Mercury 350 magnum problems please help

[badbowtie]

I am trying to help a good buddy help work on a 97 350 magnum 300 hp that he just got 3 weeks ago only had 100 hours on it. I know the previous owner and he has owned it for 13 years and had it sitting in his garage for the last 5 years unused, 3 weeks ago it ran great when they water tested it so he went ahead and bought it. The next day he had his family in it and said it just ran horrible. Barely would even get on plane and took for ever at wide open throttle to only get to like 35 mph and 3800 rpm. Then next day they put a new water separator filter on it and took it out and same thing for the first hour of the day then it coughed once and ran great again like 5000rpm and 65 mph like normal again. Thought it was all fixed went back out yesterday and ran like **** again so this morning I went over and replaced the water separator again thinking it was a fuel problem. So I unscrewed the dist screws and the terminals were all corroded so I replaced rotor and cap and figured for sure I fixed it. So went back to the lake in between the rain tonight to fins out same thing still don't run right. I drove the boat tonight and you can go wide open on the throttle and it don't even pick the bow up and go. It takes for ever to get it to finally pop up on plane. We are talking like over 100 yards and can't get any faster than 40 mph playing with trim even like 400-500 yards. Never could get above 4k rpm. It does not spit or sputter or anything. I am not sure what to try next I did pull all 8 spark plugs after running tonight and they all look good like they are all firing. I am just not sure what to try he has ran about 100 gallons of fuel and 4 bottles of sea foam thru it. Please help...

 

[Pete104]

Fuel pressure.

 

[FreeBeeTony]

If it was sitting for 5 yrs I would rebuild the carb.........assuming it has one.

 

[badbowtie]

It is fuel injected.

 

[badbowtie]

It is fuel injected and I was thinking checking fuel pressure and is there a way to check injectors without taking them out and sending them of.

 

[alldodge]

Could use a bit more info on the motor. Is it the 4bbl EFI or the MPI motor?

Being you said it is 300hp I'm guessing its the MPI model and the fuel pressure should be 43 psi

 

[badbowtie]

Mpi motor and I only have 23 psi at idle so assume that means it needs fuel pump correct.

 

[alldodge]

Mpi motor and I only have 23 psi at idle so assume that means it needs fuel pump correct.

Could be, but could also be the regulator is defective. You could test it without the motor running. Block off the return line coming from the fuel regulator going to the filter. Turn key ON and watch pressure. Turn it ON and OFF a couple times to see if pressure comes up. It should increase to 60 psi with line blocked off. Don't let it go above 60 psi.

Notice that when the key is turn OFF how long it takes for pressure to start falling back lower if not zero. It should be able to hold pressure and not drop off quickly.

Let us know what you find

 

[badbowtie]

Well unfortunately I took it all apart already today so we could get parts and put it all back together this week. I figure that way he could have the boat next weekend to use. I was going to just replace the pump and the regulator since was sitting for 5 years unused. I just figured that there would be noting else that could cause the fuel pressure problem.

 

[harleyman1975]

well if pump and regulator don't do it then injectors would be next on my list.

 

[Bondo]

I just figured that there would be noting else that could cause the fuel pressure problem.

Ayuh,.... Have ya tried runnin' it on a remote fuel tank hooked directly to the fuel filter, to rule out the boat's fuel tank havin' a problem,..??

 

[tpenfield]

Was the old fuel ever purged from the tank? If it sat for 5 years I would be running a very high concentrate of injector cleaner through the fuel system. Fresh gasoline and see where things are at. Follow the fuel pressure testing procedure that is outlined in the manual for your engine.

 

[badbowtie]

I did try remote fuel tank hooked right to fuel water seperator filter. Boat has had about 100 gallons of fuel and 4 bottles of sea foam run thru it he was thinking that it just needed fresh gas and cleaner to get it all cleaned out. I also can't believe it is injectors since regardless if injectors work or not it should still have the right fuel pressure.
I also am supposed to have new fuel pump and regulator today so hopefully I will get it all back together tonight and maybe take to lake to try it all out.

 

[Fun Times]

Don't forget that there is (should be) a small little filter screen that rests under the fuel regulator that needs replaced, cleaned or some have just removed it altogether though I personally don't do that should the screen not come new with the regulator. Item number 14, http://www.mercruiserparts.com/bam/subassembly/31786/2333/90

Good luck.:encouragement:

 

[badbowtie]

Okay for a update I did replace the regulator and also installed a new fuel pump and regulator and I only have 25 psi of fuel pressure at idle know and we took it to the lake and wide open throttle I only have 30 psi. I am completely lost know I am not sure what the issue is but I think buddy is not to happy for what pump and regulator all cost and still aint got it fixed.

 

[tpenfield]

Did you test the fuel regulator and high pressure pump according to the procedure outlined in the manual for the engine? If not, then you may just be throwing parts at the problem, but not necessarily the right set of parts.

 

[badbowtie]

I did unhook frp vac line and cycled key and could only get it to climb to 30 psi. Even with new pump and regulator that is all I am getting. If I leave the gauge hooked up and leave key and everything of holds 30 psi for 20 minutes before I unhooked it.

 

[tpenfield]

I did unhook frp vac line and cycled key and could only get it to climb to 30 psi. Even with new pump and regulator that is all I am getting. If I leave the gauge hooked up and leave key and everything of holds 30 psi for 20 minutes before I unhooked it.

What does the manual say for the pressure that the regulator is supposed to hold? Also, what pressure is the pump achieving when tested according to the manual.

I know that my MPI engine is supposed to regulate the pressure between 34 - 38 psi when the pump is activated ( engine not running). The pump is supposed to get at least the 38 psi but not over 60 psi when tested with the return line blocked off as outlined in the manual. My pump tested at 48 psi for the pump, but only 28-30 psi at the rail/regulator. so, the pump was OK, the regulator was out of spec. I got an after market regulator, and it too was out of spec. So I now have an OEM part that will be going in this weekend.

My engines are the 454's, so your specs might be a bit different.

 

[alldodge]

Suggest blocking off the return and see what pressure you can build, should see 60 psi. If you cannot get 60 then there is a leak of either fuel of air. So there are injectors leaking, regulator failing or the pump is sucking air. I'm leaning more toward injectors.

What do the plugs look like?

 

[tpenfield]

Couple more points and questions to think about . . .

Any 'codes' present on the MEFI/ECM? Is this the MEFI3 or the MEFI1 or 2 controlled engine.

Got a serial number for the engine? I think all we have is a 1997 350 Mag MPI, which narrows it down, but it is nice to confirm.

Just wondering if this is the 'VST' or the 'Cool Fuel' version of pressure pump.

Also, not clear if you have run a high concentration of fuel injector cleaner through the system.. I know "4 cans of sea foam and 100 gallons of gas", but it does not really mean much in terms of concentration. I have at times run a 50/50 mix of injector cleaner and fuel into the fuel system, about 1/2 gallon of the mix, to really clean the system and injectors.