Engine loses power while cruising, difficult to start, shuts off, etc

[vndkshn]

Thanks in advance everyone, been lurking awhile, finally have a problem I haven't found the solution to from reading and searching.

I have a 1996 Merc Bravo 3 7.4L (Serial number 0F750521). While out on the lake, it started effectively shutting off. I could cruise at 3k RPM and it would lose all power, then start running again. At times it would "backfire", other times it would completely die and be difficult to restart. It made getting the boat back on the trailer interesting as it would shut off at the worst times, you know, like those last second little corrections before you make it to the trailer!

At first I thought it might be some bad gas or water in the tank and figured it would burn off eventually, but it hasn't. And I would think that water in the gas would manifest itself a little differently than a total power loss, which led me to look at the ignition system. So I replaced the cap (which had some pretty heavy deposits on the inside terminals (basically arc residue, I've seen it on cars as well), rotor, spark plugs, and based on the advice of a local boat shop, the sensor in the distributor as they felt sure that was the culprit. I also changed the fuel filter and did a full oil change with filter. The old spark plugs looked ok, maybe a little "wet" with fuel, but I had just started the boat to test the work I had done on the cap/rotor/sensor.

Took the boat for a run yesterday and really paid attention to its behavior. Before it was fully up to temp, no real issues other than its normal "grumpiness" while cold, as I think the choke is not working. Once it came up to temp, the problem with the engine cutting out started again. I kept running it and ran for about 15 minutes across the lake without a single hiccup. Then as soon as I started turning the boat it started cutting out again. Getting it back on the trailer was a 30 minute battle filled with more cursing than I am willing to admit as it just would not keep running. I could get it running, rev it in "neutral" and let it sit there like that for 5 minutes and you could hear it shutting off from time to time. Once it was pretty stable, drop the revs to idle, put it in gear, boom... shut off. The only other thing I noticed that seemed odd, is that at idle, the battery gauge would show around 13 volts (indicating it was charging) while at cruise or WOT it would often show 11 volts or as low as 10, seemed strange to me, but I honestly have no idea how accurate the gauge is. It did not seem to have that voltage drop in neutral reving to the same RPM (say 2500 RPM).

So everyone, hopefully that gives enough information to help out. The only obvious thing I know of that I haven't tried is the coil.

 

[Eric 3.7LX Alpha 1]

Re: Engine loses power while cruising, difficult to start, shuts off, etc

I would start off by confirming that volt issue, you might be running off the battery at that point while cruising, and as volts get low giving the engine difficulty with spark.

 

[Bondo]

Re: Engine loses power while cruising, difficult to start, shuts off, etc

At first I thought it might be some bad gas or water in the tank and figured it would burn off eventually, but it hasn't.

Ayuh,.... Welcome Aboard,.... Did ya Inspect the contents of the old filter to see if there was water, or crud,..??

Why not,..?? Crud in the fuel don't burn off, it spreads to the Whole fuel system...

If yer Choke don't work, Fix it...

And,.... Insteada just throwin' parts at it because "Somebody" said whatever,...
Try Diagnosin' it...

When it Quits,....
Is there Fire at the spark plugs,..??
Is there fuel available to the carb, 'n is the carb doin' with that fuel, what it's Supposed to be doin' with it,..??

 

[vndkshn]

Re: Engine loses power while cruising, difficult to start, shuts off, etc

Ayuh,.... Welcome Aboard,.... Did ya Inspect the contents of the old filter to see if there was water, or crud,..??

Why not,..?? Crud in the fuel don't burn off, it spreads to the Whole fuel system...

If yer Choke don't work, Fix it...

And,.... Insteada just throwin' parts at it because "Somebody" said whatever,...
Try Diagnosin' it...

When it Quits,....
Is there Fire at the spark plugs,..??
Is there fuel available to the carb, 'n is the carb doin' with that fuel, what it's Supposed to be doin' with it,..??

I looked at the filter, nothing obviously nasty was in there.

How would you test to see if there was spark at the spark plugs when it quits? The problem is pretty intermittent, meaning I can't reliably make it happen. Often times it will immediately start back up (meaning the engine doesn't even fully die, it loses all power for a second or two, then catches and runs again).

I looked yesterday and didn't see a sight window on the carb, do you know of another way to see if there is fuel in the carb?

 

[vndkshn]

Re: Engine loses power while cruising, difficult to start, shuts off, etc

So I just pulled the fuel filter and drained the contents into a glass jar, no water in it. Put it back on, started the boat and ran if for a minute, pulled it again and drained again, still no water.

 

[Don S]

Re: Engine loses power while cruising, difficult to start, shuts off, etc

NEVER empty a filter and reuse it. Any dirt in it will be transferred to the other side while emptying it.
You need to always empty one and check the contents, just put a new one on so you don't add dirt to the system the filter was meant to keep clean.

How would you test to see if there was spark at the spark plugs when it quits? The problem is pretty intermittent, meaning I can't reliably make it happen.

That has been driving mechanics crazy for years. There is no easy button, if you can't force it to fail by wiggling things, then you are stuck waiting for it to fail so you can check it when it doesn't work.

 

[tpenfield]

Re: Engine loses power while cruising, difficult to start, shuts off, etc

Check the electrical connector on the wiring harness at the engine to see if it is tight or has some contact issues.

 

[vndkshn]

Re: Engine loses power while cruising, difficult to start, shuts off, etc

Yeah, its a new filter, nothing came out but gas, but I'll grab a new one anyway. I was literally trying to get a larger sample of gas to check for water.

At this point, I'm beginning to look at the coil or the ignition module as possible culprits. Since the issue doesn't really seem to be vibration or roll related (it will cut out on a glass smooth lake, or sitting there idling or reving in neutral) and the gas seems fine.

 

[vndkshn]

Re: Engine loses power while cruising, difficult to start, shuts off, etc

Check the electrical connector on the wiring harness at the engine to see if it is tight or has some contact issues.

Are you referring to where the engine's harness connects to the boat's harness? Hmm, interesting, will look at that.

The other thing that I just thought, wonder if it could be something crazy with the ignition switch? Saw something like this years ago in a former life as an auto mechanic where the ignition switch was worn to the point it would sometimes break the connection due to wear.

 

[vndkshn]

Re: Engine loses power while cruising, difficult to start, shuts off, etc

Quick question for anyone...

Should the fuel filter be full? Meaning no air at the top? It just occurred to me that when I pulled both the old and new filter off, the fuel was 1/2 to 1 inch down in the filter, not at the top as I would normally expect.

 

[vndkshn]

Re: Engine loses power while cruising, difficult to start, shuts off, etc

So I've done some more testing and some maintenance and think I've narrowed it down.

I rebuilt the carb, it was pretty nasty inside, lots of sediment and even some small black "dirt" or "grit". Either way, it was dirty, and a good cleaning was needed. The boat runs better, but still cuts out.

It seemed to be worse in choppy conditions, but once it started cutting out, it would do it in smooth/straight running as well.

This time, I was prepared. After getting the boat up to temp and adjusting the carb, I hooked a jumper from a 12volt source directly to the positive side of the coil. I then went from one end of the lake to the other and back, varying RPM from 2000 to 4000 and to idle/crawling speed..... completely smooth, no hits/misses/cuts/hiccups.... a good 10 to 15 minutes of clean running with ZERO issues... I think I'm onto something.

So, I'm starting to suspect the ignition switch (because it also hiccuped during some idle testing when I bumped the key area, but that could have been coincidence). Are there any relays or other systems between the ignition switch (while in the "run" position) and the positive side of the coil?

Other, completely unrelated things I've gotten working in the meantime. The wife and I replaced the wood for the engine hatch, I guess Baja decided that stainless steel screws were a good idea, but stainless steel T-Nuts weren't needed! Most had rusted and couldn't be tightened (or removed). We were able to reuse the vinyl (which matches the boat's graphics) and the vinyl mesh on the underside. Replaced with all new stainless hardware and used bolts with nuts instead of screws to attach to the boat... looks great and factory!

Related to that, the hatch lift motor was popping fuses, found out the previous owner (or someone else) had the switch wired wrong (switch was also missing the metal connection plate in the middle, so I replaced that as well). Once that was corrected, it was making a killer noise, it took TWO tubes of grease and still showed no signs of being full. But after several trips up and down, its quiet and smooth. Wondering if I should put some more grease in it.

 

[vndkshn]

Re: Engine loses power while cruising, difficult to start, shuts off, etc

I guess I stumped everyone.

 

[mjfink]

Re: Engine loses power while cruising, difficult to start, shuts off, etc

I guess I stumped everyone.

I had exactly the same symptoms on my 5.7TKS motor. I'll save you the buildup, it was the coil. It would heat up and then stop producing a good spark. If I let it cool down again, it would work fine for 5-20 minutes and then die again. It would sound terrible when it died, diesiling, coughing/spitting, etc.

If you haven't done the coil already, given the age of the motor, I'd just do it (pretty cheap) and see if it helps/changes the situation. In my boat, it was the cure all, changed the coil and the entire problem went away. I went through almost exactly the same steps trying to figure it out; fuel filters, carb, etc.

The test that would probably be most telling. Take the boat and run it until it quits. Open the engine compartment and let it cool for 20-30 mins. Then restart and see if everything is working correctly again. If it is, that would also suggest coil; it's getting hot and not making adequate spark. If you haven't done the plug wires, I'd change that as well when I was doing the coil.

 

[ktbarrentine]

Re: Engine loses power while cruising, difficult to start, shuts off, etc

I'll just add my 2 cents for what its worth (you may have already fixed your issue by now....hope so)... I had similar symptoms, and traced it down to a loose inline fuse holder at my ignition switch. 22 years old, just got loose over time, finally gave out (half way across the lake, of course).

 

[vndkshn]

Re: Engine loses power while cruising, difficult to start, shuts off, etc

I had exactly the same symptoms on my 5.7TKS motor. I'll save you the buildup, it was the coil. It would heat up and then stop producing a good spark. If I let it cool down again, it would work fine for 5-20 minutes and then die again. It would sound terrible when it died, diesiling, coughing/spitting, etc.

If you haven't done the coil already, given the age of the motor, I'd just do it (pretty cheap) and see if it helps/changes the situation. In my boat, it was the cure all, changed the coil and the entire problem went away. I went through almost exactly the same steps trying to figure it out; fuel filters, carb, etc.

The test that would probably be most telling. Take the boat and run it until it quits. Open the engine compartment and let it cool for 20-30 mins. Then restart and see if everything is working correctly again. If it is, that would also suggest coil; it's getting hot and not making adequate spark. If you haven't done the plug wires, I'd change that as well when I was doing the coil.

I actually had the same thoughts and replaced the coil a couple of weeks ago (hey, its what, $32?). Unfortunately no change. But I did get it all resolved this weekend. More on that below. But.. that said, the heat symptom you point out could be valid... I could have been fighting that as well, since it seemed to get worse the longer the boat ran.

Check the electrical connector on the wiring harness at the engine to see if it is tight or has some contact issues.

Good tip. I checked this as well as correcting the other things I found. It was loose as well. It looked like it was fully seated, but I pulled it off, cleaned the connectors and pressed it about 1/4 inch further in. I found it somewhat by accident actually. Had already found and fixed the loose ignition wire on the back of the ignition switch, was working on something else while listening to the stereo and bumped that connector at the engine... stereo shut off a second and came back. Started shaking it, replicated the stereo power loss... it definitely was loose. Cleaned and secured it better, problem avoided.

What I suspect was the actual problem actually makes no sense. The ignition wire was barely screwed to the ignition switch. I suspect that hitting bumps, etc would cause it to shake, interrupting the connection. What didn't make sense (and still doesn't) is why it got worse the longer I ran the boat. The only thing I can figure, the electrical connection at the motor was getting warm. Then again... it is entirely possible the coil was going bad as well, because as mjfink points out, letting it cool actually helped the issue.

All in all, its resolved, took it on a long run Saturday on VERY rough water, zero issue. Of the work I did, only the ignition sensor and possibly the coil are outside of normal maintenance, and with a boat 16 years old... while that sensor had been replaced once before, it still looked bad... well worth spending the sub $100 to cross those off of the list.