problems with Mercruiser 5.7, IO

[mstrud]

Recently replaced engine with a reman Mercruiser. When engine is hot, it does not run smooth. A couple of observations: exhaust smells rich, engine runs rough and kills at low RPM and when shifting to dock, at full throttle RPM does not get above 1000.

 

[Bondo]

Re: problems with Mercruiser 5.7, IO

Ayuh,....

We'll need Awhole Lot More Info,.....

How much of the motor was Replaced,+ How much was reused from the take-out motor,..??

Who did the Swap,..??

At the very least,...... It sounds like the Carb is AFU.......

 

[mstrud]

Re: problems with Mercruiser 5.7, IO

Sorry, my bad. The engine was a "long block" reman by Mercruiser and installed by an authorized mechanic. Essentially, all the components for the old engines (ie carb, inginition, starter, water pump, etc) were installed on the new engine.

The first run out, everything worked well. Engine cruised very well and got up to speed with no issues.

Following that first run, we then developed the issues I have been discussing. Unfortunately, those problems really presented when the engine was HOT. The mechanics test at the dock never got the engine up to operating tempreture.

I think we have isolated the problem to the ignition system. Specifically, the coil and perhaps spark plug wires. I will update once I try it out this weekend.

 

[mstrud]

Re: problems with Mercruiser 5.7, IO

OK, the plot thickens. As noted earlier, a reman "longblock" Merc, 5.7 was installed on my boat. Here are the details.

Following the replacement of the coil and spark plugs wires, engine was slow to start but seems to run fine initially. As the engine got hot and started to run under way, performance dropped significantly. Engine runs rough and does not idle well. Cant not get the engine does above 1000 - 1500 RPM when hot, almost sournd as if it may be "sucking" air. Increased throttle yields nothing.

Replacing the ignition coil and new spark plug wires seemed to help momentarily but by the end of the day, the engine was displaying the same issues.

Help!

 

[Fishermark]

Re: problems with Mercruiser 5.7, IO

As the engine got hot and started to run under way, performance dropped significantly. Engine runs rough and does not idle well.

How hot is "hot?"

 

[Firestar]

Re: problems with Mercruiser 5.7, IO

Is your warranty still in effect? I would have them look at it.

 

[SuperNova]

Re: problems with Mercruiser 5.7, IO

Make sure the choke is opening properly when hot.............only thing I can think of right off the bat that would produce those exact symptoms. On second thought............a clogged flame arrestor would do it, too.

 

[JustJason]

Re: problems with Mercruiser 5.7, IO

since nobody asked i will.....
why the new long block?
what happened to the old one?

 

[sarantis]

Re: problems with Mercruiser 5.7, IO

i would start from the basics.Use an external tank to check if yours has a problem .Usually increasing rpm means increasing fuel demand and higher temprature together.But how much hot does your engine run?And i also have the above question.Did you replace your engine because an overheating?If so start looking at the outdrive.

 

[mstrud]

Re: problems with Mercruiser 5.7, IO

HOT? What I mean is normal operating tempreture. Outdrive has been serviced. Lower unit recently rebuilt and impeller replaced. Engine was replace due to a valve the resulted two cylinders being scratched and scared.

Problem is not INCREASING RPM, problem is low RPM and poor performance

Visually, the choke appears to be opening but the flame arrestor could be something to consider. I will have it checked and get back with you.

Warranty is still in force. Will have Mercruiser get involved.

All, thanks for your comments

 

[SuperNova]

Re: problems with Mercruiser 5.7, IO

Visually, the choke appears to be opening

Shouldn't be a question, either that choke plate is standing straight up or it isn't. Also make sure it isn't sucking back down closed once you come off idle. If it isn't backfiring it isn't lean so fuel starvation shouldn't be an issue.

 

[Maclin]

Re: problems with Mercruiser 5.7, IO

No mention of year or model, so some of this may not apply...

Gotta check the basics, as in Points/dwell, timing, etc.

Has the valve lash been checked? "Tight" rockers will definitely keep the rpm's and power down.

 

[mstrud]

Re: problems with Mercruiser 5.7, IO

Hopefully this is the final resolution to what has been a trying and frustrating initial experience with the new engine. As you may recall, we replaced a various times all of the components of the ignition system. In addition, we re-built the carborator. After several sea trials, the mechanic was a wits end and finally called in the Merc technician who came up with the solution. Again, the coil, spark plug wires, distributor, etc has been replaced, this time as part of a comprehensive package. I am not sure if I will ever know if there was a mismatch of the parts or if the some of the components were just "bad" but an voltmeter never detected a problem.

That said, the "system" approach appears to have been the answer. All is well and the engine has thus far, been running fine.

I am keeping my fingers crossed.

 

[Fishermark]

Re: problems with Mercruiser 5.7, IO

Thanks for the update - and good luck from here!