Mercruiser v8 cuts out when hot

[jakwi]

Hi all, I'm hoping to get some input on the latest issue. I have a Mercruiser 357 Alpha 4v. It is essentially a Mercruiser 350 repower. Carbureted engine.

Background
For the last month or so we have been sticking relatively close to the ramp. Maybe 20 minutes of motoring, with 5 on plane. It always runs fine. Before that we have taken at least a few trips that were more than an hour of motoring with half of that on plane. So I can say for sure at that time it was running fine.

I did have an issue where after a week of being on the trailer it didn't want to start. It seemed to be the cap and rotor because after I would clean them it would fire right up and run strong. I finally replaced them with Quicksilver parts. This seemed to completely resolve the issue. Now when I start it for the first time in two weeks it maybe takes 20 seconds of cranking, but then after it gets gas to the carb it starts right up, first crank. I also swapped the prop out at this time from a 17p to a 19p. Although I think the 17p is the correct prop as max throttle is at redline. I thought the 19p would allow me to keep the rpm's down on plane. It works great for that. at 4k I was doing 25, now I'm doing 30. Didn't make a significant difference in the holeshot.


The Problem
The last time we had it out about two weeks ago we motored a little more than our standard 20 min and there was a moment when we were on plane when it felt like someone had hit the kill switch for 1/2 of a second. Just a moment and then everything was fine again. We had no other issues. I didn't know what to think but the issue was completely gone.

We decided to go for a sunset cruise last night, down to Ponce inlet. We put in at the Halifax Marina ramp, and motored down there. This is about a 45 min ride with about 3/4 on plane. The final 10-12 min or so is all low speed, and then in the inlet you can go up on plane again. It ran great the whole way, right up until the final part when we were actually in the inlet on plane. It started the same way. Momentary lapse. At this point I was thinking it was an electrical issue, bad connection to the ignition system.

It continued to get worse as we worked to get anchored. Coughing and spitting. Once we were anchored I pulled the cover made sure the spark arrestor wasn't clogged. I wiggled the wiring I could reach easily, and made sure everything was tight.
Battery voltage was 13.9 with the motor off according to my chartplotter, but 12 according to the gauge on my cluster. Voltage was 14.4 when running correctly on the chart plotter, and maybe 13 on the gauge.

We let it sit for 20 min and then decided to return early, not wanting to end up struggling after dark. Beautiful sunset btw.

We returned on the back side of the island which has a lot less idling time. It ran great for about 10 min of idling and another 10 or 15 on plane and then it suddenly cut off. Again coughing and spitting. It was at this point I noticed my temp guage seemed a little higher than normal,... I think. It was reading one line high, about 200F I think the normal is 175F. I didn't have any alarms. The water temp was 84F We fooled around for a bit, but it was progressively getting worse. The last time I shut it off here it continued to diesel for another 30 seconds after I killed it. This is what really made me think it was overheating. I checked the manifolds. Port side was normal, warm to the touch, but not hot. Starboard side was definitely hotter, which has been normal for this boat even with brand new manifolds. I could keep my hand on it for 30 seconds, but it was borderline to hot to do that.

We anchored for about 15 min.Restarted, and proceeded. It was basically dark now, my wife progressively more unhappy. Temp started at 175 but climbed. it varied between 200 and maybe 190. We kept going at at a high displacement speed. Maybe 10mph 2000 rpm. I thought maybe it might be overheating? So I thought the slower rpm might get us further. We ran another 20 minutes this way Never going on plane. One time it started to die, so I just backed off, it seemed to recover after a couple of minutes, We carried on the same way, and then it eventually died the same way coughing and spitting I was able to get us anchored before it completely died, but same deal. I also opened the fuel cap to see if it was under vacuum, but I didn't hear any suction, and it didn't make any difference. Wait 15 min. Restart and it seemed to be fine again. No changes. We decided to stop at the dunlawton ramp, which is about 5 miles closer, and have my son go get the truck and trailer from the other ramp.

So now I'm trying to figure out what to do. My cap and rotor have about an hour and a half of running time. So they seem unlikely as the culprit. My impeller and lower oil was changed 1 year and 2 months ago. So while they are on my list for this winter they aren't excessively overdue. On the muffs the exit water ports are pushing out water, so I know the impeller is at least working partially. It really seemed to be heat related, or at least time running related. I could throw a fuel filter at it. Thoughts?

Thanks for the help

 

[alldodge]

Original serial number???

 

[jakwi]

Installed in 2010
manifolds and risers changed in jan 2019
Serial Number : 1G401632
Model Number : 865108R88

I just swapped the fuel filter, no water in the filter, looked good. Running it on the muffs now

 

[jakwi]

After 20 min of running at 1500 rpm on the muffs temp is at 185 and stable. I also cleaned the spark arrestor, it was a little dirty, it's been 6 months or so since I cleaned it

 

[jakwi]

At idle
https://youtu.be/wlWDQSGks0k

And at 1500rpm
https://youtu.be/pt5QhpDA07c

while it ran fine on the muffs, my fear is out on the water. My water temp from the hose is probably 75f which is cooler than the intercoastal

 

[alldodge]

If the gauge is accurate then its running hot. Find a 160* thermostat for your motor, and would check with IR temp gun.

Can add a piece of clear hose on the line going into the thermostat from the drive. Would only be effective with it in the water, a hose would not supply enough water

Where is the TBV mounted?

 

[jakwi]

The TBV is on a bracket behind the distributor. I'll get a new tstat, and check it with a temp gun.

We are on the water now for a test. So far 15 min at plane, and 5 at idle with no issues

 

[isaacs]

My boat did the same thing though I didn't notice any temperature problems. It turned out to be the sensor in the distributor; apparently they fail all the time. Mine got to the point where it wouldn't start at all--had no spark. $50 and easy to change.

 

[jakwi]

Well I figured it out. After about 30 minutes of running great it suddenly started again, almost died, and then was coughing and spitting. I got setup to check voltages and other common things and I noticed that the coil was arching from under the lead cap to one of the wires on the coil. Clear visible spark. I shut it off and tried to push the cap down further. It seemed good, I couldn't see the spark, but when I touched it I got zapped.

So I busted out some electrical tape and wrapped it up as best I could to minimize the problem. Hoping to get back to the ramp. It ran pretty well the whole way back. Of course this isn't a fix, but I'm down to either coil or wires.

I sort of think the high tension wires are the answer, but I don't know. Is there any way to know which it could be?



Bob Thanks for the info on the sensor. That is good to know. I don't think it's my issue this time, but still good info.

 

[alldodge]

If coil isn't cracked then its the wire

 

[jakwi]

Ok, thanks, I'll examine the coil closely. I didnt see a crack, but I wasn't looking that closely either.

 

[isaacs]

Ok, thanks, I'll examine the coil closely. I didnt see a crack, but I wasn't looking that closely either.

Glad you found your problem--my daughter's boat did the exact same thing.

You could try cleaning the top of the coil and the insulator on the wire with WD40 and a rag. This will drive out any unwanted moisture and grease. There may be some (probably green) grease up inside the coil wire cap. This is ok and is there to prevent corrosion and arcing. You want to get rid of any old black grease where the insulator meets the coil.

 

[jakwi]

So unfortunately I still have an issue.

I'm starting to think that I had two separate problems. First a momentary cut out while up at speed, and second the arcing coil.

Since my last post I have replaced the Coil, Wires, and ignition sensor. The arcing at the coil issue is no more, and I can touch all of the wires without getting zapped, it also doesn't get progressively worse like it used to. Now I just have the momentary fault at speed.

In the last three months I have changed all of the above plus the cap and rotor and plugs. The only original parts of the ignition system are the ECM, knock module and the nuetral interupt switch.

Today I took it out after replacing the ignition sensor. I was convinced that was the culprit.

It ran for about 10 min before I had my first cut out. When it cuts out it is just a moment, and then it runs fine afterwards. I found a spot to anchor and looked at the schematic. I pulled the connectors of the ECM and the anti knock module and then reconnected them. Then I started to think about the neutral interrupt switch. I pulled it tested it with a meter. 1.4ohms when closed. It seemed a little high for a switch, but I don't know what to expect. It seems to me that a switch should show 0.2 ohms when closed. I adjusted the positioning and reinstalled it. I proceeded to do laps with the boat. About 5 miles each way all up on plane. Three times it had the failure. About once per lap. I ran it for about 1.5 hours mostly up on plane. 4200 rpm. The video on youtube shows the failure as reflected on the tach. You can't really hear the motor in the video, just wind noise, and you can't see the impact, but it was a definite loss of power for a second, so it isn't just an indication issue. You feel it, and hear the motor cut out.

After the first three laps, it settled down, and didn't happen again. When it did happen it was just a second. Not enough to lose plane, and then it would be fine again for another 10 or 15 min maybe. The last 45 min it ran perfectly, at this point I almost feel like it is fixed, but the only thing I touched was that switch and it failed several times after that in the first few laps.

On a previous time out I jumped power from the battery to the coil directly to try to eliminate the possibility of an intermittent connection for power, but it happened while that jumper was in place so, from my perspective the ignition switch, and kill switch and the rest of the harness must be ok, at least as far as the power to the coil is concerned.

So that is where I'm at. After 45 min of running great I decided to call it a day. Can't fix it if it isn't failing, and I was just burning gas. I feel like I must have an intermittent electrical fault, but I have no idea where to look. Could the neutral switch cause something like this? Any other ideas?

Thanks again guys for the help. I really appreciate it.

 

[alldodge]

Not the neutral safety because it is a connection for the starter engagement only

The shift interrupter could be the issue if if makes contact. It is needed only to shift out of gear (back to neutral). After getting boat out on the water, disconnect it and see if it happens. To get the motor back in neutral will need to reconnect or turn motor off

 

[nola mike]

That looks like all power to the ignition cutting out rather than a miss. I think the dead man switch could cause that behavior as well.

 

[mr 88]

Your lugging the engine with the 19" prop you put on it and probably using more fuel in the process along with excessive wear on the motor. If your WOT was a right on the max rated rpms then you should leave it that prop on it. You do realize that you have to give it a lot more throttle to reach that cruising speed of 30 mph even though the engine is turning the same rpm as it was with the 17" . I bet your top end is more with the 17 and your engine isn't working as hard vs the 19 . At 4,000 rpm your pretty much wide open and sucking gas big time with the 19.. Most V8's are geared ,propped to be more efficient to cruise around the 3,000 rpm mark ,give or take 1-200 rpms.

 

[isaacs]

I just thought of something else I found on my engine when I was trying to diagnose my identical problem. (Good job catching it on video!)

I have a wire that runs very close to the top of the dipstick. I had the ignition turned on and was poking around and was pretty sure I saw a small spark between the wire and the dipstick. Upon close examination, I noticed that the insulation had worn away and a tiny bit of bare wire was shorting out at that point. I had to look really hard; the hole in the insulation was very small.

I got all excited, feeling that I had finally found my problem. I fixed the wire but still had problems. Now I'm wondering if this little intermittent short did something to my sensor? Yours is probably different but it's worth looking at.

 

[jakwi]

Right, the Shift Interrupter switch. I'll try disconnecting it the next time I'm out. I'm also trying to figure out how to eliminate the kill switch to test that, but the wiring is pretty inaccessible. (Edit)[ I just realized while looking at the schematic that when I jumped power to the coil I was backfeeding power to the ignition system, bypassing the ignition switch and the kill switch, so that pretty much only leaves the shift interrupter switch or a random short somewhere as a potential culprit.] Part of me just wants to replace all of these switches, but at $50-75 a pop they start to add up.[so maybe replacing the shift interrupter switch is the right choice.]

On the prop, with the 19 at 4200 I'm about 3/4 throttle Full thottle on the hole shot is about 4900 and up on plane is about 5200, I don't run it like that hardly at all. The 17 was about 200 or 300 rpm faster. but as far as speed it is without a doubt faster with the 19 than it was with the 17. With the 17 my 4200rpm speed was 25mph, vs 32mph with the 19. I did try a 21 for one day, and that was as you described, lugging and struggling. With the 21 it struggled to get up on plane, it was a real endurance test. With the 19 it gets right up no problem. With the 17 it jumped up. I'm pretty confident with the 19, I think it has been a good compromise. Thanks for the input though, it's always good to be challenged, it helps to rethink your process. and to be honest that's why I put so much info in my original post.

As far as a bare wire, it's certainly possible its something like that. I've tried to look the harness over, but I'll continue to check it. I'll look in the area you described to see if it's the same for me. Thanks for the ideas. I really wish it would fail harder so that I could just identify the issue. :/

 

[isaacs]

That looks like all power to the ignition cutting out rather than a miss. I think the dead man switch could cause that behavior as well.

When he jumped from the battery directly to the coil he was bypassing the dead man switch along with just about everything else. The shift interrupter switch however works differently. Rather than cut the power to the coil, it looks like it shorts something in the ignition to ground.

Jakwi, your troubleshooting skills are good and you're running out of things to spend money on! After you rule out the shift switch and wiring to the ECM, you're left with the ECM and knock sensor. I hear the ECM rarely fails and is expensive; don't know anything about the knock sensor. Lets hope it's the switch. I would disconnect it before even starting the engine; if you have trouble getting out of gear simply turn off the ignition.

I feel your pain about not being able to catch it in the act of failing. My boat tortured me all summer 'till I finally got "lucky" and it wouldn't start at all. The only problem was, I was 30 miles from where I had launched the boat and had just spent the night in a hotel. After fighting with it for over an hour, I was somehow able to get it started and made it back. It was then that I found the bad sensor.

Good luck and keep us posted!

 

[nola mike]

I missed that the coil was jumped. So yeah, that leaves you with a short to ground, prob with the ign module, or coil (ducking). So next step is disconnecting shift interrupt, and the tach wire from the coil (missed if that has been done yet?)