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502 RUN NO RUN

throttletherapy

throttletherapy

Member
hey you guys with 502 mag mpi's. two of my buddies are having some issues with their motors. last weekend we went to a poker run and my one buddy was idling and went to get on plan and the motor stumbled then stalled and would not restart, i got on the boat and the fuel pump was running, (from hearing it atleast) and we had spark. we tried to crank it over a bunch of times and it would act like it was going to start then just stall back out. we cranked it again and the thing started he ran the rest of the day and we stopped again right before we got to our hotel and it would not start again. i had to pull him into the docks.(he had plenty of fuel)and we could not get it started after that. Also my other buddy has the same motor and on the same day, he also went to take off and his stalled out, his fired back up and he went to the dock to get fuel because he thought he might be out. (b/c you know how inaccurate the fuel gauges are)he got gas and was fine the rest of the day. We went to the river yesterday and his did it again,(with a half tank of fuel) but did not restart. any thoughts on this would be great. someone said the methenal in the fuel making it vapor lock, but i dont believe that.:brickwall:
 
I would say vapor lock. The easiest way to fix the problem would be a fuel cooler. I have one on my boat and have never had it vapor lock, a friend of mine did not have one and he had some of the same problems unitl he installed a cooler
 
its just odd that this issue just started. both buddies have owned the boats since new. ones a 98 and ones a 2001, and have never had any issues before. the boats are maticuallouslly mantained ever year. i need to check the vapor line at the regulator for fuel, the plugs and put a fuel psi gauge on it to start. i will check into the fuel cooler for them. but its still wierd, that it would just be happening to them when other than them there is 3 other guys that have the same motor in their boats and havent had any problems. thanks for the respond and info.
 
I saw a thread on OSO about the 502 MPI and the vapor looking issue. You may want to see if you can find it. I will also look and if I find it I will post the link.
 
I was on a 454 powered boat that idled out to deep water fine then he throttled up to get on plane and it died would not start for some time. Let it sit and it started come to find out he was low on oil so when he throttled up while the boat was on a large angle it tripped the sensor.
 
If those are the MPI motors of that year.....they should already have a fuel cooler on them. Look at the fuel pump housing on the port side of the motor by the front engine mount. If it says "Cool Fuel" on it then inside is the fuel pump and fuel cooler. There were two fuel systems used on those motors but I am not sure what the other one looks like. I have the Cool Fuel system on my 97. The only time that I have had a problem like that is when I shut my motor off too quick after a hard run.....then she doesn't want to start for a bit.
 
Vapor lock on an injected motor? That's strange. I doubt it would be that. On a carb motor, the best way to avoid vapor lock is to convert the fuel system to a return style set-up. My Persuader will get that next winter.
 
yes, it does have a fuel cooler on it. i talked to a couple of performance shops and called mercury racing for my buddy and they actually think it is poss. vapor lock because they are finding more ethenal in the gas and they are finding the ethenal boiling around 85-90 degrees and causing vapor lock.we are still checking into this! :confused: thanks for all input.
 
Chasing a vapor lock situation is easy, time consuming, but easy. Merc racing will instruct you to run a vaccum guage inbetween the low pressure pump and the fuel tank, and should read no greater than 3, in fact 3 is high. I always hooked up a high pressure mechanical guage for this check also have someone watch both while the engine is under load. You may have to run a while until it does it again.

With vessels of that age gunk builds up on the inside of the tank, which ethanol cleans off the side plugging up the screen at the bottom of the pick up tube, or partially plugging the pick up tube in the tank. Noteably, some pickup tubes are made of a plastic tube which is on a barbed type fitting on top of the tank. I've seen these come apart or just be loose allowing for air bubles to enter the fuel system which will cause vapor lock type symptoms. Another quick check would be to remove the fuel lift pump from the waterpump housing and see if there is oil or oil and gas in the housing. If there is gas in there, your lift pump has deteriorated and needs to be replaced.

Also, dont only focus on fuel electrical issues can cause these type problems too. The clean power side for the ecm needs 5v period. 4.9 will make it act goofey. Weak batteries, bad grounds, corroded cables, failing kill switches/key switch and the like should be checked with an ohm meter for proper operation. If you have access to a DDT this would be a beneficial tool to have on board for a diagnostic test run.
 
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