3.0 Mercruiser Hard Starting and Rough Idle

CarlJ

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Jul 5, 2013
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Hello all. I have 2001 3.0l mercruiser that just doesn't seem to want to start without being in WOT. It runs smooth above 1200rpm, but below that to idle is very rough. In fact, I cant really keep it running below 700rpm. I rebuilt the carb, still does the same thing. Im wondering if this could be a fuel delivery problem (fuel pump)???
 

alldodge

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Re: 3.0 Mercruiser Hard Starting and Rough Idle

Need to try the carb rebuild again, primary jets remain the problem. Need the carb primary/idle channels cleaned better. Above your 1200 RPM your getting into the main jets and this is why the problem seems to go away. If you had fuel supply/delivery problems it wouldn't have enough power to get much above the 1200. More speed more fuel is needed.
 

CarlJ

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Re: 3.0 Mercruiser Hard Starting and Rough Idle

Thanks for the advice. I give it another try.
 

CarlJ

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Re: 3.0 Mercruiser Hard Starting and Rough Idle

Thanks for the advice. I give it another try.

I am assuming that the jets you are referring to are the (2) small 1.55mm ones that were not replaced in my rebuild kit?
 

CarlJ

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Re: 3.0 Mercruiser Hard Starting and Rough Idle

Well, unfortunately the carb isn't the problem after all. After doing a compression check I discovered the problem:
#1 150
#2 155
#3 160
#4 20

So I guess its time to pull the head and see how bad it is. Anyone got any advice/tips on removing. Appears to be a fairly easy job??
 

Bt Doctur

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Aug 29, 2004
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Re: 3.0 Mercruiser Hard Starting and Rough Idle

loosen each bolt a bit first kinda the reverse of torqing in 3 steps. to install, clean block and head with alcohol, install head gasket CORRECTLY because it is easy to reverse it.torque in 3 steps.
Most likely stuck or burnt exaust valve from water leaking from a hole in the riser or manifold.
 

CarlJ

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Re: 3.0 Mercruiser Hard Starting and Rough Idle

Ok, so i removed the head and discovered that the exhaust valve on the #4 cylinder is unlike the others, concave where it would normally be flat. I guess I need purchase a new set of valves and take it to the machine shop. Anyone have suggestions of where I can purchase the parts?
 

alldodge

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Re: 3.0 Mercruiser Hard Starting and Rough Idle

Ok, so i removed the head and discovered that the exhaust valve on the #4 cylinder is unlike the others, concave where it would normally be flat. I guess I need purchase a new set of valves and take it to the machine shop. Anyone have suggestions of where I can purchase the parts?

Probably going to need more then valves. Need to have the head pressure tested. How many hours is one the engine? If there are alot of hours may need a rebuild. How does the piston and cylinder walls look? You can buy most all your machine parts from about any auto store, also can get them here at iboats.
 

stonyloam

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Mar 13, 2009
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Re: 3.0 Mercruiser Hard Starting and Rough Idle

Talk to your machinist, he will have access to all the parts you need. You may only have to replace the one valve. So find out what you will need before you spend a lot on parts.
 

CarlJ

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Re: 3.0 Mercruiser Hard Starting and Rough Idle

I was planning on having the head redone, no reason not to if i have the thing out. Boat has about 160 hrs on it, piston and cylinder walls look good.
 

alldodge

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Re: 3.0 Mercruiser Hard Starting and Rough Idle

I was planning on having the head redone, no reason not to if i have the thing out. Boat has about 160 hrs on it, piston and cylinder walls look good.

Dang, 160 hours and loose a valve. Like any thing else, stuff does happen, hope the rest of the engine is in better shape
 
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