I would have to cut my new fuel line from the tank insert a tee fitting to attach the vacuum gage. Regardless of the test results, I would then have to replace the fuel line (no splices in the line). Already bought the pumps and install is scheduled.

While we're all speculating, here's one more thought: Too hot a spark plug (heat range). Since this motor is ECU controlled, and has a knock sensor (usually), the ECU retards the spark when it detects preignition. That causes the motor to slow down and run hotter (from retarded spark, not preignition). The ECU eventually ends all this by shutting itself down.

Opinions?

Jeff

PS: That's a pretty big (long stroke) engine to cruise at 3,600 rpms.

Plugs are factory spec (hope they know what the right heat range is). Engines run cool - 160 deg. I don't cruise mine at 3600, usually keep them down at 3K.

Viking - if you have rubber hoses, it shouldn't be that big of a deal. If you have flared hard lines, you could check the filter heads...many have the I/O ports on 'both sides'; if so, you could pull the unused inlet plug and put a gauge there. The gauge selected would vary with the specific plumbing.

Jeff - those beasts are normally rated for close to 5000 rpm WOT. I'm inclined to think much over 3500 would start eating into the life of the engine. Your ECU shutting things down is plausible, especially with the newer MEFI4+ ECU's. Most will set the check engine light when they detect an 'out of bounds' operating condition so that cause and effect should be pretty easy to verify.