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2007 30L MerCruiser Alpha rough Idle

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mikez68

New member
"I have a brand new 3.0L MerCr

"I have a brand new 3.0L MerCruiser Alpha engine. Idle was smooth on tech ride at the dealer, but we took it out yesterday for the first time and when idling, it's nice and smooth for about 30 seconds or so, then it shakes violently for a few seconds, then goes back to smooth. Then after a minute or so it shakes again. We think we heard a hissing sound coming from the engine compartment - we thought it might be a vacuum leak, but we could not determine where it was coming from"
 
"Check some basics first, make

"Check some basics first, make sure your plug leads are correctly fitted and that all you harness plugs and earth points are fitted properly around your engine
Mike if you feel you have a vacuum leak you can use a little bit of water to help locate it.
Just fill a little atomiser/spray bottle up with fresh water and squirt it around the vac fittings, hoses and manifold while the engine is running, if there is a leak it will suck the water in and it will cause the engine to faulter a bit.
If there is a leak you should be able to locate it using that little tip.
But as the problem is intermittant I doubt it is a vac leak.
Cheers
Peter C"
 
"Thanks for the response, I sh

"Thanks for the response, I should have mentioned that we did check all of the plug wires and the are all fine. We'll try the water bottle tip - thanks for that. The other thing we thought is we might have some bad gas since it was running fine at the dealer, then 2 weeks later we put some gas in it and took it out and it was idling rough. The other thing I should have mentioned is that it runs fine at higher speeds - we had it up to 4000 RPMs and it ran like a dream.

Thanks again,

Mike"
 
"Mike: when it's idling f

"Mike: when it's idling fine, wriggle each plug wire back and forth to see if it reproduces the vilent shaking, Plug wires are notorius for intermittent failure. Are they seated tightly on the plug and cap?

I check for vacuum leaks with a small propane tank turned on LOW (not lit!) and slowly move it around carb base gaskets, intake manifold gasket, all vacuum nipples, along the entire length of vacuum lines and any other fuel/air source to the engine. When the engine revs suddenly, you found your leak. Guy"
 
"Mike, I would look at your g

"Mike, I would look at your gas. I have a 2000 Mercruiser 3.0L and had the same problem. As a short term fix try adding B-12 Chemtool to your gas. Also if you live in a sate where there is Ethanol in the gas consider installing a fuel water separator this will help alot with engine performance.

Good Luck,
Chris"
 
"We do have ethanol in our gas

"We do have ethanol in our gas, we are going burn up as nmuch fuel as we can this weekend and then try some fresh fuel and see if that helps. Thanks for all of the suggestions - I'll let you guys know what we find."
 
"It does have a carb. We took

"It does have a carb. We took it out again over the weekend, and the roughness only seems to happen after it warms up. Then it is hard to staert after its warm. Runs smooth and starts fine when cold though."
 
"I am with you Jeff. Runs ok

"I am with you Jeff. Runs ok until it warms up and begins to run rough; won't start after it's warm. Is the choke wide open when it's warm? If yes, then have the seller set the carb adjustments to merc. specs. Guy"
 
"WE took it our last night on

"WE took it our last night on a frech tank of gas, and it was idling just fine, even wehn hot...until we went in reverse a few times, then the rough idle came back - and with a vengeance. It was not intermittant anymore, but consistantly rough."
 
had the same problem with my 2

had the same problem with my 2004 3.0. Solution use only premium 91 with NO ethanol mine runs smooth even in idle.
 
"I just posted about this same

"I just posted about this same problem Mike was having...rough idle, hissing noise, and hard to start when its warm, unless giving a fair ammount of gas to restart...sounds like a common problem and i should maybe have the dealer check it out...

ever find a vacuum leak at all mike?? what was that hissing?"
 
Never found a vaccuum leak - w

Never found a vaccuum leak - we took it back to dealer and they didn't and anything wrong (they are terrible though). We adjusted the idle
speed up a little and it has been running much better.
 
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