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454 Mag Gen VI Hesitation 1500-2000rpm

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454 Mag Gen VI Hesitation 1500-2000rpm

Old 06-23-2020, 07:40 PM
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Default 454 Mag Gen VI Hesitation 1500-2000rpm

I have a Formula 252 that I am restoring with a 454 Mag EFI motor. I finally got it out on the water. Hasn't been run on the motor in years. Motor idles smoothly seems to do fine with mild acceleration. If I push it as if pulling a skier out with full throttle from idle or low rpm it bogs and sounds like its back firing through exhaust. Happened 4 times in a row so not a fluke. This does not happen above 2200 rpm and up. Cruises just fine. At sea level it does not go past 4100rpm at full throttle (Bravo III with 22P props) but it feels well into 50's (boat is very light since I removed entire interior and only had 1/4 tank of gas). I will take GPS out next time to confirm. Motor does not make any weird sounds at idle or under smooth acceleration. It does not like full throttle from start or low rpm (1500ish).

We put a fuel pressure gauge on. At idle its 40ish psi and as rpms rise goes to 70psi but is somewhat unsteady. No codes from computer.

Prior to taking it out. I did have bad fuel in the tank and removed it. Flushed with good gas and then put in 91 Octane and Octane boost. Fuel Water Separator was replaced and had old nasty fuel in it. The fuel pump is "supposedly" newer but I need to confirm.

The motor was only run for maybe 20 minutes total on the water for testing so not sure if fuel has made it trough or note.

Work that was already done. New Plugs, Wires, Dist Cap, Impeller, Fuel Separator, Oil Change, Full drive service including bellows,shift cable, etc. Starter was replaced and was very corroded when removed. Had lots of corrosion at motor mounts that was cleaned up. Compression was checked. 160 across all cylinders. 140 at far rear port cylinder. Timing was checked and seemed good.

Any ideas on what to trouble shoot? Fuel pump? Injector cleaning?

Last edited by Lafuerza; 06-23-2020 at 08:15 PM.
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Old 06-23-2020, 09:09 PM
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70 psi way too high. Could be regulator or screen clogged after/at regulator, if it has one.

Raising 30 psi beyond idle is odd though, but that’s where i’d head.

If there is junk in that screen I’d be pulling my injectors and replacing there screens along with getting injectors checked and cleaned.
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Gimme Fuel (06-24-2020)
Old 06-24-2020, 07:26 AM
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Originally Posted by Lafuerza View Post
I have a Formula 252 that I am restoring with a 454 Mag EFI motor. I finally got it out on the water. Hasn't been run on the motor in years. Motor idles smoothly seems to do fine with mild acceleration. If I push it as if pulling a skier out with full throttle from idle or low rpm it bogs and sounds like its back firing through exhaust. Happened 4 times in a row so not a fluke. This does not happen above 2200 rpm and up. Cruises just fine. At sea level it does not go past 4100rpm at full throttle (Bravo III with 22P props) but it feels well into 50's (boat is very light since I removed entire interior and only had 1/4 tank of gas). I will take GPS out next time to confirm. Motor does not make any weird sounds at idle or under smooth acceleration. It does not like full throttle from start or low rpm (1500ish).

We put a fuel pressure gauge on. At idle its 40ish psi and as rpms rise goes to 70psi but is somewhat unsteady. No codes from computer.

Prior to taking it out. I did have bad fuel in the tank and removed it. Flushed with good gas and then put in 91 Octane and Octane boost. Fuel Water Separator was replaced and had old nasty fuel in it. The fuel pump is "supposedly" newer but I need to confirm.

The motor was only run for maybe 20 minutes total on the water for testing so not sure if fuel has made it trough or note.

Work that was already done. New Plugs, Wires, Dist Cap, Impeller, Fuel Separator, Oil Change, Full drive service including bellows,shift cable, etc. Starter was replaced and was very corroded when removed. Had lots of corrosion at motor mounts that was cleaned up. Compression was checked. 160 across all cylinders. 140 at far rear port cylinder. Timing was checked and seemed good.

Any ideas on what to trouble shoot? Fuel pump? Injector cleaning?
was the fuel pressure checked at the fuel rail?
Take off fuel water seperator filters and dump out. Pour in techron concentrate put back on and run
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Old 06-24-2020, 07:34 AM
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Agree with SB, the screen on the regulator or the regulator it self
Being a Gen VI (serial numbers are better) fuel pressure should be 43 psi running.
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Old 06-24-2020, 10:57 AM
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Anytime these motors are down on power like you describe and stutter I'd lean towards a fuel issue. Specifically as SB stated, get the injectors cleaned and flow tested. I can almost guarantee that is your problem.
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Old 06-25-2020, 07:22 AM
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Default Try this 1st

Take off fuel water seperator filters and dump out. Pour in techron concentrate put back on and run for 10mins
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Old 06-25-2020, 09:16 AM
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Motor serial number.

Does anyone have a link to a good DIY tutorial to access the fuel regulator and injectors? Not familiar with these motors. I read the manual and tried finding write ups but lots of them are piece meal.

I have a place near me that cleans injectors. Once I get them out I'll have the them refurbished.

Fuel separator is new but will try techron approach.
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Old 06-25-2020, 09:23 AM
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Originally Posted by Lafuerza View Post

Motor serial number.

Does anyone have a link to a good DIY tutorial to access the fuel regulator and injectors? Not familiar with these motors. I read the manual and tried finding write ups but lots of them are piece meal.

I have a place near me that cleans injectors. Once I get them out I'll have the them refurbished.

Fuel separator is new but will try techron approach.
the techron will clean the injectors.
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Old 06-25-2020, 09:42 AM
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Unless the place near you indicates marine I would look further. Most those places do auto and you need one which can flow to to 6000 rpm.

Yes 43 psi fuel pressure
MEFI 3

Everyone is always looking for a mechanic in a can

The fuel reg is on top of the fuel cooler item 15 and is held on by 2 screws. The screen item 16 is under it
https://www.mercruiserparts.com/bam/...31812/4854/100

The problem is cool fuel is behind and under the port side motor mount. The plastic cover has press in latches on top and bottom in the middle. Cover removed there are 2 nuts item 14 which hold it in place. With plate removed cool fuel can be removed just a little. This will give you enough room to remove the screws holding the Reg. Can remove it further if the gas line feeding is removed from the pump

That said, this is saying you can get to the cool fuel module.

As for the intake, remove throttle linkage and the 10 or so torx bolts holding the intake. Lift slightly and one by one disconnect the plugs and vacuum line connected. Now have access to the fuel rail which held in place by bolts.

https://www.mercruiserparts.com/bam/...31812/4854/110

Last edited by AllDodge; 06-25-2020 at 09:44 AM.
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Old 06-26-2020, 11:34 AM
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Originally Posted by Lafuerza View Post

Motor serial number.

Does anyone have a link to a good DIY tutorial to access the fuel regulator and injectors? Not familiar with these motors. I read the manual and tried finding write ups but lots of them are piece meal.

I have a place near me that cleans injectors. Once I get them out I'll have the them refurbished.

Fuel separator is new but will try techron approach.
i would defanatly try techron before removing the injectors.
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