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500EFI loosing power, then Popping /Backfire above 2500rpm

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500EFI loosing power, then Popping /Backfire above 2500rpm

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Old 08-19-2019, 06:47 AM
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Thumbs down 500EFI loosing power, then Popping /Backfire above 2500rpm

Hello all, New to the forum so forgive me if this is an old topic...I've got a pair of 500EFI in my new to me 400ss, Port motor is fine, Starboard motor starts to labour and loose power after about 15min..followed by intermittent pop/backfire . It will run perfect with tons of power and no issues for about 15min. Then this all starts. Timing is bang on the money set to 8 Degrees in base timing mode, Timing advances to 28 degrees at 3500RPM when out of base timing mode without engine load. I switched Ignition coil motor for motor and no change. Brand new plugs in both motors, Cap and wires look perfect but may change just for piece of mind. Filters changed right before. I know the fuel is a little stale but if that was the issue, it would perform perfectly for 15min...(I wouldn't think)
Any advise would be greatly appreciated!...Mechanical fuel pumps are the next order of change (Gonna try to run a pressure test this weekend and see)
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Old 08-19-2019, 07:30 AM
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Have you done a leakdown and compression test, pulled valve covers to inspect valve train, how many hours on the engines? Changed the spin on and in line fuel filters on each engine? 500EFI are known to have seeping head gaskets and fuel injectors that need serviced/replaced.
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Old 08-19-2019, 09:58 AM
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A TechMate scan tool is a must when working on these. Since you're able to get into base timing mode, I would assume you have one (unless you're using a paperclip to jumper pins to get into base timing). Do you have any historical codes stored? That would be the first place I'd look. I'd also watch the tool while running to see if any values go funky when the issue occurs. That's where I'd start.
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Old 08-19-2019, 08:38 PM
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VoodooRob, I haven't done a leak down or compression test yet, I haven't pulled valve covers. Boat has 592hrs...Motors have 192 since valve train and light overhaul performed...They sound real nice and tight. Yes spin on changed but not inline filters.. I just find it weird that boat works great for 15min then this all starts. I am going to Run a fuel pressure gauge hopefully on the weekend and see what I am producing...
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Old 08-19-2019, 08:41 PM
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Originally Posted by TomZ View Post
A TechMate scan tool is a must when working on these. Since you're able to get into base timing mode, I would assume you have one (unless you're using a paperclip to jumper pins to get into base timing). Do you have any historical codes stored? That would be the first place I'd look. I'd also watch the tool while running to see if any values go funky when the issue occurs. That's where I'd start.
Yes I will get a scan tool hooked on it this week...I used the paper clip trick for base timing.. I've read mechanical fuel pumps may be a contributing factor ...as I am not sure the past history of them or anything really for that matter...Thanks in advance!
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Old 08-19-2019, 10:32 PM
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Definitely want to measure fuel press. Running lean can destroy an eng quickly.

This may be a long shot, but are the fuel tanks separate for each eng? If so, A clogged tank vent can cause the symptoms you described but usually takes more than 15 mins. Easy way to test is to remove the fuel cap for starboard side and re-install, then see if it runs good again for ~15 mins.

Common failure mode for a ign module is: works great cold, but not so good when hot. A lot like a coil but you covered that already. You could try swapping distributors.
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Old 08-19-2019, 11:13 PM
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first thing i would do is hook up a fuel pressure gauge and monitor it while running it,i will bet a dollar to a donut it,s a pump going south.
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Old 08-19-2019, 11:21 PM
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That would be my best guess as well. If memory serves me correctly, it should have aprox 39# at idle and pressure should rise slightly with rpm.
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Old 08-20-2019, 06:15 AM
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Originally Posted by zz28zz View Post
Definitely want to measure fuel press. Running lean can destroy an eng quickly.

This may be a long shot, but are the fuel tanks separate for each eng? If so, A clogged tank vent can cause the symptoms you described but usually takes more than 15 mins. Easy way to test is to remove the fuel cap for starboard side and re-install, then see if it runs good again for ~15 mins.

Common failure mode for a ign module is: works great cold, but not so good when hot. A lot like a coil but you covered that already. You could try swapping distributors.
yes separate tanks..I will do that as well..good point..Thanks!

Last edited by Audio Guy; 08-20-2019 at 06:22 AM.
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Old 08-20-2019, 06:18 AM
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Originally Posted by mike tkach View Post
first thing i would do is hook up a fuel pressure gauge and monitor it while running it,i will bet a dollar to a donut it,s a pump going south.
Thats what I kinda thought...It makes sense as it really seams like a lean issue. I can bounce the rev limiter easy with no load...but when put under load , a quick snap of the throttle while watching the tach will jump up and fall back off just as fast
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