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.

You won't smell anything, thats whats tricky about it. With 700 hours that is about the time some of the fuel components start to deteriorate. I'm still betting on a fuel issue similar to what I described.

Have you serviced cooling systems? Are all engines running same temp? A slightly damaged impellor will still move enough water to cool properly , but will allow water to drain from engine when shut off , causing enough radiant heat to boil fuel in fuel rail (vapor lock). Fuel pressure will still be ok , but injectors will inj air. Next time it does this , bleed fuel rail at fitting and look for air. Good Luck!!!

im going with parasail.check the impeller on the engine that wont start.had a 2000 454 mpi cool fuel with the same problem.take the fuel/water separator off and fill it with fuel and see if it will start(when it is acting up).replacing the impeller fixed the problem

you changed all the tune up parts except the coil.i think sutphin30 may be right.at this point the coil would be the easiest&cheepest thing to change.

aquaforce,please let us know what the fix was when you get it right.i can learn from this.

Before the season started this year I also replace all the impellers and water circ pumps and so far all these parts seem to be doing well. We ran the boat hard in the Pirates of Lanier poker run and the temps were stable which I expected but of course good. During the last no start testing I did check fuel pressure again and spewed out fuel from the valve and no air was seen doing this but I did not bleed it all the way down to specifically check for air. This same starboard engine had the fuel pump replaced about 100 hrs ago.

So my take away here is to induce the no start which is not hard to do sitting on the trailer, then fully bleed down the rail to check for air.

TRASH: After I do this bleed down checking for air I will check, wot, for the flooded scenario.
Thanks all.

I am going to compare coil ohms and spark test with another engine to see if a coil could be the issue. When I searched here for these hot no starts I could not find a resolved example like mine so I know this will help someone sometime. There is a Formula boater in Ohio that had a no start like this and the dist ign module solved his symptom. I am leaning in this direction partly because I have seen this in the GM auto products over the years so it doesn't surprise me to find it in a boat. I did however have more data using my scanner in the auto world to corroborate information for some concrete data for making a call. So far I have not found a smoking gun and am looking for data streams, methods tests etc to find a smoking gun. I have a module coming that I am going to install and test. There is another signal that is generated from the ign mod which is used for inputs in the auto applications. I wonder if the 5v signal above 400 rpm is failing from the mod when it gets hot and therefore all the functions for start are there but no signal to the computer for above 400 rpm operation. This is the part that I think, till I find otherwise, is failing and why it is very intermittent and only when hot.

I will pass on my findings as i'm sure it will help someone. Besides ((((( I gotta go boating )))))))) ARGH!

Hope you are getting a Delco module and are using real heatsink paste under it vs insulating paste that come with most modules.

If the paste is clear don't use it.

Yes, I agree on the paste and quality parts. It is very possible that I could remove the module, clean everything well and re-paste and make it work. I have done this before on other stuff but since the module has been hot to the point of not working, as I think, I am not willing to keep going with it. Only new good stuff for the Formula.

I planned to test today but it is raining.......argh.

FIXED !!!!!

Boating and had many hot restarts with no problem.

So the scenario is this: The distributor ignition module screws were rusty which indicates moisture at the least or heat from a bad ground contact at the module. The module would always give spark and injector signals even when it would not start; that was confusing. The no start seems to be caused by the 5V reference dropping off which carries the 400+ rpm signal to the computer to begin the start sequence. The resistor in the module would get so hot it lost it's ability to maintain the 5 volts and so the computer never knew to start up. It seems that when this was happening the 5 V could be lost while the engine was running but not shut off because the computer had synced up with the primary trigger therefore stay running until shut off but not restart because the 400+ signal was gone. This is what made troubleshooting this thing so tricky. The primary trigger through the pickup and module never quit so there was always a signal which made the testers I use say it should run and run it did, just not start up again after the 5V was gone.

I used Artic Silver #5 for the thermal paste. Top line stuff used for computer heat sinks and should far exceed anything this module will need. The original white stuff was nearly none existent and probably more of why the module failed.

It seems as if I'm having the same issues with my 1997 502 MPI. Does anyone have a part number for this module at an auto parts store, or do I just bend over and assume the position from my local marine sealer?