Why did you reroute vent line to the tank? Didn't you loose a vacuum doing that. I have different VST top. It has one more hose right on top.

thx for those pics. I will try that once I figure out my VST spring position. I don't wanna do all at once because one of those solutions may suffice.

92nsx's is the older style so it is different. I'm not sure how or why it works, but hey if it works go for it. I can't remember the exact details of what side of the diaphram the spring goes on but will grab my manual and scan the page that shows it. From memory""With the diaphram off, If you blow air through that vacuum port on the side, I think it actually goes through that Diaphram housing to the bottom side of the diphram, thus pulling the diaphram down to open the vent port once the motor starts. Alhough if your top clear/yellow hose connection/canal was blocked as you stated, you probably found the main problem.
Check to make sure you have vacuum on the side hose, but I think you can reassemble it with the spring on the bottom and be ok, now that you have the blockage clear. I will update after I look at the overhaul section in the manual.

We'll not sure if you have or not, but my 97 fastec with 502 mpi was doing exactly same thing, pull the mechanical fuel pump out of the housing and see if there is gas in there. Mine has a tube coming off the pump to let you know if the diaphram is bad but my was plugged with deteriorated rubber and didn't show any signs of being bad but it would not keep vst tank full of fuel. It's easy to do and a heck of a lot cheaper than re doing fuel system or throwing parts at it.

Yep I have the older style, 1994 GenV 502.

If It helps out here are a couple photos of the vent and inside of the VST from the last time I had it open. Spring was on the side with the black plastic nob on the diagram .

http://i212.photobucket.com/albums/c...X/IMAG0263.jpg

http://i212.photobucket.com/albums/c...X/IMAG0264.jpg

http://i212.photobucket.com/albums/c...X/IMAG0266.jpg

http://i212.photobucket.com/albums/c...X/IMAG0270.jpg

Here is the overhaul diapgram.

http://s1173.photobucket.com/user/ro...71495099570234

So I took my boat out this weekend and it vapor locked again then I just unscrewed the valve to release the fume pressure like 28Eliminator advised. The engine started right away. Now I just have to open the VST again and switch the spring above the diaphram. Problem solved. Thanks a lot everyone.
Now I can finally enjoy boating 100%

Good job Capt. Yar, glad you pulled the plug and it started right up. IMO It totally proves that the VST was vapor locked.
To bad I couldn't get you that diagram earlier, but it clearly shows what to do and where to do it. Please let us know how it works out after you change the spring location.:cheer:
Now have a happy boating season!

Spring was changed. Boat tested. All works fine. Thank you again

Vapor Lock sucks! Boat always starts/runs perfect cold. Had several "vapor lock" issues when boat was hot and shutdown for a hour or two. I ran return lines from the fuel rails to the main fuel tanks and plugged off the return inlets on the VST's. Now hot starts where better but still required some throttle to fire fire up. Now once running no more stalling. But I would love to just turn the key and have both engines just fire up and be ready to go.

Although I haven't checked fuel pressures, both mechanical fuel pumps are brand new (less than 50 hrs). Water pumps where disassembled and rebuilt. Cams where good. Fuel supply lines appear to be in good shape. New fuel water/fuel separator filters- twice. When I opened up the VST's the fuel screens on the pumps where clear.

So...I have read and re-read this entire thread several times and I still have questions. Looking at the pictures in post 17 from Dave_N and then the post 21 from 28Eliminator they seem to agree. Old Style VST (only one vent line going to intake) shows spring needs to be on top. New style VST (vent line going to intake and sight line going to throttle body) spring needs to be on bottom. But then in post 85 from 28Eliminator actually shows the opposite.

I have the old style VST's. My understanding is that with the engines running the vacuum from the intake over comes the spring pressure and closes the diaphragm inside the tank. This prevents raw fuel from being drawn into the intake while the engines are running. With the engines off the spring holds the diaphragm open to prevent pressure build up inside the tank from heat soak.


So I just took both VST's apart and can see that someone has been in there before. Both of my springs where on the bottom of the diaphragms. So my thought is to move the springs to the top and all my problems are solved-right?? Maybe?? Please???

Seemed to work for Cpt Yar so worth a try.

Or

Remove VSTs and install a high pressure pump and be done with it.