1988 mercruiser 3.7lx misfiring

ematz22

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Sep 25, 2014
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I have a 1988 carver santego 2767 with the mercruiser 3.7lx motor. Late in the season I traveled a long distance to meet up with friends approximately hour and a half trip and was there for a couple hours and headed back. When I was coming out of the bay to lake my port engine started misfiring so I throttled back. After several attempts to try and smooth it out it quit. So I got back to my dock about 3 and a half hours later on one engine. I changed the plugs and wires and it started up and after it warmed up and I headed out it started doing the same thing. I was told by the local marine part supplier that I needed to change out the distributor cap and rotor. He said I'll probably have to do this every other season or sooner. Does this sound reasonable. I'm going to change it out in the spring to see if it works. Any thoughts?
 

achris

More fish than mountain goat
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May 19, 2004
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Dissy cap should only need to be done every 5 to 10 years, depending on usage, along with spark plug leads. However, rotor should be changed along with spark plugs, contact points and condenser every year.

You need to determine why your engine is missing, fuel or spark...

Start with doing a compression test, then check the condition of points, spark plugs and rotor. If the condenser hasn't been changed for a while, change it. Then do a visual inspection on the ignition coil, make sure it's not corroded and leaking. Then check your fuel filters (for 'check' read 'change'). Also check for air leaks in the fuel lines, especially the fittings that screw into the filter housings.

As you have 2 engines playing 'swaps and gotchas' is easier... some things are easier to swap than others though.

Chris.......
 

stonyloam

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+1 on the compression check. Cap and rotor should last many years. One other thing is you could do is remove and replace the plug wires one at a time while the engine is ideling. If you remove a wire and the RPM does not change, that is you weak cylinder. As you remove the wires from the cap you should see the spark jump from the cap to the wire, if you do your cap and rotor are OK. That along with the compression check should tell you the health of the head gasket. If the compression is OK on the weak cylinder look to the plug and plug wire. If the compression is low on the suspect cylinder, chances are ( with a 3.7) it is a head gasket problem.
 

ematz22

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Why would I develop a compression problem after a just drove it with no issues?
 

achris

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Why would I develop a compression problem after a just drove it with no issues?

It's just part of good troubleshooting practices. It's easy to do, doesn't take long and rules that possibility out...

But developing a compression problem after there was no problem... Happens all the time, it's called detonation, and it takes about 8 seconds to go from 'hero to zero'..... Or a blown head gasket. Takes even less time.... Or a burnt valve... or.... .You see the possibilities?

Chris.........
 

Bt Doctur

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Aug 29, 2004
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19,093
mis-firing= cap ,rotor, plugs, plug wires, timing ,riser leaking water into #4 cylinder
back-firing= water in fuel plus all of the above
 

ematz22

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Sep 25, 2014
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Which cylinder is the #1 cylinder? Looking to the back of the boat first one or is that #4?
 

Bt Doctur

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cylinders are numbered front to back 1,2,3,4 ,The firing order is 1,3,4,2 .Remove the plugs and inspect the plug color. pure white or more white than the rest could be water ingestion into that cylinder.
 

ematz22

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Sep 25, 2014
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The spark plugs have black carbon deposits in them. After doing a compression check found #1 cylinder to be the lowest at 95#. The others were 120 to 130#'s
 

ematz22

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Sep 25, 2014
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Ran it today on muffs. Let it come up to temp. Didn't have any misfire. Would it not show signs of misfire unless it's under load in the water?
 

stonyloam

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#1 may be your problem. Run it up to temp and at idle, remove the plug wires one at a time, and note the drop in RPM. If one cylinder is weak, you will see little or no RPM loss when you pull its wire.
 

mr 88

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Nov 3, 2010
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Petronix makes a electronic kit that eliminates your points,goes for around 100.00,. might be something to look into.
 

ematz22

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Sep 25, 2014
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After cruising around yesterday then anchoring for a few hours the engines seem to be running sluggish. Can't get over 3000 rpm. Seems to bog down the more throttle I give it. I don't get it. How can it run perfectly then it sits for a couple hours then doesn't run good at all. There seems to be something going on when it sits. Any Ideas?
 

Bondo

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Ayuh,.... Welcome Aboard,.... Sounds like a Fuel delivery issue,.... Rebuild the carb,...
 

salty3rd

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Jul 13, 2010
Messages
109
It could also be a bad ignition coil. You can either replace it, or since you have twins you can swap the coils from one engine to the other. I have seen this kind of thing happen a couple times and one time it was a dirty carb, and the other was a bad coil. Just make sure you are putting the correct coil on depending on whether you have points or a petronix ignition. good luck
 
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