No power under load

Many thanks for that information. I will check those electrical connections for corrosion and use a multimeter to see if there is any significant resistance. One battery reads 11.6V but the other reads 12.6V and I normally run with the switch at the '1+2' position, so I am not sure how much voltage the pump would actually get in normal running - maybe it would only be the lower of the two, i.e. 11.6V?

I suspect that the fuel filter would only show clean fuel since I have twice changed it since this problem arose. At first the engine would not rev over about 2000 rpm, but after I changed the filter and pumped the tank to filter out the rubbish it would then rev to about 3000 rpm. So I changed the filter again hoping that it would rev fully, but there seems to have been no further improvement. Still, maybe I could check the contents of the filter again.

A battery the reads 11.6V battery is pretty much dead, and is probably damaged. When you start seeing anything below 12V you need to change the battery and if it won't charge up, it needs replaced

Thanks for that, AllDodge. But I rechecked the port battery again and it reads 12.6V, the starboard battery reads 12.8V.

I finally decided to ring the mechanic who did the work previously and he said that the first thing he would suspect is the high pressure pump. He asked me how long it was between when it worked fine and when I used it again and had trouble. I said about 8 months. He said that it would all have gummed up again. When he did the work previously he took apart the VST before and cleaned it out and he said he would have replaced the high pressure pump because it was not working, but could not get a replacement pump in Australia. So he soaked the old pump in Johnson Engine Tuner to free it up to get it to work. This is because the boat had sat for years before I bought it.

So, that is what he reckons. Which I think is what you thought too. I am thinking maybe I should buy that new pump from CPPerformance and soak the old one again and keep it as a spare.

He also said that I should run some fuel cleaner through the system to get rid of any build up in gunk and varnish.

Probably is the pump, but only question would be what caused the pump to start building pressure when wired directly. Maybe just a one time thing

In any case, I'm of the opinion that fuel cleaner of any kind is snake oil. If fuel cleaner is needed, then something in the fuel systems needs to be removed and cleaned. Old fuel can gum things up, but if it everything was removed and cleaned (looks like it was) then there should be no more gunk past the filter.

I did spend a lot of time trying to pump out the tank using a 12V pump with an inline filter to remove rubbish from the tank when the problem first arose. So although the mechanic replaced a lot of parts affected by the bad fuel, there was still rubbish in the tank, so it is possible that this has caused the problem, because it was one part of the fuel system that was never attended to, except of course that I did try to get rid of the old fuel, but in my first attempt I did not pump to the very bottom of the tank. I did in the second attempt, but by then the pump might have already got fouled, though how it might of got past the filter I would not know.

Also, I wonder whether the pump was really sound. I did not know that that was what happened - the mechanic simply could not get a replacement pump here and so he tried to rehabilitate the old one, without perhaps being completely successful. I am sure that if the work had been done in the US a new pump would have been installed.

Anyway, he had no hesitation in saying that he thought it would be the high pressure pump. He obviously knew where the weak point was in the boat. He had seen the problem with it before when I first gave it to him to work on.

That pump at cpperformance that you gave the link to is the correct part for the boat? I would buy yours if I could think of an efficient way of paying you.

Are these pumps on eBay (much cheaper than cpperformance) no good or wrong part number?

www.ebay.com/itm/NEW-MerCruiser-EFI...ash=item3cf345c1d3:g:NYwAAOSwal5YHjk2&vxp=mtr

They are two different pumps, the one on ebay is for a cool fuel II version. If you look at both links and the pics will show they are different. Search on "Mercruiser 808505T01" and other places will show up. There is a screen basket that attaches to the bottom of the pump for the VST.

Thanks for that. The part comes up on offshoremarineparts.com for $199.96 with the screen basket:
www.offshoremarineparts.com/62-14-890.html

It is not an OEM part but they claim that they have had no problems with it, so much so that they have ceased stocking the genuine mercruiser part. I am thinking that I should get the vst gasket as well - they have the gasket set listed for $76.95. I think that the mechanic just used gasket gue for the vst gasket, no doubt because he could not get the genuine part.

This part also seems to come up here as an OEM part(?) for $279 on eBay, but apparently for a Mercury outboard: www.ebay.com/itm/NEW-MERCURY-MARINE-FUEL-PUMP-LX-DX-VX-809088T-1-808505T01-99-UP-/200635077999. Wonder if it is the same pump?

Obviously I should take the VST apart. Be good to check if perhaps any of the filters in it are clogged.

Are there any good videos on how to take it apart and reassemble it that you know of? I cannot seem to find one on YouTube. Perhaps it is pretty straightforward. I saw a video for an outboard VST and thought it might be useful: www.youtube.com/watch?v=eVdJ4BvRVYQ, but have not found anything for inboards.

The pumps should work, and the disassemble is straight forward. There is a long screw at the bottom, think its a 10mm or 3/8 that holds the pump. Screws on the top are just removed and top lifted off.

Thanks for that. I have ordered the parts from offshoremarineparts. Will take a week or two to arrive. Meanwhile I will take apart the VST and see what I find.

Parts arrived and I took the VST apart. The filter at the bottom of the pump was significantly blocked. I would say only about 60% of fuel would have been getting through. So seems quite possible that the pump may be all right and that the filter was causing the trouble. Anyway, I replaced both pump and filter.

Just wondering whether I should put some fuel in the VST before putting it back together and connecting up the lines, so that when the engine is first turned over the high pressure pump is not acting on air but has some fuel to pump? Also how do I get rid of what air may be in the fuel rail when I connect things back together?

Some pictures of the new and old filter below:

You don't have to fill the VST tank, but if you have some fuel available it wouldn't hurt. There is a Schrader valve on the front of the fuel rail (underneath the plenum) that will purge any air.

Been out for a couple days. Hope you got her up an going. Filling doesn't hurt, and as 31PC mentioned, use the valve on the fuel rail to bleed the air if she has trouble firing up

Many thanks for that advice. Will let you know how we get on when we get a chance to water test the boat. But I am confident that we have finally got to the bottom of the problem.

Just took the boat out today. First opportunity to test my work on the VST. Been very busy with work and other things. Boat performed perfectly - no problems with the engine at all. It revved freely. Took a long time but I am very thankful to everyone who helped me get to the bottom of the problem. I learnt a lot. Great to have the boat running again.

Many thanks especially to you, AllDodge, for your very good and patient advice.

Glad you got her going Larry, and thanks for updating the thread.