1982 OMC 2.5L won't idle/start when hot

[Gil1982]

Have a 1982 OMC I/O, 2.5L, model # 252FTHRCNC, serial # J740693. Just replaced the plugs, plug wires, coil, points, condenser, rotor, distributor cap, and thermostat due to some starting and overheating problems. It starts and runs great when cold. Dwell, timing and WOT are all on spec. Can run it hard for an hour or so and it will run great and start fine. Then all of a sudden it gets hard to start. If I pump the gas a little, it will start but then coughs and spits some, idles rough and dies when I put it in gear.

Also, if I can get it started it will run good with the throttle up but as soon as I idle it and put it in gear it dies. Another thing that seems strange is that if it starts, at idle, the rpm will move around between 600 to 1100 and back, on its own. When it is cool it idles smoothly at 600 rpm.

If it cools for 15 minutes or so it will start and run good again for a while. The last trip in it started to run rough even at 3000 rpm, I had never noticed that before. Usually, if it ran, it ran good. Had to paddle it in the no wake zone as it kept dying at idle and/or shifting.

I did have some tools with me this last time and it had spark (at least with this kind of a spark tester Search: 25227 | Ignition Tester | AutoZone.com
) and the dwell and timing was OK. Engine temperature seems to be OK at least according to the gauge. The starter always turns the engine over OK. I can?t seem to duplicate this problem when running it on the muffs, it?s only an issue after it runs hard for an hour or so.

Any suggestions? What should I be checking when it does this to help troubleshoot the problem? Thanks.

 

[bonzoscott]

Re: 1982 OMC 2.5L won't idle/start when hot

Welcome to iboats! First thing that comes to mind is fuel delivery. When this occurs, look down throat of carb and pump couple of times make sure accelerator pump is pumping good squirts of fuel. You didn't mention any of the filters. Starting at the tank, if you can access the pick up, remove and inspect for crud. Next water/fuel separator if equipped - change it. Next, check filter at fuel pump. Finally, fuel filter in carb, where fuel line attaches - inspect clean / replace. That's just my order of checking.

 

[Gil1982]

Re: 1982 OMC 2.5L won't idle/start when hot

Hi Bonzoscott, Thanks for the tips, I'll check those things. How would the fuel filters be heat related or only act up when the engine is hot?

 

[bonzoscott]

Re: 1982 OMC 2.5L won't idle/start when hot

Hi Bonzoscott, Thanks for the tips, I'll check those things. How would the fuel filters be heat related or only act up when the engine is hot?

It's not so much that it got hot - just you been runnin hard and created a vacuum in the fuel tank, holding your fuel back from the motor. This may not be your problem but is the first thing to check. The next usual issue is the coil. A faulty coil will start to fail when heated, but based on your description, it'll fire if you pump it - coughs and spits are symptoms of either fuel or fire, but fuel is the first order. Easy check - when it fails, look down carb, pump. If it is a GOOD strong squirt, it's probably not fuel. Check filter at carb - if crud in there, it's probably time for a carb rebuild. There are other variables in ignition, but always - fuel first.

 

[bonzoscott]

Re: 1982 OMC 2.5L won't idle/start when hot

It's not so much that it got hot - just you been runnin hard and created a vacuum in the fuel tank, holding your fuel back from the motor.

Opps! What I am describing by Vacuum in tank is a clogged Tank Vent. Check that it is clear as well.

 

[kbibeau1]

Re: 1982 OMC 2.5L won't idle/start when hot

I had a problem similar to this. After using the boat for about 40 mins the boat would start to misfire but not die. I found the problem to be that the resistor going to the + side of the coil was not working, causing the coil to overheat after being under a load with too much voltage.
Replaced the coil with one w/ internal resistor and good to go. Worth a shot. Also be sure you are using ethanol treatment. I have had ethanol create all sorts of problems in the past.

 

[Gil1982]

Re: 1982 OMC 2.5L won't idle/start when hot

Thanks kbibeau1, I think I'm on the same track. The voltage on the plus side of the coil is 12.7 to 14v which is high by some other posts. I ran it tonight for about 45 minutes until it started to die at idle and wouldn't start (which is what it typically does). This was with a new coil (the right one for this engine that I just got from iboats) that said on it that it required an external resistor.

When it wouldn't start I pulled that coil out, it was very hot to the touch, and put in a "cold" spare coil that I had from Advance Auto (which said on it that it didn't require an external resistor) and it started right up and ran fine. Put in the "hot" coil again and it wouldn't start. Put the "cold" coil in again and it started right up and ran fine. Swapped them out two more times and it ran good with the "cold" coil and not with the "hot" coil.

I then ran this "cold" coil about 45 minutes until it was hot and it had the same problem as the first coil.

As a side note, I tried a spark tester where you can set the gap. I set it at about 1/4" and when it was cold it would jump that gap with a good strong spark. When it got hot, it would still jump that gap but not every time, it missed maybe half of the time.

So, it certainly seems like a "hot" coil issue. How do I fix that? Replace the external resistor or resistance wire? I haven't found that yet, where is it? Is there a different coil to use? I'm stuck about the next step. Thanks for any help.

 

[Gil1982]

Re: 1982 OMC 2.5L won't idle/start when hot

I called Sierra Marine tech support and asked them about the external resistor. It is a 2 ohm wire imbedded in the wiring harness. They recommended a coil that doesn't require an external resistor. The original coil was a Sierra 18-5437. They recommend an 18-5466 which has a built in 1.5 ohm resistor. Any thoughts? Do you think this should work? Thanks much, Gil

 

[Howard Sterndrive]

Re: 1982 OMC 2.5L won't idle/start when hot

you need to follow the wire that is feeding your coil + terminal and see where it is coming from . If it has been replaced with a non-resistive wire, then yeah, you can go to a coil designed to run full 12V.
The thing is, when the old resistance wire has a problem, it wouldn't result in supplying full voltage to the coil, so someone has done some re-wiring, or your voltmeter is not accurate.
Your solution may be to just hook the resistance wire back up if it's laying there disconnected somewhere, and get rid of the 12V supply that has been added.

 

[bryancurit]

Re: 1982 OMC 2.5L won't idle/start when hot

I'm also having the exact same issue, I measured the voltage to ground on the + side of the coil and am getting 12.39 volts when the engine is running. I have verified that on my boat that the wiring appears to be original. I can't explain how the 12 volts is getting there through the original resistor wire, but it is. I have already gone and purchased a new coil with an internal resistor. I'll let you guys know how it works out.