Vapor lock?

Horton177 · May 27, 2024

It would appear the starboard 7.4 on my 2000 sea ray likes to vapor lock. I run all morning fine, run the blowers entire trip, run blows for few min after I anchor and before I fire. Then when I leave it fires up 10 seconds or so dies and won’t fire again until a long setting period.
Very annoying as my last ski boat had this issue.

Anyone found a true fix? My port motor is fine.

Jimmy Buoy · May 27, 2024

Does your 7.4 engines have two fuel pumps - one for high volume/low pressure near the fuel filter and one for low volume/high pressure in a "cool fuel" unit on the other side of the engine? If so, it could be that your high volume pump is not operating properly. Have a friend cycle the ignition on each engine while you hold the low volume pump and see if it seems to be vibrating the same as the working engine.

Horton177 · May 27, 2024

Jimmy Buoy said:
Does your 7.4 engines have two fuel pumps - one for high volume/low pressure near the fuel filter and one for low volume/high pressure in a "cool fuel" unit on the other side of the engine? If so, it could be that your high volume pump is not operating properly. Have a friend cycle the ignition on each engine while you hold the low volume pump and see if it seems to be vibrating the same as the working engine.

I’m not sure, when I go back up Friday I will look. After tons of research I was assuming it didn’t have the low volume pump. It seems like that was a fix mercruiser suggests. I plan to hook a FP gauge up too when it acts up. That’s how I determined my old ski boat had the same issue

Jimmy Buoy · May 27, 2024

Horton177 said:
I’m not sure, when I go back up Friday I will look. After tons of research I was assuming it didn’t have the low volume pump. It seems like that was a fix mercruiser suggests. I plan to hook a FP gauge up too when it acts up. That’s how I determined my old ski boat had the same issue

My last boat had 4.3 V6 MPI's which only had the one high pressure pump. Not an issue here in Canada with our shorter/cooler seasons, but apparently vapor lock was an issue in hotter climates like Florida. The solution was to add a low pressure pump like the V8's had (or so I thought).

If your engine serial # is
[HEADING=2]0L002006 THRU 0L674613[/HEADING]
it looks like there is only the one high pressure pump. Not sure why one of your engines would have an issue while the other does not.

When the engine will not start, hook up a fuel pressure gauge on the schrader valve (top of the engine) and see what the pressure is (if anything) before you crank and while you crank. If there is a vapor lock at the pump you will see no pressure until the fuel cools and condenses back to a liquid.

Horton177 · May 27, 2024

Jimmy Buoy said:
My last boat had 4.3 V6 MPI's which only had the one high pressure pump. Not an issue here in Canada with our shorter/cooler seasons, but apparently vapor lock was an issue in hotter climates like Florida. The solution was to add a low pressure pump like the V8's had (or so I thought).

If your engine serial # is
[HEADING=2]0L002006 THRU 0L674613[/HEADING]
it looks like there is only the one high pressure pump. Not sure why one of your engines would have an issue while the other does not.

When the engine will not start, hook up a fuel pressure gauge on the schrader valve (top of the engine) and see what the pressure is (if anything) before you crank and while you crank. If there is a vapor lock at the pump you will see no pressure until the fuel cools and condenses back to a liquid.

What you said about hooking up the FP gauge when it won’t start was exactly how I figured out my last boat last year had the same vapor lock issue. Was a 5.0 mpi and only had one pump. I had to open the hatch when I got to the beach to beat the issue. However with this boat everyone sets above the hatch so that’s not an option.

Horton177 · May 28, 2024

From a lot of reading the cheapest easiest route is installing the check valve kit form mercruiser. Anyone confirm this fixed there issue on similar engines as mine?

bobeast · May 28, 2024

Horton177 said:
From a lot of reading the cheapest easiest route is installing the check valve kit form mercruiser. Anyone confirm this fixed there issue on similar engines as mine?

The fix for mine was to install the check valve in the water line between the engine and the cool fuel module. Hot water from the engine was syphoning back into the fuel cooler causing vapor lock. This completely solved the issue for me.

Jimmy Buoy · May 28, 2024

bobeast said:
The fix for mine was to install the check valve in the water line between the engine and the cool fuel module. Hot water from the engine was syphoning back into the fuel cooler causing vapor lock. This completely solved the issue for me.

I assume that the hot engine water would only reverse back to the cool fuel AFTER the engine was shut down? While the engine and water pump are running it would be hard to see how water flow would be able to reverse.

Apparently fuel can start to boil at 140 degrees so 160-170 degree hot water bleeding back down from the engine's water hoses may just cause vapor lock right were the fuel pump is located. The pump can't move vapor - just fluid fuel.

Never heard of this fix - makes sense!

bobeast · May 28, 2024

Jimmy Buoy said:
I assume that the hot engine water would only reverse back to the cool fuel AFTER the engine was shut down? While the engine and water pump are running it would be hard to see how water flow would be able to reverse.

Apparently fuel can start to boil at 140 degrees so 160-170 degree hot water bleeding back down from the engine's water hoses may just cause vapor lock right were the fuel pump is located. The pump can't move vapor - just fluid fuel.

Never heard of this fix - makes sense!

Right. Only after shutdown. Heat soak increases the water temp in the block which then drains back down into the fuel coolers having the exact opposite of the intended effect. In my case, I had to wait 6 to 8 hours before I was able to restart the affected engine.

Horton177 · May 28, 2024

bobeast said:
The fix for mine was to install the check valve in the water line between the engine and the cool fuel module. Hot water from the engine was syphoning back into the fuel cooler causing vapor lock. This completely solved the issue for me.

This makes sense to me as well. Bc if I restart my motor within 5 min or so it runs fine. I’ll order the check valve kit today. Thanks!

Oddly enough it doesn't list 7.4 motors for the check valve kit. only small blocks. Hmm

370Dancer · May 28, 2024

7.4 was discontinued before the raw water check valve thing came along. I wound up having to put low pressure fuel pumps inline to "overcome" the vapor lock in Florida and Alabama. I threaten to also put the check valves in, but have been chasing too many other issues for it to bubble to the top of the list.
Hose size from the cool fuel to the oil cooler should be the same. If so, the 5.7 kit should work. Can't tell where you boat, but if in salt water, it will sooner or later fail open or closed, unless you get a gravity ball without the spring.

Horton177 · May 28, 2024

370Dancer said:
7.4 was discontinued before the raw water check valve thing came along. I wound up having to put low pressure fuel pumps inline to "overcome" the vapor lock in Florida and Alabama. I threaten to also put the check valves in, but have been chasing too many other issues for it to bubble to the top of the list.
Hose size from the cool fuel to the oil cooler should be the same. If so, the 5.7 kit should work. Can't tell where you boat, but if in salt water, it will sooner or later fail open or closed, unless you get a gravity ball without the spring.

i'm in oklahoma. No salt here!

did you make your own low pressure pump kit, if so have any details>? or did you buy the outrageous Indmar kit, i'm finding for around 600 but it does look good.

bobeast · May 29, 2024

Horton177 said:
i'm in oklahoma. No salt here!

did you make your own low pressure pump kit, if so have any details>? or did you buy the outrageous Indmar kit, i'm finding for around 600 but it does look good.

FWIW, I tried the LP pump kit from Bakes Marine before I did the check-valve. It seemed to help a little, but didn't solve the issue. The pump attempted to overcome the issue. The valve prevented it from happening in the first place.

370Dancer · May 29, 2024

bobeast said:
FWIW, I tried the LP pump kit from Bakes Marine before I did the check-valve. It seemed to help a little, but didn't solve the issue. The pump attempted to overcome the issue. The valve prevented it from happening in the first place.

What was your source for the check valve?

Horton177 · May 29, 2024

So after reading more I’m thinking it could be from winter fuel since I put 3/4 tank in my boat march 8 on just starboard tank and it’s just that engine.
Is there a fuel additive to help with the rsv values of winter gas?

BillK2632 · May 29, 2024

A trick, for vapor lock - pour some cool freshwater on the fuel lines and fuel pump if you can - this will cool the "vaporized" fuel in the lines and usually get you going. Once the pump can pull even a little cool fuel from the tank, the vapor lock incident is over.
Not much to do about the winter fuel except work the throttle!

Horton177 · May 29, 2024

BillK2632 said:
A trick, for vapor lock - pour some cool freshwater on the fuel lines and fuel pump if you can - this will cool the "vaporized" fuel in the lines and usually get you going. Once the pump can pull even a little cool fuel from the tank, the vapor lock incident is over.
Not much to do about the winter fuel except work the throttle!

Darn! I’m down to half tank, I’ll try to get to 1/4 then fill it up to full to delute it. Hopefully that solves the issue.

BillK2632 · May 29, 2024

Horton177 said:
Darn! I’m down to half tank, I’ll try to get to 1/4 then fill it up to full to delute it. Hopefully that solves the issue.

A dockmate had a 2000 340 with 7.4's - he had vapor lock troubles and used the water trick often. He would use the transom shower to spray down the fuel pump and off he would go. Otherwise it was a great boat.

Horton177 · May 29, 2024

BillK2632 said:
A dockmate had a 2000 340 with 7.4's - he had vapor lock troubles and used the water trick often. He would use the transom shower to spray down the fuel pump and off he would go. Otherwise it was a great boat.

Yeah that doesn’t fly with me. It’s too nice a boat for Everytime I wanna leave making everyone on the boat shift so I can go down and water it down lol I don’t accept those kinda temporary fixes as permanent. Just crazy on such an expensive boat

bobeast · May 29, 2024

370Dancer said:
What was your source for the check valve?

I got it on ebay, but you can just search for Mercruiser 21-862271A-1

Vapor lock?

Horton177

Jimmy Buoy

Horton177

Jimmy Buoy

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Horton177

bobeast

Jimmy Buoy

bobeast

Horton177

370Dancer

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bobeast

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BillK2632

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