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Author Topic:   OMC V6 Missing at Idle

pglein posted 07-07-2008 02:02 PM ET (US)
The saga of my rebuild continues: Ok, so the rebuilt 1985 (I've decided it's an 85, not an 84) 150-HP Johnson V6 now has about 50 hours on it. It's through the break-in fuel, and running well on the VRO. I've replaced the plugs and plug wires, and it runs much more reliably now. I fixed some gremlins in the electrical system (of the boat), and the batteries seem to hold their charges now, so I can actually turn off the engine when not in use, which is a huge relief.
However, one nagging problem seems to remain. When motoring around at idle, every so often (I'd say every 5-10 seconds, occasionaly more often) it "misses". That is, the engine noticably slows for a second, then resumes normal speed. Once I speed up, the engine seems to run smoothly and with plenty of power.
I have a couple theories:
Fuel: Perhaps my fuel tank has water in it, and the hiccups are occuring when some water gets past the Racor filter/water seperator. Or, perhaps the fuel filter is clogged. I did replace it when I bought the boat, but I haven't replaced it since rebuilding the engine, or since running the break-in fuel.
Electrical: The wire harness is all original. The connectors that attach to the coils sometimes come loose. It's possible one or more of these connectors is not making a good connection. I've tried tightening them up as good as possible, and from what I can tell, they should work fine, but they're old. Who knows. I've been unable to find a replacement harness. I suppose it's also possible that the coils or powerpacks are faulty. I used the original ones.
Any idea where I should start? Fuel filter? Wiring harness? Drain the bowl more often?
Chuck Tribolet posted 07-07-2008 03:12 PM ET (US)
A clogged fuel filter will cause problems at high power, not
low.

Chuck
pglein posted 07-07-2008 03:34 PM ET (US)
That's kind of what I thought. Thanks for the input. I guess I can check that one off the list.
Sal DiMercurio posted 07-07-2008 04:52 PM ET (US)
First of all, you can never run just off the engine, without having your battery hooked up to the engine, because you will blow your rectifier. Even a loose battery cable will blow your rectifier. As for water in your fuel, it would miss steadily, not just a stutter every 10 seconds.
Sal
pglein posted 07-07-2008 06:37 PM ET (US)
Sal, I'm not sure what you're refering to. My batteries (at least one) are always connected when running. What I was saying was that before they were draining their charge within minutes when I shut off the engine. Turned out that the bilge pump float switch was stuck in the "on" position, and there was a piece of rope stuck in the blades, so it was just trying to run constantly, but not making any noise, so I couldn't tell.
One thing I have noticed, and this may just be the alternator that is on the engine, is that the voltage meter never goes above 12 volts, even when the engine's running. I would expect it to read around 14 when the alternator is charging. Any chance I need to replace/rebuild the alternator, or is this just the normal output for this engine? It does recharge up to 12 volts after being drawn down, so it's not like the alternator is just plain not working.
Sal DiMercurio posted 07-07-2008 07:14 PM ET (US)
PG, I was thinking you were saying you could just turn the batteries to off and run off the engine juice, but now I re-read it and see what your saying. Sorry about that. Your rectifier won't put out 14 volts until your [engine speed] reaches at least 3,000-RPM At idle or low rpm the charging system in your engine can barely keep the battery maintained, let alone charged. I think if not mistaken your rectifier only puts out 9-amperes , so you really have to put some revs on the engine to produce much juice. Whatever you do, take the wing nuts off on the batteries and replace them with double stainless steel nuts and use a wrench to tighten them. If even one of the cables gets loose, it will blow your rectifier and your tach will stop working.
Sal
jimh posted 07-07-2008 11:39 PM ET (US)
A misfire could be due to a spark ignition problem. The ignition coils are driven by the Power Pack. These devices seem to be often replaced. I had to put a new Power Pack on my 1992 V6 OMC motor when it became increasingly intermittent about firing cylinder number five. The Power Pack is somewhat expensive to replace without some strong evidence pointing to it as the fault.
Check the ignition coil ground connections. Typically the ignition coils have a metal tab that is bonded to the engine block as the ground circuit path. My master mechanic recommended that I move the star washers so that they were positioned between the engine block and the metal tab. The points of the star washer will protrude into the block and the metal tab and help insure a very positive connection.

pglein posted 07-08-2008 11:48 AM ET (US)
Jimh,
Thanks for moving the discussion, and thanks for the input. I will check all the grounds and connections and see what I come up with. Given that everything was removed and replaced, it's entirely possible that I neglected to secure every ground properly.
JMARTIN posted 07-08-2008 01:58 PM ET (US)
Did you have the carburetors synchronized? Wait, this is a 1985, does it have six carbs? John
Don88outrage posted 07-08-2008 02:40 PM ET (US)
I have a similar problem with a 1988 OMC V6, it misses from 2 to 5 minutes at the start of the day regardless of air temperature. Once it clears up it won't miss again all day long. I'm religous about fuel additives, clean filters and preventing carbon build up and have talked to mechanics for 3 years now and do not have an answer.
pglein posted 07-08-2008 02:56 PM ET (US)
JMARTIN,
No, it has three dual-barrel carbs. They are not adjustable. The linkages are all fixed.

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Author	Topic: OMC V6 Missing at Idle
pglein	posted 07-07-2008 02:02 PM ET (US) The saga of my rebuild continues: Ok, so the rebuilt 1985 (I've decided it's an 85, not an 84) 150-HP Johnson V6 now has about 50 hours on it. It's through the break-in fuel, and running well on the VRO. I've replaced the plugs and plug wires, and it runs much more reliably now. I fixed some gremlins in the electrical system (of the boat), and the batteries seem to hold their charges now, so I can actually turn off the engine when not in use, which is a huge relief. However, one nagging problem seems to remain. When motoring around at idle, every so often (I'd say every 5-10 seconds, occasionaly more often) it "misses". That is, the engine noticably slows for a second, then resumes normal speed. Once I speed up, the engine seems to run smoothly and with plenty of power. I have a couple theories: Fuel: Perhaps my fuel tank has water in it, and the hiccups are occuring when some water gets past the Racor filter/water seperator. Or, perhaps the fuel filter is clogged. I did replace it when I bought the boat, but I haven't replaced it since rebuilding the engine, or since running the break-in fuel. Electrical: The wire harness is all original. The connectors that attach to the coils sometimes come loose. It's possible one or more of these connectors is not making a good connection. I've tried tightening them up as good as possible, and from what I can tell, they should work fine, but they're old. Who knows. I've been unable to find a replacement harness. I suppose it's also possible that the coils or powerpacks are faulty. I used the original ones. Any idea where I should start? Fuel filter? Wiring harness? Drain the bowl more often?
Chuck Tribolet	posted 07-07-2008 03:12 PM ET (US) A clogged fuel filter will cause problems at high power, not low. Chuck
pglein	posted 07-07-2008 03:34 PM ET (US) That's kind of what I thought. Thanks for the input. I guess I can check that one off the list.
Sal DiMercurio	posted 07-07-2008 04:52 PM ET (US) First of all, you can never run just off the engine, without having your battery hooked up to the engine, because you will blow your rectifier. Even a loose battery cable will blow your rectifier. As for water in your fuel, it would miss steadily, not just a stutter every 10 seconds. Sal
pglein	posted 07-07-2008 06:37 PM ET (US) Sal, I'm not sure what you're refering to. My batteries (at least one) are always connected when running. What I was saying was that before they were draining their charge within minutes when I shut off the engine. Turned out that the bilge pump float switch was stuck in the "on" position, and there was a piece of rope stuck in the blades, so it was just trying to run constantly, but not making any noise, so I couldn't tell. One thing I have noticed, and this may just be the alternator that is on the engine, is that the voltage meter never goes above 12 volts, even when the engine's running. I would expect it to read around 14 when the alternator is charging. Any chance I need to replace/rebuild the alternator, or is this just the normal output for this engine? It does recharge up to 12 volts after being drawn down, so it's not like the alternator is just plain not working.
Sal DiMercurio	posted 07-07-2008 07:14 PM ET (US) PG, I was thinking you were saying you could just turn the batteries to off and run off the engine juice, but now I re-read it and see what your saying. Sorry about that. Your rectifier won't put out 14 volts until your [engine speed] reaches at least 3,000-RPM At idle or low rpm the charging system in your engine can barely keep the battery maintained, let alone charged. I think if not mistaken your rectifier only puts out 9-amperes , so you really have to put some revs on the engine to produce much juice. Whatever you do, take the wing nuts off on the batteries and replace them with double stainless steel nuts and use a wrench to tighten them. If even one of the cables gets loose, it will blow your rectifier and your tach will stop working. Sal
jimh	posted 07-07-2008 11:39 PM ET (US) A misfire could be due to a spark ignition problem. The ignition coils are driven by the Power Pack. These devices seem to be often replaced. I had to put a new Power Pack on my 1992 V6 OMC motor when it became increasingly intermittent about firing cylinder number five. The Power Pack is somewhat expensive to replace without some strong evidence pointing to it as the fault. Check the ignition coil ground connections. Typically the ignition coils have a metal tab that is bonded to the engine block as the ground circuit path. My master mechanic recommended that I move the star washers so that they were positioned between the engine block and the metal tab. The points of the star washer will protrude into the block and the metal tab and help insure a very positive connection.
pglein	posted 07-08-2008 11:48 AM ET (US) Jimh, Thanks for moving the discussion, and thanks for the input. I will check all the grounds and connections and see what I come up with. Given that everything was removed and replaced, it's entirely possible that I neglected to secure every ground properly.
JMARTIN	posted 07-08-2008 01:58 PM ET (US) Did you have the carburetors synchronized? Wait, this is a 1985, does it have six carbs? John
Don88outrage	posted 07-08-2008 02:40 PM ET (US) I have a similar problem with a 1988 OMC V6, it misses from 2 to 5 minutes at the start of the day regardless of air temperature. Once it clears up it won't miss again all day long. I'm religous about fuel additives, clean filters and preventing carbon build up and have talked to mechanics for 3 years now and do not have an answer.
pglein	posted 07-08-2008 02:56 PM ET (US) JMARTIN, No, it has three dual-barrel carbs. They are not adjustable. The linkages are all fixed.