Mercruiser Vapor Lock Kit Install

I recently purchased a 1996 Sea Ray 280BR with a 7.4 Mercruiser EFI. It's been having some issues with the heat that the prior owner claims to have not experienced in the past. I'm fairly confident as this point it is the vapor lock issue Mercury released a bulletin for back in the day. I've purchased the vapor lock kit Mercury released as a fix after trying the other suggested solutions without any reliable luck. Has anyone installed one of these before and have any advice on where the wiring harness connects? I looked up the wiring diagram for the 7.4, but when I'm actually digging around on the engine I'm not certain where the harness splices in. Thanks in advance!

Ayuh,..... Welcome Aboard,..... I haven't done the kit, but tell me 'bout it, 'n we can figure it out,....

Thanks, I appreciate it! The motor is a 7.4 EFI/MP. It runs well but on hot days it has trouble starting back up after it sits for a bit after a run. It will turn over and then sputter and die back out and never get going again. I’ve been running the blower and opening the hatch and that’s seemed to help. I also added some sea foam as well. Based on other threads, I’m guessing this is vapor lock as described in this mercury bulletin. I’ve ordered the vapor lock kit it mentioned which I think is part number 862264A4. It is supposed to add a low pressure fuel pump before the mechanical fuel pump and includes a harness that wires it into the oil pump circuit is my understanding.

I’ve added some pictures of the motor if that helps.

It is supposed to add a low pressure fuel pump before the mechanical fuel pump and includes a harness that wires it into the oil pump circuit is my understanding.

Click to expand...

No, that's not what it does or how to connect

My original motor was a 502 with VST and did VL. The Merc is a boost pump which is installed after the fuel filter and before the Gen 2 high pressure pump. There is no fix for the VST, and as to what is actually happening a boost pump before the VST will not help

The fix for VL fix is return to tank line.

Thanks for the help getting me pointed in the right direction. Is my motor definitely the VST? The serial number pulls up what I thought was the correct engine for the mercruiser kit.

MERCRUISER : 7.4LX BRAVO (MPI)(GEN. V) GM 454 V-8 1996​

Yes your motor has a VST (vapor separation tank)


Your mechanical belt driven fuel pump is keeping the tank filled but the fuel rail is is heating up and vapor is building. You could bleed the fuel rail each time it does it to get going, but that puts a gas soaked rag in the boat

Your motor parts listing
https://www.mercruiserparts.com/7-4lx-bravo-mpi-gen-v-gm-454-v-8-199-1

Thanks I’m following you now. I read through your post history and it makes sense what you’re saying. If I’m tracking, I need to plug the VST return and run it into my fill line or into the tank? My boat’s manual doesn’t show any extra ports on the tank so thinking may have to tee.

Is the booster pump a complete miss? I’ve seen conflicting posts discussing the VST and suggesting it may help too but I may be mixed up on if it’s the same system. Only reason I ask is I may not be able to return what I ordered so if it is a viable alternative to a return line I may still go down that route.

The booster pump was installed on early Gen cool fuel. This was done to help keep more fuel pressure feeding the cool fuel pump. Original cool fuel only used the one pump.

The VST is feed by the mechanical pump so no need to add booster. The Mech pump is doing the same thing. The issue is the Mech pump is filling a tank like a carb fuel bowl. The HP pump inside the tank is drawing fuel from inside the tank, so no added value installing another pump

In my other post I mention about plugging the return coming back to the tank, but it also needs to have the line feed back to the tank, kind of a bad news thing could happen if not done right. Fuel rail on my current engine was the same rail which was on my VST motor. VST was removed and cool fuel installed. The change was the rail was cut down on the rear and 2 new fittings installed. The lower fitting is where the Regulator feeds excess fuel out to a line attached going back to my fuel tank

This has been a very enlightening thread. I'm having the same issue. Same engine, my serial no. is within 100 of yours. What was the final outcome / solution? Did you install the fuel pump kit? Did it work? Did you do anything else to solve the VL problem? Thanks for your help.

kwells114, please read are rules start your own thread, this one is closed