2008 Mercruser. 8.1 misfire

N

Nmor830

Member
Sep 12, 2016
90
Channel Islands
Boat Info
2008 340 Sundancer
Engines
8.1l Mercruisers
2008 Mercruiser 8.1l misfire/knocking. Seems to get worst as engine heats up. It’s coming from the port side of the starboard engine. If I hold my hand on the exhaust I can feel it. Last time out didn’t notice any loss of power but didn’t go over 3K rpm. I only heard it after changing oil and running engine with hatch open. So far replaced plugs, checked plug wires and coils on that side of engine. The injectors and cool fuel module were replaced about six months ago.
Any suggestions before I get someone out with a computer. I’m told the code will at least get me started in the right direction.
Thanks
 
Mine was doing that,.,, it was bad fuel from winter storage. Please let me know what you find.
 
You did the obvious items. Software will grab a code if it exists. But anything fuel related (cf3, injectors, regulator, etc) won’t produce a code.

Might need to use the software and kill a cylinder at a time to see which one doesn’t change anything then go search that cylinder for issues.

Wires go bad on those motors- how did you check them?
 
I had the same issue ( Popping out the exhaust ) we did put a computer on it that showed nothing, I have Vesselview and that show all good. After chasing this around for two weeks ( did what you did ) I decided two have the fuel injectors cleaned.
I ask them to test them first before they clean them.
There was two of them at 60% clogged and rest were 20% clogged.
This fix my problem, No more popping
 
Spark600 said:
I had the same issue ( Popping out the exhaust ) we did put a computer on it that showed nothing, I have Vesselview and that show all good. After chasing this around for two weeks ( did what you did ) I decided two have the fuel injectors cleaned.
I ask them to test them first before they clean them.
There was two of them at 60% clogged and rest were 20% clogged.
This fix my problem, No more popping
Click to expand...
Friends Chappy with the 8.1 - same thing. We chased the issue, scanned the computer - nothing. Replaced the injectors - perfect again.
 
if u had cool fuel issues with the paint going through the fuel system, the fuel rail and the injectors need to be either replaced or cleaned out.
 
Thanks for all the reply’s. First, the cool fuel module was replaced less then 6 months ago, checked filters and they look clean. Replaced the injectors as well all new. I guess there still may be a possibility they may have gotten clogged again with some bad fuel but the other engine runs fine on same batch. Finally found a mechanic to work with. These guys are really busy around here. Anyhow he thinks The popping is from water getting into one of the cylinders from a leaking riser gasket. Sure hope that’s not the case. In the middle of taking things apart to see. I’ll give an update when we know more. Should mention I’m the third owner and these are the original manifolds and risers. From what I read they are good for about 10 years so it might be time to replace.
Thanks,
Nick
 
For anyone interested. The popping sure enough was caused by a water leak between the riser and the manifold. To be sure the head was pulled to check condition of the valves. # 4 cylinder exhaust valve showed signs of running hot but fortunately wasn’t damaged. I’m told I was lucky that it was caught early. With the engine hatch closed you really couldn’t hear it. It was only when I changed the oil and did a run up it was noticeable. Lesson learned open up that hatch and take a close listen every once in a while. This was an expensive lesson but have could have been much worst.
Thank you all for the input.
Nick
 
That's surprising to me since the manifold/elbow joint on the 8.1 is a "dry" connection. No Seawater passes through that joint. There are two hose fittings on the top of the manifold (one in front and one behind the exhaust elbow) which have tubing going to two hose fittings on the elbow itself. This is a purposeful design to avoid previous problems with seawater leaking around the old joints that relied on gaskets to seal the area. With the 8.1 if the gasket leaks all you'd get is exhaust gasses escaping to the engine room.

It is more probable for the manifold itself to have an internal leak where the sea water is allowed to pass into the exhaust chamber and run downwards to the exhaust valve area.
 
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