Engine loading up at idle MPI
 

I have a 257 mirage with a 98 454 mag MPI GEN 6. it runs great except for at idle. when its been idling the motor acts like it is loading up on fuel. it will run good at idle for about 3 mins or so then you can here the engine start sound like its having a hard time keeping 600 rpm it will not stall its self out when just idleing but it has stalled a few times when I was shifting from forward to reverse while I was docking. you can also smell fuel when standing behind the boat on the dock. when you go to get on plane the engine will stutter a little bit but it dose clear its self out and runs great on plane. another issue I noticed is that if the engine is hot and you shut it off and let it sit for 15 mins, it wont start back up unless you give it 1/2 throttle, cold starting is good, and starting it right after you shut it off when it was hot is also normal.

things I checked

1. checked all plugs, #4 and 5 were wet with fuel but all others looked good.
2. TPS voltage, it was reading .85v at idle so I re-adjusted it to .55v at idle
3. cap and rotor look good.
4. Map sensor was reading
5. water temp sensor was also reading accurate (replaced last year)
6. IAC was functioning, was about 40 at idle
7. No water in the oil
8. Fuel pressure is about 37 pounds, when I shut the boat off it drops to 33 or so and did not drop for over 30 mins

I am out of ideas of what could be causing this, any help would be appreciated, thanks

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These are the readings I was getting when the engine was warm.

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Engine running



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Engine off for about 25 mins or so

Is the timing changing a lot or fairly steady?
Have you checked compression?

With engine idling, pull plug wires one at a time to see if engine stumbles with each plug
Get a noid light and see if each injector is firing

The compression readings were between 150 and 160, (that made me happy). The timing numbers are constantly changing but not by very much, I believe that's because the 454 mag mpi some how use timing to keep idle rpm (from what I've heard).

the injectors were flow tested and matched 2 years ago when I had the heads rebuilt.

The boat runs normal when it's not idling. It will run 60 with 2 ppl and a full tank. The fastest I've ever had it is 62. I've had it for 5 years.

This is a classic case of the VST fuel pump diaphragm going bad. Check the yellow fuel vent lines off the VST pump assembly. They are likely percolating fuel into the TB or manifold causing your idle / hot start issue. Basically you are flooding out the engine. The VST assembly or likely needs a new diaphragm kit installed.

^^^Good advice if it has VST. I assumed (bad to assume right) that a 98 GEN VI is cool fuel.

It is cool fuel. When I was researching, I found some posts describing symptoms of of a bad VST (I didn't know what a VST was) I got real excited. Then i researched what a vst was and realized I don't have one since I have a cool fuel system, then my heart sank. Lol

I did not try unplugging the spark plugs to see if it stumbles but I did plug a spark plug light in-line and all checked out good.

a few months ago when this issue first started, my neighbor plugged in a tool that tests the injectors, its a higher tech version of a noid light from what I understand and all tested good. I never herd of noid lights before you mentioned them. I will try to get a set a check them again.

I bought all new spark plugs, cap, and rotor. I'll try changing them and see what happens, I'm not very hopeful though because the current ones only have 1 season on them.

I was once told that spark plug wires are replaced way to often and that 99% of the time, if they are not visually dry rotted, cracking, or damaged then they are fine. The guy also stated that spark plug wires don't "wear out" (besides dry rot) and that they either work or don't work. Has anybody ever had any other first hand experience with this? Or can anybody confirm this advice?

Thanks

ID replace the IAC, might have to open the butterflies just a tad with the idle screw, I had one that looked normal on the laptop but would not idle right and stall when put in gear, replaced it and new plugs and was fine....wheres the TPS volts suppose to be set,

If a plug is not firing, or not firing very well the ECM will change the timing frequently (milliseconds) to try and keep the rpm stable. The spark can be making it down the wire but not jumping the spark plug gap. This is why I asked for the individual plug wires to be pulled. Pulling individually also can listen if the change is about the same or different

Yeah, I didn't pay attention to the year. Regardless, this is a fuel issue, and as such I would look at how it is supplied, returned and vented.

when running one thing off the bat IF YOU HAVE MEFI 3 ECM then you have low fuel pressure unless your gauge is not accurate. And is that a bouncing needing in the pic as looks to be ? or is the needle steady eddie ??

I do not know which MEFI you have because I did not see any engine serial number posted. That is a must for all marine engines.. Engine serial numbers is how all marine engines are set up, correct service procedures and all related specs - Year model really means nothing

That's called power balance testing ^^^^^

and you use a digital tach to help with testing. You have to be quick watching the tach cause the IAC will compensate and help pick up the rpms fairly quickly in most cases.

how old is the ignition coil ? and the correct one needs to be used - the primary and secondary resistance needs to match the OEM coil. They all look the same from all different sellers out of the box but they are not internally. The coil windings and so forth are important for spark output for a jist of info.

Totally wrong info about spark plug wires actually in marine apps they ARE OFTEN NOT CHECKED NOR CHANGED ENOUGH, must have been an auto mechanic that mentioned all that for they rarely ever need changing. .. .

There is many reasons why spark plugs wires in marine apps need to be changed and correct wires as well. The main issues for a jist of it ----- For one is the heat soak in an marine engine compartment last for hours and is way hotter than an auto would ever be. Also the gas fumes breaks down rubber and plastic components over time. And if one does not think gas fumes are not an issue - why is it USCG mandated that all boats have a bilge blower -- to evac gas fumes before engine start up. Marine engines compartments breed gas fumes 24 hours a day,

Anyways -- I did a lot of testing with Moroso for spark wires and another issues with off the shelf spark plug wires is the suppression needed against RMI and EMI. Basically electrical noise that interferes with efi systems and other components.

marine techs on top of their training -- ohm test spark wires all the time and check for corrosion inside the boots, spark tracking, discolorations and other things

ok I see what your getting at. how much should the timing be changing under normal condition's (ever second or couple seconds?) I will pull them individually next time I pull it out of my garage, probably tomorrow.


It is MEFI 1 according to the scanner. The serial number is OLO14516.

could I use the tach that is on the scanner? or do I need the mechanic style that connects to a spark plug?

Im not sure of the age of the coil. I know I never changed it so its at least 4-5 years old.

For the most part it would not stop moving, changing constantly

i was able to get the boat out today and run it (on muffs). I did not notice a difference pulling the spark plug boots off one at a time. I did shut it off and re-start it.

I installed all new plugs, a new cap and rotor. I did not replace the plug wire's because I did plugs, wire, cap and rotor last year also. I also checked the timing, it was at 6.5-7 BTDC. I changed it and got it as close to 8 as I could. I on the muffs I am not sure if any of this made much of a difference.

here is a video I took of the fuel pressure gauge while the engine was running to show how much the needle move's. I thought this was normal but honestly this is the first time ive ever check fuel pressure. I could not figure out how to upload it like I did with the pics so here is the you tube link.
here is a link to a video I took to show how much the spark advance is changing. I thought it was normal but that was normal but again I don't have a ton of experience with this scanner yet to be able to say for sure.

so i was right about a bouncing fuel pressure needle/

Yes you were, dose the needle bouncing indicate an issue with the fuel pressure? I guess I assumed it was from the vibration of the motor and it was normal, but that is the first time ive ever had to check fuel pressure so I really don't know if it is normal.

im starting to lean towards replacing the IAC also since everything else seems to be checking out fine. I think im going to remove it, clean it and try it and if there is no change, then replace it. I hate to throw parts at it since I just bought that scanner with the hopes that it would save me money by not having to throw parts at it but I guess it was not a win this time. I could not find any info telling me were the volts should be set at for the TPS in my mercruiser books, only info about install and removal. I did a search and found somebody stating that for a 454 mag mpi gen v it should be between .45 and .65 volts so I set it at .55 volts.

I opened the secondary butterfly valve enough so that when the engine was warm and idling, the IAC was hovering around the #40 position. one time we were out on the boat and going threw a long channel before getting to the ramp, and the dying issue became a major issue while I was trying to dock, I opened the engine compartment and turned the idle up and it seemed to help the issue a lot. when I put the scanner on it (next time we went out), the IAC was hovering around #3 or #5 position. I know that is not correct because it doesn't give the IAC enough "room" to do its job but it helped the issue, that is a "+1" on making me think maybe the IAC is faulty in a way that its not moving fast enough to do its job. of course im just brain storming.

Was hoping you would pull and reinstall the plugs with the motor running. Doing this method you can here the difference before/after and compare.
The timing is moving around, but this can also be to compensate for other issues like the IAC.

Would like to add a correction. Heat and heat soak is way worse in cars. Far, far worse.

I had a bad experience one time while I was trying to shut off a snowmobile that started dieseling, it was late at night but that shock gave me the energy to power on for another couple hours until I go my sled running. I was about 15 mins from loading it up and going home LOL. assuming that you have tried this before and snowmobiles must be different, is it safer to take the plug off at the cap? or should I be taking it off at the spark plug its self?