Brief Power Loss at Full Speed

[cjohnson66]

Hi - I have a 2005 Cobalt 250 (340 Hours) with a Volvo Penta 8.1 L 375HP Gi.

When I have the boat planing at full speed (~55MPH) the engine burps or has a sudden lag for a split second. It then keeps going normally as if nothing is wrong.

Any ideas? I'm concerned it something serious -- hopefully it is not a major issue. My boat is not very close to any Marinas.

[AsLan7]

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Hey CJ66. Welcome to the Cobaltchat forum. So glad you found us!

Could be any number of things. Bad plug. Plug gapped wrong. Water in the fuel. Bad gas. Corroded plug wires. You’ll have to answer questions regarding when these items were last serviced.

Service Tech can also pull codes to help diagnose.
Any pics?

Again welcome aboard mate!

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[cjohnson66]

Thanks for the gracious welcome. I'm going to change the plugs this weekend. I had fuel stabilizer in the gas over the winter months and just added about 36 gallons of premium fuel -- topped up to the brim . New oil and fuel filter installed a couple of weeks ago. I have not checked the intake (where the flame arrestor is located?) or the distributor cap as of yet. There is also a slight smell of gas in the boat which is odd. I'll take a look at the gas tank, but don't really know what to look for other than the obvious.

Thanks for your help!

Chris

[AsLan7]

Nice pic.

There is no distributor cap. Coil packs on each cylinder.

We had a bad plug and a bad coil pack with similar symptoms that yours exhibits. Replaced plugs and the pack and good to go. The tech can shut down each pack with computer to diagnose the bad one.

Let’s get you fixed.

[bruceb58]

I have not checked the intake (where the flame arrestor is located?) or the distributor cap as of yet.

Don't look very hard for a distributor cap...you won't find one!

EDIT: Aslan beat me to it...and I thought I was being clever!

[cjohnson66]

Thanks! (Clearly I'm not a mechanic based on my distributor cap comment).

I'll change the plugs this weekend and see how that works out. If the engine lags/burps I'll need to have it towed to my marina (which unfortunately is far from my summer house).

Thanks again - appreciate the quick response. I'll let you know how it goes.

Chris

[Big Block Power]

So what rpm are you hitting? Maybe your just bouncing off the Rev limiter?

[cjohnson66]

I'll check the RPM this weekend. Do you know what rpm the limiter might kick in at? I'm not sure what the "pre-set" is for my engine. It also happens intermittently so this might indicate something else. I'm pretty new at diagnostics (obviously) but this is all good data for me to troubleshoot the issue.

Thanks again,

Chris

[Big Block Power]

I'll check the RPM this weekend. Do you know what rpm the limiter might kick in at? I'm not sure what the "pre-set" is for my engine. It also happens intermittently so this might indicate something else. I'm pretty new at diagnostics (obviously) but this is all good data for me to troubleshoot the issue.

Thanks again,

Chris

Your wot is 4600 rpms. I'm thinking at 55 your bouncing off of that. What props are on your boat? F8?

[AsLan7]

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Normally that year model will also sound a loud engine alarm beep from the speaker under the dash if the rev limiter is being reached as the engine backs off to protect itself. This is normally the case if the boat is under-propped.

[cjohnson66]

The prop is original - although was repaired once (after my ex-wife decided to try to dock the boat - we won't talk about that...).

I'd need to double check; but I believe it is an F8.

Regarding the loud engine alarm -- I have not heard it.

[cjohnson66]

Quick Update: I installed new spark plugs on the weekend. This seems to have solved the problem for the time being. Max RPM at WOT on my boat was 4500 so it doesn't look like that is the issue (also not getting any alarms at wot). One issue I did uncover was that one of the spark plug internal threading was problematic. Each spark plug threaded on easily -- however the problematic one took me 10 minutes to find a thread and seat the plug. This may be an issue -- at minimum I'll need to get the thread repaired. I'm assuming this may be a problem even though she seems to be running fine right now.

Pics of Spark Plugs:

[rabiddawg]

Spark plugs on these 8.1 engines are an issue. When I bought mine last summer, it ran fine on the water hose but had No power under a load. I pumped the tank out, changed fuel pumps, had injectors serviced. Changed the plugs and that cured the issue.

Recently I removed the manifolds and risers to repair the butterfly in my captains call. Now it seems to be missing on one cylinder. I changed plug wires but have not run it.

If the wires don’t clear up the problem I’ll change the plugs again. Research on the net says the 8.1’s eat plugs for some reason.

[cjohnson66]

Interesting -- thanks for the info. That is a lot of work! Hopefully, your boat will run well after going through all that maintenance. I'll keep monitoring over the Summer. The Fall will be bellows, gimbal bearing and power steering maintenance (I've got a small leak that I can't find).

[Big Block Power]

I just Chavez my plugs in under 30 minutes easy.
Maybe run a thread chaser down that 1 hole.
My plugs had 3 summers on them from me and who knows how many before me. They looked like brand new that why I never changed them. I'm chasing something else and is the only reason I changed them this year. I disagree that plugs are a issue on this engine.