Mercruiser 8.1L 'popping' above 3400rpms

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copb8

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34005
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I have an 05 8.1L Mercruiser inboard that starts to 'pop' under load or above 3400rpms. So far I've replaced the plugs, wires and fuel injectors. All removed parts looked to be in good condition.

Fuel is shared with a second engine so I've eliminated that as an issue. Also fuel filters are new.

I was considering whether the ignition coils could be an issue. There are 8 of them and each are around $90 so I really don't want to just change them out to see.

Do you think the coils could cause this issue? The boat idles and runs up to 3400 smoothly.
 
Distributorless engine. I would scan both engines with Marine Diacom to simultaneously compare parameter readings, and also check fuel rail pressure above 3400 rpm.

What happens when you exceed 3400 rpm? When "popping", will it refuse to gain rpm but resume smooth running as soon as you reduce throttle to just below 3400 under load, ...or do you have to reduce throttle to ~~ idle for a bit to get going again?

If you suspect just 1 cylinder is the popping source , the scan device will allow you to command a power balance test , dropping either spark or injection at each cylinder sequentially so you can check for one or more which do NOT cause very roughly a 12% rpm drop.

I couldn't say whether a coil could be failing but you could measure & compare coil resistance values.

FWIW- My 1st suspect would be insufficient fuel pressure at 3400rpm demand rate .Any loose or corroded connections at the LP & HP pumps or either pump very hot at 3400 ? Plenty of RW flow at the fuel cooler & exhaust risers not too warm to comfortably keep your hand on indefinitely?

Is there any abnormal steam or smoke or odor at the exhaust? Coolant and oil level steady?

Hopefully this is not related to the infamous 8.1 water inversion issue.
 
other things can be a (depending where you hear the backfire) a intake or exaust valve, or broken/weak valve spring(s)
 
Bt Doctur- Certainly a weak spring could cause that, but wouldn't a burnt, "tuliped" or bent valve be more likely to allow backfire at all rpm?
 
A tulipped or bent valve would give a rough run at all speeds and would probably bend or break a pushrod, as well. I’ve had both experiences on the Mercruiser 7.4 engines on our boat. It’s more likely a weak valve spring or an electrical problem. Rather than guess and waste money simply replacing parts, hire a marine technician run a diagnostic using a marine diagnostic computer which can test all the systems. Hy
 
I know I'm stupid on these fuel injected engines, but did you check for corrosion on wiring to the knock sensor and/or crankshaft position sensor? I always think of timing or poor fuel with a popping symptom.
 
I have 8.1s Mercs in my Carver. Symptoms included popping / feels like a miss or bogging at higher RPM, drop it back to idle, no problems in idle or up to the higher RPM. Chased many things, fuel, ignition, but ended up being the High pressure fuel pump.
 
God gave you two engines for a reason....start swapping parts.
I'd begin with the coil packs then the high pressure fuel pump.

Good luck.
 
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