Engine surge Help! 350 mag mpi

[Illiana]

Hello, busted old blue out of the garage today and had a great maintenance run. Changed all fluids, filters, impeller and backing plate on impeller. Put in fake a lake to check for any issues. She fires right up but starts to surge a little every for to 5 second and eventually gets worse and starts hissing pretty bad and dies after about 30 seconds. Never had this issue before so I'm guessing I messed something up.

Hissing sounds like it is coming from the flame arrestor. Motor will crank right back up and do the same things. During my maintenance, I was unable to remove the flame arrestor after loosening the clamp so I just wiped it off with a rag. I thought maybe I broke a gasket or seal between the filter/arrestor and the intake. How do I get the damn arrestor off? Was putting quite a bit of force on it and it will turn but not come up off the intake. Should have left the thing dirty. Arrestor was covered in dirt/belt dust from prior owner. I. appreciate any thoughts or feedback.

1998 merc. Mag mpi 350 fuel injected at 550 hours. 0L008530. Velvet1:1

Thanks for any help.

[cbath]

Sounds like it's running out of fuel pressure... do you have a fuel pressure gage you can put on it to see what kind of reading you're getting? Can probably rent one from a local auto parts store if not.


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[freeheel4life]

Grab a screw driver and see if you can open up tabs on bottom of flame arrestor. Don't be afraid to pull up hard a bit on it..It's not attached to anything
Once you get it off you will see that around the rim of intake there is a small square hole. That's where IAC intakes air to control idle, it's the source of your hissing which is normal. Sounds like you possible have an issue with it. It controls airflow to keep engine idling when throttle plate is closed.
Easiest was to confirm is put boat in neutral and throttle up to 1500 RPMS or so(while fake a laking obviously). If boat runs fine but dies when you return to idle then it's IAC for sure. Relatively cheap part and easy fix.
Good luck

[Illiana]

Thanks to both of you. Really hope it's not fuel pressure/pump related. I got the arrestor off and it looks good and clean (after it sucked all of the debris I loosened up into intake probably). Cleaning up the intake, removed the iac and it's really difficult to move up and down. Also removed what must be throttle pod. Sensor and it looks good. Going to grab both parts hopefully today.

I'm going to clean up the intake best I can. With cleaner and hope a new iac does the trick. I did put a new fuel filter on and replaced with mercruiser part for the motor. Had to screwdriver all filters to get off but everything looked good and flange were solid and clean. Thanks for any further feedback. Will let you know if valve does the trick.

However I did think the fuel pump sounded strange before cranking a couple times.

[Illiana]

Well, the iac valve did not do the trick. I put new valve in, got everything hooked back up tight, cleaned intake and iac ports real good. Ran good for about 2 mins then started surging low and ultimately died.

I don't have fuel pressure gauge to check that. When I turn the key to run, before cranking/start, the noise seems different than last year. I think that is the fuel pump kicking on. Anyway to further diagnose if it is the fuel pump? Any other suggestions?

Thanks for any help/feedback.

[freeheel4life]

Need to hook a gauge up to to fuel rail at Schrader valve. Sorry I led you astray. Hopefully cbath nailed it.

[Illiana]

Thanks and no worries. My first thought was iac and would have done that anyways. From the looks of the intake and age of boat I'm sure it wouldn't hurt to replace every sensor on intake.

Will get a gauge and report readings.

However, the pump is now not running at all when turning the key. About every fourth or fifth turn I hear a slight gurgle of death. Any opinions on whether to still run pressure test vs. proceeding with fuel pump replacement. Used to get a solid couple of seconds of pump noise before cranking. I did put a small amount of fuel in new fuel filter. Fuel is fresh. Thanks for any feedback.

[cbath]

The IAC allows more or less air to enter the intake to balance the fuel coming in... when it's lean on fuel it opens fully hence the air rush noise. I would like to know fuel pressure readings to narrow down and be certain, but it really sounds like a fuel delivery issue to me, either pressure or volume...


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[Illiana]

Ok, getting zero reading on schrader valve at fuel rail. Now the damn thing won't even start. When I first started today it would run for a minute or two and die out. Now when I turn key on I hear nothing from the fuel pump. Maybe once every 10 times do I hear a little activity from the fuel pump but not anything like what it sounded like last year or when it ran good. If I hear the pump engages a few times in a row it gets enough fuel to fire for just a few seconds but was still getting zero on gauge.


Any other fuses, relays, regulators I can check before replacing pulp?

Thanks for any feedback.

[cbath]

Do you have a test light or volt/ohm meter handy? We're going to have to get to the fuel pump and check for power and ground.


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[Illiana]

Thanks again. I do have voltometer but had to shut it down for the night. It's now covered in driveway. Glad I decided not to drop into lake today. Will get back on it tomorrow.

[boatwakes]

See my write up on how to change that fuel pump, she's toast. Change the whole unit including the pressure regulator and fitting.

[Wakeman22]

What fuel pumps do you have on that engine. I fought the same gremlins before. Mine has 2. A low pressure and a high pressure on. There is a wire connector on the low pressure one that has a tendency to work itself loose. Others have found this to be a problem. If you change you fuel filter this wire gets hit and knocked loose. Check these connectors. And get a fuel pressure gage. They are cheap and indispensable when needed.

[Thegerman618]

Did you change the fuel filter?

If you did, did you fill the new one up with fuel before you put it back in?

If not then you have vapor lock, there is a Schrader valve on the port side fuel rail push it in till a little fuel comes out. Start the boat, it will likely die again. Keep bleeding the air out of the system till only fuel comes out when push the Schrader Valve.

This is all assuming you changed the fuel filter.

[Illiana]

Thanks for the feedback guys. Will be tomorrow before I can get back to it mostly because I left my voltometer at my garage and my boat is in my driveway. Going to see if I have power at the pump and if I do then I know the pump is done. I had some intermittent pump noise yesterday which gives a small glimmer of hope that a relay or connection somewhere is bad since I'd think the pump is either dead or not. However, I think cbath and boatwakes nailed this one and this pump is done.

Any other thought on relay location or elective testing?

I did change the fuel filter and filled it about half way up. Turned key a few times before cranking and it fired right up. Get no fuel or air at all from the Schroeder vale and zero pressure on a gauge. As the day went on yesterday, I stopped hearing the pump engage when turning the key and ultimately the boat would not even start for a couple seconds. When the pump did engage, it sounded very weak/not normal. However if I did that a couple times in a row the boat would run for a second or two and immediately die.