1998 502 Mag Wont Start
 

Hello, I am new to working on boat engines so go easy on me.


I have a boat with a 1998 502 mag serial # 0L017XXX. It is suddenly extremely hard to start. Hot or cold makes no difference. Throttle position makes no difference. It will start only if I use some starting fluid. It seems to run and idle fine once started. It will not restart once turned off. I can hear the fuel pump whine when I first turn on the key.


Things that were just performed:

New Plugs (CR43TS properly gapped)

New Cap & Rotor (OEM)

Oil and Filter Change

Fuel Filter Change

Impeller and Housing Replaced


I have pulled the plugs and verified spark. However, the plugs were wet with gas and very black but not oily. They looked fairly bad for brand new plugs that don't even have an hour on them. Any ideas on where I should check first?

Fuel Pressure? Just because the pump is running doesn't mean that there's correct pressure.

What is the easiest way to check the fuel pressure? Im not too familiar with this engine and I am still learning. I can go borrow a pressure gauge from the local auto parts store.

There is a brass cap over the fuel pressure test port, just above and behind the thermostat housing, under the intake plenum.

its a 502 mag mpi ? if so there should be a schader valve on the fuel rail hook the gauge on there

Was everything ok before the maintenance items above were done? Or did you change those things trying to correct this problem?

Yes, it is a 502 mag mpi. I will test the pressure as soon as it stops raining and report my findings.

Dr feelme and I where typing at the same time...........:lolhit:

:party-smiley-004:

Not the current time, and might be a type-O but should be MR43T (point on the M) gap .040
Serial number area is a MEFI -1

When you replaced your fuel filter, did you fill it COMPLETELY up with fuel? I've seen my 454 MPI be very cranky, and even not want to start and run, if there was ANY 'air' left in the filter. I always thought it would sort of self-purge by objective evidence indicates that isn't so.

I read on the forum that the CR43TS was a good alternative to the MR43T as it has a projected nose. Should I just stick with the MR43T? I tried to post the url but I don't have enough posts.

cr43ts ‘s are great on non vortec small and big block black engines.C= commercial )coating for less corrosion. Ts is projected tip. They don’t make the ts’s in marine. They do on regular r43ts and commercial cr43ts.

We typed at same fine. :) look above. They are Great plug :thumbs

I believe so, but I will check it again.

Bajaman makes a great point. The first year I had my 502 mag mpi, I fired it up to change oil and fuel filter for winterization. After changing, I fired the engine up, it ran for about 30 seconds and quit, could not get to keep running. Pulled the fuel filter back off and it wouldn’t fill up with fuel pump. Filled up to the top, made sure all air bubbles were out, put back on, and she fired right up. Do it every season and haven’t had a problem. I am not an engine guy, but my previous carb engine never had that problem......assumed the electric fuel pump would fill it? Not saying that is your problem, but a good thing to remember to do.

While waiting for fuel psi tests…..

He said engine runs fine until shut off again, so filter would be full while running. Filling filter is not his issue, but you guys made great point for all. Electric pumps will typically cavitate and not pull fuel when filter/separator not filled.

OP- does engine crank over fine or does it labor ? Are batteries fully charged and holding charge ? They should measure around 12.6 with engine not running and not charged for atleast few hrs.

https://cimg1.ibsrv.net/gimg/www.off...ce0de482e.jpeg

If its cranking slower then normal i'd be looking at getting the battery tested to make sure its up to par and then inspect the main batt cables for corrosion...

Consider looking at gas to see if you see any seperation or rust from tank. Really a 1998 boat probably at end of fuel system life. If not been done already I'd replace all the fuel lines.

Thanks for all of the replies so far. I have noticed that it seems to be cranking more slowly/labored than normal. I will put the batteries on the charger and see if that makes a difference. The battery cables look good, I recently installed a new battery switch and cleaned up any corrosion.

I should be able to check the fuel pressure this afternoon, the weather is finally clearing up.

Any ideas why the plugs are so black? Could that be related to this issue?

Dont just look at the connection joint where it k=hooks to the battery.....look at the cable strands where it goes into the connector.....if its green or black then id say replace the cables cause you don't know how far back the corrosion goes

My bet is fuel pressure-related.

I had an issue in the past that was similar. I had low fuel pressure due to cracks in the fuel feed hose coming from the tank. It wasn't leaking fuel, but was pulling air into the systema and in the end the engine wouldn't even run. Fuel pressure was down about 8 PSI and the plugs were black. The initial thought was that it was getting plenty of fuel but in reality the injectors were not atomizing the fuel properly.

You mentioned that yours will run fine until you shut it off; is this in a loaded condition or just on the hose? A no-load test condition will mask a lot and if it is a fuel pressure issue, it's important that you do not run it until resolved. It's easy to burn down an MPI engine - the injection system is rather basic and not smart enough to keep the engine from grenading itself.

*edited*

I was able to test the fuel pressure and it definitely seems to be the issue. When I first turn the key on it shoots up to 60psi and then goes down and holds steady at 18 psi. Would that be the fuel pressure regulator? If so what does everyone recommend? Getting an adjustable or aftermarket one or just sticking with oem?

Is that running?

No, that is just when turning on the ignition. When I first turn it on and the pump whines, it jumps to 60 psi for a second. When it stops whining, it drops down to 18 psi.

60 psi is higher than it should be capable of reaching unless the return line is restricted or the regulator is faulty. I'd be curious to see what pressure it supplies while the engine is running and the fuel pump is continuously "on".

Alternatively, you can jumper 12V+ to the gray wire at the fuel pump relay, this should allow for constant running of the fuel pump for testing purposes. The fuel pump relay is located in the electronics box at the top port side of the motor.

I haven't wanted to start the engine again as I really don't like using starter fluid. I just did it the one time as a test. I can try your suggestion and have the pump run continuously.

Understandable. shouldn't take more than 4-5 seconds of jumper-wire pump operation to see the needle stabilize at running pressure.

Id stay with the OE stuff, replace the regulator and pump if it was me...............if you boat in summer high temps and humidity you might want to consider an after market pump and regulator to eliminate the risk of vapor lock.

I live in Texas so it gets really hot in the summers here.

I did as you suggested and ran a red + wire to the grey wire to the fuel pump and let it run for about 5-10 seconds. After about 5 seconds it was holding steady at about 10 psi.

thats about 30 psi low

Okay sounds like a new pump is needed. Any recommendations for a model pump to purchase? Looks like the oem part isnt available anymore. I'll probably replace the regulator while I'm in there.

I used a pump from the ROP shop , $65, it's generic so it's a roll of the dice. You can buy an entire new cool fuel assembly with pump regulator and cooler from Merc if you are so inclined. $500+

Seems steep. I may just have to roll the dice. I saw some generic ones from QFS as well.

Also, there's another test you can do to check for pump vs. regulator being faulty.

With 12V to the gray wire, pump running, carefully and slowly pinch off the return line with a pair of pliers (use a rag on the jaws to prevent damaging the return line). As the line is occluded, if the pump is good but reg is bad, the pressure should be able to spike to 60+ psi (immediately release if you see this happening). If the pressure can't easily exceed 45psi with the return line pinched off, then the pump is weak for sure.

I've got a pump from a 98 Cool Fuel setup. It's used but was working well when pulled (I built new engines - carbureted). 400 hours per the ECM, but some of that could have been key-on-only time. You can have it for $50 plus the ride (I'm in VA). I'll throw in the regulator too (green stripe).

I also have a great set of better-than-OEM injectors that have maybe 20 hours on them if you're interested.

If on key up the pressure shoots to 60 but then drops off to 10 doesn’t that indicate a leak down? Once it achieves pressure when keyed up it should hold.

We eliminated that by jumping 12V to the pump supply lead. even while running continuously it only generated 10psi.

That could still be regulator bleeding off. Someone said block off the return. Did that get done?

I suggested that. Haven't heard back from the OP as of yet.