502 MPI intermitant no start when hot.... not fuel prob
 

I am running 97 model 502 Mag MPI's and B1's. The engines have around 700 hrs and a very strange intermittent no start has begun to form a pattern in the starboard engine. Last season there was one incident where it did not start but that was brief and it didn't repeat the rest of the year. Went to Iowa and it did the no start threat again but finally did after three tries and all was fine. This started again this season and now seems to be more regular; only hot after rafting for a while when the engines have been run. The starboard engine will not start until completely cooled.

When I tested fuel psi all is well around 40 psi.. The other two engines are not doing this so I don't believe it has anything to do with the hot fuel business that these systems are known for. Testing spark and injection signals they are present, strong and consistent. With these signals strong still the engine will not start. At times when cranking it may hit a time or two but still not start. I am using a high energy 80KV tester for ignition and it is strong and consistent all the way to the plug. The noid for the injection never stops flashing but the engine does not start. Now what could keep the condition of a no start while ignition, injection and fuel are all good???

Do I need a scanner to see something here? Can the Merc side cause this no start apart from the ignition side? Can the ignition side work but not signal the merc side? Can a shorted injector cause this?

I have never seen such a situation. Anyone else seen something like this? What else can I check to pin down the prob?
Right now it will start when cold but not restart when hot and the ignition, injection and fuel all test fine during the no start condition.
TIA

maybe some reversion from a bad exhaust manifold or riser.

Ok, if that is the case then how would it give the no start hot but not cold? When the engine cranks there is nothing different or changing during crank function. It stays steady and normal with the starter.
This condition can be duplicated in the water or on the trailer and both no start conditions respond the same. Is the test on the trailer subject to reversion too or could this way of testing rule out reversion?

Are they VST or Cool Fuel engines???

Check the yellow fuel lines off the VST and mechanical pump. They usually dump into a port on the intake plenum. The inner diaphragm on the mechanical fuel pump or inside the VST can deteriorate allowing fuel to permeate through. This fuel is dumped via those yellowish fuel lines into the plenum effectively flooding the motor, and typically only when the motor is hot.

First thing is to verify this is the case. You will see residue or remnants of liquid (fuel) in those yellowish lines. It will tend to percolate, sometimes slowly into the manifold.

Replacing the diaphragm will solve the problem. In the meantime the work around is to put the motor in flood clear mode while starting. Typically this is 50-75% throttle which triggers the injectors to shut off while the throttle blades are open.

The above mentioned scenario will also look like vapor lock, but is instead merely a flooded motor.

if the engine is cranking but not starting it is not reversion.most likely a fuel issue.

coil

Thanks for all the responses, help and ideas.
I have the cool fuel MPI engines and there have not been any fuel issues in the past 700 hours so I have a hard time rationalizing a fuel issue on one out of three engines. Wouldn't all of them seem to have this issue if the conditions are present for one? There must be something unique to the starboard engine is what I think.
The spark is strong through an HEI 80KV tester all the way to the plug or if this test is at the coil it is perfectly consistent and strong according to how the tester readings work. That tells me that the "trigger" (pickup coil) is functioning and the injector signal, as seen by the noid light is a steady blink which also uses the input from the trigger. What can keep the start sequence from happening when these vitals are strong?



P.S. I replaced ALL of the tune up components, cap, rotor, plugs, wires, this spring before the season and ALL filters throughout the boat systems. This symptom has not changed at all before or after the tune work.

I have the MPI cool fuel system but I understand the "clear flood" function to be the same. I did not try this during testing so that is another parameter I can check. I have tried to be attentive to a fuel smell to tip me off in this area but I don't smell anything. I also have the Formula S/T which could hide some smell maybe.Thanks

Also: I have cool fuel, V belts and it is a 97. I understand how this kind of blurs some lines but to the best of my knowledge it is all original as produced and no alterations.