1999 Hallett 240 with a Mercury Racing 500 EFI - 404 Hours

The boat runs good consistently for 10 minutes and then backfires through the exhaust randomly through all cylinders. Once it backfires, I roll out of the throttle and it will idle perfectly but once under load it begins backfiring again. Yesterday I took the gas cap off for a few minutes, went for a swim and it runs like a bat out of hell again for almost the exact amount of time and then it backfires again. I'm at a loss...anyone else experience this?

Also, the tachometer does not match what the scan tool is saying...it shows no more than 2300 RPM and the scan tool showed 3500 for example.

Compression test showed compression at 145-150 on six (6) cylinders and 125-130 on the other two (2)...I know, its low, but it's not zero.
New fuel and oil filter
New distributor cap
Re-traced spark plug wires, new plugs
Scan Tool showed timing was good, temperature was good, no codes registered

Watch cam and crank sensor signals on scan tool for any engine exhibiting your issues. Obv compression and fuel psi tests first, as it seems you did.

edit in: faulty tachs have been known to cause wacky issues.

Id get a fuel pressure gauge hooked up, should be a place on the fuel rail, might be time for an injector service also

its a 500 not a 525.....

No sensor codes came up when the symptoms begin.

Thats my next test...just ordered a fuel pressure gauge.

500 efi

that’s fine. I was covering any (all ) engines, as noted, even honda and Toyotas.
wink: lol :

might be the coil going south ??

Could be. Could be distributor mag pick up too. Heh, just like a cam sensor. And 1/2 a crank sensor.
Seriously. As the tachometer reading bad could be effected by that (It's the only thing counting engine revolutions on these engines.) or even the tach itself.

https://www.mercruiserparts.com/bam/.../31335/4889/60