Friend's problem - His write up follows:
I am having a unique problem with a 1986 5.0 L 230HP 4 BBL mercruiser. I have owned the boat since it was new and utilize it year round (the engine has no hour meter but based on my other boats hours I would estimate a staggering 4000 hours on the powerplant, that said it has never presented a problem, can’t say the same for the sterndrive which has been rebuilt multiple times). This past March I went to take it out and it failed to start. Quick troubleshooting revealed there was no spark coming from the coil. Removal of the distributor cap revealed a fair amount of wear on the cap conductors and rotor but the ignition sensor was engrossed with corrosion (sensor was OEM, not a potted unit). The part had done its job impressively for several decades so I was glad to replace it along with cap and rotor. I also replaced the coil (just due to age). The boat fired up immediately and ran smoothly. I took it out on a trial run of about 25 miles and it ran fine (it should be noted that I remained on plane for the entire run until returning to port). I noticed a roughness when reducing throttle but thought nothing of it since the idle was smooth during docking. Unfortunately on my next run out I was wakeboarding the kids and it ran fine for about 20 minutes but when the boarder fell and I throttled down I felt a roughness in the transition back to idle. When the next boarder was ready the boat would not accelerate past 1200 rpm, it would bog significantly but regain smoothness when rpms were reduced below 1000. I was able to return home at idle speed. I replaced the spark plugs, plug wires, alternator and battery. Amazingly the boat repeats this same phenomena every time, it will accelerated and plane multiple times within a 20 minute timeframe, it will run fine at all speeds even WOT, but after 20 minutes if you let the engine speed fall below 2000 rpm it is like the engine loses all advance and goes into base timing mode. I verified that this is not carb/fuel related by spraying ether into carb during the bog phenomena only to have no effect. Once again, if the boat is left on plane (above 2000 rpm) it will continue to run indefinetly. I assume the timing aspect (i.e. 20 minutes) of this is thermally driven by one of the electrical components (I even left engine cover off to see if it would have an effect, it did not). BTW, my further troubleshooting included:
I am having a unique problem with a 1986 5.0 L 230HP 4 BBL mercruiser. I have owned the boat since it was new and utilize it year round (the engine has no hour meter but based on my other boats hours I would estimate a staggering 4000 hours on the powerplant, that said it has never presented a problem, can’t say the same for the sterndrive which has been rebuilt multiple times). This past March I went to take it out and it failed to start. Quick troubleshooting revealed there was no spark coming from the coil. Removal of the distributor cap revealed a fair amount of wear on the cap conductors and rotor but the ignition sensor was engrossed with corrosion (sensor was OEM, not a potted unit). The part had done its job impressively for several decades so I was glad to replace it along with cap and rotor. I also replaced the coil (just due to age). The boat fired up immediately and ran smoothly. I took it out on a trial run of about 25 miles and it ran fine (it should be noted that I remained on plane for the entire run until returning to port). I noticed a roughness when reducing throttle but thought nothing of it since the idle was smooth during docking. Unfortunately on my next run out I was wakeboarding the kids and it ran fine for about 20 minutes but when the boarder fell and I throttled down I felt a roughness in the transition back to idle. When the next boarder was ready the boat would not accelerate past 1200 rpm, it would bog significantly but regain smoothness when rpms were reduced below 1000. I was able to return home at idle speed. I replaced the spark plugs, plug wires, alternator and battery. Amazingly the boat repeats this same phenomena every time, it will accelerated and plane multiple times within a 20 minute timeframe, it will run fine at all speeds even WOT, but after 20 minutes if you let the engine speed fall below 2000 rpm it is like the engine loses all advance and goes into base timing mode. I verified that this is not carb/fuel related by spraying ether into carb during the bog phenomena only to have no effect. Once again, if the boat is left on plane (above 2000 rpm) it will continue to run indefinetly. I assume the timing aspect (i.e. 20 minutes) of this is thermally driven by one of the electrical components (I even left engine cover off to see if it would have an effect, it did not). BTW, my further troubleshooting included:
- I put another Thunderbolt IV ignition module on and had identical signature.
- I removed the tach lead from the negative side of coil and disconnected the shift interrupt switch wire from distributor. I also removed the ignition switch wire from coil positive side and utilized a homemade wire between the alternator output (orange) and coil positive terminal (after a 20 minute ride and same failure signature I forgot that with this arrangement I could only turn engine off by removing coil wire!).