FliesAndFloats
Seaman Apprentice
- Joined
- Sep 6, 2015
- Messages
- 30
Short version:
Installed new distributor (Merc 5.0 MPI with crab style cap -- shaft separated from pressed fitting on top). Boat ran great, but chugged at idle. Friend came and looked at it and believed it was off by a tooth, so moved it by a tooth. Boat now idles great, but misfires above 2000 RPM and struggles to get to 3500 RPM.
Long version:
Had a failure in my distributor on my Mercruiser 5.0 MPI (crab style). The rotor button came apart, bound itself up and broke the head of the distributor free from the shaft. The shaft that's pressed into the sheetmetal part that the rotor button screws into can spin inside of that piece of sheetmetal. Sea Tow towed me in and left me in a spare slip that they had at the marina just before Easter weekend. I figured it was just the rotor button that failed and was out there first thing the next morning with a new OEM cap and rotor. That's when I discovered that the top of the shaft could be turned by hand.
The mechanic that I use was busy, but sourced a new part for me and explained how to find TDC and install it. Of course I had to wait until a marina resident was entering or leaving before getting in there, which seemed to take about an hour each time. At that point I discovered that the socket set I kept on the boat did not have deep-well sockets above 1/2". Screw it, I've got a 50/50 chance of installing the distributor correctly. The batteries were nearly dead from trying to get the thing started while out on the water before getting towed. I install the distributor. Won't start, but the starter was really struggling with the nearly dead batteries. Rotated distributor 180 degrees. Boat starts immediately and I take it home, opening it up at one point along the way. It ran great and the repair that I did got me out of there prior to their busy Easter weekend. I noticed when I first started it that it chugged a little, but I figured it might've just been the low system voltage so I ignored it. Upon getting home and putting it in the lift, I noticed the chugging was still there and I thought that I heard some spark knock during the time that I was running wide open.
I call a mechanic friend of mine who has the diagnostic tools and he offered to come help out with it. He connects a timing light to #1 and notes that he can't see the mark on the harmonic balancer. We then set the harmonic balancer to the alignment "V" on the block and he pulls the cap and notes that the rotor appeared to be pointing one cyclinder off (closer to the front of the engine than to #1). So he pulls the distributor and rotates it one tooth. Now the boat idles great and the line on the harmonic balancer showed up under the timing light, but when we took it out to trial, it had a misfire above 2000 RPM and would struggle to get to 3500 RPM. I wanted to rule out the notoriously unreliable cap/rotor, so, having the one that came with the replacement, still unused, I installed that combination with no change in performance. My friend checked that each cylinder was firing at low RPM by dropping each one offline and noting the RPM drop. The codes that were logged by the ECM were standard and seemed to be pretty unrelated. Fuel level sensor error, trim position sensor error, pitot error (I assume for a non-existent speed input).
In all honesty, I was surprised that it would run at all if it were off by a tooth. My suspicions are that either I'm 180 degrees off and it is still able to run, or possibly the distributor position sensor on the new one may not be working correctly and the behavior that I'm seeing is the result of the timing not advancing at all from idle. My mechanic friend believes it's a fuel delivery issue that just happened to come up at the same time as this distributor replacement. He verified the continuity of the vacuum line from the intake manifold to the fuel pressure regulator and noted there was another vacuum line that went somewhere to the back of the engine, but couldn't find where it went with the engine hot.
I'm curious what those of you who are familiar with these systems can suggest that I try.
Installed new distributor (Merc 5.0 MPI with crab style cap -- shaft separated from pressed fitting on top). Boat ran great, but chugged at idle. Friend came and looked at it and believed it was off by a tooth, so moved it by a tooth. Boat now idles great, but misfires above 2000 RPM and struggles to get to 3500 RPM.
Long version:
Had a failure in my distributor on my Mercruiser 5.0 MPI (crab style). The rotor button came apart, bound itself up and broke the head of the distributor free from the shaft. The shaft that's pressed into the sheetmetal part that the rotor button screws into can spin inside of that piece of sheetmetal. Sea Tow towed me in and left me in a spare slip that they had at the marina just before Easter weekend. I figured it was just the rotor button that failed and was out there first thing the next morning with a new OEM cap and rotor. That's when I discovered that the top of the shaft could be turned by hand.
The mechanic that I use was busy, but sourced a new part for me and explained how to find TDC and install it. Of course I had to wait until a marina resident was entering or leaving before getting in there, which seemed to take about an hour each time. At that point I discovered that the socket set I kept on the boat did not have deep-well sockets above 1/2". Screw it, I've got a 50/50 chance of installing the distributor correctly. The batteries were nearly dead from trying to get the thing started while out on the water before getting towed. I install the distributor. Won't start, but the starter was really struggling with the nearly dead batteries. Rotated distributor 180 degrees. Boat starts immediately and I take it home, opening it up at one point along the way. It ran great and the repair that I did got me out of there prior to their busy Easter weekend. I noticed when I first started it that it chugged a little, but I figured it might've just been the low system voltage so I ignored it. Upon getting home and putting it in the lift, I noticed the chugging was still there and I thought that I heard some spark knock during the time that I was running wide open.
I call a mechanic friend of mine who has the diagnostic tools and he offered to come help out with it. He connects a timing light to #1 and notes that he can't see the mark on the harmonic balancer. We then set the harmonic balancer to the alignment "V" on the block and he pulls the cap and notes that the rotor appeared to be pointing one cyclinder off (closer to the front of the engine than to #1). So he pulls the distributor and rotates it one tooth. Now the boat idles great and the line on the harmonic balancer showed up under the timing light, but when we took it out to trial, it had a misfire above 2000 RPM and would struggle to get to 3500 RPM. I wanted to rule out the notoriously unreliable cap/rotor, so, having the one that came with the replacement, still unused, I installed that combination with no change in performance. My friend checked that each cylinder was firing at low RPM by dropping each one offline and noting the RPM drop. The codes that were logged by the ECM were standard and seemed to be pretty unrelated. Fuel level sensor error, trim position sensor error, pitot error (I assume for a non-existent speed input).
In all honesty, I was surprised that it would run at all if it were off by a tooth. My suspicions are that either I'm 180 degrees off and it is still able to run, or possibly the distributor position sensor on the new one may not be working correctly and the behavior that I'm seeing is the result of the timing not advancing at all from idle. My mechanic friend believes it's a fuel delivery issue that just happened to come up at the same time as this distributor replacement. He verified the continuity of the vacuum line from the intake manifold to the fuel pressure regulator and noted there was another vacuum line that went somewhere to the back of the engine, but couldn't find where it went with the engine hot.
I'm curious what those of you who are familiar with these systems can suggest that I try.
