'96 252 454 Magnum performance benchmark

[akorcovelos]

Remind me to never buy a boat with that engine set up. Seems like merc was just trying stuff to see if they could get it to work. I'm sure for a year or 2 they were fine but what a mess.

Yeah, the VST system wasn't around long, maybe 2 years, not a great design. When it's working it's okay, but seems to be the only real common failure point on these engines. Pretty sure I could delete it and add cool fuel but mine was rebuilt right before I bought the boat, well...obviously they just rebuilt the actual VST unit and didn't replace all the lines or they would have seen these issues. Not surprised, it was a trade in at a Chap dealer so they weren't going to tear the top of the engine off. Plus, other than cold start it ran great so they knew it would sell.

[Big Block Power]

Remind me to never buy a boat with that engine set up. Seems like merc was just trying stuff to see if they could get it to work. I'm sure for a year or 2 they were fine but what a mess.

Yeah, the VST system wasn't around long, maybe 2 years, not a great design. When it's working it's okay, but seems to be the only real common failure point on these engines. Pretty sure I could delete it and add cool fuel but mine was rebuilt right before I bought the boat, well...obviously they just rebuilt the actual VST unit and didn't replace all the lines or they would have seen these issues. Not surprised, it was a trade in at a Chap dealer so they weren't going to tear the top of the engine off. Plus, other than cold start it ran great so they knew it would sell.

Right. Didn't the 502 use the same set up?

[akorcovelos]

Remind me to never buy a boat with that engine set up. Seems like merc was just trying stuff to see if they could get it to work. I'm sure for a year or 2 they were fine but what a mess.

Yeah, the VST system wasn't around long, maybe 2 years, not a great design. When it's working it's okay, but seems to be the only real common failure point on these engines. Pretty sure I could delete it and add cool fuel but mine was rebuilt right before I bought the boat, well...obviously they just rebuilt the actual VST unit and didn't replace all the lines or they would have seen these issues. Not surprised, it was a trade in at a Chap dealer so they weren't going to tear the top of the engine off. Plus, other than cold start it ran great so they knew it would sell.

Right. Didn't the 502 use the same set up?

yep, sure did.

[AsLan7]

Yeah, the VST system wasn't around long, maybe 2 years, not a great design. When it's working it's okay, but seems to be the only real common failure point on these engines. Pretty sure I could delete it and add cool fuel but mine was rebuilt right before I bought the boat, well...obviously they just rebuilt the actual VST unit and didn't replace all the lines or they would have seen these issues. Not surprised, it was a trade in at a Chap dealer so they weren't going to tear the top of the engine off. Plus, other than cold start it ran great so they knew it would sell.

Right. Didn't the 502 use the same set up?

yep, sure did.

.
Trying to remember the year the 454/502’s went to a roller cam vs a flat tappet guide. 96?
That was an issue for awhile.

.

[akorcovelos]

Right. Didn't the 502 use the same set up?

yep, sure did.

.
Trying to remember the year the 454/502’s went to a roller cam vs a flat tappet guide. 96?
That was an issue for awhile.

.

Yeah, '96 was a split year production as they changed to roller. I keep meaning to pull the serial number off the block to see which mine is.

[Big Block Power]

yep, sure did.

.
Trying to remember the year the 454/502’s went to a roller cam vs a flat tappet guide. 96?
That was an issue for awhile.

.

Yeah, '96 was a split year production as they changed to roller. I keep meaning to pull the serial number off the block to see which mine is.

While your at it let's pull the intake to look.

[akorcovelos]

.
Trying to remember the year the 454/502’s went to a roller cam vs a flat tappet guide. 96?
That was an issue for awhile.

.

Yeah, '96 was a split year production as they changed to roller. I keep meaning to pull the serial number off the block to see which mine is.

While your at it let's pull the intake to look.

Hard pass on that, lol. I just need to look back by the distributor for the casting numbers when I’m back in there next time. I keep forgetting to snap xlint a pic of my blower tubes too...

[xlint89]

I don't think that's a vacuum fitting on your fuel pump. On my 7.4L with the similar set up, that's an indicator if your fuel pump diaphragm ruptures. That transparent hose would have fuel in it. It should be going to someplace on the engine that will introduce it to the air fuel mixture. Coast Guard doesn't want raw fuel dripping in to the bilge. So that's a "catch can" if you will for a fuel pump failure.

Also, does your set up have an inspection plug on it? There's gear lube inside the raw water sea pump to lubricate the fuel pump lever and cam it rides on. Often times if the lube is watered down, a seal has failed and fuel is diluting the gear lube.

[xlint89]

I can see the inspection plug in your pics. Directly below the fuel pump. It's the round plug with the flat slot cut into it. Open it up and check the lube if you haven't.

VST simple fix I believe was to just run a return line back to the fuel tank instead of going back to the water seperator or something like that. If you ever have the vapor lock problem

I've seen someone sells a "T" that installs into the fuel tank vent line for returning fuel back to the tank for that very purpose.

[akorcovelos]

I can see the inspection plug in your pics. Directly below the fuel pump. It's the round plug with the flat slot cut into it. Open it up and check the lube if you haven't.

VST simple fix I believe was to just run a return line back to the fuel tank instead of going back to the water seperator or something like that. If you ever have the vapor lock problem

I've seen someone sells a "T" that installs into the fuel tank vent line for returning fuel back to the tank for that very purpose.

That hose barb on the fuel pump is plumbed into the lines that go to the throttle body and VST. It’s definitely under vacuum because when I broke through that crud plug I could hear the hissing of air being sucked into the pump.

Good call on the pump oil, I’ll crack it open and take a look.

[xlint89]

Interesting. Everything I've ever seen about that translucent hose is to show if fuel is in it. If it is, your mechanical fuel pump diaphragm has crapped out. I wonder why you'd have vacuum going to it?

[akorcovelos]

I can see the inspection plug in your pics. Directly below the fuel pump. It's the round plug with the flat slot cut into it. Open it up and check the lube if you haven't.

VST simple fix I believe was to just run a return line back to the fuel tank instead of going back to the water seperator or something like that. If you ever have the vapor lock problem

I've seen someone sells a "T" that installs into the fuel tank vent line for returning fuel back to the tank for that very purpose.

Good call on checking the pump oil!

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[Big Block Power]

Better than the camshaft in the engine. Buy a new pump I guess and back in action.
This is a great 7.4L thread
Cool to see you 2 getting this going so good.

[akorcovelos]

Interesting. Everything I've ever seen about that translucent hose is to show if fuel is in it. If it is, your mechanical fuel pump diaphragm has crapped out. I wonder why you'd have vacuum going to it?

You are correct, but with the nipple clogged it must have just been creating vacuum on the back side of the pump diaphragm. Maybe that’s what ruptured the diaphragm seal to begin with? But yes, after some research I saw the marine mechanical fuel pumps require the vapor release port run to the throttle body, and in my case the VST too.

More pics! I had to do an autopsy on the pump.

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[akorcovelos]

Better than the camshaft in the engine. Buy a new pump I guess and back in action.

New sea pump and fuel pump...B.O.A.T...

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