An old subject, but one I?ve toiled over for a couple of months now. Vapor Lock!! Or is it? I?ve logged hours pouring over forum posts (many on this web-site), visiting with Merc Technicians, performance boat guys and even speaking with Mercury Technical Support on a couple of occasions. Not surprising, some of the ?experts? misdiagnosed my problem, probably because of all the known ?issues? with VST systems from back in the day. Most advised ditching the MPI and VST for a carburetor suggesting ethanol boiling points too low for reliable operations. While research supported some of what they were saying, I also realized there are likely hundreds?thousands of boats still operating successfully with VST systems. BTW, I took fuel samples and didn?t find much (if any) ethanol in my fuel. I performed several component swaps, some that were legitimately questionable. After all my boat is a 1995, so having components operating out of spec wouldn?t be exceptional.
My motor?a GEN 5, 454Mag MPI with a large VST, WITHOUT Cool Fuel. So, the same would likely apply to the bigger brother, 502MPI.
I?m coming from a layman perspective so if I don?t use perfect terminology or if just WRONG on something, please forgive me...and correct me. I?m only trying to provide some solid info for the next unfortunate chap with the same problems.
My big block started and ran flawlessly at the beginning of each new days outing. However, once stopped it heat soaked and developed reliability issues for the remainder of the day. Yes, under the advisement of the Merc Service Bulletin on vapor lock conditions, I ventilated the engine compartment (open hatch), blowers on all the time at idle, idled excessively (relative to previous boats I?ve owned), etc. Nothing seemed to have any affect on the problem. When I described my particular problem to Merc Technical support they relayed that my particular vapor lock condition did not match they?re historical data. My problem was inconsistent and ambient temperatures, water temps, didn?t seem to have any affect on the condition. Merc?s service bulletin described a non start condition after heat soaking. My condition was that my boat would start after heat soak but would only operate 3 to 5 minutes and simply shut down. Restarts were frustrating to say the least. The engine would eventually restart after several attempts requiring sometimes ten minutes. Local Merc mechanics prescribed (without looking I might add) replacing the IAC as the most likely suspect. Because mine is not easily accessible, I replaced it along with the TPS for the same reason.
Over time, the condition worsened (happened more and more often) and I replaced component after component, each time really thinking I had found my smoking gun. Junk in my fuel system, anti siphon valves replaced, fuel lines changed, MAP sensor replaced, fuel/water separators x 4, cap and rotor. All of these things were suspect and easily changed, eliminated as a concern or replaced. I had considered a Marine ? USCG approved electric fuel pump. But I didn?t want the ?key-on? pump. It seemed legit without fuel system molestation, that this boat although new to me, likely operated well the first 20 years of it?s life?so why now?
After all of this, at my end, I finally found a Merc Tech that wasn?t too busy as to look at it. Spring time on the lake is not the time to have boat problems!! Almost immediately the real issue presented itself?the mechanical fuel pump. Merc says this pump produces between 2 and 8 psi on the low side of the system before reaching the VST?where it is boosted to right around 40psi. I?m sure the 2 ? 8 is linear with RPMs. Mine was only producing ? psi at idle and only 2 psi at 1500 RPM.
So why didn?t I look to this component earlier? I?m no mechanic but I do know that mechanical pumps either work or they don?t! Right? Even my car mechanic friends supported my thoughts. NOT SO FAST!! I never thought about it until an obscure forum post from long ago?this mechanical pump, the one in this particular application IS NOT attached to the motor. It?s belt driven and attached to the sea water pump. After years of ignorance, the seals or gaskets?whatever, can allow water to invade the fuel pump?s drive system. What?s more the mechanical fuel pump has a lubricant that is supposed to be changed. Who ever heard of a closed oil system (like a car rear-end) on a fuel pump!? I?m not being critical here?just showing my ignorance.
Now I speculate that MANY people have replaced their older warn out mechanical pumps with new electric ones, figuring the more consistent and likely ?slightly higher? pressures were ?beating? the vapor lock bubble. All along though, their mechanical pump just needed rebuilt.
If I can keep just one guy from having the same frustrations, I feel I?ve done a good thing.
FYI?I buy Valve Tech fuel on the water, supposedly guaranteed to be alcohol free. After fixing my mechanical fuel pump, I?ve never had an issue again. All the performance boat guys critical of my clunky VST?it?s never missed. Even on the hottest days (100plus) and nearly 90deg surface water temp, with the fuel lines too hot to touch?no vapor lock. No boiling fuel in the VST.
My motor?a GEN 5, 454Mag MPI with a large VST, WITHOUT Cool Fuel. So, the same would likely apply to the bigger brother, 502MPI.
I?m coming from a layman perspective so if I don?t use perfect terminology or if just WRONG on something, please forgive me...and correct me. I?m only trying to provide some solid info for the next unfortunate chap with the same problems.
My big block started and ran flawlessly at the beginning of each new days outing. However, once stopped it heat soaked and developed reliability issues for the remainder of the day. Yes, under the advisement of the Merc Service Bulletin on vapor lock conditions, I ventilated the engine compartment (open hatch), blowers on all the time at idle, idled excessively (relative to previous boats I?ve owned), etc. Nothing seemed to have any affect on the problem. When I described my particular problem to Merc Technical support they relayed that my particular vapor lock condition did not match they?re historical data. My problem was inconsistent and ambient temperatures, water temps, didn?t seem to have any affect on the condition. Merc?s service bulletin described a non start condition after heat soaking. My condition was that my boat would start after heat soak but would only operate 3 to 5 minutes and simply shut down. Restarts were frustrating to say the least. The engine would eventually restart after several attempts requiring sometimes ten minutes. Local Merc mechanics prescribed (without looking I might add) replacing the IAC as the most likely suspect. Because mine is not easily accessible, I replaced it along with the TPS for the same reason.
Over time, the condition worsened (happened more and more often) and I replaced component after component, each time really thinking I had found my smoking gun. Junk in my fuel system, anti siphon valves replaced, fuel lines changed, MAP sensor replaced, fuel/water separators x 4, cap and rotor. All of these things were suspect and easily changed, eliminated as a concern or replaced. I had considered a Marine ? USCG approved electric fuel pump. But I didn?t want the ?key-on? pump. It seemed legit without fuel system molestation, that this boat although new to me, likely operated well the first 20 years of it?s life?so why now?
After all of this, at my end, I finally found a Merc Tech that wasn?t too busy as to look at it. Spring time on the lake is not the time to have boat problems!! Almost immediately the real issue presented itself?the mechanical fuel pump. Merc says this pump produces between 2 and 8 psi on the low side of the system before reaching the VST?where it is boosted to right around 40psi. I?m sure the 2 ? 8 is linear with RPMs. Mine was only producing ? psi at idle and only 2 psi at 1500 RPM.
So why didn?t I look to this component earlier? I?m no mechanic but I do know that mechanical pumps either work or they don?t! Right? Even my car mechanic friends supported my thoughts. NOT SO FAST!! I never thought about it until an obscure forum post from long ago?this mechanical pump, the one in this particular application IS NOT attached to the motor. It?s belt driven and attached to the sea water pump. After years of ignorance, the seals or gaskets?whatever, can allow water to invade the fuel pump?s drive system. What?s more the mechanical fuel pump has a lubricant that is supposed to be changed. Who ever heard of a closed oil system (like a car rear-end) on a fuel pump!? I?m not being critical here?just showing my ignorance.
Now I speculate that MANY people have replaced their older warn out mechanical pumps with new electric ones, figuring the more consistent and likely ?slightly higher? pressures were ?beating? the vapor lock bubble. All along though, their mechanical pump just needed rebuilt.
If I can keep just one guy from having the same frustrations, I feel I?ve done a good thing.
FYI?I buy Valve Tech fuel on the water, supposedly guaranteed to be alcohol free. After fixing my mechanical fuel pump, I?ve never had an issue again. All the performance boat guys critical of my clunky VST?it?s never missed. Even on the hottest days (100plus) and nearly 90deg surface water temp, with the fuel lines too hot to touch?no vapor lock. No boiling fuel in the VST.
