502 Mag MPI Gen VI Hot start issue
 

Twin engine and both engines exhibit the same problem. Go out for a run and shut down to grab lunch. Hour later, they try to start and will run for a second or two before dying. Both act like they are starved for fuel. I checked the fuel/water separator and no water in system. Let it sit for another hour and after a few cranks, they fire up like it never happened. Get home, tie up and restart to flush without issue. Its just that magic 'hour' after I let it sit. Happened many times.

Fuel tank vent line? Ill check and replace both fuel filters in the AM (8 months old), but it seems like a heat related issue. Fuel and heat is the only thing the two engines share? Both are bone stock with about 30 hours on rebuilds.....nothing fancy like blowers or aftermarket EFI/Carbs.

Tonight I almost had to call the Tow Of Shame in front of a packed boaters bar. I just hid down below for a few hours and came home alone in the dark after I got one engine to start up. And this time I verified I had about a half tank of gas (Yea...I did that once after a faulty fuel sender).

Ill try anything as mechanics are backed up at least a month.

Thanks in advance.

Willing to bet vapor lock.

Vapor Lock. My 1997 MPIs have the same issue. More so the stbd side. Avoid ethanol fuel. Akways run the blower. Always allow some idle time to cool down before shutting down. Leave hatch cracked and blower on when parked.

Oh, and im all ears for any other remedies. There is a kit for the cool fuel equipped engines (check valve and electric supplemental pump) , but not applicable for my v-belt VST era engines.

I have threatened my engines to carburetor swaps! About ready for freshening at 850hrs. Been looking for a fair priced pair of HP500 carb motors, or even better Bulldogs! If i build these up, on the fence with keeping the efi

I have the cool fuel system and the little plastic covers are in tact (see...I used the search function after I knew where to start). So thanks to my fellow sufferers.

Is this the correct anti vapor lock kit for the cool fuel engines?
https://www.ebay.com/itm/MerCruiser-...-/201385686370

Corrective options are Cool fuel evap kit as shown above, or idling for five minutes with the blower running and hatch open or?

Vapor lock for sure. The hotter and more humid it is, the more likely it is to happen.

Not running ethanol fuel and opening hatch and running the blower for 5-10 mins after you dock helps a lot. I rarely have it happen when I do this.

Run the fuel return lines back to the tank.

I’ve seen a few people put clothes pins on the metal fuel lines to act as heat sinks.... I shook my head when I saw it, but the people swear it worked to get by in a pinch. :party-smiley-004:

Don't see that kit helping much. The issue is there is vapor in the fuel rail developed from the heat. Your mechanical pump is working, as been said, the best method is run the return back to the tank.

A quick fix can be done with a rag, small screw driver and pair of safety glases.
Remove the cap off the schrader valve on the fuel rail, put on saftey glasses, turn key until pump stops. Place rag around the schrader valve and press in with screw driver. Repeat the step a couple times till you start getting fuel come out

Is this the first time this has happened? Is the boat new to you? The reason Im asking this is maybe you got some bad gas. I know the refineries had to change their ethanol formula because of the vapor lock issues and small engines having trouble with it. Mostly gumming up in the carbs. Ive also noticed that on the new boats they have a low pressure fuel pump and a high pressure fuel pump. If this is the first time and its never happened before maybe your pump is getting weak. I would definitely check your tank vent also.

Ill also have to respectfully disagree with the idle open hatch blower thing. In this day and time you just shouldn't have to do that to be able to restart your engine in this day and time. You cant tell me in this day and time engine builders cant come up with a solution for vapor lock.

Fuel injection should be our modern answer to fix this.

These are fuel injected engines we're referring to.

Some manufactures have fixed the issue like Ilmor which does have a return to tank. Merc just keeps changing the pump type and location instead of fixing it.

Thanks...this will be my short term solution.

And this will be my final solution as I have an unused fuel barb off the tank for a generator that isnt installed. I take it I need to re route the lines that take the unused fuel from the injectors and instead of going back to the cool fuel pumps, run them back to the tank. Im also assuming I need to put in a check valve somewhere?

And this will be my '****..forgot to keep the hatch open and run the blower after shutdown' fix.

Ive had the boat for about 7 months..so newish to me I guess. This is is about the 9th time its happened and ive been pulling my hair out because its usually only happened to one engine. Every time, I investigated, drained filters, pulled plugs, check wiring, only to have it fire up the next day like nothing happened. Last night it happened to both at the same time which was a blessing in disguise.

Agreed 100% I think we can put this to bed gents. Big thanks for the solutions and education. Final question...About how long can I run the blower without draining the batteries assuming they are healthy?

The collective knowledge here is staggering.

Is it correct to assume the vapor lock between the tank and vapor tank? I would assume the high pressure pump can force any vapor out and I assume 45psi should compress fuel vapors. Tho OP stated that it fired then quit, is that the fuel getting used up in the vapor tank? How about insulating the low pressure fuel hose and maybe a way to return return some hot fuel back to the tank?

This is one of two things. both common to the Merc engines (big and small block) that use the VST system.


The most likely issue, as mentioned is vapor lock. During sits after running the stainless high pressure fuel lines tend to percolate fuel due to heat soak from the manifold and surrounding engine heat. The engine will typically start and run for a few seconds on the fuel on top of and near the injector. It then quits. It takes a surprisingly LONG time to recharge the lines and get the motor running again. Problem is the lines are now hot and mostly void of liquid fuel.


The other issue that can occur is deterioration of one of the rubber diaphragms in the VST assembly will allow fuel to bleed out through the yellow fuel vent line and into the intake manifold. This will bleed down VST tank pressure exacerbating the vapor lock issue AND it also floods the engine. For the small blocks this fuel enters at the throttle body. For the big blocks it enters the manifold near #7 if memory serves me correctly. Just trace the yellow line off the VST and see where it connects. Also if there is any trace of fluid in this line you have VST diaphragm issues.


One workaround is to bring the throttle up to 50-75% and attempt a start. This will force 'flood clear mode', which shuts off the injectors. Doing this will allow the flood to clear (throttle blades open / high air flow) and allow the fuel feed lines to re-pressurize easier (injectors not trying to pulse). Work the motor in this matter for a few iterations making sure to not kill the battery or starter with prolonged attempts. While doing this advance the throttle to greater than 75% or less than 50% (exiting flood clear mode). The injectors will start firing and the motor should light off. Keep the RPM up around 2000 or so in neutral for 30 seconds. You need to flow more cool fuel into the lines quickly to help bring line temperature down. Idle rpm is insufficient fuel flow to adequately cool the fuel lines in this case.


Once running get the boat up on plane and cruise as this is the best bet to get the most fuel into the lines for cooling purposes. If you go right back to idling you stand a chance of getting right back into vapor lock.


Obviously this is all a work around. I have found that fuel makes a big difference. Good fuel, preferably no or low ethanol helps (more than I had imagined). Make sure the VST internal diaphragm is not deteriorated (there are rebuild kits). Finally reroute the fuel return from the second injector rail to the main fuel tank rather than back into the VST. This will keep a constant flow of cooler fuel supplied to the injector rails rather than returning it to the VST only to be immediately reheated down the fuel rail assembly.


The fact that both motors exhibit this issue it is likely vapor lock. However, with the age of the motors that internal diaphragm could be bad on both VST assemblies.


Hope this helps.

Thanks Trash. I have Cool Fuel II systems so I dont think the 50% throttle trick would work. I have all sorts of goodies in the mail tomorrow including silicone jackets to wrap fuel lines, heat tape and thermal paste. I read an thread here about how the Cool Fuel internal mounting bracket should have thermal paste on the side that faces the engine...mine dont. Makes sense as the internal plate is mounted to the block and heat would transfer to the Cool Fuel pump and lines in the black housing. Short term plan is to wrap fuel rails with the silicone jackets, thermal paste on the side of the brackets and maybe wrap the Cool Fuel housings in thermal tape. Its a low cost experiment. If it doesnt work, I get to try the schrader valve/rag method to test it out. But I think the final fix is to run return lines back to the fuel tank.

Ill also take a look at my blower motor to see about upgrading it to a newer and or larger unit to pull more air out.

The OP doesn't have a VST. He has cool fuel and the excess fuel (which is warm) just runs right back into the fuel system. The fuel gets heat soaked from the hot engines when you dock and essentially turns into vapor. E10 is worse because alcohol evaporates faster and at lower temps. It will run at idle for about 20-60 seconds from the fuel still left in the rails. Until the vaporized fuel gets through the rails, it won't run right. Cycling the key to kick on the fuel pump several times helps.

If you have the cool fuel plastic covers around your fuel filters, take them off.
I wouldn't waste your time wrapping the fuel rails or lines. That silicone jacket will hold heat in as well as keep it out.
Heck, just opening the hatch and dumping a bottle of cold water on the fuel filter and lines when you dock will probably cure it.
I have never had to open my schrader valve when I've had an issue. Just cycle the key about 5 times to the run position and on every 5th time try and start it. Repeat a few times if needed.

Is it against coast guard regulations to return the warm fuel back to the tank?

Nothing in the regs, and diesels have always done its. That said, Ilmor started doing it years ago

https://www.uscgboating.org/assets/1...YC.1002.01.pdf

Good info. Did a test run to recreate the Vapor lock and we succeeded but this time we were at our dock. Schrader valve fix worked like a charm so thats my ace in the hole. Ill try the key on off method next time. Do you know about the Mercury check valve fix? It goes between the TStat and the cool fuel to prevent hot water from draining back into the cool fuel system after shut down.
https://nuwavemarine.com/oem-mercury...t-21-862271a1/

Never heard of that as a fix for vapor lock.

Im going to make a call or two to investigate the check valve in the AM. Im also looking into a wireless remote for the engine latch. Problem with my boat is you cant really get on or off with the hatch/bench up. A remote would solve that issue from the dock.

Same here have not heard that as a fix for VL
Merc did start putting in check valves in the pump feed, but at the same time the pump was moved to the starboard side and it became a box with high priced filters. It may help keep the fuel cooler in the box, but don't see how that will keep the heat away from the rail

I never hear of it before investigating this issue and reading through the Mercury Service Bulletin. They talk about it in section 6.c of the below document...........
"EFI and MPI engines with the ‘Cool Fuel’ system should have the fuel cooler temperature measured after the engine is shut off. The coolant hose going to the ‘fuelcooler’ should not get much hotter to the touch after the engine is shut off for 10-20minutes than what it is with the engine running. If it gets hot after the engine is shutoff, hot water from the cylinder block might be siphoning back. Installing the CheckValve Kit will stop this backward water flow."

https://stu-in-flag.net/blog/wp-cont.../vaporlock.pdf

Ill hit this hose with a infared thermomoter after an engine flush today to see if and how much a rise I get. If I get a significant rise, I might look further into this check valve. Adding Stabil is another cheap/easy recommendation in the Service Bulletin but my local fuel source said their fuel does not contain Ethanol.

Hell, Im leaps and bounds ahead of where I was a few days ago chasing down this hard start issue thanks to you guys.

Good to know, thanks
Was put out in 99 and looks like this is Merc first try's in trying to solve the problem. Some of the later model small blocks used the check valve on some open and closed cooling models but later was only used on closed. Then when box pump/filter was used on 3 point drain and not on 7 point.

The new 8.2 use closed and no check valve

So after much thought and wading through various solutions, I came up with a game plan. Put rubber washers under the bilge blower (MUCH quieter) and start using it after shutdown, a Bottle of Sta-Bil every fill up, removed the Cool Fuel covers as per Griffs suggestion, and adding a wireless controller to the engine hatch actuator. I found an industrial wireless controller for the lifting actuator, made in the USA for $150. Its designed for dump trucks, heavy lift gates etc and has internal relays. The receiver also has UP/Down buttons and will be mounted in an easy to get to location for when I loose the remote. 4 wires, 2 to the ram, Pos and Neg. Easy peasy.

If that doesnt work, Ill look into adding pre pumps or reroute the vent lines back to tank or? So yea, dock, turn on blower, hop off boat, raise hatch, come back to a boat that will start. Sounds good according to the bar napkin last night. Ill post a link to the wireless controller once I get it hooked up and working.

If you want the problem gone for good, just run the return back to the tank. It is an easy fix and it doesn't require any sort of check valve. Just unscrew the return line from the filter head and plug the hole in the filter head. Hook a hose to the return line and hook it to a barb on top of your fuel tank. This works 100% of the time in my experience. No need to reinvent the wheel for an easy fix

^^ Agree
and keep the Sta-Bil on the shelf for winterization

Would #6 be the return line in the diagram below? Cap that off and just run new line up to the tank? Thats it? How does the fuel go up hill..pressure?
Genuine Mercury & Mercruiser parts. FUEL FILTER(FUEL COOLED SYSTEM) S/N 0K147350 & UP

Op have one tank two engines ? 1 tank per engine ? Or ?

One tank with separate fuel pickups (one for each engine) and an unused pick up for a generator that is missing. The basics concept is easy to understand but the details are what would get me. Just need to find the port to cap off and what line to run back to the tank. Im assuming I can put in a T Fitting so both return lines lines could share the Gen pickup.

The filter head will have arrows pointing out and in, or list IN and OUT. One of the 2 lines will be connected to the inlet (arrow in), this is the return line.
The other side of the filter head will be the line coming from the tank with a arrow in

Depends how big that 1 generator port is. I doubt big enough for two pumps.

At idle nearly 99% of your fuel will be going thru that 1 port. Remember, electric pumps pump 10O% their volume all the time. Little fuel goes thru injectors at idle/low speed, rest goes thru return.

If your going to return into fuel fill hose, i’d fo something like this but with two return connections.
https://www.cpperformance.com/c-500-...s-returns.aspx

Yes, that #6 fitting is where you will plug it. You need a 1/4 npt plug. A 3/8 rubber fuel line will slip right over the line that you disconnected. An easy check to see if the 1 return port is big enough for both engines, check your fuel pressure at idle before and check it again after with both engines idling. If the fuel pressure is the same, you are good. If the fuel pressure is higher, you will need a bigger or 2 separate returns. I have returned 2 engines to a single 3/8 npt return in the top with no issues, but you will just have to try it. If that fitting in the top of your tank has a pickup tube on it, I would remove it and just screw in a fitting to dump the fuel in tank without pushing it through a pickup tube.

^^^Good advice^^^^

Update. Ran the boat kind of hard/easy for about an hour and went to dinner last night. Raised the engine hatch and came back to a nice normal start. The wireless kit came in so I did the install this afternoon. It looks like its really well made for outdoor use as all the electronic guts were hot dipped in some sort of hard rubber. Hid the receiver in an easy to get to location and tested it to abut 100ft. Up and down as advertised. The return fuel line is still on the books when its not so perfect outside.

https://shop.atcomp.com/g3-e01

Thanks again to everyone who helped me find the exact problem and various solutions.

It is a possible heat soak issue with winter gas? It vapor locks easier if the fuel system gets hot. Try opening the hatches a little the next time you shut it off after a hard run. This fixed it for my neighbors Formula SS.
He changed fuel source and went to no alcohol fuel.

Vapor Lock "No - More"
 

Good afternoon fellows. I bought a 32 Hustler over 6 years ago with twin 502 MPI's, this issue happened to me the first time I took the boat out, and of course it happened to one engine all the time until it happened to the other engine one hot day in Hot Miami. The problem drove me crazy for one whole year! It occurred to me that vapor lock happened more often when it was really hot out. Apparently the issue is that the fuel gets hot and causes the VST box to lock-up, so my solution was to run the engines as cold as possible. I removed the factory water pumps and thermostat housings
https://cimg8.ibsrv.net/gimg/www.off...930365c6b3.jpg
with "thermostats" and installed crossovers with stainless steel thermostat housing, (without thermostats), and run my engines at very cool temperatures, less than 100 degrees. Guess what, PROBLEM SOLVED"!!! Not one engine cut-off or failure to start after running hard, for the last 5 years.....by the way I also run REC 90 fuel which reduces water vapor in the fuel tank and the engines run a lot cleaner.