454XL TBI Throttle Body Injection problem

1997 Crusader 454's. I put the boat into the water, both engines ran great. STBD engine had 20 gallons of fresh gas, port had 1/4 tank of last years gas. Got to the slip fine. Took a few weeks to get decent weather to get to the fuel dock. taxied over and Put on 150 gallons and it ran great, idled great. Left the fuel dock and headed out of the harbor. Went only 1/4 mile at no wake speed and stbd engine dies. Quickly restarts and dies. Will not stay running. Anyone that has tried to dock a 40ft boat into a slip with one engine has to feel sorry for me. I unfortunately am not that hi tech to know how to operate some of these scanning tools.

Gotta be a fuel issue- yeah? Wisconsin marina that just opened for the season, figured one of the first boats fueled, must be water in the gas. Winter lay up -All three fuel filters replaced, each side. Pulled the water separator. I pumped out a bunch of gas from the tank, all looks pretty good. Restarted, ran and idled nice for a little while...then died. Valved the fuel shut offs such that the STBD engine is drinking off the Port tank... no difference. Port motor still runs perfect, starboard turns off. Fuel pressure gauge flat at 20psi...then falls to about 10psi as the engine stumbles before it dies. Original 1997 fuel pump, so i replaced the pump. No difference. Pulled and checked the new fuel filters, all clean. Took out the original 1997 injector bodies and soaked, cleaned, re-installed. Visually watching them at idle - they have a nice spray cone...right up until the motor dies. STBD motor will throttle up ...then die. I been reading the many great posts here that have been extremely helpful to me. Might be the fuel pump relay? Swapped port to starboard relays...same problem. Might me the oil pressure switch killing the fuel pump? Swapped port to starboard...same problem. Checked the ignition switch, cleaned terminals, no difference. Port engine continues to run good as stbd stumbles and dies. Swapped the air idle control valve....no difference. The wire snap connection to the injector body looked suspect...replaced those- with no difference.

Maybe the problem is on the spark side?? Swapped the ignition coil, port to starboard- no difference. Swapped ignition control units...no difference. Pulling out what little hair i have left. Maybe the ECU...swapped left and right...no difference. I replaced all the spark plugs over winter layup. Finally I pull a spark plug...covered in black soot!! Running extremely rich.

I am about to swap the entire throttle body now, when I thought what about the throttle position sensor? I swapped that -port to starboard. Turned the key and the stbd engine runs and idles PEFECT NOW! The bad throttle sensor must have been telling the ecu that the throttle was way past the idle position, dumping a ton of gas into the motor, more than it could handle at idle.

So i bought a new throttle position sensor for the port motor. The problem now is that the port motor with the new throttle position sensor idles at 1100 rpm. There is no idle screw. Historically some cars i owned had throttle position sensors that had slotted mounting holes for adjustment. The boat one does not. Is there a way to calibrate the new throttle position sensor to get the proper idle now? The port engine will return to idle correctly with it's original throttle position sensor. Thanks! (and sorry for the long winded post).

Will an identical Looking NAPA throttle position sensor for a GM 454 truck motor work on the Crusader 454XL TBI motor?

OK, I am not an expert in TBI systems (mine is the next gen, the MPI), but the principles are, I believe, the same. First, the engine computer does not use the throttle position sensor to determine fuel flow or idle speed, at least not directly. The injector on times (fuel flow) are determined by the manifold air pressure (MAP) and the rpm. The throttle position sensor is there to tell the computer about changes in the throttle position so it knows what is coming. Rather than wait for the MAP to to drop as the throttle opens it will warn the computer to start the extra fuel flow immediately and prevent bogging down. Does the same thing in reverse when you quickly shut the throttle down. So on quick acceleration it replaces the function of the accelerator pump but its effect on fuel flow is transitory.

But, you do seem to have excess fuel flow based on the plugs. Most likely, this would be the MAP sensor reading incorrectly too high a pressure making the system think the there is much more air available to burn than there actually is, so it puts in more fuel to keep the mixture correct. Or, of course, the computer could be sending the correct information to the injectors, but one of them is sticking open and flooding the engine.

Now the idle speed is supposed to be determined by the idle air control (IAC) valve that operates by making a small, controlled air leak to adjust and control the engine rpm. Your observation of too high of idle would suggest a bad IAC or perhaps also be caused by an actual vacuum leak.

Lots of possibilities and I am not sure any one of them alone could explain all of your observations. (A sticky injector or bad MAP sensor AND and manifold leak could, however)

I believe your engine has the feature that allows you to shut down the injectors when the engine is flooded by advancing to full throttle. This will allow you to turn over the engine and clear the flooded condition so that you can restart. Useful to know if that happens again as you test things out.

Using a vacuum gauge to compare engine vacuums at idle, using a timing light or strobe to look for a stuck injector, swapping injectors between engines, using a probe and a 9v battery to insure each injector is "clicking" off and on, looking into the manifold (after shutdown) for puddled fuel with a borescope (cheap (>$50) and shows pictures on your smart phone with bluetooth). swapping IACs and MAP sensors, are all things you could consider.

Good luck and I would guess you will get a few more suggestions.

Does the fuel pressure drop and then the engine stumbles or the other way around? Is the fuel pressure maintained, under load, thru-out the RPM range?

As far as the TPS goes, on the MEFI1 & 2's it does drive the injectors' pulse width. not know what is was providing for the ECU to read, hard to say definitively. You can easily download an MEFI manual and it will also show you what most sensor values should be.

As far as calibrating the TPS, shouldn't be needed. there is nothing magical about the TPS's used in the marine world so it is very possible the one at NAPA will work...

the ECT sensor can drive fuel delivery issues too, especially if the sensor is still working (but no longer accurate)...

Yes a throttle position sensor out of adjustment can cause a high idle.Sounds to me that you found the problem and skipped right past it.Fuel pressure dropped to 10 psi and idled rough and stalled.My 454xli tbi engines have 3 fuel filters on each motor.Did you check them all and anti siphon valve might deserve another look.My fuel pressure is rock steady at 22 psi.Chase the fuel pressure dropping and you will fix your problem.Good luck.