Mercruiser 7.4MPI Stuttering/Skipping after long runs

[Lucrative]

Engine info:
[FONT=&quot]7.4 L MPI BRAVO GEN VI - L29 Serial number: 0L609594[/FONT]

So, I have had this issue the past 4 or 5 times I've gone out fishing. Usually when we fish, It's a 20 mile trip one way to the spot and 20 miles back. I've only had this issue on the return trip in the last 5 miles or so, until yesterday when it happened about 15 miles into the trip on the way out. I have not had this issue when I take my family to the beach, which is only a 5-7 mile run one way.

It almost seems as if the engine is not getting fuel for a split second, or like an injector just forgets to inject or a spark plug isn't firing. It seems completely random when it starts happening, only that it happens after longer runs (I run between 3k and 3.5k rpm). It will continue to do this every 30 seconds or so, then become more frequent until I have to pull back on the throttle because it gets too herky jerky. When I pull back on throttle, it will run fine for a while at that rpm, then begin to do the same thing. If I run at a very low rpm or at idle this will not happen. Also, not sure if this means anything, but it seems that it will coordinate sometimes when I hit a rough spot, it will "skip", but it will also do it in the bay as well. I stopped the engine when I got in the bay yesterday and let it sit about 2-3 mins, then went again and I got about 4 miles (1 mile from the marina) and it acted up again, but I was able to push through and get back. I haven't had to deal with any fuel issues or injector issues with an engine yet, so I'm not sure where to start for a diagnosis on this....I'm kind of assuming it's a fuel issue because I can't think of anything else.

Any ideas on what this could be and/or what I should do to start looking?

Thanks,

Luc

 

[alldodge]

What kind of maintenance has been done prior?
Things like filters, check distributer cap, rotor, plugs and wires. Just looking for the status of the motor.

Could use if any codes are present, but without that need to see what your fuel pressure is (43 psi)

 

[Lucrative]

Hey AllDodge, this engine was actually a complete top end rebuild last year, and you've helped me a lot throughout the process (THANKS!). It has been running great for a while now, but this has popped up recently. I changed fuel filter and plugs at the end of the year last year, did oil change at the beginning of this year, but I have not done any serious engine maintenance since the rebuild. I've been fixing smaller issues here and there over the winter and into the season........gauges, temp sender, knock sensor issue, rebuilt trim rams, power steering leak....but nothing that seems relevant.

I suppose I should just change out the fuel filter, right? I could just go and change spark plugs.....I don't know when the spark plug wires were replaced last. What am I looking for on the wires? What would I be looking for on the distributor cap?

It wouldn't be water in my gas, right? Wouldn't it be doing it all the time, and not just after long runs?
Doesn't seem like it would be fuel injectors either, for the same reason^^. Is that a reasonable assessment?

 

[alldodge]

Just trying to get a thought of where the motor maintenance is. I replaced the plugs, cap, rotor in my 21 year old Rinker two years ago for the first time and had no change. Did it just because. I think of the cap and other items because your having issues and because the cap and other items may have some kind of build up in it.

So with that its probably none of those things. Now it could be an issue with the fuel pump heating up and starting to reduce pressure. Could be a vacuum in the fuel tank caused by the tank vent line getting clogged. If its the vent line you could open the gas cap and it would run great again for a while.

The thing about your motor, the MEFI-3 and other sensors is there is a lot going on and need to see what is happening to the ECM.

Your could make your self a code reader with a resistor led and some wire, or an led and 470 ohm resistor hooked up in the pic. Then turn key to ON and count flashes. If it flashes 14 then this is there is no codes

 

[Lucrative]

I am running at around 170+. Every once and a while it probably goes up to 175, then back down. The venting thing seems to make some sense to me...as I make a long run and am burning up a bunch of gas, if it's not venting, it would be trying too hard to pull gas through? What is the fix for that? Also, I've got a Chaparral 270 and my gas cap is on the side of the boat, I don't really want to leave it open at all for fear of getting water in, is there another option for this?

I want to try a few fixes myself before I take it in, but there is a marina not far from me that has hooked me up to their diagnostic and I just paid them for hooking me up and giving me a reading.

 

[alldodge]

Your gas cap will either have another line going to it for venting or the vent hose will be close to the cap. my Formula has a chrome fitting net to the cap which is also on the side. Run the motor until it starts, so and stop motor so its quite. Open the gas cap and listen for the sound of air rushing in. If you hear air, you should be able to fire back up and go another ways until it starts again.

The fix if this is the issue is to clear the vent line, most use air. Around hear we can find mud dobbers filling the hole

 

[Lucrative]

Now that I'm thinking of it, there have been times when I've gone to fill up that there's been a lot of pressure built up inside the tank. I'll take the cap off, and as I unscrew it, a ton of pressure is released from the cap. The venting line would vent both ways, right? So if it's holding in a bunch of pressure, it would indicate a clogged vent line?

 

[alldodge]

Possibly and if it is building pressure there could be an issue. The vent should be full open, no check valve of such

 

[Lucrative]

It was only building pressure when the boat sat for a little while. I'd go to put gas in, and when I unscrewed the cap it would push a ton of air out. I could even hear the gas tank reforming once or twice when this happened.

Anyways, I went through all the lines to/from the tank. It doesn't really appear that there's any way for the gas tank to vent other than when the cap is off and I'm fueling up. There are 3 lines that run from the tank. 1 goes to the fuel filter, another is the big fuel inlet line, and the 3rd runs right along side the fuel inlet and meets up inside the gas cap with a little vent nozzle on the end. How is the tank supposed to vent if the fuel cap is closed??

I took apart basically all the lines to make sure that there wasn't a clog somewhere, and took out the fuel level sender to try and access the inside where the "vent" line runs into the gas tank. The only issue that there *may* have been was that the metal part of the vent line that runs down into the gas tank has a 90 degree curved bend, and I tried to run a heavy gauge wire down through it, and I was having a hard time getting it through. Not sure if I wasn't getting all the way past the 90, or if it was somehow blocked off there. I don't know what those lines typically do once they run into the tank. Is is just straight into the tank and it's open or does the metal piping run down into the tank?

Anyways, after I did all that, I took the boat out yesterday and on my way back in, it started stuttering again. I slowed down and opened the gas cap, and sure enough, air was getting sucked in. I got back up to speed and ran the rest of the way in with no issue.

So, the question here for me is how the heck is it supposed to vent when the gas cap is closed? The system is 100 % completely sealed at at that point. Also, is there a chance the metal portion of the vent line (at that 90 curved bend I was talking about) somehow rusted closed or something?

 

[alldodge]

The cap should be able to vent the air into the tank. There are several different designs other then the pic below. The cap should allow air in. Now if the tank end is clogged this to will cause an issue. Suggest removing the vent line and blowing sir thru it to see if it is clear.

 

[Lucrative]

I took the boat out again on Tuesday and it gave me issues, so I took the cap off, let air in, and was able to run in for a while longer until it happened again. On the way in it was giving me issues still, but removing the cap periodically helped again. I took the cap off for the last 5 miles (it was glassy in the bay) and it didn't stutter one time. This is driving me nuts.

The vent line seems to be clear from one end to the other. My gas cap looks like that one except it's a threaded cap and has a gasket around it. It's definitely not venting out the cap, b/c when I take it off after running it's sucking air in, and it builds pressure if the boat has been sitting out on a hot day. Isn't there supposed to be some other way for it to vent?

 

[Bondo]

Ayuh,..... All the boats I work on, are fitted with a vent fittin' like These,....

Just make sure the hose hooked to the inside, goes Up, before it drops down into the tank, a splash water brake,...

 

[Lucrative]

So, here's where I'm at with this. I believe the previous owner of the boat lost the gas cap in Davy Jones' locker and replaced it with a non-venting cap that would normally be used for water/waste. He only ran the boat about 3 miles from dry storage at the marina to the island, so he never had a problem. I've only realized it somewhat recently because I've been making runs long enough to create adequate suction in the tank to give the fuel pump hell.

So, my question is......what fuel cap should I go with at this point? Some are better than others at keeping water out apparently, and I don't want that issue. I'm not opposed to reworking the fuel inlet fixture and vent lines if that would be better. My boat is a 2000 Chaparral 270, so I think this way of venting may be outdated, but I don't really know.

 

[alldodge]

Looks like you have two choices, add a vent fitting like the ones Bondo posted and keep your current cap, or pick up one with the vent included. The vent can be added in another location but best to keep it near the fill cap. As for the type I would think what it's made out of is more important then type. My Rinker is plastic like the one posted, and was replaced early by the dealer for a recall