MerCruiser model 496 MAG (H.O.) 425 HP loss of power

[RubyLily121]

Hi Everyone,

We have a Four Winns 2008 SL262 with a MerCruiser model 496 MAG (H.O.) (Serial #1A350233) 425 HP. We have hand a brand new Fuel Rail and spark plugs installed (thanks to Larry at Raylar Engineering) and we have then Fuel Pump replaced with a brand new Mercury Pump and the stern drive and lots of other bits and pieces. There can’t be much more to swap out (probably could have purchased a new boat for what we have spent so far!

Anyway, we seem to have this intermittent issue that we get in the boat and have a great day out to getting in the boat the following week and can not get the engine to go past 1500rpm so we idle back to the marina… Mechanic goes out to inspect next day and running fine. This has happened a few times now and we are at a loss what could be causing it. If you can imagine it sounding like a blockage and not enough fuel getting to the engine but nothing seems to be blocked. There is no overheating, there is no beeping alerts - nothing just wont pick up speed. It was also a full tank of fuel so was not low on fuel!

Mechanic is doing its annual service next week and will also change the impeller over.

Any ideas would be greatly appreciated as it is really hard to diagnose when the mechanic can’t replicate the trouble.

Ideas welcome and thank you.
Amy

 

[Scott Danforth]

inspect the contents of your fuel filter to ensure you have only clean fuel

check your anti-siphon valve

check your fuel tank pickup

have fuel pressure gauges along for next time it acts up so you can read fuel pressure

have Rinda plugged in to see engine parameters next time

 

[alldodge]

Do you have digital or mechanical throttle control?

 

[stresspoint]

had a semi broken wire to the fuel pump cause a similar fault on a LS chev .

wire was intact but where it went into the connector was only 3or4 strands , probably caused from some lame ass pulling the wires to unplug the fuel pump .

 

[Scott06]

Do you have digital or mechanical throttle control?

YEah was thinking it may not see it is in gear vs neutral

 

[paulswagelock]

Do a cylinder drop test via software when it is acting up. We found 3 partially plugged injectors on a 2006 496 HO that started as an occasional issue like yours then eventually became a full time problem. We chased it for about a month. Eventually did the cylinder drop test and identified 3 bad firing cylinders. Cleaned the injectors and all was good for another 6 years or so until it happened again.

 

[QBhoy]

Was it the high pressure fuel pump that was changed out ?
I’d watch the voltage indication when this occurs too..see if anything is happening there.
When you say it won’t go past 1500 rpm. Is it dead on 1500 and as if it’s being electronically limited at that..or more like lack of fuel and bogging

 

[RubyLily121]

Ok so we took it out today. Started perfect. Drove for about 15 min at 8 kmph till we entered the harbour and then full speed for another 15 minutes. Not an issue then instantly the engine just pulled right back. This time we both notice a flooding smell of fuel. Then shut the boat down. Restarted 10 min later. Drove fine for 10 min then again flooding fuel smell and pulled right back. Drove back to the Marina for 10 min at 1500 rpm as that’s where it seems to drive without it causing the issue. Got to the Marina and it was fine again.

Fuel pump that was installed was the Mercury-Mercruiser 8M0047215 Fuel Pump Module Service Kit Complete.

Fuel Rail was also a 2001-2004 supplied and capped by Raylar Engineering.

injectors were 42 Lb Marine injectors from 5-O motor sports in California

If it was a car I would say it felt like too much juice and it flooded. But struggled to recover.

Mechanic also stumped but will take it out again next week.

 

[RubyLily121]

Do you have digital or mechanical throttle control?

I’m going to say manual it’s a 2008 boat.

 

[RubyLily121]

Was it the high pressure fuel pump that was changed out ?
I’d watch the voltage indication when this occurs too..see if anything is happening there.
When you say it won’t go past 1500 rpm. Is it dead on 1500 and as if it’s being electronically limited at that..or more like lack of fuel and bogging

Fuel pump that was installed was the Mercury-Mercruiser 8M0047215 Fuel Pump Module Service Kit Complete.

All the gauges are normal when it happens. It’s like a flood of fuel then dropped right down with a blocked feeling.

 

[RubyLily121]

Do a cylinder drop test via software when it is acting up. We found 3 partially plugged injectors on a 2006 496 HO that started as an occasional issue like yours then eventually became a full time problem. We chased it for about a month. Eventually did the cylinder drop test and identified 3 bad firing cylinders. Cleaned the injectors and all was good for another 6 years or so until it happened again.

We will have the mechanic do this week again. The injectiors are new (well 6 ish months) with not a lot of use over winter.

 

[RubyLily121]

inspect the contents of your fuel filter to ensure you have only clean fuel

check your anti-siphon valve

check your fuel tank pickup

have fuel pressure gauges along for next time it acts up so you can read fuel pressure

have Rinda plugged in to see engine parameters next time

Yep mechanic will try again this week.

 

[tpenfield]

have Rinda plugged in to see engine parameters next time

This ^^^^ Hopefully, your mechanic has the computer (CDS or Rinda) so the engine data can be viewed/recorded when the loss of power actually happens.

All too often 'we' see/hear about a plethora of parts being thrown at fuel injected engines to no avail. More IACs, ignition, and fuel system components seem to get sold that way

 

[RubyLily121]

This ^^^^ Hopefully, your mechanic has the computer (CDS or Rinda) so the engine data can be viewed/recorded when the loss of power actually happens.

All too often 'we' see/hear about a plethora of parts being thrown at fuel injected engines to no avail. More IACs, ignition, and fuel system components seem to get sold that way

Thanks pretty sure he has the CDS dongle - left in our boat once. But agree when we are not handy ourselves we are reliant on the mechanic telling us what we need.

 

[alldodge]

I’m going to say manual it’s a 2008 boat.

Your serial number shows it can have digital or mechanical controls, even in 2008

injectors were 42 Lb Marine injectors from 5-O motor sports in California

If the issue is flooding then might be the injectors. Been many places stating they "rebuild" injectors which cannot be done. Not saying this is the issue but needs to be checked.

Gen III module should be 39 PSI at idle and 43 PSI at WOT. Test pump pressure high level by removing vacuum line off regulator.

There is a 10 micron screen below basket.
Fuel rail regulator part number 8M0046753

 

[dubs283]

Thanks pretty sure he has the CDS dongle - left in our boat once. But agree when we are not handy ourselves we are reliant on the mechanic telling us what we need.

Could be time for a new mechanic seeing as how you are seeking advice online.

Based on symptoms and work performed I'd start looking at ignition. 496 is notorious for coil and ignition driver failures

 

[RubyLily121]

Your serial number shows it can have digital or mechanical controls, even in 2008


If the issue is flooding then might be the injectors. Been many places stating they "rebuild" injectors which cannot be done. Not saying this is the issue but needs to be checked.

Gen III module should be 39 PSI at idle and 43 PSI at WOT. Test pump pressure high level by removing vacuum line off regulator.

There is a 10 micron screen below basket.
Fuel rail regulator part number 8M0046753

Confirmed that the Throttle is mechanical and not digital.

Injectors were brand new and cost a lot of money each - pretty sure they were not rebuilt.

 

[RubyLily121]

Could be time for a new mechanic seeing as how you are seeking advice online.

Based on symptoms and work performed I'd start looking at ignition. 496 is notorious for coil and ignition driver failures

Oh and I forgot to mention it has a brand new starter motor just installed.

 

[alldodge]

For us to help we need more info and best method is with a scanner. Without a scanner do some checks like measure the fuel pressure at the rail and see if its correct. Turn key ON and see, then turn OFF to see if it holds.