502 mag mpi cutting out
 

Hello,

I have a 1996 gen VI 502 mag mpi in a 97 fountain. It has started cutting out above 4,000 rpm intermittently.
Gauges aren’t doing anything weird, besides the tach falling a few hundred rpm. I did do new ignition system parts about 30 hours ago. New plug wires, coil, pick up coil, module, jumper harness cap rotor.
This season I had the injectors tested and cleaned, replaced the rubber line from the tank pick up to the fuel shut off valve and from there to the filter head. New ac delco mr43t plugs put in maybe 10 hours ago. The previous owner of the boat had the heads, gone through, new cam, emi exhaust, ecu was tuned by Arizona speed and marine and a new cool fuel pump and adjustable fuel pressure regulator down under the block by the fuel pump where all put on about 100 hours ago. I’m planning on putting another new fuel filter on, pulled spark plugs tonight and all of them look pretty white/lean(picture attached) leading me to believe this maybe be a fuel delivery issue? Filter or fuel pump possibly?
I did do the grab and wiggle and shake test with the boat running on the wiring harness etc around the engine, kill switch and ignition switch at the helm Looking for any insight or something I am over looking.

thanks

https://cimg3.ibsrv.net/gimg/www.off...d31b9477b.jpeg

Start with the basics. Plugs, wires, cap and rotor. Do a compression test and check the fuel pressure. You will find 95% of the issues by doing this.

yea, I did cap rotor wires etc about 25-30 with all of boats unlimited parts. I did do a compression test just 10 hours ago when I did plugs, all within a few pounds of each other and 160psi. I have not check fuel psi yet.

what is the spec on the fuel pressure?

im also going to grab a friends scanner and see if there are any codes that pop up.

I should also add what I forgot. The boat did do this in early July, at which I did do the new plugs, comp test and new fuel filter which cured the issue then. Kind of leaning to fuel issue with the color of the plugs, and if the new fuel filter clears it up I’m wondering the cool fuel screens are plugged or weak mechanical pump

Fuel pressure 43 psi

what coil did you install

oem coil, was part of BUP’s ignition kit

Let me guess he did the injectors also ?

correct

Describe “cutting out” in a clear/concise way so we may better understand what could be happening.

When it starts getting above 4,000 it’s almost like you were to hit the ignition or kill switch for a split second. The tach will fall back 2-300 rpm. Sometimes it not as pronounced, but you can feel the boat pulling back, as if you were almost hitting weed patches or something.

Has it always done this since you owned it ?

Does it always do this at same exact rpm ?

you mention fuel filter change and was better for a bit. How old is any of the gas and guess how much of it ?

do a fuel psi test under wide f’n open at/past the rpms during issue ?

what exhaust ? Factory Y pipe still there ?

It just started this mid season. I’ve owned the boat since 2018. When I did the filter and new plugs the issue went away. Now is back. Doesn’t do it at the same rpm, it is sporadic, it’ll get up around 5,000 but will cut in and out through the rpm. Only tried twice. Didn’t want to push it. Haven’t ran any tests with fuel psi gauge yet, going to get a scanner on it as well. I’ve ran a few tanks through this season, all non eth gas, filled with gas sunday am. Emi thunder exhaust, factory y pipe in place with the silent choice still

Have been working on it for a little bit tonight, popped the cap off and noticed the rotor seems
to have some wear into it. Not sure if it’s enough to cause this issue?

https://cimg4.ibsrv.net/gimg/www.off...9934add32.jpeg

I'm having a VERY similar issue with one engine(502mpi). It was random last year but happened nearly all the time this year. Most of the time it was doing it at 3400rpms for me and pulling back to 3100ish. Did all the normal basic stuff with no changes. Finally ran it under load with a Techmate hooked up. The end result is something is causing it to think its hitting the rev limiter. Everything else on the Techmate was normal except MAP sensor was slightly low on voltage. Swapped ECM's and no change. I didn't have much time mess with after that, but I did check the wires coming off the coil and moved them around a little. The upper wire harness for the coil seemed very tight and I got it to free up some. I'm assuming the ECM gets its rpm signal from this harness. Went out and ran the boat after that and it was holding 4400rpms evenly on both engines. The props (spares and +2" pitch from my normal ones) I have on will only spin to 4600rpms at best and due to water conditions, I really couldn't trim out enough to hit max WOT rpms. Bottom line, is I'm pretty sure I have short in that section of my wire harness. I'll be digging more into it over the next couple weeks.

I will say though that I have the MEFI 3 and Cool Fuel and you most likely have an MEFI 1 and VST system

I have cool fuel. No mechanical fuel sea water pump, no vst tank.

Sounds like the tank vent or anti-siphon valve. Sounds like a fuel starvation problem.

Mine was doing this a few weeks ago and the only thing I found was one of the main harness grounds was broken out of the connectors. I fixed it and it’s been normal since. Not sure if it’s a coincidence but it certainly seemed more electrical than a fuel issue to me

.

are you talking the ground connections on both sides rear of the engine block? Or a ground in a harness connector?

Vent should be clear, I always put my finger by it when fueling and check for air pushing out.

That's exactly what I thought for mine originally as well. I thought I had a cool fuel pump getting week, but my fuel pressure held steady at 41psi.

Mine is still having the same issue and with my regular props on that spin to 5000rpms, its happening at 4000-4100rpm and pulls back 200ish rpms.

BTW, I FB messaged with BUP last night and he is pretty sure its my EST module and/or pick up coil going out.

What about the anti-siphon valve?

I did buy a new cap, rotor and ignition module for mine to be safe, gojng to run the boat this weekend.

I hooked up a scanner for codes, nothing. I also ran a fuel pump test with the scanner with a fuel pressure gauge on the rail, 48-49 psi while the pump was cycling, held about 44-45 when it was turned off.

unsure on that, or how you would go about testing/checking that

If your gauge is accurate
Pressure is to high, check/clean the Regulator screen

If it's clean its the Reg

there is a adjustable Hardin pressure reg on mine. Was done with the cam, exhaust, and tuned ecu

OK, I'll retract my statement

no worries, I appreciate your input!

If pressure holds, then anti siphon valve is not an issue

It is likely culprit for bogging/fuel starvation. Seen it a bunch. It is at your tank hose barb that leads to the fuel/water separator on the engine.

Looks like this:
https://cimg5.ibsrv.net/gimg/www.off...4c1dd51364.jpg

I just noticed that it looks like all fuel pressure tests were not done in the water and under load while running the boat. Fuel psi needs to be checked under load.

BTW, changed my EST module today and no change for how mine runs.

.

yea I just checked to see if it were obvious. Planning on taking scanner and gauge with this weekend.

so that’s the fitting on the fuel filter housing where the line from the tank runs to?

No, this valve fitting is on the tank itself and every inboard boat is required by law to have one.

It's called a antisiphon valve. Stops fuel from siphoning into bilge if there is a leak between tank and filter

Is it something you simply can clean? Or is it a replace only?

I put in a new ac delco pick up coil this AM, just to be safe, one I pulled out smelled a little burnt actually. Had the boat running warming up just to check timing, had the scanner on it just watching everything. Every time I refire the boat I get a code 23 manifold air temp malfunction(which is a new sensor) and code 14 coolant temp malfunction. When I watching those numbers on the scanner both are pretty accurate, I shot the hose with my laser temp gun and was with in a few degrees of the scanner coolant temp, and the air temp seemed about right at 78° and rose to low 80’s after 30 mins of idling. I unplugged both on the air temp went to 0 and back to it previous reading when plugged back in and coolant went to -39 when in plugged and back to 165 when plugged back in. Pins look good in the plugs, I removed the ecm harness connections don’t see any signs of corrosion or damage. Don’t think those plugs could have been swapped under the plenum when I did the injectors? I have the gray harness plug on the air temp and and the black harness plug on coolant. Sound right?

Did you use the white grease under the pickup ?

not under the pick up coil, but did on the ignition module

It's like $10...replace it unless the ball valve is freely moving.

Check this before continuing to troubleshoot.