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Malibu Hammerhead 380 Running Rich

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Hello all, I have a 2004 Malibu with the 380 Hammerhead 5.7L motor in it.  Last time I had it on the lake it was bogging down so much it was barely getting on plane and running really really rich.  I pulled the plugs and they were heavily carbon fouled.  This is new to me boat this year and don't have much service history but this issue was not present the first few times I had the boat on the lake, the engine was running very smooth.

I did have the issue of low oil pressure the first time on the lake.  It was running 15W-40 but I changed to a straight 40 weight along with replacing the oil pressure sensor and oil pressure switch which seems to have corrected that issue.

Since the engine was running rough and I don't know much of it's past service history, I went ahead and replaced the, cap, rotor, plug wires, spark plugs, fuel filter, and knock sensor (which was not connected when I bought it).  The K&N air filter was cleaned and oiled properly.  The intake of the motor did seem a bit dirty when inspecting it through the throttle body as well.

I have been reading through these forms trying to troubleshoot this but no success yet.  Going to check fuel pressure tomorrow.  Does anyone have any other thoughts on this?

Thanks in advance.



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I did a bunch of searching of the forums today but still not finding a good lead.  Fuel pressure will be checked in the next day or two.  Does anyone have any other items to check or possible causes?  Any ideas are appreciated.  Thanks again.

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It’s a simple ecu.  Uses engine temp, map, rpm, and throttle position for fueling.  It just varies pulse width with no feedback on how much fuel is flowing.  Rich would be too high fuel pressure.  Could be a bad thermostat that’s stuck open.  Common on outboards and it causes rich running.  

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The fuel pressure should tell you a lot.  If it isn't constant at key on, idle, and WOT, fix the regulator or pump.  Keep in mind that your problem could also easily be bad fuel lines if yours are still original.

Otherwise, the temp sensor could also be bad, causing the ECM to enrich what it thinks is a cold engine.

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Thank you for the replies.  I replaced the temp sensor as well a few days ago but forgot to put it above in my list.  I checked the fuel pressure yesterday and saw that I was getting approximately 80psi when I believe it is supposed to be 60psi +/- 2psi for fuel pressure so I believe the regulator is likely the culprit.  I have a new one on order and will swap it out and re-test it and go from there.  I also checked the return line back to the tank and it is not blocked.


On an unrelated note, I am wondering if anyone has a picture of the heads used on the hammerhead 380?  Mine have the Chevrolet logo on the front and rear of the heads which I can't imagine that is how they came from the factory?  It is my understanding that historically there has been some issues with the heads used on the Hammerhead motors in 04 and 05.  I am wondering if a previous owner may have swapped them out.


Thanks again!

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