454 Mag Fuel Pressure Low
08-05-2019 | 03:25 PM
#1
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From: Maine
454 Mag Fuel Pressure Low
Back again. I was collecting debris in my fuel filters and one of my motors kept dropping RPMs above 4K it would drop 5-700RPM. Got the tank polished and haven’t been seeing anymore debris in filters but motor still continues. Got fuel pressure readings and when motor bogs down it’s going from 32 PSI to 24 and back to 32. I’m being told to check the VST that the screen could be clogged. But I’m also being told that even both motors running 31/32 PSI they’re still running lean. Is this true? Was told 40 is normal. These were carbed 420s turned 454 Mags; light top end work. Early 90s Completely rebuilt. Any help at all would help or ideas.
08-10-2019 | 08:50 AM
#2
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Joined: Jul 2015
Posts: 1,201
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From: Atlantic Southeast
Per the manual OE fuel pressure at idle is 34-36 psi with the vacuum line off. You're vacuum, if within spec will pull the number down as much as 6 psi, so you can be as low as 28 psi and still be within spec. My LX's which have the same pressure specs as your mag's run around 30 vac line on. If you pull the vac line you should see the 34-36 psi range as I do.
Check for fuel pressure line kinks or collapses as well, that's free and easy.
If you check the VST screen and it's clean your next bet would be to check the screens on each injector. If they're clear then the fuel pressure regulator and/or VST fuel pump may be on the fritz. Instead of buying a new fuel pressure regulator I got an add on hat to make it adjustable. If you can't clean the VST screen new one's can be had on ebay MUCH cheaper than Merc. I recommend an entire VST rebuild if you're in there.
Keith
Check for fuel pressure line kinks or collapses as well, that's free and easy.
If you check the VST screen and it's clean your next bet would be to check the screens on each injector. If they're clear then the fuel pressure regulator and/or VST fuel pump may be on the fritz. Instead of buying a new fuel pressure regulator I got an add on hat to make it adjustable. If you can't clean the VST screen new one's can be had on ebay MUCH cheaper than Merc. I recommend an entire VST rebuild if you're in there.
Keith
08-11-2019 | 09:03 AM
#3
Thread Starter
Registered
Joined: Jun 2019
Posts: 23
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From: Maine
Per the manual OE fuel pressure at idle is 34-36 psi with the vacuum line off. You're vacuum, if within spec will pull the number down as much as 6 psi, so you can be as low as 28 psi and still be within spec. My LX's which have the same pressure specs as your mag's run around 30 vac line on. If you pull the vac line you should see the 34-36 psi range as I do.
Check for fuel pressure line kinks or collapses as well, that's free and easy.
If you check the VST screen and it's clean your next bet would be to check the screens on each injector. If they're clear then the fuel pressure regulator and/or VST fuel pump may be on the fritz. Instead of buying a new fuel pressure regulator I got an add on hat to make it adjustable. If you can't clean the VST screen new one's can be had on ebay MUCH cheaper than Merc. I recommend an entire VST rebuild if you're in there.
Keith
Check for fuel pressure line kinks or collapses as well, that's free and easy.
If you check the VST screen and it's clean your next bet would be to check the screens on each injector. If they're clear then the fuel pressure regulator and/or VST fuel pump may be on the fritz. Instead of buying a new fuel pressure regulator I got an add on hat to make it adjustable. If you can't clean the VST screen new one's can be had on ebay MUCH cheaper than Merc. I recommend an entire VST rebuild if you're in there.
Keith
