Hello all

I’m experiencing an erratic idle after taking the my boat out with 502s.

First time I experienced this we went out for a short boat day about 3/4 hours. I stopped for lunch after a 40 min drive and let the boat idle while I ate lunch. I did shutdown the boat at times and drifted and cycled through start and shutdown a few times.

Boat started up but experienced bad idle then cleared up. This was probably vapor lock. It was a cool day about 72 degrees out but the hatch was down. We decided to head home. This was another 40 min drive back and loaded the boat to head home for an 45 min drive home.

Got home went to flush motor and noticed starboard engine was idling very poorly. I tried to give it throttle and it would stumble then surge to 1500 rpm, so I backed off and it stumbled again.

Next morning started the motor and everything was perfectly fine.

Fast forward, couple weeks later we went out again and pretty much did the same thing. This time I shut the motors down for about an hour and the motors stumbled and stalled but cleared up in in less than 2 min.

i we went home same deal went to flush motor and port motor was fine but starboard again stumbled and could not idle very well. The IAC sounds like it was wide open since hear loud hissing sound. The hatch was open for about 15 min .

I have been reading of vapor lock with these motor but never experienced this before. Looks like I have Mercruiser cool fuel.

problem seem to be after a run and shutting down but figure with the drive home unloading this would have been enough time for motors to cool.

Has anyone experienced this before and what would be some things to check since now when I go to start it (next morning) it’s probably going to be fine.

I have read of people installing a return line to help with this issue but I really don’t want to do this if I don’t have to.

Also, some people in other post have commented that the covers on the water / fuel separator should be installed. This helps reduce with vapor lock. Mine do not have it.

Thanks

i am no pro at that, but vapor lock the engine does not start at all if I am not mistaken
i would look for something else

i would at least try to swap IAC from engine to engine see if the trouble follows the IAC

but great minds on here should be able to help more than I did !

Thanks for the reply Jeff32.

i have never experienced vapor lock ( at least i do not think).

However, I have seen where the fuel in the engine area starts to boil and turn vapor / steam which causes the fuel to not flow correctly into the combustion chamber. This then cause the motor to not start or stumble when start. After some time the new fuel then cools the boiling fuel and the engine returns to normal.

This seems to be the case of what I am experiencing.

I have heard it blamed on the ethanol fuel that some are now using.

What I don’t understand is why is this now happening. I have been running ethanol fuel for some time now so what has changed. I do know that there are different blends of fuel, winter / summer. Both times this has happen to me this year in the winter (south Florida).

Prior to these issues I would run premium fuel and most people tell me to run 87 octane. Since switching to 87, I have experienced this issue.

im not sure if the detergents that are in the 93 are giving the fuel a higher boiling point or if it is just a coincidence.

i look forward to others reply.

I dont understand this comment, 502 what ?.........unless they've been modified 87 is all you need, any thing else is a waist of money

Also, some people in other post have commented that the covers on the water / fuel separator should be installed. This helps reduce with vapor lock. Mine do not have it.

Thanks

These are 98 502 mpi. I understand 87 is all I need, but to tell the story accurately I felt the need to disclose this started after switching to 87. I’m not saying this was the cause but nothing else was changed.

hearing that you have the covers on your fuel filter and mine does not, this maybe an issue.

i do have covers on the cool fuel system, im just missing the covers on the filter.

Something else that is strange is it’s only the starboard motor.

I have never seen nor heard of covers on the water separator filters, what boat is this ? 72* ambient could be well over 100* in the engine bay without proper ventilation, maybe you got some bad fuel.

EDIT: Being the starboard engine does make perfect sense, think about it..... the cool fuel pump sets below the port side engine mount and in between the two engines where the hottest temps are, where as the port engine the cool fuel is towards the gunnel side where cooler air can circulate

[QUOTE=F-2 Speedy;4922000]I have never seen nor heard of covers on the water separator filters, what boat is this ? 72* ambient could be well over 100* in the engine bay without proper ventilation, maybe you got some bad fuel.[/QUOTE
its a 312 fastech

Here are the parts:
Part numbers are : 807956T, 807806T, 807807T
502 MPI vapor lock-fuelcovers.bmp

However you do make a good point about the starboard engine sandwiching the cool fuel system.

You’re right. There definitely would be more heat. So maybe the fuel pump is going bad or breaking down with heat?

you need to check the fuel pressure at the rail before shotgunning parts at it, could be the regulator

F-2, I totally agree. However, since it only happens after a run and did not have a fuel pressure gauge with me at the time, I will have to recreate the run again and see what the pressure is.

I was looking for feedback to see if others may have experienced a similar situation. I would not purchase parts just because someone says they think it might be “X”.

just looking for some discussion / guidance before my next run on the water to not disappoint the family and waste a day on the water.