So i've had a surging issue on my 97 330 SS with 7.4L MPI motors. left motor idles fine, but when it gets to 2500 or 3000 RPMs it surges, dying occassionally. i've had our mechanic out troubleshooting, and we were getting low pressure. my initial thoughts were check and replace lines, check and replace filter, since thats the cheap stuff.

He came out, pressure tested, and we were showing like 30-32 so it had low fuel pressure. i'm tryin to understand how this fuel system works based on the merc service manual. Mechanic says there is a low pressure pump, located on the lower left of the motor, and a high pressure pump, located on the back right of the motor. is this correct?

After reading, it looks like there is a pump located in the VST assembly on the low left, so i'm assuming that's low pressure? when he came out the first time, he replaced a pump 818383T, for $270 or so in parts, plus labor. I got a call this morning, saying he ran the boat out and it was still surging, so he thinks its the high press pump, and wants to replace it with #808505 for a cool $700.

A quick google returns 818383T looking like a mechanical pump, not the electronic pump located in the VST assembly like i thought. so, can someone explain how this works, and what i'm missing??

IMO If there is only 30lbs pressure at the rail it would not be the lift pump on the VST assembly, the high pressure pump provides the fuel to the fuel rail where the injectors are and the pressure should in the 42-45 range ( I think ) eventually he will find the problem ( at your expense ) if he changes out enough parts. he sounds like a parts swapper instead of a trouble shooter, good luck

I agree F2, and normally he's a trouble shooter. I thought he tested the pressure before high pressure pump, after the low pressure pump, to isolate the LPP as the issue.

so can you explain how this all works to me? the lift pump pushes fuel to the high pressure pump on the back right of the motor assembly? is there an electronic pump in the VST assembly like i think there is? so mech pump pushes fuel to VST pump, which pushes fuel to the High pressure pump?

Found this on another post, it may be the same pump...
Edit: others on here are more familiar with vst than I am

The low pressure mechanical pump fills a "bowl" the high pressure pump sits in. The high pressure pump then sends the fuel from the bowl to the rails/injectors.

Electric pumps like to push, mechanical pumps like to pull.

The price for the high pressure pumps is in the 600-700 dollar range...there is a cheaper alternative and the one Speedy mentioned is it.. There is also a micron filter on the bottom of the high pressure pump that can get plugged up. Also be very carefull with the oring on the vst pump, that sucker is not cheap either.

yep, $712 to be exact. unfortunately our mechanic already ordered that setup....so i guess that's what i'll be paying. so the high press pump is right next to the low press pump, down on the front of the motor. nothing on the back right, like I was told.

You need to get a basic understanding and look at system diagrams on line. the low pressure pump (depending on gen) is mounted on the sea water pump of conventional bbc location. The vst tank is on the top back of motor. and if your mechanic hasnt installed or recieved the pump yet, you can still save yourself 600... (or your mechanic ordered the hundred dollar pump and told you it was the 700 dollar pump)

top front left is the VST

Hate it, but its typical
Call mechanic about a problem, he replaces a good low pressure pump and it doesn't fix the problem. There is no mention about putting the old pump back on, just another part to pay for. Also with a HP pressure being low, why was the automatic go for the low pump

Now if is getting a HP pump and if that doesn't fix the problem, he will order something else.

If it doesn't bring the pressure up it just might be the regulator (item 15)
so far I haven't heard about much troubleshooting from the Mech
https://www.mercruiserparts.com/bam/.../31810/2336/10