'03 Launch SSV Spitting/coughing after 1 hour

[tbs156]

Hi all,

I have an issue that started after being on the lake for several hours.

Boat: 2003 Supra Launch SSV 350 TBI (5.7L Vortec)

It consistently follows the same pattern:

1) Go on lake for around 1 hour, everything runs great, can go WOT without problems even with a wakeboarder.
2) After 1 hour (hot engine), engine will pop/spit after pulling a wakeboarder or going under some type of load. When it begins spitting, it will continue to do so every few seconds until the boat is throttled back to idle.
3) If we try to go again, it only takes 2 minutes to start acting up again. Once it is acting up, don't need to pull anyone to start having problems. Any speed over 8 mph will do it.
4) If we wait for 20 minutes with the engine off, it seems to reset/cool down or and take 10 minutes to start acting up again (not confirmed).
5) Will occur at any speed over 8 mph once it starts. Has never occurred under this speed.

Have performed the following steps and diagnosis.

- New distributor cap and rotor
- New spark plugs and wires
- Have now been through 3 new half tanks of fuel. (Note: It started the 2nd weekend since being in the garage during winter. The first was a 2 day camping trip with no problems and running hard all day both days).
- New fuel/water separator. No water inside in the old one.
- Mechanic ran it for 45 minutes hooked up to computer couldn't replicate or see issues. Seems to need full ballast bags/towing wakeboarder to be initiated. Saw a bad cap on a wire and figured that was it. Still had problems afterwards.
- Checked fuel line connections and hoses
- Checked all fluids
- Checked gas cap

Next steps:

- Throttle Position Sensor from this thread (https://forum.supraboats.com/archive...p/t-17067.html) where the person had the exact same issue
- Run external fuel tank (difficult because I have to replicate an issue that takes 1 hour to test)
- Pull fuel tank and lines and inspect inside
- Compression test

Any input would be appreciated. Thanks!