Mercruiser 5.7l Cuts out at cruising speed

[thormx11]

The engine is a 1996 5.7l 2 barrel, with an alpha one gen 2 drive. This was my first summer with the boat, and it has been fine up until my last day out. I kept the boat in a covered slip on a lift and used the boat every weekend. I usually don't go too far from the marina with it. On our last day out we went to a bay maybe 8-9 miles from the marina. I cruise at about 3400 rpms while running about 30-35 mph.

On out way there I noticed the boat would bog down every once in a while, but would clear up and get back to cruising speed. On the way back it got a lot worse, it would bog down every few seconds. It would backfire also. If I let off the throttle a bit I could get it to clear out and get back up to speed for a few seconds, then it would start bogging again. The engine never completely died, and it would idle in gear all day long without any problem. There have been a few times throughout the summer when I would have the engine idling in our slip while loading the boat and it would just die. It always started right back up, so I wasn't very concerned at the time. Not sure if these 2 issues are connected or not.

I changed the fuel/water filter and the backfiring seemed to be gone, but it would still bog badly at cruising speed.

I removed the anti-siphon valve from the fuel tank and it seems to be working fine. I tried to blow through my fuel tank vent while the anti-siphon valve and fuel line (to the engine) was disconnected, and could not blow through the vent. If I opened the fuel filler cap then I could blow through the vent fine. Im not sure if I should have been able to blow through the vent hose with the fuel filler closed or not.

I read online to try to run the boat off an external gas can to see if that helps, but I have not been able to do that yet. Once the boat started messing up I brought it home for the winter. I live about an hour from the lake, so I would like to try a few small things at the house before I take it to the lake again. I'd like to fix the issue before winter comes and I have to winterize it.

Sorry for the long post, but hopefully I gave enough info (an then some!) to help point me in the right direction.

 

[alldodge]

Check fuel pressure and while idling look down the throat of the carb to see if you see fuel dripping. Can also shut the motor off and check again

 

[thormx11]

How can I check fuel pressure? There is no port that I see. I have thought about cutting the metal fuel line right before the carb and splicing in a piece with a fuel port, but that was going to be a last resort. I was thinking the fuel pump may be getting weak when it warms up, the boat (and pump) sat for about 7 years before I got it. I know with cars now-days, when they sit that long it is not good for them, especially the fuel pumps for some reason.

The fuel pump is also electric, not the manual type. I forgot to mention that in the first post.

 

[achris]

How can I check fuel pressure? There is no port that I see.

With one of these....

...The fuel pump is also electric, not the manual type. I forgot to mention that in the first post.

In that case, you may have an electrical problem. Not unusual for the connectors on the oil pressure switch to corrode and start failing. To test for that, just make a short (1") jumper cable with a male spade connector on each end. Remove the 2 wires from the oil pressure switch and join them with the jumper. When you turn the key ON, you should hear the pump running... This jumper is ONLY for testing purposes and not a long-term fix...

One more thing, electric fuel pumps weren't fitted to the 5.7 until 1998, can you post an engine serial number please.

Chris.......

 

[thormx11]

Now that you mention it, I did have a couple electrical problems one day (a different day) While cruising, sometimes when I hit the trim button on the throttle the engine would bog a little bit. Also, another day when I shifted into forward/reverse the radio would cut out. Each of these only happened a couple times each day, and didn't happen again the next time I was out.

As for the engine, I do not know the exact model, the boat is a '95 model, and when I got it the block was cracked. We had a spare truck block (1991 model I think) laying around and sent it to a machine shop to have it rebuilt and I put it in the boat. I know it is a vortec engine , and I know it has an electric fuel pump behind the alternator. I believe I have the original engine cover on the engine though, I will check the serial number on it tomorrow.

 

[thormx11]

Sorry guys, I got busy at work and completely forgot to go get my serial number from the engine.

The SN is: 0M358755

I have not used this number to order any parts, so I am not sure if it is even the correct cover to the engine, it was on there when I got it.

 

[alldodge]

That SN number was for 1998 to 2001 305 2 bbl, so its not adding up

Guess we go forward as we don't know

 

[achris]

That serial number is also showing the engine as Bravo configured. Given the problems, have you considered an electrical problem inside the control box? A frayed wire in the kill switch circuit being rubbed by something in the control box?

Chris......

 

[thormx11]

It must be a different cover then. I will trace the wiring and see if I can see any bad spots first. By control box do you mean the throttle/shifter? If so, i installed it new this spring. It is a Seastar CH1751P. I installed it, and had a marine shop hook up the tilt/trim wiring when I had the boat there for bellows.

Another thing, my voltage gauge has never been correct, it reads about 10 volts idling, and if I turn on any accessories it'll drop a little bit more. I have checked the alternator output and it was charging good. I've used the boat all summer and never had any dead batteries, except for when I left the battery switch and the radio on for 2 weeks without using the boat. I tried to do a little research on this issue in the spring, and was pointing towards an electrical issue as well, but I never figured it out.

 

[achris]

You have a voltage drop somewhere between the engine and the instrument panel. This could explain your engine run problems too. Get a digital multimeter and start tracking through looking for lost voltage.

Chris......

 

[thormx11]

Sorry for the long delay, I finally got some time to mess with the boat today. While idling, I am getting about 14.3/14.4 volts out of the alternator, and at each battery. I started by cleaning all of the connections at the battery, and the voltage gauge is finally reading correctly, or a lot closer than it used to. It always read around 12 volts idling, and would drop to 10-11 volts when running the blower and bilge pump, although the actual voltage never changed from the alternator.

At the engine, everything is reading 14+ volts idling, but at my ignition switch I am only getting about 13.7/8 volts idling. If I rev the engine up to about 2k RPMs the voltage drops to 13.3/4 volts at the ignition. If I turn on any accessories the voltage will drop a little bit more. Where/what should I look for between the engine and ignition that I could be losing the voltage?

I am taking the boat back to the lake tomorrow to test and see if I at least solved my issue with the engine cutting out on plane.

I did notice one of my 2 batteries is only showing about 10.6 volts when the battery switch is turned off. It is an older battery, and the other battery I bought new this spring, was showing about 12.3/4 volts with the switch off. Could this have anything to do with the voltage issue?

 

[alldodge]

Check connects at the 10 pin engine connector

Power goes from Starter post, to 50 amp breaker, to 10 pin connector (red/purple), to key switch, from key switch (purple), to kill switch, to pin pin connector (an gauges), to coil

 

[thormx11]

One of the things I did notice was the main connector at the rear of the engine was not all the way plugged in. If I wiggled it, the engine would cut out. I pulled it apart, added some dielectric grease, and pushed it all the way in. I think that solved my running problem.

Between that and cleaning my battery connections, the boat ran great today. Probably better than it has ran all summer. I am going to have to winterize it next weekend, but I will try to follow the wiring as you stated above and see if I can find where the voltage drops.