Mercury 40 HP 402 Runs on one cylinder

[Nazmen]

Merc 40 HP 402 <br /><br />Problem:<br />Engine starts but runs on one cylinder. Will run up to 2500 RPM.<br />After engine heats up running this way, the 2nd cylinder will kick in, then the <br />engine runs fine at full speed and idle. <br />Next day the engine is very hard starting. When engine does start it runs again on<br />one cylinder, heats up and then runs fine.<br /><br />Testing that I have done:<br />While engine is running on one cylinder, pulled spark plug wire from<br />top cylinder, no change in running condition, bottom cylinder is the one running OK.<br />Concluded must by be top cylinder with the problem.<br /><br />Switched spark plugs and same condition - - ruling out spark plugs.<br />Could not exchange wires since shorter wire cannot reach when exchanged.<br /><br />Service manual I have suggested this problem could be from leak in fuel pump <br />diaphram.<br /><br />My guess would be faulty ignition coil, except that the 2nd cylinder fires when the <br />engine warms up. Could still be ignition coil if there is connection that makes <br />itself when the engine block heats up on one cylinder.<br /><br />Did not try to exhange coils. <br />Considered purchasing new coil except they are greater than $50 and I was on vacation<br />with the boat for only 1 week and the Merc dealer said he could get me the coil in 2 weeks.

 

[Chinewalker]

Re: Mercury 40 HP 402 Runs on one cylinder

Hi Nazmen,<br /> I would definitely swap the coils top and bottom and see if the problem follows the coil. The green wires that go to the coils - one has a white tracer which helps you tell the apart. Can't remember off the top of my head which is the + or -, but you'll see when you do the swap - just make sure they get on the right posts on the other coil.<br /> Check the wiring for cracks, breaks - not uncommon on those older Mercs to have the coating deteriorate. Check under the coils for corrosion and make sure the little ground wire is making good contact with the coil mount. Being that yours does kick in points to it being a coil issue. I've had several with cracks in the plastic casing allowing it to arc out to the mounting bolts... You can sometimes hear the "snap" of the arcing spark as the motor is running.<br /><br />- Scott

 

[Nazmen]

Re: Mercury 40 HP 402 Runs on one cylinder

Thanks for the info.<br /><br />I removed the coil from its mount and there was some varnish like material between the ground wire and the engine case. Cleaned this up but I was unable to determine if this was the cause of my problem since when I went to start my engine, it would crank but would not start at all.<br /><br />It is not a fuel problem. Shot some WD 40 into the carb and there was absolutely no pop from the engine.<br /><br />Removed the spark plug and cranked the engine. There was no spark. <br /><br />Need your help on how to trouble shoot the problem from here. How to determine what in the upstream electical system is causing the problem. I checked all the wiring in the upstream components and found them to be tight.<br /><br />Thanks for you help thus far.

 

[dilligafda]

Re: Mercury 40 HP 402 Runs on one cylinder

I had exactly the same issue with my 402 and found it to be a bad stator. To be sure, get a peak reading volt meter and check the output of the stator. When cranking the engine, the red lead should read a minimum of 30 volts and the blue lead should read at least 190 volts. If those check out, check the trigger output. THe white and brown leads should read at least .5 volts. Next check the output leads to the coils, the reading should be a minimum of 190 volts. If the stator and trigger check out, replace the switchbox. Otherwise, replace the components that fail the inspections. <br /><br />Hope this helps!

 

[Nazmen]

Re: Mercury 40 HP 402 Runs on one cylinder

dilligaf <br /><br />Thanks for your reply.<br /><br />I think my next trip is to get a manual for my engine. Until then...<br /><br />Can you tell me what the function of each of the components are until I do. I looked at some of your other posts and you provide some of that information on rectifiers.<br /><br />Stator<br />Trigger<br />Switch box.<br /><br />Thanks Nazmen

 

[dilligafda]

Re: Mercury 40 HP 402 Runs on one cylinder

The Mercury service manual does not give the voltage values for this engine. It does give ohm readings for the trigger and stator but they are not as accurate as a voltage output test.<br /><br />What do you want to know about rectifiers? The basic function is to convert AC voltage to DC voltage. Your engine has a rectifier, however it has nothing to do with the ignition system. It provides voltage to keep the battery charged. <br /><br />Stator:<br />Provides voltage to the switchbox for the ignition and charging systems<br /><br />Trigger:<br />Provides a low voltage signal to the switchbox telling it to release the stored voltage (provided by the stator) to the coils, thus firing the plugs.<br /><br />Switchbox:<br />Stores voltage in a capacitor for ignition. A low voltage signal from the trigger releases the voltage to the coils to provide spark. <br /><br />This is a very simple explaination of the Thunderbolt Ignition System. There is no specific test for the switchbox. the procedure is to eliminate all the components around it.

 

[Nazmen]

Re: Mercury 40 HP 402 Runs on one cylinder

While testing the resistance of the stator I discovered a short to the block in one of the wires. Upon further inspection the wire started to crumble and I noticed an arc mark where the wire crosses the CD body.<br /><br />I spliced in new wires from a location on the stator wires where the insulation seemed solid. One troubling thing when I spliced the wire, the stripped wire had corrosion and I had to scrape the corrosion off to solder. The solder joint did not look good. But I continued and taped it up.<br /><br />Did get a resistance reading at the terminal block. BUT the engine still did not start.<br />Did not have a peak volt meter to make readings. I was hoping engine would start.<br /><br />Could the corroded wires pose a problem?<br /><br />I will pursue voltmeter to get readings.<br /><br />Any thoughts.