1988 Mercruiser 3.0 - Random stall / start issues

[dadof3]

Good evening.

Long time watcher, first time posting.

1988 Mercruiser 3.0
SN: 0B900xxx

## Here is a brief summary of what is happening:
Issue started a couple of years ago. With a cold engine, the boat will start without issue. After approximately 30-45 minutes, boat will either randomly stall out, or if we shut it down after running for some time, it will not start back up. The engine will turn over, but does not start. I have removed the flame arrestor to confirm the carb is squirting gas. Notice that the coil is very hot to the touch. Once we let it cool down for a good 10-20 minutes, the boat will start. I have confirmed with an in-line spark tool that the plugs are indeed getting spark.

## Troubleshooting steps taken:
*Replaced coil with OEM Mercruiser coil. Did not help.
*Drained fuel tank and replaced with fresh Premium gas (have been running this way for 2 years now)
*Removed Mercruiser coil, points, etc and replaced with brand new Pertronix 3 ohm coil and Pertronix electronic ignition, new distributor cap, rotor, wires, plugs, and battery. It is my understanding that this model coil calls for the removal of the resistor wire, since it is a true 12v coil.
*Replaced all gauges
*Again, after 30-45 minutes the boat either stalls out, or will not start if we shut it down (to take the kiddos swimming or water skiing). Again, if we let it cool off for a good 20 minutes, it starts again.
*Removed and taped up the gray wire from the negative side of the coil in an attempt to rule out the tach and shift interrupter switch. With a cold engine, took boat back out. Still, after 30-45 minutes, boat would not start after shutting down.

***Something very peculiar that must be noted: When we have had the issue, we have discovered that when we connect the in-line spark tester in place, the engine starts!!! It will then run fine, without hesitation or misfires until being shut down again. I have duplicated this multiple times, over multiple outings. How crazy is that???

Thank you in advance to anyone who can help point me in the right direction.

 

[Scott Danforth]

check your vent on the fuel tank

check for +12 volts at the + side of the coil when it wont start

BTW, it is almost never ever the coil.

 

[alldodge]

See if the carb is flooding, see if fuel is dripping. Have you tried giving it more throttle while starting?

 

[dadof3]

Scott, thank you very much for your prompt reply.

I will check the vent tonight when I am at the dock, barring any rain.

Curious, what should my readings be at the + side of the coil when the engine is running?

If memory serves me correctly, I am getting approximately 12v when key is on and approximately 13.4v when running.

I have checked these readings on all the purple wires (purple wires behind dash, purple wire on electronic choke at carb, and purple wire on alternator) and I am getting a fairly consistent reading of 13.4v when running.

Does 13.4v when running sound about right?

check your vent on the fuel tank

check for +12 volts at the + side of the coil when it wont start

BTW, it is almost never ever the coil.

 

[tlewis1]

Yes 13.4V would be ok when the engine is running. It is odd that the engine would start when the inline spark tester is installed. When the issue arises you need to make sure there is sufficient spark going to each plug not just to the distributor. I would also be looking at the carb as AllDodge said. Could be weeping fuel when shut down. Put the throttle all the way down when starting when hot (be ready to pull it back tho). What do the spark plugs look like after attempting to start it.

 

[dadof3]

Thank you for your responses everyone. Rain is in the forecast, so it may be a few days to report back. Will let you know my findings.

 

[dadof3]

Good morning,

With nicer weather in store today, I got the shift interrupt switch re-installed this morning along with the gray tach wire. I did not get to take it out as we had some family obligations.

I checked the vent on the tank and there was a little bit of dry wax stuck in there. It didn't seem like much, but I cleaned it out nonetheless.

I was able to to take a few measurements with the voltmeter:

At the + side of coil I am getting 12.7v key on. With the engine running I am getting a very random and sporadic reading between 14.x volts down to 5.x volts. I swear I even saw the digital voltmeter drop down to 2.x volts a few times. It happens very quickly, almost instantaneously.

There is no sputter or hesitation from the engine.

I took a reading from the purple wire at the back side of the ignition key switch and it was a steady 14.x volts. No sporadic reading.

Shut the engine down and repeated these readings three time with similar results.

Does this sound normal for the readings on the + side of the coil to jump around so randomly? Do I possibly have a short somewhere in the harness?

 

[alldodge]

Right now I don't see anything your finding that makes me thing of other issues. We need it to die and then see what the voltage is at the coil

 

[dadof3]

Thank you. Just got back from our family function. I'll head on down to the dock and take it out. Give me a couple of hours to report back.

Right now I don't see anything your finding that makes me thing of other issues. We need it to die and then see what the voltage is at the coil

 

[dadof3]

Alright, got to take the boat out tonight. It took about 50 minutes this time and then started to sputter again and die out. As requested, I checked the voltage at the + side of the coil and here are my findings with a dead engine:

Key on: 11.7v (as previously noted, with a cold engine, was getting 12.7v)
Key on, and attempting to turn engine turning over: 10.5v (maybe even a little bit lower)

Thoughts?

 

[alldodge]

Do you have points and if so has the dwell angle been measured (28-34)?

The 10.5V while cranking is showing there is either a high current draw or a weak battery.

 

[dadof3]

It does not have points. It has been converted to a pertronix electronic ignition and pertronix 3 ohm coil. The issue started before it was converted.

(Just to note, last night when I took it out for a test run and it died, I pulled the spark plugs, grounded to the block, and confirmed I was getting blue spark. I also pulled the flame arrestor and had a passenger pump the throttle 3-4 times and confirmed 2 solid streams of fuel squirting in the carb.)

Other then the battery, where should I test or look at next next?

Do you have points and if so has the dwell angle been measured (28-34)?

The 10.5V while cranking is showing there is either a high current draw or a weak battery.

 

[alldodge]

The motor dies and you have spark, fuel and air and it will not start, I have no idea unless its flooding. Would say may the pertronix but you have spark

 

[dadof3]

If I understand correctly, you are leaning towards either the electronic ignition or fuel related (flooding, etc)? I am not seeing fuel dripping into the carb when shutdown, fyi.

I did have the filter replaced and the carb inspected last year by a marine mechanic.

Another person recommended replacing the fuel pickup tube and anti siphon valve on the top of the tank. I ordered them and should be here soon. I'll let you know of my findings.

Please let me know if I should be looking at anything else?

The motor dies and you have spark, fuel and air and it will not start, I have no idea unless its flooding. Would say may the pertronix but you have spark

 

[tlewis1]

What do the spark plugs look like when the issue happens? Any chance the head or gasket is allowing water through into the cylinders when the engine is warmed up.?

 

[dadof3]

Good question! Now that you mention it, it seemed like the threads on the plugs were very oily. I cleaned each with a rag before reinstalling. The gap, however, seemed dry (i.e not fouled).

What do you think this signifies, if anything?

What do the spark plugs look like when the issue happens? Any chance the head or gasket is allowing water through into the cylinders when the engine is warmed up.?

 

[dadof3]

Good morning,

Made it down to the dock last evening before the rain and took a closer look at the plugs. I believe the residue was antiseize from when they were installed (I believe I used copper anti-seize at the time). I looked at the base of the valve cover and gasket and did not see any oil dripping down, so no visible leaks there. When I took it in to a mechanic previously I believe one of the first things they performed was a compression test. I didn't get numbers, but was told it checked out ok.

It concerns me that the coil is running hot. I'm not a mechanic, nor an electrician, however, my job requires that I employ a range of troubleshooting skills. While I will be the first to admit I am wrong, my gut tells me it may not be fuel related.

With this said, I am going to look for some sort of short or ground fault, ruling each device out, one at a time.

While out on Sunday night, I noticed the trim gauge needle bounce a few times. It did this a few times a couple of years ago, but has been fine since replacing all of the gauges.

Last night we took the boat out and put the drive down. We then disconnected the purple wire from the back of the trim gauge along with the heavy gauge red/black wires that go directly from the trim pump to the battery.

We then drove the boat for a good 50 minutes without it dying. I shut it down and started back up about 10 times with success ever time.

We rode the boat hard with multiple bouts of acceleration and deceleration, up to WOT and back down to idle, cruising, etc. Trying to simulate a typical day of taking the kiddos water skiing and tubing.

When we got back to the dock I then grabbed the coil and it felt noticeably less hot. I say less hot as it was still very warm, but this time you could grab it and hold on for more then 5 seconds.

 

[tlewis1]

http://boatinfo.no/lib/mercruiser/manuals/mercruiser8.html#/0

In section 4E of the above manual for your engine gives a wiring diagram for the trim system and engine electrical. I am not too sure why disconnecting the trim pump from the battery would change the running issue with your engine. Possibly bad battery or connections...??

 

[alldodge]

The purple wire feeds every gauge, the coil and distributer. If anything was pulling to much current it would blow the 20 amp fuse, unless the fuse has been replaced with a larger fuse. Power goes to the coil and only the distributer connection and tach have any effect or it. If something else on the purple wire was pulling more current it has no effect on the coil.

If the trim pump and or relays were intermittently shorting out it can kill the motor IF the battery and alternator can not keep the voltage up.

 

[dadof3]

My apologies for the delay in responding. Between work, family, and the constant rain we've been having it's been difficult to get down to the dock.

Well, I think it's running much more reliable now and am fairly confident that I can say it is fixed.

Wow, so where do I begin!

Wiring corrosion in the engine compartment. And lots of it! I started to file the ring connectors, but ended up replacing a bunch of the harness, including the battery cables. I went 1 gauge larger with custom made wires and heavy duty connectors.

Starter relay. I don't know if this was necessary, but for 20 bucks, I thought I might as well and just start new.

Trim pump. Do I believe this was the problem? I don't know. The trim gauge did randomly bounce a few times as previously noted. It also made an occasional weird grinding noise and seemed to cut out a time or two, so thought it was time to replace. When I bled the new pump, there was only about a half a quart of darkish fluid before it ran clear. A benefit is that the trim is running smoother now.

Resistor wire. When I installed the pertronix I bypassed this per the instructions, since it is a 3 ohm coil and spliced into a switched 12v purple wire. I decided to go back to the resistor wire, but cut it so that it is about 9" in length. Enough to give me 12v, but still enough resistance to keep the amps down.

Fuel pickup tube and anti siphon valve. Pulled and replaced. It did not have an anti siphon valve. Do I think this contributed to the issue? I guess I will never know, but as previously noted, I got plenty of gas when pumping the throttle when the stall issue emerged.

Going back to the trim pump: Here is the interesting part about the trim pump: when tracing the wires from the pump, there were (I believe) two wires that terminated on the Mercathod from the pump. I then traced a pair of wires from the Mercathod down to the anode on the bottom of the outdrive. One, the anode was completely dissipated or broke off, but what was more interesting was that both wires were severed and clearly exposed bare copper. Could this have been grounding out the trim pump, thus going all the way back to the comment about voltage?

In the meantime, I have removed the Mercathod and all wiring relating to it. I know this is not recommended, however the boat is on a lift, fully out of water every night, no shore power etc.

I took it out today for one last hurrah! (Yes our seasons are very short here up north.). She ran like a champ for close to two solid hours with no stalling or issues with starting. Stopped and shut down the engine about a dozen times and it started up just fine every time. I grabbed the coil when heading back to the dock and was able to hold on to it for well over twenty seconds before my hand even became uncomfortable. Previously, my hand wound have been burned after 5 seconds.

It could have been any one of these, or a combination of. I guess we'll never know. I do believe in sharing what I have done, with hopes that It will somehow give others with similar symptoms a reference point somewhere down the road.

Was all of this work overkill? I really don't think you can't put a price tag enjoying time on the water with family, especially the kiddos who love to waterski and wakeboard so much!

Thank you very much for everyone's input.