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 Mercruiser 8.1L 'popping' above 3400rpms
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Author Previous Topic: fuel pump leaked into oil Topic Next Topic: Antifreeze change.  

copb8

RO# 34005

Posted - Nov 25 2017 :  14:33:21  Show Profile  Reply with Quote
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.
Bart
2005 Sea Ray 420DA 8.1L

Homeport: Texas

Sandy

RO# 1159

Posted - Nov 25 2017 :  17:34:56  Show Profile  Reply with Quote
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.




Sandy

Edited by - Sandy on Nov 25 2017 17:54:45

Homeport: The Vineyard Go to Top of Page

alien1952

RO# 32487

Posted - Nov 26 2017 :  11:37:27  Show Profile  Reply with Quote
other things can be a (depending where you hear the backfire) a intake or exaust valve, or broken/weak valve spring(s)

aka Bt Doctur

Homeport: nj Go to Top of Page

Sandy

RO# 1159

Posted - Nov 26 2017 :  17:40:15  Show Profile  Reply with Quote
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?

Sandy

Homeport: The Vineyard Go to Top of Page

littlebookworm

RO# 27413

Posted - Nov 27 2017 :  20:58:52  Show Profile  Reply with Quote
A tulipped or bent valve would give a rough run at all speeds and would probably bend or break a pushrod, as well. Iíve had both experiences on the Mercruiser 7.4 engines on our boat. Itís 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



Homeport: New Rochelle, NY Go to Top of Page

Stephen

RO# 14

Posted - Nov 27 2017 :  23:13:37  Show Profile  Reply with Quote
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.


Homeport: Matts Landing, NJ Go to Top of Page

Nautiville

RO# 30016



Posted - Nov 28 2017 :  08:35:15  Show Profile  Reply with Quote
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.

Bob!
More Nautiville 2004 Carver 360 SuperSport

Nautiville 2005 Cruisers 280 CXi *Sold*
Nautitown 2012 SeaDoo 180 SP *Sold*

Homeport: Cumberland River, Tennessee Go to Top of Page

WALSHIE

RO# 2124



Posted - Nov 28 2017 :  09:16:55  Show Profile  Reply with Quote
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.


Favorite Quote: Don't sweat the petty things...AND...Don't pet the sweaty things!! - Steven Tyler

Homeport: Hudson River Go to Top of Page

Sandy

RO# 1159

Posted - Nov 28 2017 :  23:44:31  Show Profile  Reply with Quote
It appears we lost copbait.

Sandy

Homeport: The Vineyard Go to Top of Page
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