1997 Stringray 3.0 Mercruiser - ignition issue

Thoma Ck

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Joined
Jul 27, 2020
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3
Refurbished motor, new outdrive new distributor cap and rotor, new ignition module and coil, new spark plug wires.

I am wondering if anyone might have any suggestions that I can try. So here is my problem. All cylinders have spark (spark jumps over an inch), all cylinders have compression above 150, assuming all cylinders are getting fuel (I know #1 and #2 definitely are), when the engine is running and we pull the spark plug wire for #3 cylinder there is no change in the motor rpm (acts like a cylinder is running 10% or even a dead hole), when we replace the wire for #3 and pull #4 there is very little change in the rpm (maybe running at 25% of potential). Unlike #1 and #2 which have distinct rpm drops.

Also in order to start the engine, the coil has to be hot-wired from a 12v source to start. I am thinking it has something to do with the shift interrupt switch.

Any suggestions besides shooting it? :grumpy:
 

chs marine service

Petty Officer 3rd Class
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Jun 29, 2019
Messages
89
did you do the compression test with all the spark plugs out or 1 at a time? there may be a head gasket failure between #3 & 4 which is giving a false reading and allowing the compresion in 1 cylinder to retain the pressure. its hard to put into words, but try the compression with all plugs out. `


i would also do some digging into the hot wiring issue, if th shift interrupt is engaged, the spark will be deminished which will get a real crappy spark.
 

Thoma Ck

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Jul 27, 2020
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I will check again with the mechanic but I recall the last compression check was done with all the plugs removed. we had thought the same thing so we tried that but I will verify. As for the hot wire issue, yes I believe there is something related there but we have tried the pull the wire and see if there is any rpm drop both with the shift interrupt disengaged and engaged, as I recall there is no change. The shift interrupt has been checked and we do have continuity across when it is engaged.
 

Thoma Ck

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Compression without any plugs - Cylinder #1 155, Cylinder #2 168, Cylinder #3 175, Cylinder #4 162
 

Bondo

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As for the hot wire issue, yes I believe there is something related there but we have tried the pull the wire and see if there is any rpm drop both with the shift interrupt disengaged and engaged, as I recall there is no change. The shift interrupt has been checked and we do have continuity across when it is engaged.

Ayuh,...... Welcome Aboard,...... Engagin' the switch, Should kill the ignition, the motor Should die,......

You got a wrin' problem somewhere in the ignitions primary circuit,......
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Recheck the firing order. Good compression and getting fuel, but near no drop in the when the plug leads pulled on 2 cylinders makes me thing firing order...

And I totally agree with Bondo, you also have a primary wiring problem...

Chris...
 

dubs283

Vice Admiral
Joined
Jul 27, 2005
Messages
5,343
Recheck the firing order. Good compression and getting fuel, but near no drop in the when the plug leads pulled on 2 cylinders makes me thing firing order...

And I totally agree with Bondo, you also have a primary wiring problem...

Chris...

my thoughts exactly
 
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